Péroline Helbling

Fabien Hermant

Morgane Petit

Thomas Vidil

Henri Cramail

Design of plurifunctional

Keywords: Poly(hydroxyurethane), poly(cyclic carbonate), networks, curing conditions, gelation, room temperature

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The industrial synthesis of polyurethanes (PUs) is based on the polyaddition between polyisocyanates and polyols, a method developed in 1930 by Otto Bayer. Although this synthesis is rapid, quantitative and involves available and inexpensive reagents, the toxicity of isocyanates is a real threat to health and to the environment. [START_REF] Bucher | Methyl isocyanate: A review of health effects research since bhopal[END_REF] Indeed, most isocyanates used in the industry are both petro-sourced and registered as harmful chemicals by the REACH regulation. In addition, a recent amendment to this regulation, published by the European Commission, will limit the use of the registered isocyanates by the end of August 2023, if the safety measures necessary for their handling are not implemented in factories producing PUs. [START_REF]Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards diisocyanates[END_REF] Therefore, the development and optimization of alternative syntheses for isocyanate-free polyurethanes, also called NIPU for non-isocyanate polyurethane, has been the object of intense research over the past decades.

Among the different synthesis pathways of NIPU, the aminolysis of cyclic carbonates leading to polyhydroxyurethanes (PHUs) is a promising strategy to replace the isocyanate-alcohol route. Indeed, the multitude of synthesis routes to cyclic carbonates of various sizes (X-membered cyclic carbonate, noted XCC, with X varying from 5 to 8) enables the design of PHUs with various thermo-mechanical properties. Nevertheless, the synthesis of thermoplastic PHUs with molar masses adapted to the targeted applications remains an unsolved challenge, even with reactive cyclic carbonates such as 6CC or 8CC. Despite tremendous improvements over the past decade, notably thanks to the use of reactive extrusion, [START_REF] Magliozzi | Benefit of the Reactive Extrusion in the Course of Polyhydroxyurethanes Synthesis by Aminolysis of Cyclic Carbonates[END_REF] the molar masses of thermoplastic PHUs are usually too small to be competitive with isocyanate-based PUs. When it comes to the synthesis of thermosets, these materials are no longer characterized by their molar masses. Indeed, they are obtained by reacting monomers mixtures whose average functionality, fav, is strictly larger than 2, fav > 2 (Figure 1). After their gelling point, the resulting crosslinked networks can be seen as an infusible and insoluble giant macromolecule. Providing that the reaction is efficient enough, the soluble fraction (i.e. the monomers and polymeric chains that are not incorporated to the network) and the number of dangling chains will remain small, and the thermomechanical properties of the resulting network will qualify for the numerous applications of thermosetting materials. with the use of carbonated soybean oil as a cyclic carbonate precursor. 5 Since then, many authors have been interested in the study of PHU networks obtained from multifunctional 5CCs. Many biosourced substrates, in addition to vegetable oils, have also been implemented in the synthesis of thermosetting PHUs. [START_REF] Carré | From the Synthesis of Biobased Cyclic Carbonate to Polyhydroxyurethanes: A Promising Route towards Renewable Non-Isocyanate Polyurethanes[END_REF][START_REF] Nohra | From petrochemical polyurethanes to biobased polyhydroxyurethanes[END_REF] In 2015, Fortman et al. reported the first synthesis of PHU networks with vitrimeric properties, paving the way to new application perspectives for PHUs. [START_REF] Fortman | Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers[END_REF] In all these works, the resulting thermosetting PHUs exhibit low soluble fraction (typically < 5wt%), and their thermomechanical properties are competitive with those of isocyanate-based PUs, proving that the aminolysis of cyclic carbonate has a bright future in the field of thermosetting chemistry. In many respects, the recent developments of the carbonate-amine chemistry suggest that it behaves similarly to the epoxy-amine chemistry, which also fails in providing high molar masses thermoplastic polymers, and which is essentially used in the field of thermosetting materials.

For all these reasons, there is a growing demand for poly(cyclic carbonate)s and/or polyamine precursors with functionality ≥ 3 within the PHU community. This is a prerequisite to obtain carbonateamine mixture with fav > 2. If different triamines are commercially available, plurifunctional cyclocarbonates with f ≥ 3 are more uncommon. Over the past 20 years, numerous strategies have been reported for the synthesis of plurifunctional cyclocarbonates (Figure 1). Recently, there have been an increasing number of references reporting the synthesis of precursors with f ≥ 3 that are directly implemented in the synthesis of thermosetting PHUs. To the best of our knowledge, this paper provides the first overview of the synthesis strategies of plurifunctional cyclocarbonates with a functionality higher than or equal to 2, necessary for the synthesis of cross-linked PHUs. A special emphasize is accorded to the safe and eco-friendly synthetic approaches. In particular, a notable attention is devoted to the use of CO2 and to the carbonating agents that are not derived from phosgene. Subsequently, the synthesis of thermosetting PHUs under different temperature conditions as well as their potential industrial applications are described. A special emphasize is accorded to the synthesis of thermosetting PHUs at low temperature, down to room temperature. Indeed, it has long been assumed that PHUs thermosets can only be formed at high temperature (typically T > 70 °C). However, recent examples are proving the contrary, opening new avenues in the field of PHU-based coatings, adhesives, or sealants. Overall, this review provides new guidelines for both academic and industrial researchers interested in the field of PHUs thermosets.

Synthesis of multifunctional monomers

The synthesis of thermosetting PHUs requires the use of at least one trifunctional monomer, or of higher functionality, in order to obtain a monomer mixture with fav > 2, and then a three-dimensional network. While many diamines and triamines are commercially available, most commercial cyclic carbonate precursors are monofunctional and do not, as such, allow the synthesis of PHU networks. The synthesis of original multifunctional cyclic carbonates has therefore been studied and different synthetic strategies have been reported. The carbonation of polyepoxides is one of the most used methods for the synthesis of multifunctional cyclic carbonate monomers containing 5CCs (Figure 2). Indeed, polyepoxy substrates can be easily obtained by oxidation of polyunsaturated compounds or by glycidylation of polyols with epichlorohydrin.

Carbonation of polyepoxides and poly(diol)s

Carbonation of polyepoxides

In the first case, carbonated vegetable oils obtained from their epoxidized version are reported as precursors of choice for the synthesis of thermosetting PHUs and, in particular, carbonated soybean oil. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Panchireddy | Bio-based poly(hydroxyurethane) glues for metal substrates[END_REF][START_REF] Javni | Soy-Based Polyurethanes by Nonisocyanate Route[END_REF][START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF][START_REF] Hu | Biobased Reprocessable Polyhydroxyurethane Networks: Full Recovery of Crosslink Density with Three Concurrent Dynamic Chemistries[END_REF][START_REF] Poussard | Non-Isocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines to Achieve Thermosets or Thermoplastics[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: From bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF] The latter is obtained by carbonation of epoxidized soybean oil through CO2 addition in the presence of catalytic amounts of an onium salt such as tetrabutylammonium bromide or iodide (Figure 3). The reactions are generally carried out at high temperature, 110 °C ≤ T ≤ 140 °C, under high CO2 pressure, 10 bar ≤ P ≤ 100 bar and the complete conversion of epoxides into carbonates is obtained after 1 to 3 days of reaction. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF] Other carbonated oils with different functionalities, such as sunflower, [START_REF] Doley | Solvent and catalyst-free synthesis of sunflower oil based polyurethane through non-isocyanate route and its coatings properties[END_REF] linseed, [START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF] jojoba or castor oil [START_REF] Mokhtari | Reactive jojoba and castor oilsbased cyclic carbonates for biobased polyhydroxyurethanes[END_REF] are also used as carbonated precursors. Terpenes, such as limonene, can also be oxidized and then carbonated by absorption of CO2, as described by Bähr et al., [START_REF] Bähr | Cyclic limonene dicarbonate as a new monomer for nonisocyanate oligo-and polyurethanes (NIPU) based upon terpenes[END_REF] Schimpf et al. [START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF] or Maltby et al. [START_REF] Maltby | Selective Catalytic Synthesis of 1,2-and 8,9-Cyclic Limonene Carbonates as Versatile Building Blocks for Novel Hydroxyurethanes[END_REF] The conversion of epoxidized limonene into limonene dicarbonate is also complete in less than 50 h at 140 °C under 30 bar of CO2 and in the presence of 3 mol% of TBAB. [START_REF] Bähr | Cyclic limonene dicarbonate as a new monomer for nonisocyanate oligo-and polyurethanes (NIPU) based upon terpenes[END_REF] Many polyepoxide substrates are derived from the glycidylation of polyols and, in particular, the trimethylolpropane triglycidyl ether (TMPTE). It is obtained from trimethylolpropane and its carbonated form is widely used for the synthesis of PHUs. [START_REF] Lambeth | Mechanical and adhesive properties of hybrid epoxypolyhydroxyurethane network polymers[END_REF][START_REF] Monie | Divergent Aminolysis Approach for Constructing Recyclable Self-Blown Nonisocyanate Polyurethane Foams[END_REF][START_REF] Asemani | Design of hybrid nonisocyanate polyurethane coatings for advanced ambient temperature curing applications[END_REF][START_REF] Cornille | A new way of creating cellular polyurethane materials: NIPU foams[END_REF][START_REF] Liu | Carbon dioxide derived carbonized polymer dots for multicolor light-emitting diodes[END_REF][START_REF] Coste | Cascade (Dithio)carbonate Ring Opening Reactions for Self-Blowing Polyhydroxythiourethane Foams[END_REF][START_REF] Panchireddy | Reinforced poly(hydroxyurethane) thermosets as high performance adhesives for aluminum substrates[END_REF][START_REF] Pronoitis | Structurally Diverse and Recyclable Isocyanate-Free Polyurethane Networks from CO2-Derived Cyclic Carbonates[END_REF][START_REF] Gomez-Lopez | Synergetic Effect of Dopamine and Alkoxysilanes in Sustainable Non-Isocyanate Polyurethane Adhesives[END_REF] As with carbonated oils, the tricarbonate (TMPTC) is obtained via the carbonation of TMPTE by CO2 fixation in the presence of an onium salt type catalyst (Figure 3). Syntheses are performed at high temperature, 80 ≤ T ≤ 140 °C and high CO2 pressure, 10 ≤ P ≤ 80 bar, leading to quantitative conversion of epoxides into carbonates in approximately 1 day. [START_REF] Liu | Carbon dioxide derived carbonized polymer dots for multicolor light-emitting diodes[END_REF][START_REF] Coste | Cascade (Dithio)carbonate Ring Opening Reactions for Self-Blowing Polyhydroxythiourethane Foams[END_REF][START_REF] Panchireddy | Reinforced poly(hydroxyurethane) thermosets as high performance adhesives for aluminum substrates[END_REF][START_REF] Blattmann | Flexible and Bio-Based Nonisocyanate Polyurethane (NIPU) Foams[END_REF] Other polyols are also used as precursors such as poly(propylene glycol), [START_REF] Asemani | Design of hybrid nonisocyanate polyurethane coatings for advanced ambient temperature curing applications[END_REF][START_REF] Cornille | A new way of creating cellular polyurethane materials: NIPU foams[END_REF][START_REF] Cornille | Promising mechanical and adhesive properties of isocyanate-free poly(hydroxyurethane)[END_REF][START_REF] Gomez-Lopez | Monocomponent Nonisocyanate Polyurethane Adhesives Based on a Sol-Gel Process[END_REF] tri(4-hydroxyphenyl)methane [START_REF] Hu | Reprocessable polyhydroxyurethane networks reinforced with reactive polyhedral oligomeric silsesquioxanes (POSS) and exhibiting excellent elevated temperature creep resistance[END_REF][START_REF] Chen | Recyclable polymer networks containing hydroxyurethane dynamic cross-links: Tuning morphology, cross-link density, and associated properties with chain extenders[END_REF] or ethoxylated trimethylolpropane. [START_REF] Blattmann | Flexible and Bio-Based Nonisocyanate Polyurethane (NIPU) Foams[END_REF] Biosourced polyols, such as sorbitol, [START_REF] Hu | Biobased Reprocessable Polyhydroxyurethane Networks: Full Recovery of Crosslink Density with Three Concurrent Dynamic Chemistries[END_REF][START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF][START_REF] Wu | Sorbitol-based aqueous cyclic carbonate dispersion for waterborne nonisocyanate polyurethane coatings via an environment-friendly route[END_REF] glycerol, pentaerythritol, [START_REF] Fleischer | Glycerol-, pentaerythritol-and trimethylolpropane-based polyurethanes and their cellulose carbonate composites prepared via the non-isocyanate route with catalytic carbon dioxide fixation[END_REF] or derivatives of ferulic acid [START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF] can also be glycidylized and then carbonated. 4). [START_REF] Ecochard | From multi-functional siloxanebased cyclic carbonates to hybrid polyhydroxyurethane thermosets[END_REF] The resulting polyepoxides were carbonated under CO2 pressure to provide polysiloxanes with pendent cyclic carbonate functions which were used for the synthesis of hybrid thermosetting PHUs. By using polysiloxanes of variable functionality (3 < f < 6), the authors were able to modulate the thermo-mechanical properties of the PHU networks. While all polyols can be glycidylized and then carbonated by CO2 absorption, poly(1,2-diols) (also called poly(vicinal diol)s) and poly(1,3-diols) can be directly carbonated with carbonating agents such as dialkyl/diaryl carbonates, to yield plurifunctional cyclocarbonates, containing 5CCs and 6CCs respectively (Figure 5).

