Frédéric Meunier

Luis Cardenas

Dr Helena Kaper
email: helena.kaper@saint-gobain.com

Dr Břetislav Šmíd

Dr Mykhailo Vorokhta

Rémi Grosjean

Dr Daniel Aubert

Kassiogé Dembele

Dr Thomas Lunkenbein

Frederic C Meunier

Kassiogé Dembélé

Dr F C Meunier

Dr Cardenas

D K Dembélé

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO Oxidation on Pt/Ceria

Keywords: room temperature CO oxidation, operando DRIFTS, structure-reactivity relationship, NAP-XPS, platinum

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Understanding metal speciation is essential for the design of atom-efficient catalysts. [1][2][3] The nature of the active form of Pt for CO oxidation has been widely discussed, with a renewed recent interest following the preparation of so-called single atom catalysts. [4][5][6] Surface science [7][8][9] and X-ray absorption spectroscopy techniques [10][11][12] have been instrumental in unravelling the changing nature of Pt single crystals and nanoparticles depending on experimental conditions.

In short, two different regimes can be observed depending on the partial pressure ratio P(CO)/ P(O2) and the temperature. The low activity regime is associated with a COpoisoned metallic surface characterized by a high apparent activation energy (between 80 and 176 kJ mol -1 ) with the ratedetermining step being CO desorption from surface sites to enable O2 adsorption. Recently, Gänzler et al. showed that the CO poisoning on Pt can be circumvented by oxygen supply from the ceria, thus explaining the better performance of ceria supports at low temperatures. [13] Typical reaction orders are negative in CO (often -1) and positive in O2 (often +1). [9,14,15] The metallic or oxidized nature of the high activity regime, often observed after a so-called light-off at higher temperature, is more controversial. This controversy partly arises from (i) the difficulty in controlling temperature and the potential presence of mass transfer limitations in the highly active regime and (ii) the role of the support in the stabilization of a more active form of Pt, which has been proposed to be oxidic. [10,11,16] The latter point contrasts with earlier reports that oxidic Pt exhibited a lower activity than that of metallic Pt surfaces. [12,17] Whatever the exact nature of Pt in the high activity regime, this domain is commonly characterized by a lower activation energy (30-60 kJ mol -1 ) and fractional positive reaction orders for both CO and O2. [8,9] IR spectroscopy is commonly used to investigate the nature of Pt surfaces through the analysis of the wavenumbers of adsorbed CO, although spectral interpretation is not trivial. [18][19][20] The stretching frequency ν(CO) is affected by the Pt coordination number [21] and oxidation state, [22][23][24] CO adsorption modes, [20,25] and lateral interactions. [4] Somorjai and co-workers noted several types of adsorbed CO on Pt(111) surface during CO + O2 reaction. [8] Linear species at 2100 cm -1 are the main species present in the low activity metallic regime and are reaction inhibitors rather than spectators or reaction intermediates. The reaction intermediate is a species present at defect sites absorbing at ca. 2045 cm -1 , which is present both in the low and high activity regime. The 2100 cm -1 inhibiting species are not observed at all in the high activity regime, in contrast to new bands at 2130 and 2240 cm -1 that indicate the presence of oxidized Pt sites. Oxide-supported single atom Pt species, presumably positively charged Pt n+ , lead to bands around 2105 ± 10 cm -1 . [1,2,5,[26][27][28] In some cases the CO adsorbed on single atom species can be discriminated from those adsorbed on Pt nanoparticles by the fact that the latter are readily removed through oxidation by O2 even at room temperature, while the CO adsorbed on single atoms is unreactive. [1] The poor activity of Pt single atoms on ceria is further stressed by several groups. [16,27,29,30] In contrast, metallic Pt nanoparticles interacting with oxygen from ceria [31][32][33][34] and supported PtOx clusters [16,35] are proposed as highly reactive species. The present investigation focuses on Pt/CeO2 catalysts composed of Pt nanoparticles supported on nanosized ceria prepared according to a method reported earlier. [36] These materials are active at room temperature for CO oxidation and the activity is even increased in the presence of water. Our main aims are to investigate the natures of (i) the active Pt phase and (ii) the main reaction intermediates. We shall show, firstly, that the most active form of the catalyst actually involves the simultaneous presence of metallic and oxidic Pt. Secondly, we demonstrate by quantitative IR spectroscopy that the main reaction intermediates are carbonyls adsorbed on the metallic Pt phase. These carbonyls may react with oxygen from the Pt oxide phase or ceria, depending upon availability. Our observations help to rationalize apparently conflicting reports on similar catalysts about the origin of the oxygen used to oxidize CO, e.g. from ceria [32] or from oxidic Pt, [16] as both routes may occur simultaneously.