Carbonation of polyols

Several examples of sorbitol and mannitol carbonation have been described to obtain di-and tricarbonates. [START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF][START_REF] Furtwengler | From D-sorbitol to five-membered bis(cyclo-carbonate) as a platform molecule for the synthesis of different original biobased chemicals and polymers[END_REF][START_REF] Schmidt | Isocyanate-Free Route to Poly(carbohydrate-urethane) Thermosets and 100% Bio-Based Coatings Derived from Glycerol Feedstock[END_REF][START_REF] Mazurek-Budzyńska | Bis(cyclic carbonate) based on D-mannitol, D-sorbitol and di(trimethylolpropane) in the synthesis of non-isocyanate poly(carbonate-urethane)s[END_REF] The direct carbonation of sorbitol and mannitol using dimethyl carbonate (CDM) and K2CO3 yields only dicarbonates with relatively low yields, ~ 40 %, because of the unfavourable stereochemistry of the diols for the formation of 5CC (Figure 6). [START_REF] Furtwengler | From D-sorbitol to five-membered bis(cyclo-carbonate) as a platform molecule for the synthesis of different original biobased chemicals and polymers[END_REF][START_REF] Mazurek-Budzyńska | Bis(cyclic carbonate) based on D-mannitol, D-sorbitol and di(trimethylolpropane) in the synthesis of non-isocyanate poly(carbonate-urethane)s[END_REF] [START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF] After 19 hours of reaction and recrystallization, the tricarbonate is obtained with a yield of 77 %. Another way to obtain this tricarbonate is to dimerize acrolein, carbonate the resulting vicinal diol, then oxidize the double bonds and carbonate the epoxides (Figure 6). If the pure tricarbonate is obtained, this synthesis is tedious, with many successive synthesis and purification steps and the overall yield is strongly affected, reaching only 31 %. [START_REF] Schmidt | Isocyanate-Free Route to Poly(carbohydrate-urethane) Thermosets and 100% Bio-Based Coatings Derived from Glycerol Feedstock[END_REF] Tetraols containing two couples of vicinal diols can also be carbonated for the synthesis of cyclic dicarbonates such as diglycerol, [START_REF] Bossion | Unexpected Synthesis of Segmented Poly(hydroxyureaurethane)s from Dicyclic Carbonates and Diamines by Organocatalysis[END_REF][START_REF] Magliozzi | Hydrolyzable Biobased Polyhydroxyurethane Networks with Shape Memory Behavior at Body Temperature[END_REF][START_REF] Van Velthoven | Poly(hydroxy urethane)s based on renewable diglycerol dicarbonate[END_REF][START_REF] Younes | Study of Moisture-Curable Hybrid NIPUs Based on Glycerol with Various Diamines: Emergent Advantages of PDMS Diamines[END_REF] erythritol [START_REF] Dannecker | Facile and Sustainable Synthesis of Erythritol bis(carbonate), a Valuable Monomer for Non-Isocyanate Polyurethanes (NIPUs)[END_REF][START_REF] Salvado | Sequence-controlled polyhydroxyurethanes with tunable regioregularity obtained from biobased vicinal bis-cyclic carbonates[END_REF][START_REF] Schmidt | Erythritol Dicarbonate as Intermediate for Solvent-and Isocyanate-Free Tailoring of Bio-Based Polyhydroxyurethane Thermoplastics and Thermoplastic Elastomers[END_REF] and threitol 49 (5CC). Interestingly, polyols containing 1,3 diols can be carbonated to obtain plurifunctional cyclocarbonate, containing 6CCs. They include ditrimethylolpropane [START_REF] Mazurek-Budzyńska | Bis(cyclic carbonate) based on D-mannitol, D-sorbitol and di(trimethylolpropane) in the synthesis of non-isocyanate poly(carbonate-urethane)s[END_REF][START_REF] Matsukizono | Mono-and bifunctional six-membered cyclic carbonates synthesized by diphenyl carbonate toward networked polycarbonate films[END_REF][START_REF] Matsukizono | Reworkable Polyhydroxyurethane Films with Reversible Acetal Networks Obtained from Multifunctional Six-Membered Cyclic Carbonates[END_REF] and pentaerythritol. [START_REF] Ousaka | One-Pot Nonisocyanate Synthesis of Sequence-Controlled Poly(hydroxy urethane)s from a Bis(six-membered cyclic carbonate) and Two Different Diamines[END_REF] However, in those cases, the carbonating agent is always diphenyl carbonate or ethyl chloroformate which are both derived from phosgene in their commercial versions.

Figure 6 : Synthesis pathways to obtain sorbitol di-and tricarbonate according to Mazurek-Budzynska et al., [START_REF] Mazurek-Budzyńska | Bis(cyclic carbonate) based on D-mannitol, D-sorbitol and di(trimethylolpropane) in the synthesis of non-isocyanate poly(carbonate-urethane)s[END_REF] Furtwengler et al. [START_REF] Furtwengler | From D-sorbitol to five-membered bis(cyclo-carbonate) as a platform molecule for the synthesis of different original biobased chemicals and polymers[END_REF] and Schmidt et al. Conveniently, several monofunctional 5CCs and 6CCs are commercially available. However, as such, they cannot be used for the synthesis of thermosetting PHUs. In many instances, these monofunctional CCs contain another reactive function, whose reactivity is orthogonal to those of the CC, and which can be used in a coupling reaction with a polyfunctional backbone (Figure 7). The resulting poly(cyclic carbonate)s can then be implemented in the synthesis of PHUs thermosets. The different grafting (or coupling) strategies developed in the literature are presented hereafter with a special emphasize on the mild synthetic approaches such as click-chemistry.

Grafting of monofunctional cyclic carbonates on plurifunctional backbones

Precursor synthesis via thiol-ene reaction

The thiol-ene reaction, which mechanism is represented in Figure 8, is one of the methods to synthesize multifunctional monomers. It usually consists in reacting monocarbonates containing terminal unsaturation with multifunctional thiols via a free-radical mechanism. This technique is particularly interesting since the thiol-ene reaction is a so-called "click" chemistry reaction, leading to almost total conversions under mild reaction conditions and generating no by-products. [START_REF] Hoyle | Thiol-ene click chemistry[END_REF] Various authors have thus taken advantage of this reaction to synthesize bis-cyclic carbonates by grafting cyclic carbonates on polythiols backbones of different structures.

Thermal initiation

Besse et al. introduced the synthesis of 5CC and 6CC dicarbonates based on the thiol-ene reaction between allyl cyclic carbonate and dithiols (Figure 9). [START_REF] Besse | Access to nonisocyanate poly(thio)urethanes: A comparative study[END_REF] In the first case, the allyl glycidyl ether (AGE) was first carbonated with CO2 and the resulting allyl 5CC was grafted on dithiols. In the second case, trimethylolpropane allyl ether (TMPAE) was carbonated with ethyl chloroformate and the resulting allyl 6CC was grafted on the same dithiols. : Synthesis strategies of plurifunctional monomers through thiol-ene reaction described by Besse et al. [START_REF] Besse | Access to nonisocyanate poly(thio)urethanes: A comparative study[END_REF] and Tomita et al. [START_REF] Tomita | Polyaddition Behavior of bis (Five-and Six-Membered Cyclic Carbonate)s with Diamine[END_REF][START_REF] Tomita | Polyaddition of bis(seven-membered cyclic carbonate) with diamines: A novel and efficient synthetic method for polyhydroxyurethanes[END_REF] Tomita et al. also used the thermo-activated thiol-ene reaction for the synthesis of 5-, 6-, and 7membered cyclic dicarbonates. The corresponding alkene-terminated cyclic monocarbonates were reacted with ethanedithiol at T = 60 °C (Figure 9). [START_REF] Tomita | Polyaddition Behavior of bis (Five-and Six-Membered Cyclic Carbonate)s with Diamine[END_REF][START_REF] Tomita | Polyaddition of bis(seven-membered cyclic carbonate) with diamines: A novel and efficient synthetic method for polyhydroxyurethanes[END_REF] In their case, the synthesis yields are much lower, between 50 and 60 % in the case of 5CC and 6CC and only 31 % in the case of 7CC dicarbonate. This is likely due to the lower reactivity of regular alkene as compared to allyl during the thiol-ene reaction.

Photochemical initiation

The thiol-ene reaction triggered by photochemical activation has also been exploited for the synthesis of cyclic dicarbonates. Maisonneuve et al. reacted an alkene-terminated 6CC with butanedithiol under UV-irradiation for 4h (λ = 254 nm) at room temperature without any photoinitiator (Figure 10). [START_REF] Maisonneuve | Fatty acid-based (bis) 6membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors[END_REF] They obtained the corresponding 6CC dicarbonate with a yield of 37 %. According to the authors, the low yield is due to secondary reactions, in particular their experimental data are consistent with the ring opening of the cyclic carbonate. Recent work by Monie et al. teach us that thiols can react with cyclic carbonates by decarboxylative S-alkylation. [START_REF] Monie | Chemo-and Regioselective Additions of Nucleophiles to Cyclic Carbonates for the Preparation of Self-Blowing Non-Isocyanate Polyurethane Foams[END_REF] Thus, during the thiol-ene grafting two reactions are in competition: (i) the reaction of the thiol with the alkene, (ii) the reaction of the thiol with the carbonate. The latter is undesirable. In the case of Maisonneuve et al. study, the low yield of the synthesis is likely due to the combination of a highly reactive carbonate (6CC) and the absence of a free-radical photoinitiator thus limiting the efficiency of the thiol-ene reaction. In the end, the S-alkylation of the carbonates is more favored than the hydrothiolation of the alkenes.

Figure 10 : Synthesis strategies of plurifunctional monomers through thiol-ene reaction described by Maisonneuve et al. [START_REF] Maisonneuve | Fatty acid-based (bis) 6membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors[END_REF] , Rix et al. [START_REF] Rix | Synthesis of fatty acid-based non-isocyanate polyurethanes, NIPUs, in bulk and mini-emulsion[END_REF] and Lamarzelle et al. [START_REF] Lamarzelle | A thioglycerol route to bio-based bis-cyclic carbonates: Poly(hydroxyurethane) preparation and postfunctionalization[END_REF] Rix et al. also synthesized a bis-5 membered cyclic carbonate by thiol-ene reaction via a photochemical activation (Figure 10). [START_REF] Rix | Synthesis of fatty acid-based non-isocyanate polyurethanes, NIPUs, in bulk and mini-emulsion[END_REF] The mixtures of carbonate, dithiol and DMPA, used here as a photo-initiator, were irradiated at a wavelength λ = 365 nm at room temperature. The dicarbonate is obtained with a yield of 100 % after 6 h of irradiation. In this case, the quantitative yield indicates that there are no secondary reactions. Indeed, the decarboxylative S-alkylation of the 5CC is very unlikely in the absence of a base. Moreover, the use of DMPA as a photoinitiator results in a dramatic increase of the thiol-ene kinetics. Overall, in this case, the hydrothiolation of the alkenes is much more favored than the decarboxylation of the cyclic carbonates.

Alternatively, the synthesis of a difunctional vicinal diol from thioglycerol and a fatty acid-derived diene in the presence of DMPA has been described by Lamarzelle et al. (Figure 10). [START_REF] Lamarzelle | A thioglycerol route to bio-based bis-cyclic carbonates: Poly(hydroxyurethane) preparation and postfunctionalization[END_REF] Complete conversion of the double bond is achieved after 1 hour of reaction at room temperature (λ = 365 nm). The corresponding cyclic dicarbonate is then synthesized using dimethyl carbonate. In this example, the thiol-ene reaction appears as a tool for synthesizing a vicinal polyol which is further used in a synthetic strategy that falls in the scope of the paragraph 2.1.2 (i.e. carbonation of poly(vicinal diol)).

In the end, the thiol-ene reaction appears as a versatile tool for the synthesis of plurifunctional cyclocarbonates in mild conditions. This is well illustrated by its recurrent occurrence in the patent literature related to PHU synthesis. [65][66][67] Plurifunctional cyclocarbonates with functionnality up to f =4 were synthesized which can be easily implemented in the synthesis of PHU thermosets.

Precursors synthesis via esterification and amidation

Thanks to esterification and amidation, it is possible to couple mono-and multi-functional carboxylic acids to alcohols and amines respectively. These reactions are therefore particularly used for the synthesis of dicarbonates from biosourced fatty acids and their derivatives. For example, Carré et al. described the synthesis of cyclic dicarbonates by esterification of sebacic acid or fatty acid dimers with glycerol carbonate (Figure 11). [START_REF] Carré | Solvent-and catalyst-free synthesis of fully biobased nonisocyanate polyurethanes with different macromolecular architectures[END_REF] They used thionyl chloride to activate the carboxylic functions in the form of acyl chlorides. Yields above 99 % demonstrate the efficiency of this approach. Similarly, Rix et al. synthesized cyclic dicarbonates from glycerol carbonate and Pripol 1009 or one of its derivatives. [START_REF] Rix | Synthesis of fatty acid-based non-isocyanate polyurethanes, NIPUs, in bulk and mini-emulsion[END_REF] The synthesis yields are very high, ≥ 90 %, highlighting once again the good performances of this synthetic pathway.

Tryznowski et al. also applied this technique to synthesize a 6-membered cyclic dicarbonate from sebacoyl chloride and a 6CC bearing a hydroxyl group (Figure 11). [START_REF] Tryznowski | Synthesis, characterization and reactivity of a six-membered cyclic glycerol carbonate bearing a free hydroxyl group[END_REF] The 6CC dicarbonate was isolated with a 48 % yield. In this case, the efficiency of the reaction is limited by the isomerization of 6CC into 5CC via an intramolecular transcarbonation reaction. More generally speaking, the basic conditions necessary for the neutralization of the acid chloride released during the coupling reaction, can be problematic in the presence of 6-membered cyclic carbonates, less stable than their 5-membered counterparts. Therefore, this synthetic pathway seems less suitable for the synthesis of pluri-6CC.

Maisonneuve et al. presented the synthesis of a difunctional diene thanks to the reaction of 1,4butanediamine with methyl undecylenate (Figure 11). [START_REF] Maisonneuve | Novel green fatty acid-based bis-cyclic carbonates for the synthesis of isocyanatefree poly(hydroxyurethane amide)s[END_REF] This new difunctional molecule is epoxidized and then carbonated, leading to a cyclic dicarbonate. This example demonstrates that an amidation reaction can be a useful tool to synthesize a polyepoxide and return to a more conventional strategy based on CO2-carbonation such as that mentioned in paragraph 2.1.1. Different diamines are also used by the authors in order to modify the structure of the monomer. The syntheses were made on several grams with yields ≥ 44 % after the three synthetic steps.

Figure 11 : Synthesis strategies of plurifunctional monomers through esterification and amidation described by Carré et al. [START_REF] Carré | Solvent-and catalyst-free synthesis of fully biobased nonisocyanate polyurethanes with different macromolecular architectures[END_REF] , Tryznowski et al. [START_REF] Tryznowski | Synthesis, characterization and reactivity of a six-membered cyclic glycerol carbonate bearing a free hydroxyl group[END_REF] and Maisonneuve et al. [START_REF] Maisonneuve | Novel green fatty acid-based bis-cyclic carbonates for the synthesis of isocyanatefree poly(hydroxyurethane amide)s[END_REF] Mokhtari et al. also took advantage of this strategy to synthesize a precursor from jojoba and castor oils with a functionality f ≥ 2. [START_REF] Mokhtari | Reactive jojoba and castor oilsbased cyclic carbonates for biobased polyhydroxyurethanes[END_REF] First, thioglycolic acid is grafted onto the unsaturations of the triglycerides via thiol-ene reaction. Then, the resulting poly(carboxylic acid)s are transformed into plurifunctional cyclocarbonates through an esterification reaction with glycerol carbonate. Thanks to this two-step synthesis, it is possible to obtain vegetable oils containing terminal cyclic carbonate, which are much more reactive than the pluri-5CC obtained through the CO2-carbonation of epoxidized vegetable oils (c.f. paragraph 2.1.1).

In addition to being an efficient synthetic route to obtain multifunctional monomers, the electronwithdrawing effect of the ester and amide functions on cyclic carbonates contributes to a significant increase of their reactivity, as it has been demonstrated during model reactions by Cornille et al. [START_REF] Cornille | A study of cyclic carbonate aminolysis at room temperature: Effect of cyclic carbonate structures and solvents on polyhydroxyurethane synthesis[END_REF] and Lamarzelle et al. [START_REF] Lamarzelle | Activated lipidic cyclic carbonates for non-isocyanate polyurethane synthesis[END_REF] The degradability of the polymers can also be improved by the presence of the hydrolysable ester functions.

To conclude, esterification and amidation are very efficient strategies for the synthesis of pluricyclic carbonates. They have been implemented for the synthesis of precursors of high functionality (> 3), essentially in the case of 5CC. Unfortunately, the syntheses usually rely on the use of harmful, and potentially explosive, chemicals such as acyl chloride and SOCl2. The future studies using these synthetic approaches will have to investigate milder coupling conditions to make it more suitable with the latest safety and environmental regulations.

Precursor synthesis via metathesis

The metathesis reaction between two carbonates containing a terminal double bond can be used to synthesize dicarbonates. Maisonneuve et al. described the synthesis of a 6CC biscarbonate by metathesis reaction of undecene-6CC (Figure 12). [START_REF] Maisonneuve | Fatty acid-based (bis) 6membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors[END_REF] The reaction is carried out at room temperature in anhydrous pentane with a third generation Grubbs catalyst. After 4 h of reaction and purification by column chromatography, the reaction yield reaches 50 %. Lamarzelle et al. also reported the synthesis of 5CC biscarbonates, activated by ether and ester groups (Figure 12). [START_REF] Lamarzelle | Activated lipidic cyclic carbonates for non-isocyanate polyurethane synthesis[END_REF] In this case, the reactions carried out at room temperature with a Grubbs I type catalyst lead to complete conversions of double bonds, and a yield of 53 % after purification.

If this reaction may lead to quantitative conversion of monocarbonates into dicarbonates, the limited yields (~ 50 %) indicate that the purification steps necessary to eliminate the Grubbs catalysts result in a substantial loss of product. In addition, the inert synthesis conditions and the high cost of the catalysts can be an obstacle to the use of this technique in an industrial context. Finally, it should be noted that this method essentially leads to symmetrical dicarbonates. It is not possible to consider the synthesis of poly(cyclic carbonate) with a functionality ≥ 3, i.e. the precursors that are the most adapted to the synthesis of thermosetting PHUs.

(Co)polymerisation of monocyclic carbonate

Mono-cyclic carbonate containing another function of orthogonal reactivity (e.g. alcohol, CC double bond) are classically used for the synthesis of pluri-cyclic carbonate via a grafting approach, as demonstrated in the previous section. However, they were essentially coupled to difunctional backbones, thus providing pluri-cyclic carbonate with a functionality of f = 2. When the function of orthogonal reactivity can be engaged in a (co)polymerization reaction, it is possible to synthesize oligomers and polymers with dangling carbonate functions along their chains (Figure 13). This is especially the case with mono-cyclic carbonates containing radically polymerizable functions such as (meth)acrylate or allyl. The resulting oligomers and polymers are pluri-cyclic carbonates with f ≫ 3, that can be straightforwardly implemented in the synthesis of thermosetting PHUs. For instance, Bourguignon et al. synthesized water-soluble polymers carrying cyclic carbonate functions through radical copolymerization of glycerol carbonated methacrylate with various comonomers (Figure 14). [START_REF] Bourguignon | Water-Borne Isocyanate-Free Polyurethane Hydrogels with Adaptable Functionality and Behavior[END_REF] These highly functional monomers have been used to synthesize networks in water at room temperature, with gel times between 1 and 35 min depending on the pH. These results are particularly interesting considering that PHU syntheses usually take place at high temperature (T ≥ 70°C) for several hours.

Scholten et al. have copolymerized two cyclic carbonates having an allylic and α-alkylidene double bond, respectively, with ethylene (Figure 14). [START_REF] Scholten | Functional Polyethylenes by Organometallic-Mediated Radical Polymerization of Biobased Carbonates[END_REF] Depending on the reaction parameters of the radical copolymerization (e.g. type of initiator, ethylene pressure), polymers with variable molar masses have been synthesized. The resulting pluri-5CC present different functionalities, > 3, and can be used to obtain PHU networks after reaction with plurifunctional amines.

Summary and outlooks

It is possible to consider numerous substrates, in particular biosourced, for the synthesis of thermosetting PHUs thanks to the different strategies yielding multifunctional cyclic carbonate. If the carbonation of polyepoxides and polyols is the most common technique, the other synthetic methods, i.e. grafting and (co)polymerization of cyclic monocarbonates, are particularly interesting for the synthesis of multifunctional monomers with terminal cyclic carbonate functions, which are more reactive than the internal ones.

In addition, all these synthetic strategies can be exploited to create highly reactive monomers, i.e. 6 (or more)-membered and/or chemically activated cyclic carbonates. The diversity of monomer structures obtained through these methods can also improve the compatibilization with certain diamines, thus favoring the reactions according to mass processes, and possibly at moderate temperature.

Eventually, it is noteworthy that epoxides and polyols are the elementary bricks behind all the pluricyclic carbonates, independently of the synthetic strategy. Advantageously, epoxides are very easily converted into 5CC under CO2 pressure, the latter being a renewable resource. However, in many cases, they are derivatives of epichlorohydrin, which is known for being a highly toxic reagent.

Polyols precursors are safer surrogates. Conveniently, the vicinal diols and the 1,3-diols, necessary to synthesize 5CC and 6CC respectively, can be bio-sourced. Glycerol, erythritol or serinol are a few representative examples. However, they have long been carbonated with toxic phosgene derivatives including diphenyl carbonate or ethyl chloroformate. Dialkyl carbonate (e.g. dimethyl carbonate) are innocuous alternatives which are increasingly used in the literature. [START_REF] Dannecker | Facile and Sustainable Synthesis of Erythritol bis(carbonate), a Valuable Monomer for Non-Isocyanate Polyurethanes (NIPUs)[END_REF][START_REF] Salvado | Sequence-controlled polyhydroxyurethanes with tunable regioregularity obtained from biobased vicinal bis-cyclic carbonates[END_REF] Despite several reports regarding the CO2 carbonation of vicinal [START_REF] Linsalata | Application of pervaporation membranes to the direct carboxylation of ethene glycol using CeO2-based catalysts-Comparison of the batch reaction to a flow reaction in SC-CO2[END_REF][START_REF] Tomishige | Reaction of CO2 With Alcohols to Linear-, Cyclic-, and Poly-Carbonates Using CeO2-Based Catalysts[END_REF][START_REF] Mcguire | Synthesis of 5-to 8membered cyclic carbonates from diols and CO2: A one-step, atmospheric pressure and ambient temperature procedure[END_REF] and 1,3 diols , [START_REF] Honda | Direct cyclic carbonate synthesis from CO2 and diol over carboxylation/hydration cascade catalyst of CeO2 with 2-cyanopyridine[END_REF][START_REF] Brege | En route to CO2based cyclic carbonates and polycarbonates from alcohols substrates by dirct and indirect approaches[END_REF][START_REF] Hedrick | Simple and Efficient Synthesis of Functionalized Cyclic Carbonate Monomers Using Carbon Dioxide[END_REF][START_REF] Gregory | Polymers from sugars and CO2: Synthesis and polymerization of a d-mannose-based cyclic carbonate[END_REF][START_REF] Gregory | Synthesis of 6-membered cyclic carbonates from 1,3-diols and low CO2 pressure: A novel mild strategy to replace phosgene reagents[END_REF][START_REF] Brege | The coupling of CO2 with diols promoted by organic dual systems: Towards products divergence via benchmarking of the performance metrics[END_REF] very few works have been using these strategies to synthesize PHUs precursors so far. [START_REF] Liu | Chemically Recyclable Biobased Non-Isocyanate Polyurethane Networks from CO2-Derived Six-membered Cyclic Carbonates[END_REF][START_REF] Miao | Mechanically Robust and Chemically Recyclable Polyhydroxyurethanes from CO 2 -Derived Six-Membered Cyclic Carbonates[END_REF] In this context, the next generation of greener PHUs thermosets will rely on the latest progress regarding the CO2 carbonation of biosourced polyols.