Results and Discussion

The evolution of the reaction rate with time is monitored isothermally at 30 °C in the absence and presence of a small concentration of steam (0.6 kPa) on powdered catalysts (Figure 1(A)). The reactions rate decreases rapidly within the first 100 min, albeit slower in the wet case. Overall, the reaction rate in the wet case is ca. 2-6 times higher than that observed in the dry case. Water-promotion of CO oxidation at low temperature is well documented in the case of Au-based catalysts, [37,38] and was also recently reported in the case of ceria-supported Pt. [28] In the following, our investigation focuses on the use of a wet feed, not only because of the observed higher activity and stability, but also because of its greater relevance to real-life applications. Any "dry" case data always remains ambiguous because water control at the ppm level is not trivial, while ppm-levels of water were shown to have dramatic effect in the case of Au. The faster deactivation noted in the dry feed (Figure 1A) could actually be due in part to a gradual water removal from the catalyst surface with time. In order to analyze the spent catalysts, the reaction was also carried out on a catalytic bench (see SI for details). The X-ray diffraction patterns of the as-prepared and spent Pt/CeO2 (after CO oxidation for 12h at 30 °C in the presence of humidity) indicate the presence of cubic ceria (space group Fm-3m) only (Figure 1B). The crystallite size of ceria particles determined using Scherrer's equation for the diffraction peak located at 2Θ = 29° was 7.4 nm. The surface area as derived from nitrogen physisorption (Figure S1) at 77 K is 112 m 2 /g, which agrees with the X-ray diffraction results. No diffraction peaks from neither metallic Pt nor platinum oxide are visible, in both the as-synthesized and spent sample. This indicates that a high platinum dispersion is obtained.

The analysis of the HAADF-STEM images of the sample show that the fresh sample reduced at 300 °C contains mostly Pt nanoparticles, which sizes are comprised between 0.6 and 1.5 nm (Figure 2.A-C), corresponding to a Pt dispersion of about 100 %. The particle size distribution is essentially identical after use for the CO oxidation in wet conditions at room temperature (Figure 2.D-E). A few single Pt atoms can also be observed (Figure 2B and E), as typical for this type of samples. Selected area electron diffraction (SAED, Figure S2) does not show Pt diffraction peaks on the reduced sample. This further confirms a high dispersion of the platinum species. However, the SAED analysis of the spent catalyst (Figure S3) evidences the presence of crystalline metallic Pt, indicating that part of the Pt crystallizes and undergoes changes under reaction conditions, even though the overall dispersion is nearly unchanged. No evidence of oxidized Pt clusters can be obtained, though those may have been reduced in the high vacuum and under the electron beam of the electron microscope. [31] These potential effects limit the informative depth of ex situ TEM studies to determine the structure of the catalytic active Pt species under the applied reaction conditions, warranting the use of other in situ or operando techniques to clarify further the nature of the active surface.