Synthesis of thermosetting PHUs : high-versus low-temperature curable systems

Preamble

Historically, most thermosetting networks are obtained at high temperature for several reasons. 91 First of all, to obtain a 3D network, it is necessary to succeed in mixing the precursors homogeneously and, in the case of solid and/or viscous and/or incompatible precursors, this generally requires an input of thermal energy. Moreover, from a thermodynamic point of view, if the activation energy Ea of the elementary reaction is too high, then thermal energy is mandatory to accelerate its kinetics. Finally, working at high temperature is necessary to overcome the limits induced by the physico-chemical transformations of the growing network. For example, vitrification results in a drastic reduction of the molecular mobility and, in most cases, in the interruption of the reaction. It is then necessary to increase the reaction temperature above the Tg of the growing network to pursue the polymerization. The relationships between temperature and the physico-chemical transformations of the growing network over time (e.g. gelation, vitrification) is fundamental in thermosetting technology and it is often rationalized using TTT diagrams, for Time -Temperature -Transformation. This type of dynamic phase diagram, introduced by Gillham et al., is a useful tool to distinguish the limits of the different phases of the growing thermosetting network -viscous or vitreous liquid and rubbery or vitreous gel -as a function of time and temperature. [START_REF] Enns | Time-temperature-transformation (TTT) cure diagram: Modeling the cure behavior of thermosets[END_REF] Yet, for highly exothermic crosslinking reactions, the final Tg of the network can be higher than the setpoint temperature of the crosslinking reaction as, in this case, the actual temperature of the system can rise well above the setpoint temperature.

Considering these basic knowledges about thermosetting networks, the ideal room temperature curable system would be composed of liquid precursors, compatible in any proportion and reacting according to a low activation energy reaction. Moreover, the Tg of the final network would be lower than room temperature. If all these criteria cannot be met, there are methods to bypass the various abovementioned obstacles:

• The use of a solvent to mix solid and/or incompatible precursors. The solvent can also plasticize the growing network during the crosslinking reaction and therefore facilitate species mobility, or even prevent the vitrification of the network. However, a drying step at the end of the crosslinking will be necessary to get rid of the solvent, potentially at high temperature.

• The use of a catalyst to lower the activation energy Ea of the elementary crosslinking reaction.

• The use of an exothermic system to ensure that the temperature of the system is always higher than its theoretical glass transition temperature, even if the setpoint temperature is not.

With the first examples described in the early 2000s, the literature concerning thermosetting PHUs is still scarce compared to that of epoxy-amine or isocyanate-alcohol systems. Interestingly, the 5CCamine reaction has often been described as being too slow and inefficient to synthesize PHUs in bulk at room temperature. [START_REF] Tomita | Polyaddition Behavior of bis (Five-and Six-Membered Cyclic Carbonate)s with Diamine[END_REF][START_REF] Cornille | A study of cyclic carbonate aminolysis at room temperature: Effect of cyclic carbonate structures and solvents on polyhydroxyurethane synthesis[END_REF][START_REF] Lamarzelle | Activated lipidic cyclic carbonates for non-isocyanate polyurethane synthesis[END_REF][START_REF] Maisonneuve | Isocyanate-Free Routes to Polyurethanes and Poly(hydroxy Urethane)s[END_REF][START_REF] Tomita | Reactivity comparison of five-and six-membered cyclic carbonates with amines: basic evaluation for synthesis of poly(hydroxyurethane)[END_REF][START_REF] Yuen | Room temperature synthesis of non-isocyanate polyurethanes (NIPUs) using highly reactive N-substituted 8-membered cyclic carbonates[END_REF] Thus, syntheses of thermosetting PHUs are generally carried out at high temperature, i.e. T ≥ 70°C, in order to promote the homogenization of the monomers and the carbonate-amine reaction. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Panchireddy | Bio-based poly(hydroxyurethane) glues for metal substrates[END_REF][START_REF] Javni | Soy-Based Polyurethanes by Nonisocyanate Route[END_REF][START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF][START_REF] Hu | Biobased Reprocessable Polyhydroxyurethane Networks: Full Recovery of Crosslink Density with Three Concurrent Dynamic Chemistries[END_REF][START_REF] Poussard | Non-Isocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines to Achieve Thermosets or Thermoplastics[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: From bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF][START_REF] Doley | Solvent and catalyst-free synthesis of sunflower oil based polyurethane through non-isocyanate route and its coatings properties[END_REF][START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF][START_REF] Monie | Divergent Aminolysis Approach for Constructing Recyclable Self-Blown Nonisocyanate Polyurethane Foams[END_REF][START_REF] Cornille | A new way of creating cellular polyurethane materials: NIPU foams[END_REF][START_REF] Liu | Carbon dioxide derived carbonized polymer dots for multicolor light-emitting diodes[END_REF][START_REF] Coste | Cascade (Dithio)carbonate Ring Opening Reactions for Self-Blowing Polyhydroxythiourethane Foams[END_REF][START_REF] Panchireddy | Reinforced poly(hydroxyurethane) thermosets as high performance adhesives for aluminum substrates[END_REF][START_REF] Pronoitis | Structurally Diverse and Recyclable Isocyanate-Free Polyurethane Networks from CO2-Derived Cyclic Carbonates[END_REF][START_REF] Blattmann | Flexible and Bio-Based Nonisocyanate Polyurethane (NIPU) Foams[END_REF][START_REF] Cornille | Promising mechanical and adhesive properties of isocyanate-free poly(hydroxyurethane)[END_REF][START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF][START_REF] Fleischer | Glycerol-, pentaerythritol-and trimethylolpropane-based polyurethanes and their cellulose carbonate composites prepared via the non-isocyanate route with catalytic carbon dioxide fixation[END_REF][START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF][START_REF] Magliozzi | Hydrolyzable Biobased Polyhydroxyurethane Networks with Shape Memory Behavior at Body Temperature[END_REF][START_REF] Carré | Solvent-and catalyst-free synthesis of fully biobased nonisocyanate polyurethanes with different macromolecular architectures[END_REF][START_REF] Carré | Original biobased nonisocyanate polyurethanes: Solventand catalyst-free synthesis, thermal properties and rheological behaviour[END_REF][START_REF] Chen | Reprocessable polyhydroxyurethane networks exhibiting full property recovery and concurrent associative and dissociative dynamic chemistry: Via transcarbamoylation and reversible cyclic carbonate aminolysis[END_REF][START_REF] Świderska | Energy dissipating poly(hydroxyurethane) elastomers -Synthesis, characterization and comparison with shear thickening fluid materials[END_REF][START_REF] Dong | Assessment of Plant and Microalgal Oil-Derived Nonisocyanate Polyurethane Products for Potential Commercialization[END_REF][100][START_REF] Yang | Preparation of non-isocyanate polyurethanes from epoxy soybean oil: Dual dynamic networks to realize self-healing and reprocessing under mild conditions[END_REF][102][103] However, the following part of this literature review shows that an increasing number of references are reporting the synthesis of PHU networks at low temperatures, using several of the levers previously mentioned. Arbitrarily, the limit to discriminate the high-from the low-temperature curable systems was set at 70 °C. Historically, the first PHUs thermosets were crosslinked at high temperatures. This is for two main reasons: (i) authors were using highly viscous or solid pluricyclic carbonates and (ii) the carbonates were not reactive enough in mild thermal conditions (Figure 15). Very quickly, it was realized that these curing conditions can be detrimental due to the initiation of many undesired side reactions that provoke changes in the polymer structure, e.g. by favoring the formation of urea linkages. However, high curing temperatures are also essential when high Tg networks are targeted (or at least superambient Tg). The next section introduces the examples of the literature where crosslinked PHUs are obtained at high temperature. A special emphasize is dedicated to the reasons why a large input of thermal energy is needed, and to the strategies to limit the undesirable impacts of side reactions that come along.

Synthesis of thermosetting PHUs at high temperature, T ≥ 70 °C

Solid and/or highly viscous precursors

In many cases, the synthesis of PHUs thermosets involves one or several solid precursors. A thermal energy input is then necessary to ensure their homogeneous mixing. Moreover, the resulting materials are usually high Tg networks, and elevated crosslinking temperatures are also required to avoid the vitrification during the crosslinking reaction. Thus, the crosslinking scheme of these materials usually relies on two steps: (i) one step at moderately elevated temperature (T ~ 70-80 °C) to ensure the mixing of the precursors, (ii) a second step at higher temperature (T > 100 °C) to ensure the completion of the crosslinking reaction. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Panchireddy | Bio-based poly(hydroxyurethane) glues for metal substrates[END_REF][START_REF] Javni | Soy-Based Polyurethanes by Nonisocyanate Route[END_REF][START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF][START_REF] Doley | Solvent and catalyst-free synthesis of sunflower oil based polyurethane through non-isocyanate route and its coatings properties[END_REF][START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF][START_REF] Cornille | A new way of creating cellular polyurethane materials: NIPU foams[END_REF][START_REF] Panchireddy | Reinforced poly(hydroxyurethane) thermosets as high performance adhesives for aluminum substrates[END_REF][START_REF] Cornille | Promising mechanical and adhesive properties of isocyanate-free poly(hydroxyurethane)[END_REF][START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF][START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF][START_REF] Ecochard | From multi-functional siloxanebased cyclic carbonates to hybrid polyhydroxyurethane thermosets[END_REF][START_REF] Dong | Assessment of Plant and Microalgal Oil-Derived Nonisocyanate Polyurethane Products for Potential Commercialization[END_REF] Different strategies are implemented to guarantee the efficient mixing of the precursors during the first step. Here, they are classified in three different sections:

Conventional mechanical stirring in bulk

In many instances, the simplest strategy to ensure the efficient mixing of a solid pluri-cyclic carbonate with a polyamine at high temperature is conventional mechanical stirring.

A representative example is sorbitol tricarbonate (STC, Figure 16). Indeed, it is a crystalline compound with a very high melting temperature (Tm ~ 210 °C). In order to valorize STC, Schmidt et al. mixed it with a diamine according to two different procedures. In a first case, they grind STC at room temperature with the chosen diamine, for example Priamine 1074 or IPDA (both liquid), in order to obtain a liquid prepolymers. The latter is then degassed at 60 °C and the crosslinking reaction is completed at 80 °C for 16 h. [START_REF] Schmidt | Isocyanate-Free Route to Poly(carbohydrate-urethane) Thermosets and 100% Bio-Based Coatings Derived from Glycerol Feedstock[END_REF] During a second study, a mixture of STC and sorbitol ether carbonate (a liquid pluri-cyclic carbonate) is heated beforehand to 100 °C in order to obtain a liquid mixture and thus facilitate the homogenization with the diamine. The material is crosslinked at 80 °C for 16 h and then at 100 °C for 4 h. [START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF] Another biosourced carbonate that requires working at high temperature is limonene dicarbonate, (Figure 16). For example, Schimpf et al. dissolved the grinded dicarbonate in pentaerythritol tetracarbonate (a liquid pluri-cyclic carbonate) at 130 °C. [START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF] Pentanediamine (5DA) is then added to the mixture of carbonates at 100 °C using a mechanical stirrer and the whole is crosslinked at 80 °C for 16 h then at 100 °C for 4 h.

Ménard et al. described the synthesis of thermosetting PHU from carbonated derivatives of ferulic acid (Figure 16) which are viscous liquids. [START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF] The preheating of the cyclic carbonates between 60 and 70 °C is required to lower their viscosity in order to homogeneously mix them with the chosen diamine. The authors indicate that the temperature of the system must be sustained until the beginning of the crosslinking procedure to avoid both vitrification and phase separation. The system is crosslinked in two stages, 5 h at 80 °C and 10 h at 100 °C, these two temperatures being respectively lower and higher than the temperature of the maximum of the exothermicity of the crosslinking reaction measured through DSC analysis.

Carbonated vegetable oils are also highly viscous and they usually need to be preheated to ensure their homogeneous mixing with diamines. For example, Tamami et al. reported that carbonated soybean oil (CSBO), whose structure is shown in Figure 16, must be preheated at 60 °C to be mixed with ethylenediamine (2DA) or hexanediamine (6DA). [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF] The authors specify that, without this preheating step, homogenization with the diamines is not possible, even if the latter are liquid. Panchireddy et al. also reported the need to degas the carbonated soybean oil at 60 °C and then to mix it with the diamine at 50 °C for 3 min at 500 rpm to obtain a homogeneous reaction mixture. 11 Use of solvents as diluent and plasticizer

The dissolution of the reactants in a minimum of solvent can also be a solution to ensure their homogeneous mixing, the solvent then being evaporated during the polymerization. This technique was implemented by Chen et al. who pre-dissolved all of the PHU thermoset precursors at room temperature, i.e. cyclic carbonates such as tri(4-hydroxyphenyl)methane tricarbonate (a solid carbonate), amines such as mxDA and the catalyst DMAP, in THF or DMF. [START_REF] Chen | Recyclable polymer networks containing hydroxyurethane dynamic cross-links: Tuning morphology, cross-link density, and associated properties with chain extenders[END_REF] In these examples, the solvent represent 50 wt% to 80 wt% of the total mixture. The mixture is then heated at 80 °C for 12 h under a flow of argon in order to partially remove the solvent while synthesizing the polymer. The gel obtained at the end of this step is then placed under vacuum at 80 °C for 48 h to evaporate the last traces of solvent. This technique facilitates the mixing of the reagents at room temperature, thus limiting the loss of diamine, which is generally volatile at high temperature. Hu et al. have also applied this technique for the synthesis of PHU networks from sorbitol ether carbonate. [START_REF] Hu | Biobased Reprocessable Polyhydroxyurethane Networks: Full Recovery of Crosslink Density with Three Concurrent Dynamic Chemistries[END_REF] DMF is added to the tricarbonate-diamine mixture in order to reduce its viscosity and to ensure its homogeneity. In this example, the solvent represents approximately 60 wt% of the total mixture. After 24 h of crosslinking at 80 °C, the resulting gel is placed under vacuum for 24 h at 80 °C in order to evaporate the remaining DMF. Recently, Liu et al. reported the synthesis of crosslinked PHUs from TMPTC and 2DA via a solvothermal route. [START_REF] Liu | Carbon dioxide derived carbonized polymer dots for multicolor light-emitting diodes[END_REF] The crosslinking takes place in an autoclave at 230 °C in ethanol for 12 h and the latter is then distilled under vacuum.

Despite its effectiveness, this technique is not relevant from an industrial point of view, because it relies on the use of potentially flammable and/or toxic solvents. Moreover, the additional drying step results in a more complicated curing process.

Kneader reactor or twin-screw extruder

In some cases, the homogeneous mixing of the PHU precursors can remain complicated due to the viscosity of the precursors, even at high temperature, or the insolubility of the monomers in usual organic solvents. Kneader reactors or twin-screw extruders can be alternative solutions to facilitate the melt processability of the mixtures.

Poussard et al. as well as Grignard et al. used a kneader reactor to mix a viscous carbonated soybean oil with a solid amino end-terminated oligoamide (melting point ~ 65 °C). [START_REF] Poussard | Non-Isocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines to Achieve Thermosets or Thermoplastics[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: From bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF] The later was first introduced in the kneader. When molten, the carbonated soybean oil was added, and the reaction was conducted for 3 h at 120 °C (rotating speed 30 rpm). This process facilitates the homogeneization of the systems, which is initially extremely difficult, and therefore offer to use new formulations for the synthesis of PHU.

More recently, Magliozzi et al. described the synthesis of high molar mass linear PHUs by reactive extrusion of diglycerol dicarbonate (DGDC), a solid dicarbonate derived from diglycerol, with different diamines. [START_REF] Magliozzi | Benefit of the Reactive Extrusion in the Course of Polyhydroxyurethanes Synthesis by Aminolysis of Cyclic Carbonates[END_REF] The same approach was used for the synthesis of thermoset PHUs. [START_REF] Magliozzi | Hydrolyzable Biobased Polyhydroxyurethane Networks with Shape Memory Behavior at Body Temperature[END_REF] To do so, the extruder is preheated at 90 °C and the DGDC is introduced first in order to melt it. Next, tris(2-aminoethyl)amine, a trifunctional amine, is introduced in a stoichiometric amount. The polymerization in the extruder is stopped after 5 min because the machine is no longer able to mix the reaction medium due to gelation. The crosslinked polymer is then removed from the circulating channel of the extruder and post-cured at 90 °C for 3 h to reach the complete conversion of the carbonate functions. The resulting polymer is a densely crosslinked network with a swelling ratio in THF of 15 % and a gel fraction of 94 %. Thanks to this extrusion process, the implementation of mass polymerization was greatly facilitated and it was possible to synthesize a highly crosslinked and homogeneous network in a short period of time at a rather low temperature (90 °C). The extent of side reactions was greatly limited in these conditions.

It is important to note that, in these examples, it is necessary to finely tune the kinetics of the reaction if one wants to inject the growing network in a mold before the gelling point, and thus obtain well defined final shapes.

Low-reactivity precursors

Many thermosetting PHUs syntheses must be carried out at high temperature because the carbonated precursors are not reactive enough at T < 70 °C. This is especially the case with carbonated vegetable oils. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Javni | Soy-Based Polyurethanes by Nonisocyanate Route[END_REF][START_REF] Bähr | Linseed and soybean oil-based polyurethanes prepared via the non-isocyanate route and catalytic carbon dioxide conversion[END_REF][START_REF] Hu | Biobased Reprocessable Polyhydroxyurethane Networks: Full Recovery of Crosslink Density with Three Concurrent Dynamic Chemistries[END_REF][START_REF] Poussard | Non-Isocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines to Achieve Thermosets or Thermoplastics[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: From bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF][START_REF] Doley | Solvent and catalyst-free synthesis of sunflower oil based polyurethane through non-isocyanate route and its coatings properties[END_REF][START_REF] Dong | Assessment of Plant and Microalgal Oil-Derived Nonisocyanate Polyurethane Products for Potential Commercialization[END_REF][START_REF] Yang | Preparation of non-isocyanate polyurethanes from epoxy soybean oil: Dual dynamic networks to realize self-healing and reprocessing under mild conditions[END_REF] Although these oils are widely used for the synthesis of PHUs, they are poorly reactive because of the internal position of the cyclic carbonates.

For example, carbonated soybean oil (CSBO) has been extensively used for the synthesis of cross-linked biobased PHUs. In the first publication reporting the synthesis of thermosetting PHUs, Tamami et al. synthesized CSBO by CO2-carbonation of epoxidized soybean oil (ESBO) and obtain crosslinked PHUs by reacting CSBO with diamines (e.g. ethylenediamine or hexamethylenediamine). [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF] After preheating CSBO at 60 °C to lower its viscosity and adding the diamine, the mixture is cured in two stages: 10 h at 70 °C and 3 h at 100 °C. These two high-temperature crosslinking steps are necessary to ensure the complete conversion of cyclic carbonates and to obtain highly crosslinked networks, with gel fractions varying between 89 and 94 %.

Since then, CSBO has been used in a lot of studies for the synthesis of crosslinked PHUs. They all involve crosslinking schemes of several hours in two stages at T ≥ 70 °C11,13 or in a single stage at higher temperature, T =120 °C, in a kneader reactor. [START_REF] Poussard | Non-Isocyanate Polyurethanes from Carbonated Soybean Oil Using Monomeric or Oligomeric Diamines to Achieve Thermosets or Thermoplastics[END_REF][START_REF] Grignard | CO2-blown microcellular non-isocyanate polyurethane (NIPU) foams: From bio-and CO2-sourced monomers to potentially thermal insulating materials[END_REF] In the latter case, the kinetics of reaction remain long, ~ 3 h, when compared to precursors containing terminal carbonates. For instance, in the case of trimethylolpropane triglycidyl carbonate (TMPTC), the carbonates are fully converted after 10 min at 70 °C. [START_REF] Fleischer | Glycerol-, pentaerythritol-and trimethylolpropane-based polyurethanes and their cellulose carbonate composites prepared via the non-isocyanate route with catalytic carbon dioxide fixation[END_REF] Thus, it seems difficult to work with vegetable oils at low temperature for both processing and reactivity issues.

However, at too high temperatures, side reactions can take place, limiting the extent of the crosslinking reaction. Javni et al. investigated the impact of these side reactions. [START_REF] Javni | Soy-Based Polyurethanes by Nonisocyanate Route[END_REF] The authors worked on the synthesis of crosslinked PHUs from CSBO and aliphatic diamines following the same crosslinking protocol as Tamami et al. [START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF] They observed that the networks are well crosslinked, with soluble fractions between 6 and 11 %. However, the analysis of the latter reveals the presence of amide and glycerol functions, resulting from the aminolysis of the esters. This secondary reaction was clearly highlighted by infrared spectroscopic monitoring of the crosslinking reaction (Figure 17) but also by model reactions with unmodified soybean oil. The results indicate that, even at T ≤ 100 °C, the esters of vegetable oils are sensitive to aminolysis. Thus, in the case of PHU synthesis from carbonated vegetable oil, the selectivity of the crosslinking reaction is limited due to the initiation of the amidation side reaction at temperatures as low as 70 °C. A compromise in terms of temperature must then be found in order to ensure both the homogenization of the reagents and the aminolysis of the carbonates while limiting that of the esters. Recently, Helbling et al. further investigated the relative reactivity of carbonates and esters, but also of epoxides, by studying the crosslinking reaction of a partially carbonated ESBO with different diamines. 104 A dual-spectroscopic monitoring (infrared and Raman) indicates that cyclic carbonates are more reactive than epoxides, the latter being more reactive than esters. Based on this comprehensive knowledge of the functions reactivity, the authors demonstrate that the extent of ester aminolysis can be minimized by using a two-stage curing schedule with a first step at 80 °C for 1 h and a second step at 120 °C for 5 h. Figure 18a represents the conversion of the different moieties as a function of time for the two-stage curing schedule while Figure 18b represents the same conversion profiles for a onestep curing schedule at 120 °C for 6 hours. Clearly, the final ester conversion is of about 20 % in the first case against 30 % in the latter case. Furthermore, the gel content of the resulting thermosets decreases from 85 % for the two-stage curing schedule to 80 % for the single stage approach. This curing methodology appeared as a good technique to limit the side reactions by favoring first the reaction cyclic carbonates with amines at 80 °C, and then the reaction of epoxides at 120 °C.

Super-ambient and high Tg networks

Networks which Tg is higher than the reaction temperature can be tough to synthesize due to the vitrification of the reaction medium, limiting the molecular mobility. When targeting high Tg networks (or at least super-ambient Tg), it is thus essential to work at high temperature to promote the growth of the network.

For example, Ecochard et al. obtained networks with T g > 35°C by reaction of siloxane D4 monomers bearing four 5CC functions with an aliphatic diamine, 2-methylpentane-1,5-diamine (Dytek-A) and a cycloaliphatic diamine, 1,3-Cyclohexanebis(methylamine) (CBMA) (Figure 19). [START_REF] Ecochard | From multi-functional siloxanebased cyclic carbonates to hybrid polyhydroxyurethane thermosets[END_REF] The networks were cured 4 h at 80 °C then 1 h at 100 °C. With the diamine Dytek-A, the Tg of the polymer is 37 °C while with CBMA it is increased to 60 °C. Although the crosslinking temperatures are higher than the Tg of the final networks, the conversion of carbonate is limited to 86 % in the case of Dytek-A. According to the authors, this conversion limit is due to the high viscosity of the reaction medium resulting from strong hydrogen bonding. The authors stress that it is mandatory to use high crosslinking temperatures to increase the molecular mobility of the growing network. But this is accompanied by a larger extent of side reactions that might explain the limited 5CC conversion as well. [START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF] The latter was reacted with increasing amount of pentaerythritol tetracarbonate (from 10 wt% to 50 wt%) and with stoichiometric amount of pentamethylene diamine (5DA). The Tgs of the resulting networks are comprised between 45 and 62 °C. The mixtures were all cured according to a two-step scheme, i.e. 16 h at 80 °C then 4 h at 100 °C, in order to ensure the completion of the crosslinking reaction of the rigid and brittle networks. Similarly, Ménard et al. developed high Tg networks by reacting an aromatic pluricyclic carbonate obtained from ferulic acid with a series of diamines, including IPDA, 1,10decanediamine (10DA) and EDR-148. [START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF] The networks were cured at 80 °C for 5 h then at 100 °C for 10 h, and their Tgs vary from 38 to 72 °C depending on the amine. The authors stress that they have intentionally slowed down the reaction kinetics by initially working at 80 °C in order to avoid a too fast vitrification of the reaction medium, which would have limited the aminolysis of the carbonates.

In another representative example, Schmidt et al. synthesized very high Tg PHU networks by reacting mixtures of sorbitol tricarbonate (STC, solid) and sorbitol ether tricarbonate (STEC, liquid) (Figure 16) with isophorone diamine (IPDA). [START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF] The Tgs of the networks increase from 140 °C to 180 °C by increasing the amount of STC, the stiffest precursor, from 0 wt% to 20 wt%. For 0 wt% of STC, the network was cured at 80 °C for 16 h and then at 100 °C for 4 h. The first curing step is necessary to avoid the instantaneous gelation of the network that would limit the aminolysis of the carbonates. This step also facilitates the shaping of the polymer since at 100 °C, the gel point is reached in less than one minute. Remarkably, in this case, the Tg of the final network (140 °C) is larger than the highest curing temperature, suggesting that the reaction is exothermic enough to be self-sustained. When STC is added to the carbonate mixtures, the Tgs are larger than 160 °C and a third crosslinking step at 170 °C for 1 h is necessary to complete the crosslinking reaction. For 20 wt% of STC, the Tg reach the top value of 180 °C, suggesting once again that the reaction is completed thanks to the in-situ input of thermal energy due to its exothermicity.