The evolution of the reactive species is further monitored by the corresponding IR bands of species adsorbed at the surface of the catalyst (Figure 3) over time at 30 °C and varying gas atmosphere with a view at unraveling structure-activity relationships. The corresponding rates measured in the DRIFTS cell is given in Figure 1 (A). For the DRIFTS experiment, the exsitu reduced sample is first exposed to the O2 + H2O stream for 30 min before recording the IR background. CO is then introduced, taken as the origin of time.

The main bands observed (Figure 3.A) are located at 2055 cm -1 and 1822 cm -1 , corresponding to linear (L-CO) and bridged (B-CO) carbonyls adsorbed on Pt 0 . [21,25] The 2055 cm -1 band is typical of Pt 0 sites with a coordination numbers of 5 or 6, [21] consistent with the many edge sites present on our 1 nm diameter particles (Figure 2). The intensity of the bridged carbonyl band at 1822 cm -1 is unusually high in comparison to that of linear species for this size of nanoparticles. In addition, the band was asymmetric, exhibiting a shoulder below 1800 cm -1 , suggesting the presence of various sites. A weak band at 2169 cm -1 typical of linear CO of Pt 1+ is observed only at short times on stream, indicating that this oxidized species was readily getting reduced upon CO introduction. [20,[22][23][24] It must be stressed that CO adsorption on Pt n+ , n>1 (observed in the range 2220-2140 cm -1 ) is usually too weak to be observable at 30 °C. [22][23][24] Therefore, the absence of such bands in Figure 3.B should not be taken as a proof of the absence of highly oxidized Pt cations (single atoms or oxidic particles), i.e. the corresponding CO surface coverage could be too low to be observed. Other bands below 1700 cm -1 are typical of carbonate/carboxylate/formates and are difficult to resolve and not discussed further here.

Apart from the first few minutes on stream relating to some induction process, CO conversion gradually decreases, while the carbonyl signal gradually increases (Figure 3.A, section I. See Figure S4 for the individual contributions of L-CO and B-CO). A higher activity could yet be obtained after having removed CO for short durations, although deactivation rates were then faster (Figure 3, periods III and VII).

CO conversion is plotted versus the IR band intensity of L-CO and B-CO to determine whether or not any unequivocal correlation existed between activity and these carbonyls (Figure 4). No unequivocal relation is observed between CO conversion and the concentration of neither carbonyls, as the four periods considered led to different curves. This result indicates that either the expression of the rate is not a direct function of the surface concentration of CO adsorbed on metallic Pt sites or that the state of the catalyst surface was markedly modified over the various periods considered. We favor the latter explanation for the three following observations. Firstly, the overall decays of CO conversion with increasing Pt 0 -CO signals for each of the reported periods (Figure 4) suggests that these carbonyls and Pt 0 sites form at the expense of more reactive sites. Since carbonyls need to react with oxygen to form CO2, it is reasonable to propose that the disappearing reactive species was an oxygen-supplying entity such as PtOx. This suggestion is supported by recent work showing that PtOx clusters were highly reactive for low temperature CO oxidation. [16,35] Secondly, the possible presence of PtOx species is evaluated by removing O2 from the feed, leaving the catalyst in the presence of steam and CO. The intensity of both carbonyl bands L-CO and B-CO gradually increases to reach a level about double that obtained before removing O2 (see periods IV in Figure 3.A and S5 and Figure 3.C). This result indicates that these new carbonyls are formed on Pt sites previously not available for CO adsorption, either because of the presence of an adsorbed O on Pt 0 or because the Pt is present as an oxide phase that does not adsorb CO at this temperature.