Remarkably, in all the above-mentioned examples, a curing step at high temperature is necessary to complete the crosslinking reaction, however it must be preceded by a first curing step at a moderate temperature to avoid the instantaneous gelation of the network. Just like vitrification, premature gelation can be detrimental to the formation of homogeneous and densely crosslinked networks.

Summary and outlook

To conclude, the synthesis of thermosetting PHUs has long been conducted at high temperatures for several reasons. First, the carbonate precursors are often viscous liquids or solids that must be meltprocessed at high temperature. Furthermore, the carbonate-amine reaction is often considered as a slow reaction that requires high temperature to reach full completion over descent periods of times. On top of that, high curing temperatures are mandatory when high Tg PHU thermosets are targeted.

In their comprehensive study of the curing reaction of thermosetting PHUs, Blain et al. compared the carbonate-amine reaction to other conventional chemistries used in thermosetting technologies such as the epoxy-amine and the isocyanate-alcohol reactions. 105 Figure 20 represents the DSC thermograms for three curing systems using precursors of similar structures and based respectively on the epoxy-amine, the isocyanate-alcohol and the carbonate amine chemistry. Clearly, the exotherm of the carbonate-amine system is more diffused than the two other systems. More importantly, it is bimodal with a first peak at rather low temperatures (78 °C) and a second peak at high temperature (172 °C). The authors demonstrated that both peaks can be attributed to the reaction of carbonate with amine. The reaction proceeds in two steps with a second peak at high temperature because an extra input of thermal energy is necessary to compete with the inter-chain hydrogen bonds formed during the first stage of the curing reaction. Unfortunately, Blain et al. also observed that side reactions, such as urea formation, are initiated during the second step at high temperature. It is thus critical to finely tune the curing temperatures of PHUs thermosets to obtain a good balance between the kinetics of the carbonate-amine reaction and those of the undesirable side reactions. Interestingly, in the same study, Blain et al. stress that the aminolysis of activated carbonate can be initiated at room temperature as emphasized in Figure 20. Indeed, the onset of the exotherm of the carbonate-amine reaction is observed for Tonset = 15 °C (against Tonset = 32 °C for the isocyanate-alcohol system and Tonset = 74 °C for the epoxy-amine system). This result indicates that, providing a solution is found to circumvent the decrease of the reaction kinetics due to hydrogen bonding, it is in theory possible to form a PHU network at room temperature. This is highly desirable for two reasons. First, low curing temperatures guarantee a lesser extent of side reactions. Second, room temperature curable systems are highly desirable in a myriad of applications such as adhesives, sealing, coating, 3Dprinting, etc.

Recently, several authors have introduced carbonate-amine systems that can be fully crosslinked at low temperature (T < 70 °C), including room temperature. They are presented hereafter with a special emphasize on the technical approaches as well as the chemical features of the precursors allowing low temperature curability.

Synthesis of thermosetting PHUs at low temperature, RT ≤ T < 70 °C

The synthesis of PHU thermosets at moderate temperature, such that RT ≤ T < 70 °C, reduce the extent of side reactions but is also interesting from an energy point of view. In addition, room temperature curable PHU thermosets could be used by non-professional users for outdoor and "do it yourself" applications (e.g. painting, coating, sealing). The idea that the low reactivity of cyclic carbonates is the main obstacle to their use at moderate temperature has long been maintained. However, some chemically activated monomers appear to be reactive enough in mild temperature conditions. Examples of syntheses of thermosetting PHUs at moderate temperature from 5CC 78,106-109 but also from 6CC [START_REF] Fortman | Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers[END_REF][START_REF] Matsukizono | Reworkable Polyhydroxyurethane Films with Reversible Acetal Networks Obtained from Multifunctional Six-Membered Cyclic Carbonates[END_REF][START_REF] Coste | Non-isocyanate polyurethane foams based on six-membered cyclic carbonates ☆[END_REF]106,[110][111][START_REF] Hua | Isocyanate-Free, UV-Crosslinked Poly(Hydroxyurethane) Networks: A Sustainable Approach toward Highly Functional Antibacterial Gels[END_REF] and 8CC 113 precursors, are described in the literature and are presented hereafter (Figure 21).

Polymerization in solution

Blain et al. demonstrated that the reaction of activated cyclic carbonates with amines can be initiated at room temperature (RT) but that an increase of the curing temperature well above RT is mandatory to compete with inter-chains hydrogen bonding and fully complete the crosslinking reaction. 105 One possible alternative to the temperature increase is to use a solvent that breaks the hydrogen bonds, thus providing sufficient molecular mobility to the growing network to sustain the reactivity until the completion of the crosslinking reaction.

Hydrogels

Bourguignon et al. obtained crosslinked PHUs at RT in water with the aim of synthesizing hydrogels. [START_REF] Bourguignon | Water-Borne Isocyanate-Free Polyurethane Hydrogels with Adaptable Functionality and Behavior[END_REF]108 They first developed a one-step synthesis of PHU hydrogels at RT by reacting a dicarbonate derived from polyethylene glycol with polyethyleneimine (PEI) in water as shown in Figure 22. 108 For this, an aqueous solution of PEI at a pH between 9 and 12.5 is added to the cyclic dicarbonate and the mixture is left for 24 hours at room temperature until crosslinking. The optimum pH was determined thanks to model reactions with the diamine 2,2'-ethylenedioxy bis(ethylamine). Thus, a high pH, ~ 12.5, leads to the hydrolysis of 5CCs and limits the formation of hydroxyurethanes, which only corresponds to 60 % of the conversion of carbonates after 16 h of reaction. At a lower pH, ≤ 9, the aminolysis of the carbonate is no longer operative due to the protonation of the amine, whose pKa is 9.7. Finally, the intermediate pH of 10.5 leads to the aminolysis of carbonates while limiting the hydrolysis of 5CC to only 9 mol% after 16 h of reaction. The investigation of the curing reaction at pH = 10.5 indicates that the gel point is reached in only 15 minutes for an optimum functional ratio of [NH2]:[5CC] = 0.8 (Figure 22). Furthermore, analysis of the dried sample by infrared spectroscopy indicates that the conversion of cyclic carbonates is complete in that case. The resulting hydrogel is rigid and exhibits the highest gel fraction of the series of hydrogels synthesized in this study, i.e. 85 % (measured after 48 h of immersion in water).

In another study Bourguignon et al. describes the synthesis of a new series of hydro-soluble poly(5CC)s by copolymerization of glycerol carbonated methacrylate with different hydrophilic co-monomers (both neutral and ionic). [START_REF] Bourguignon | Water-Borne Isocyanate-Free Polyurethane Hydrogels with Adaptable Functionality and Behavior[END_REF] The poly(5CC)s were then reacted with commercial diamines at RT in water.

After pH optimization for each diamine, gel times shorter than 5 min were measured in rheology and the full conversion of cyclic carbonates was confirmed by infrared spectroscopy. Interestingly, the poly(5CC)s obtained from the copolymerization of glycerol carbonated methacrylate with polyethylene glycol methyl ether methacrylate leads to the synthesis of thermoresponsive gels with lower critical solution temperatures in water.

Fanjul-Mosteirin et al. took advantage of the high reactivity of N-substituted 8-membered cyclic biscarbonates to synthesize semi-crystalline hydrogels with the triamine TAEA and poly(ethylene glycol) diamine at RT (Figure 23). 113 The reagents, mixed in water, react overnight at room temperature to give hydrogels. The complete conversion of the 8CC was confirmed by infrared spectroscopy. Moreover, the curing reaction is monitored by rheology in order to investigate the impact of the reactant concentrations and ratios on gel times. The greater the amount of water in the formulation, the slower the gelation, with a gel point reached in 5.8 h for 50 wt% of water against 12.5 h for 70 wt%. Furthermore, an increase in the quantity of triamine TAEA from 20 mol% to 60 mol% (as compared to the diamine) results in a sharp decrease of the gel time from ~30 h to 3.3 h (70 wt% water). Interestingly, the hydrogels containing less than 35 wt% of water are thermoresponsive. An increase in temperature from RT to 60 °C leads to a significant decrease of their storage modulus G', from 1 MPa to ~ 2 kPa with a transition between 35 °C and 45 °C, while a decrease in temperature results in a sharp increase of G' back to its initial value for T = 27 °C. The correlation between the melting-crystallization transition of the poly(ethylene glycol) skeleton of the diamine and the reversible changing of the mechanical response was confirmed by DSC analysis. This thermoresponsivity is then exploited to 3Dprint the hydrogel via an extrusion process. Finally, the cytocompatibility of these 3D-printed structures was investigated to confirm their potential use in biomedical applications. Curing in organic solvents PHUs thermosets have been synthesized at moderate temperature in volatile organic solvents. [START_REF] Fortman | Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers[END_REF][START_REF] Matsukizono | Reworkable Polyhydroxyurethane Films with Reversible Acetal Networks Obtained from Multifunctional Six-Membered Cyclic Carbonates[END_REF]106,111,114 In these examples, the role of the solvent is to homogenize the initial reaction medium, but it also acts as a plasticizer that will mitigate the impact of hydrogen bonding, and the potential vitrification of the growing network.

For example, Fortman et al. synthesized two PHU thermosets by reacting butanediol glycidyl carbonate, (a bis(5CC)) and di(trimethylolpropane) carbonate (a bis(6CC)), respectively, (Figure 24) with a mixture of hexamethylenediamine and the triamine tris(2-aminoethyl)amine. The reactants were initially dissolved in dichloromethane (typically 75wt% of solvent). 106 The mixture is left at room temperature for 24 h, resulting in a gel after only 1 h of reaction. The latter is then placed under reduced pressure at 90 °C for 2 days to fully evaporate the solvent. The infrared analysis of the gels obtained after 24 h at room temperature reveals the total conversion of the carbonates, for both the 5CC and the 6CC precursors (Figure 24). The final networks are highly crosslinked, with a gel fraction ≥ 94 %. They are thermally stable up to 270 °C, with super-ambient Tg comprised between 35 and 55 °C and elastic modulus between 1.2 and 4.5 MPa at 100 °C. Interestingly, the PHUs based on the 6CC are vitrimers. They can be reprocessed at high temperature (160 °C) via compression molding thanks to the transcarbamoylation reaction between the primary hydroxyl and the urethane moieties contained in the network. On the contrary, the reprocessed PHUs based on the 5CC precursor exhibit lower mechanical properties and coloration, likely due to irreversible degradation of the network via initial reversion of the PHU linkage and subsequent side reactions of the liberated amine and cyclic carbonate. In a similar fashion, Feng et al. synthesized PHUs vitrimers by reacting the same bis(6CC) precursors with tris(2-aminoethyl)amine via a solvent-mediated process. 111 The bis(6CC) and the triamine were dissolved separately in dichloromethane (a total amount of approximately 80 wt% of solvent) and the two solutions were combined to provide a crosslinked system after 12 h at room temperature. The carbonate conversion and the gel content were not investigated but the resulting polymer is a transparent, self-standing material, with a stress and a strain at break of 1.5 MPa and 40 % respectively, as measured by tensile testing. After a 18 h post-curing step at 90 °C, the stress and the strain at break become 37 MPa and 4.4 %, respectively. This was attributed to further elimination of residual traces of solvent and to the completion of the crosslinking reaction, but again it was not confirmed by infrared or swelling test investigation. Interestingly, the authors demonstrated that these materials exhibit intrinsic luminescence under UV irradiation (λ = 365 nm). This is unusual for nonconjugated polymers, and it is attributed to electron cloud overlapping in clusters of carbamate moieties. The luminescence of these PHUs was exploited to prepare a composite with paper that could be of interest for anticounterfeiting applications.

To conclude, solvent processing is a very effective approach to observe the crosslinking reaction of PHU thermosets at room temperature. However, it relies on the use of large quantities of volatile solvents that can be dangerous for human health and the environment. Moreover, this approach is not viable for most industrial applications.

Bulk polymerization

Mass polymerization is preferred to solvent mediated processes in an industrial context, notably for adhesive and coating applications. 115 In principle, this is a simple approach, but it can be constrained by several phenomena including an insufficient homogenization of the reactants or the interruption of the crosslinking reaction due to excessive viscosity or vitrification. Each of these constraints can be circumvent by a temperature increase. However, in some cases, it is possible to obtain fully crosslinked PHUs using mild curing temperatures (< 70 °C). To do so, the authors used highly reactive (activated) and easily processable liquid carbonates. [START_REF] Coste | Non-isocyanate polyurethane foams based on six-membered cyclic carbonates ☆[END_REF]116 In some cases, the carbonate-amine reaction was further promoted by using a suitable catalyst. [START_REF] Cornille | Room temperature flexible isocyanate-free polyurethane foams[END_REF]117 Another approach consists in using hybrid systems, i.e. functional (oligo)hydroxyurethanes that can be crosslinked according to another chemistry (thiol-ene, Michael addition, etc.). 109,110,[START_REF] Hua | Isocyanate-Free, UV-Crosslinked Poly(Hydroxyurethane) Networks: A Sustainable Approach toward Highly Functional Antibacterial Gels[END_REF] Liquid precursors bearing activated cyclic carbonates and catalysis of the carbonate-amine reaction

Capar et al. worked on the development of low-viscosity two-pack formulations with potential application in the field of coatings and dental fillings. They carried out the CO2 carbonation of various di-and trifunctional glycidyl ethers, thus obtaining polyfunctional 5CC in which the carbonate is activated by the electron withdrawing effect of the ether moieties. 116 In particular, they worked with trimethylpropane triglycidyl carbonate (TMPTC) and hexanediol glycidyl carbonate (HGC), respectively noted compound 1 and 2 in Figure 25. The two poly(5CC) are liquids with viscosity of 300 Pa s and 1 Pa s respectively, at 25 °C. They were mixed with the liquid triamine TAEA and the crosslinking reactions were monitored by rheology for 12 h at 40 °C, i.e. a temperature close to body temperature. In both cases, the gelation is observed over a short period of time: 39 min in the case of carbonate 1 and 215 min for carbonate 2 (Figure 25). Moreover, it was confirmed by infrared spectroscopy and DSC that the carbonate conversion is total after 12 hours of reaction at 40 °C. The Tg of the resulting network is 50 °C in the case of carbonate 1, which is slightly higher than the curing temperature. It goes down to 9 °C for carbonate 2. By combining carbonates 1 and 2 (50/50 wt%), it is possible to obtain formulations with intermediate gel times (90 min, Figure 25). The authors used other polyamine including the hydrophilic polyethyleneimine and the hydrophobic Priamine obtained from di-and trimerized fatty acid. Interestingly, this study demonstrates that it is possible to form fully cured PHUs over decent period of time (~ 12 h) but also that the shape of the object, i.e. gelation, can be set very quickly (gel time ~ hour). It indicates that the crosslinking of pluri-5CC at moderate temperature without catalyst is possible and relevant for practical applications, when using chemically activated carbonates with low viscosity and good miscibility with the amines. Wu et al. developed a room temperature curable formulation containing bisphenol A diglycidyl carbonate, a dicarbonate which is known for being a solid. 109 To that purpose, they first reacted the dicarbonate with isophorone diamine (IPDA) in DMF at 150 °C for 3 h to obtain a series of liquid telechelic prepolymers bearing cyclic carbonate (Figure 26). By varying the dicarbonate to diamine ratio, they were able to obtain telechelic prepolymers with molar masses comprised between 470 and 2280 g mol -1 (Dispersity ~ 1.5) and increasing viscosity. These tailor-made liquid dicarbonates were then reacted with polyethyleneimine at room temperature for 5 days. The conversion of the cyclic carbonates is complete after this period of time according to infrared spectroscopy (Figure 26). Furthermore, the reactions were monitored by rheology, indicating that the gel time, tgel, is an increasing function of the molar mass of the prepolymer, with tgel = 3 h for Mn = 470 g mol -1 and tgel = 14 h for Mn = 2280 g mol -1 . In all cases, the films are tack free in less than 40 min. The Tg of the film increases from 35 °C to 51 °C by increasing value of the molar mass of the dicarbonate prepolymer. It suggests that the exothermicity of the curing reaction is sufficient to obtain super-ambient T g materials, but this point is not discussed by the authors. Interestingly, crosslinking trials with other conventional liquid amines (e.g. polyether amine) were unsuccessful at room-temperature. For this reason, the authors hypothesized that the room-temperature curability of the formulation based on polyethyleneimine relies on the numerous secondary and tertiary amines of the polyamine, which catalyze the carbonate-amine reaction. Moreover, it is worth noting that the carbonates are chemically activated by the ether function contained in the starting bisphenol A glycidyl carbonate. In a study describing the synthesis of thermosetting PHU foams, Cornille et al. developed room temperature curable formulations based on the use of ether-activated carbonates in combination with a catalyst. [START_REF] Cornille | Room temperature flexible isocyanate-free polyurethane foams[END_REF] Mixtures of TMPTC and poly(propylene oxide) diglycidyl biscarbonate (PPOBC, M n = 380 g mol -1 or 640 g mol -1 ) were reacted with a polyether amine (EDR-148) in the presence of a thiourea as a catalyst (1 mol% relative to carbonate) and a polymethyl(hydrogensiloxane) as a foaming agent (PMHS, 5 mol% relative to amine) (Figure 27). All the compounds are liquids except for the catalyst. It is first homogenized with the cyclic carbonates and the other reagents are then added to the first mixture. The thiourea catalyst was chosen over TBD, another conventional catalyst of the carbonate-amine reaction, because of its solubility into the carbonates. [START_REF] Cornille | A new way of creating cellular polyurethane materials: NIPU foams[END_REF] Crosslinking takes place in a beaker without stirring at room temperature. According to infrared spectroscopy, the conversion of cyclic carbonates is complete after 3 days of reaction (Figure 27). The authors demonstrated that both the catalyst and the foaming agents are necessary to observe the full conversion of carbonates. Catalyst-free mixtures are highly viscous liquid after 3 days of reaction and, in the absence of foaming agent, the materials remain sticky. It is hypothesized that the foaming agent helps in homogenising the reactants mixture thanks to the local stirring induced around the growing gaseous bubbles. Very interestingly, the authors investigated the exothermicity of the reaction and measured a 10 °C increase of the system temperature. This temperature increase also plays a crucial role in the completion of the curing reaction. In the end, stable foams with Tg varying from 0 °C to 11 °C and gel fraction > 90 % were synthesized according to this protocol. et al. synthesized catalyst-free PHU foams in mild temperature conditions, by using a liquid 6CC tetrafunctional monomer, which is known for being more reactive than 5CC monomers (Figure 28). [START_REF] Coste | Non-isocyanate polyurethane foams based on six-membered cyclic carbonates ☆[END_REF] The tetracarbonate, synthesized by thiol-ene reaction beforehand, is manually mixed with PMHS, the same foaming agent than the one used by Cornille et al. [START_REF] Cornille | Room temperature flexible isocyanate-free polyurethane foams[END_REF] The amine is then added to the mixture before injection in a silicone mould. Particular attention was paid to the selection of liquid diamines, in particular isophorone diamine (IPDA), m-xylylenediamine (mxDA) and hexamethylene diamine (5DA), in order to ensure the homogeneity of the medium. The final shape of the foams is obtained after only 4 hours at 50 °C, indicating that the crosslinking reaction is well operative in this mild conditions. The foams are further maintained at 50 °C for an additional period of 8 h and a post-curing step is carried out at 120 °C for 2 h. Indeed, after curing at 50 °C, DSC analysis indicates that the curing reaction is not fully completed. After high-temperature treatment, the 6CC carbonyl absorption band is no longer observed in infrared, proving the full conversion of cyclic carbonates. The good reactivity of these systems in mild conditions was confirmed by the rheological investigation of the crosslinking reaction for formulations free of foaming agent. Very short gel times were recorded for all the amines with tgel = 247 s, 411 s, 1012 s for 5DA, mxDA and IPDA respectively (Figure 28). In the end, stable foams with Tg varying from 7 °C to 27 °C and gel fraction > 80 % were synthesized according to this protocol.

Liquid (oligo)hydroxyurethanes crosslinkers bearing new functional groups: hybrid systems

A different strategy to obtain PHU networks at room temperature is to design liquid crosslinkers containing hydroxyurethane moieties and bearing functional groups that can be involved in a crosslinking reaction operative at room temperature. In this approach, the hydroxyurethane moieties are formed during the synthesis of the liquid crosslinkers, i.e. prior to the crosslinking reaction. The latter is no longer the carbonate-amine reaction. It is another classical reaction of thermosetting technology, such as the epoxy-amine reaction or the free radical polymerization of alkene. For this reason, the resulting materials are often considered as "hybrid" thermosets. 120 For instance, liquid (oligo)hydroxyurethane crosslinkers can be diacrylates that will be crosslinked via photoinitiated free radical polymerization or diamines that will be used to crosslink epoxy resins.

Two main strategies can be distinguished in the literature for the synthesis of these liquid (oligo)hydroxyurethanes crosslinkers. The first one consists in reacting a di-or tri-amine with a monocyclic carbonate containing a polymerizable function, usually an alkene. The resulting compound is a di-or tri-hydroxyurethane bearing two or three alkene functions that can be involved in a crosslinking reaction. For example, Hua et al. synthesized an allylated tri-hydroxyurethane by reacting the 6CC obtained from carbonation of trimethylolpropane allyl ether with the triamine TAEA (structures represented in Figure 29). [START_REF] Hua | Isocyanate-Free, UV-Crosslinked Poly(Hydroxyurethane) Networks: A Sustainable Approach toward Highly Functional Antibacterial Gels[END_REF] The mixture of 6CC and TAEA is heated at 60 °C for 6 h to provide the trihydroxyurethane crosslinker with a quantitative yield. The latter is then mixed with a dithiol and a photoinitiator (benzophenone) in a small amount of diethyl ether. The solvent is evaporated under reduced pressure and the mixture is crosslinked at room temperature via thiol-ene reaction under irradiation at 365 nm (Figure 29). 95 % of the double bonds are converted after 20 min of irradiation. The gel fraction of the resulting material is around 90 %, in agreement with the formation of a well crosslinked thermoset. In another work, Schimpf et al. synthesized low viscosity di-hydroxyurethane methacrylates by reacting methacrylated glycerol carbonate with a series of diamines (Figure 30). 121 The resulting compounds exhibit viscosities comprised between 7 Pa.s and 200 Pa.s. They were used as crosslinkers in photocurable formulations composed of the di-hydroxyurethane methacrylates (39 wt%), 4-Acryloylmorpholine (59 wt%) and a radical photoinitiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, TPO, 1 wt%). Materials were obtained after curing under UV irradiation for 20 min at RT. Interestingly, the authors demonstrate that these formulations can be used in commercial 3D-printers based on stereolithography technologies. A second strategy to obtain liquid (oligo)hydroxyurethane crosslinkers is based on the synthesis of amino-terminated PHU prepolymers. The latter are obtained by reacting dicarbonates with an excess of diamine. The resulting (oligo)hydroxyurethane diamines can then be used as crosslinkers for epoxy or acrylic resins to afford hybrid PHUs, usually at room temperature. [START_REF] Asemani | Design of hybrid nonisocyanate polyurethane coatings for advanced ambient temperature curing applications[END_REF]122,[START_REF] Cornille | Synthesis of hybrid polyhydroxyurethanes by Michael addition[END_REF] For instance, Cornille et al. developed a series of liquid amino-telechelic PHU oligomers by reacting a polypropylene glycol biscarbonate (Mn = 640 g mol -1 ) with an excess of diamine, either 1,3-cyclohexanebis(methylamine) (CBMA) or 2,2′(ethylenedioxy)diethylamine (EDR-148) (Figure 31). [START_REF] Cornille | Synthesis of hybrid polyhydroxyurethanes by Michael addition[END_REF] The catalyst-free reactions were conducted in bulk at room temperature overnight and the resulting prepolymers were used without any purification. They exhibit viscosities comprised between 0.2 and 500 Pa.s depending on both the amine used for the synthesis and the carbonate/amine ratio. Eventually, the amino-telechelic PHUs were used to crosslink mixtures of a commercially available triacrylates and various polypropylene glycol diacrylates. The curing reaction is based on the Michael addition of the amine onto the acrylate functions and is conducted at room temperature for 48 h. Infrared measurement indicates that the acrylates are fully converted after this period of time. Moreover, the hybrid PHU thermosets exhibit high gel fraction (92-94%) and Tg varying from -59 °C and -17 °C. In another example, Asemani et al. used a similar approach to synthesize amino-telechelic PHUs that were further used to crosslink polyepoxides at ambient temperature during 7 days. [START_REF] Asemani | Design of hybrid nonisocyanate polyurethane coatings for advanced ambient temperature curing applications[END_REF] The resulting hybrid PHU thermosets are tack-free after periods of time varying from 4 to 16 h and they exhibit gel content of about 80 %.

Summary and outlook

All these examples of low-temperature curing of PHU thermosets demonstrate that the carbonateamine chemistry can be suitable for many ambient applications including coating, sealing or 3Dprinting. The growing number of references dedicated to these curing conditions can be used to determine the typical characteristics of the ideal carbonate-amine systems for ambient curing in solvent-free conditions. First, pluricyclic carbonates with viscosity lower than 500 Pa s at 25 °C seems to be the most adapted for an implementation at room temperature (i.e. mixing with the amine and further processing such as coating, foaming or injection). Another important point is the reactivity of the carbonate. 5CCs activated by the electron withdrawing effect of an ether function seems to be the best compromise in terms of reactivity versus availability of the precursors. Further activation of the carbonate-amine reaction by a catalyst seems to be essential in most cases. Interestingly, the catalyst can be a tertiary amine moiety present in the backbone of the polyamine precursors, thus avoiding the incorporation of a "free" catalyst that could leak out of the crosslinked network afterwards.

Even if the profile of the ideal ambient curable system is getting clearer, many questions are still unaddressed. In particular, the exothermicity of the carbonate-amine reaction has been scarcely investigated. When compared to the abundant literature regarding the detailed thermodynamic analysis of the epoxy-amine reaction, the data related to the carbonate-amine reaction are very poor. Thus, it is extremely difficult to predict the exothermicity of typical carbonate-amine combinations in ambient conditions. To our knowledge, there are no existing TTT diagram (Time-Temperature-Transformation) for a typical crosslinkable carbonate-amine system. This is another indication of the lack of knowledge regarding the thermodynamics of this reaction.

Last but not least, hydrogen bonding has been well identified as one of the main obstacles to lowtemperature curing. However, the correlation between the chemical structure of the carbonate and the amine precursors and the extent of hydrogen bonding during the growth of the PHU network has never been investigated. This would be a critical information to determine the ideal precursors for room temperature curability. Torkelson et al. investigated the relationship between hydrogen bonding and the chemical structure of the precursors in fully polymerized PHUs. 124,125 These informations should be used as a source of inspiration for future investigations.

Conclusion

The synthesis of thermosetting PHU is widely documented and different strategies can be applied to obtain these polymers. Various synthesis methods of plurifunctional cyclic carbonates have been described and extensively used in the literature. Most of the poly(cyclic carbonates) have been synthesized through a grafting method via the thiol-ene reaction or by co-polymerization in the case of highly functionalized monomers. It is interesting to note that most of the thermosetting PHUs syntheses involve classical curing conditions, i.e. high temperature for long reaction times, in order to avoid any physical limitation from the growing network but also to foster the carbonate-amine reaction.

If the resulting networks meet the expected thermomechanical properties, those reaction conditions are not adapted to all the targeted applications, e.g. coatings and adhesives, that are mostly synthesized at room temperature. Even though 5-membered cyclic carbonates are considered as barely reactive at room temperature, catalysis or even chemical activation have been highlighted as solutions to use them in such temperature conditions. Highly reactive X-membered cyclic carbonates, with X ≥ 6, have also been used in the case of syntheses at mild or even room temperature. Finally, various authors have shown that, in water or in bulk, it was possible to use 'classical' 5-membered cyclic carbonates to synthesize thermosetting PHUs at moderate temperature, ≤ 50 °C, or even at room temperature, paving the way to new applications for such polymers.

Figure 1 :

 1 Figure 1 : From the design of plurifunctional cyclocarbonates to the synthesis of poly(hydroxyurethane) thermosets. Overview of the scope of the review. Isocyanate-based thermosetting PUs represent a large part of the world production, particularly for applications such as foams or coatings. It is therefore necessary to design alternatives to these polymers, i.e. thermosetting PHUs. The first examples of thermosetting PHUs were reported in 2000 by Figovsky et al., with the synthesis of hybrid PHU networks from cyclic carbonate, epoxide and amine oligomers of functionality ≥ 3, 4 and in 2003 by Wilkes et al.with the use of carbonated soybean oil as a cyclic carbonate precursor.5 Since then, many authors have been interested in the study of PHU networks obtained from multifunctional 5CCs. Many biosourced substrates, in addition to vegetable oils, have also been implemented in the synthesis of thermosetting PHUs.[START_REF] Carré | From the Synthesis of Biobased Cyclic Carbonate to Polyhydroxyurethanes: A Promising Route towards Renewable Non-Isocyanate Polyurethanes[END_REF][START_REF] Nohra | From petrochemical polyurethanes to biobased polyhydroxyurethanes[END_REF] In 2015, Fortman et al. reported the first synthesis of PHU networks with vitrimeric properties, paving the way to new application perspectives for PHUs.[START_REF] Fortman | Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers[END_REF] In all these works, the resulting thermosetting PHUs exhibit low soluble fraction (typically < 5wt%), and their thermomechanical properties are competitive with those

Figure 2 :

 2 Figure 2 : General scheme for the synthesis of poly(5CC) from polyepoxides

Figure 3 :

 3 Figure 3 : General synthesis pathways of CSBO and TMPTC

Figure 4 :

 4 Figure 4 : Synthesis of polysiloxane based plurifunctional cyclic carbonate proposed by Ecochar et al.[START_REF] Ecochard | From multi-functional siloxanebased cyclic carbonates to hybrid polyhydroxyurethane thermosets[END_REF]

Figure 5 :

 5 Figure 5 : General scheme for the synthesis of poly(5CC)s and poly(6CC)s from pol(vicinal diol)s and poly(1,3diol)s respectively

Figure 7 :

 7 Figure 7 : General scheme for the synthesis of poly(5CC)s and poly(6CC)s by grafting of monofunctional cyclic carbonates on plurifunctional backbones

Figure 8 :

 8 Figure 8 : Mechanism of the thiol-ene reaction

Figure 9

 9 Figure 9 : Synthesis strategies of plurifunctional monomers through thiol-ene reaction described byBesse et al.[START_REF] Besse | Access to nonisocyanate poly(thio)urethanes: A comparative study[END_REF] and Tomita et al.[START_REF] Tomita | Polyaddition Behavior of bis (Five-and Six-Membered Cyclic Carbonate)s with Diamine[END_REF][START_REF] Tomita | Polyaddition of bis(seven-membered cyclic carbonate) with diamines: A novel and efficient synthetic method for polyhydroxyurethanes[END_REF]

Figure 12 :

 12 Figure 12 : Synthesis strategies of plurifunctional monomers through metathesis described byMaisonneuve et al.[START_REF] Maisonneuve | Fatty acid-based (bis) 6membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors[END_REF] and Lamarzelle et al.[START_REF] Lamarzelle | Activated lipidic cyclic carbonates for non-isocyanate polyurethane synthesis[END_REF]

Figure 13 :

 13 Figure 13 : Synthesis strategies of plurifunctional monomers through the radical homopolymerization or copolymerization of alkene-containing carbonates

Figure 14 :

 14 Figure14: Synthesis of plurifunctional monomers through monocyclic carbonates co(polymerization) described by Bourguignon et al.[START_REF] Bourguignon | Water-Borne Isocyanate-Free Polyurethane Hydrogels with Adaptable Functionality and Behavior[END_REF] and Scholten et al.[START_REF] Scholten | Functional Polyethylenes by Organometallic-Mediated Radical Polymerization of Biobased Carbonates[END_REF]

Figure 15 :

 15 Figure 15 : Typical monomers and curing conditions used in the literature for the synthesis of crosslinked PHUs at high temperatures, 70 °C ≤ T.

Figure 16 :

 16 Figure 16 : Structure of solid or viscous liquid pluricarbonates used for the synthesis of crosslinkedPHUs[START_REF] Tamami | Incorporation of Carbon Dioxide into Soybean Oil and Subsequent Preparation and Studies of Nonisocyanate Polyurethane Networks[END_REF][START_REF] Javni | Polyurethanes from soybean oil, aromatic and cycloaliphatic diamines by nonisocyanate route[END_REF][START_REF] Bähr | Cyclic limonene dicarbonate as a new monomer for nonisocyanate oligo-and polyurethanes (NIPU) based upon terpenes[END_REF][START_REF] Schimpf | High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics[END_REF][START_REF] Schmidt | Liquid sorbitol ether carbonate as intermediate for rigid and segmented non-isocyanate polyhydroxyurethane thermosets[END_REF][START_REF] Ménard | Chemo-enzymatic synthesis and characterization of renewable thermoplastic and thermoset isocyanate-free poly(hydroxy)urethanes from ferulic acid derivatives[END_REF][START_REF] Schmidt | Isocyanate-Free Route to Poly(carbohydrate-urethane) Thermosets and 100% Bio-Based Coatings Derived from Glycerol Feedstock[END_REF]

Figure 17 :

 17 Figure 17 : Infrared spectra from the monitoring of the crosslinking reaction between CSBO and 4DA at different CSBO : diamine ratio according to Javni et al. 12

Figure 18 :

 18 Figure 18 : Conversion curves of cyclic carbonates, epoxides and esters as a function of time in the case of the reaction between partially carbonated epoxidized soybean oil and octanediamine a) following a two-step curing schedule; b) at 120 °C by Helbling et al. 104

Figure 19 :

 19 Figure 19 : Tetrafunctional cyclic carbonate D4 C5 developped by Ecochard et al. and used for the synthesis of thermoset PHU with two different diamines, Dytek-A and CBMA 40

Figure 20 :

 20 Figure 20 : Study of the enthalpy of reaction of several reactive systems monitored by DSC: epoxy/amine, isocyanate/alcohol, cyclocarbonate/amine according to Blain et al. 105

Figure 21 :

 21 Figure 21 : Typical monomers and curing conditions used in the literature for the synthesis of crosslinked PHUs at mild temperatures, RT ≤ T < 70 °C.

Figure 22 :

 22 Figure 22 : Evolution of gel points measured during PHU hydrogels syntheses at room temperature as a function of a) the ratio [NH2] : [5CC] at pH = 10.5 b) the pH at a ratio [NH2] : [5CC] = 0.8 according to Bourguignon et al. 108

Figure 23 :

 23 Figure 23 : Synthesis of crystalline PHU hydrogels at room temperature according to Fanjul-Mosteirin et al. 113

Figure 24 :

 24 Figure 24 : IR spectra of bis-carbonates a) 5CC and b) 6CC but also of the PHU networks synthesized by the reaction of 5CC or 6CC with TAEA in 24 h at room temperature according to Fortman et al. 106

Figure 25 :

 25 Figure 25 : Rheological monitoring of the crosslinking reaction at 40 °C of bis-carbonate 2, tricarbonate 1 and the equimolar mixture of both carbonate with TAEA, respectively, according to Capar et al. 116

Figure 26 :

 26 Figure 26 : PHU prepolymer and PEI structures used for the synthesis of crosslinked PHU and stacking of the infrared spectra recorded during the crosslinking reaction at RT according to Wu et al. 109

Figure 27 :

 27 Figure 27 : Reactants used for the synthesis of PHU foams at RT in bulk and comparison of the IR spectra of the reactants with those of the PHU foams (F2, F4 and F9) obtained after 3 days of crosslinking reaction at room temperature (Cornille et al.) 107

Figure 28 :

 28 Figure 28 : Structure of tetra-6CC and evolution of G' and G'' moduli as a function of the crosslinking time of tetra-6CC with various diamines according to Coste et al. 63

Figure 29 :

 29 Figure 29 : Synthesis of an allylated tri-hydroxyurethane and its photocrosslinking reaction with a dithiol to yield a thermosetting PHU. The plot represents the evolution of the double bond C=C conversion as a function of crosslinking time according to Hua et al. 112

Figure 30 :

 30 Figure 30 : Synthesis of di-hydroxyurethane methacrylates and their subsequent photocrosslinking with 4acryloylmorpholine at room temperature, as proposed by Schimpf et al. 121

Figure 31 :

 31 Figure 31 : Synthesis of amino-telechelic PHUs and their subsequent use as crosslinkers of acrylic resin via Michael addition at room temperature, as proposed by Cornille et al. 123

  The corresponding 5CC and 6CC dicarbonates were obtained with 90 % and 77 % yields respectively. Alternatively, Besse et al. obtained the 5CC dicarbonates by first grafting AGE on the dithiols and then performing the CO2-carbonation but this strategy falls into the scope of paragraph 2.1.1. (i.e. polyepoxyde carbonation). More recently, Coste et al. synthesized a 6CC tetrafunctional monomer by reacting TMPAE with a tetrathiol, the pentaerythritol tetrakis(3mercaptopropionate), using similar conditions and achieving 87 % yield.[START_REF] Coste | Non-isocyanate polyurethane foams based on six-membered cyclic carbonates ☆[END_REF]