Thirdly, the marked modification of Pt oxidation state when varying feed composition was confirmed by in situ NAP-XPS. O2, CO and mixtures of those are injected inside the NAP-XPS analysis and reaction chamber at a total maximal pressure of 5 mbar. The samples were reduced ex-situ in order to be coherent with HAADF-STEM and DRIFTS analysis. The Pt/CeO2 sample measured in the sole presence of O2 exhibits a mixture of oxidation states Pt 0 , Pt 2+ and Pt 4+ (Figure 5 A). Metallic Pt 0 represents a minority species under these conditions (Figure 5 B). The corresponding NAP-XPS Ce 3d levels spectra shows mostly Ce 4+ species (see Figures S5-S7 in the NAP-XPS section in the supplementary). The proportions among Pt species were modified once CO was added to O2: that of Pt 4+ was reduced at the benefit of Pt 0 , while Pt 2+ proportion was unaffected. Only Pt 0 was then observed under CO once O2 is removed from the chamber. Bringing back O2 alongside CO leads to a strong reoxidation of the sample, with the Pt 0 being now almost absent. This final distribution of Pt states under CO + O2 is quite different from that observed earlier, indicating that the sample oxidation state is history-dependent. The origin of this historydependent structure is yet unclear and may arise from various factors, including thermal effects (e.g. CO oxidation is highly exothermic) and the known ability of Pt to wet ceria to different extents depending on redox treatments. [12,31,33] The NAP-XPS data (Figure 5) thus confirm our hypothesis that the structure of our Pt/CeO2 greatly varies depending on reaction cycles, as suggested by the operando DRIFTS data (Figure 3 and4). It is crucial to note that the fraction of oxidized Pt (Pt 2+ and Pt 4+ ) observed by NAP-XPS in the presence of the reagents, i.e. CO + O2, is always higher than that of metallic Pt (Figure 5). This observation supports the conclusion obtained from the operando DRIFTS data that at steady-state reaction conditions a large fraction of Pt is in an oxidized state or covered with oxygen, and therefore electro-deficient.

The conclusion that the catalyst surface under reaction condition shows both metallic and oxidic Pt raises the question of which of this phase is most responsible for the activity, or whether a synergy exists. The measured apparent activation energy was 49 ± 5 kJ/mol and the reaction orders in CO and O2 were 0.35 ± 0.04 and 0.19 ± 0.03, respectively (Figure 6). These values are fully consistent with those reported on oxidized Pt surfaces [10,11] and not with those on metallic Pt. [9,14,15] Therefore, we must conclude that the rate-determining step involves oxidized Pt. CO pressure and (C) O2 pressure used to determine the apparent activation energy and the reaction orders. Data were obtained at pseudo-steady-state at conversion lower than 20 % using the DRIFTS cell. T = 30-45 °C. The CO and O2 feed composition was slightly varied around that of the typical feed (0.2 % CO + 0.6 % water + 2 % O2/He) at atmospheric pressure according to the pressure reported in the graph.

An important question remains on the role of metallic Pt and the associated carbonyl species that were observed by DRIFTS. It is possible that these carbonyls could react with oxygen from the PtOx species or from ceria, as proposed on similar materials. [29,36,37] The data collected while removing CO from the feed (period II in Figure 3.A) were used to determine the rate constant of decomposition of L-CO and B-CO. Both species decompose uniformly with identical rates (Figure 7 and Table 1). This apparent similar reactivity could possibly be an artefact due to a fast surface transfer between the L-CO and B-CO forms, as observed on cobalt-based catalysts during CO hydrogenation. [41] The specific decomposition rate of linear and bridged carbonyls can be calculated based on a Pt dispersion of ca. 100 % and assuming that about 50 % of the Pt atoms are in a metallic state (vide supra). The total concentration of carbonyls is thus half that of Pt, also assuming a ratio 1:1 between surface metallic Pt and both linear and bridged carbonyls as a first approximation. The calculated decomposition rate of carbonyls, i.e. 6.1 ± 3 µmol s -1 gcata -1 , is similar to the rate of CO2 formation, i.e. 4.0 ± 0.4 µmol s -1 gcata -1 , (Table 1). This observation strongly suggests that these CO species are main reaction intermediates in the formation of CO2 (or at least one of those, while the other could readily be surface-transferred to the other type of site to react).

This conclusion is consistent with the model proposed by Safonova and co-workers [32] on a similar catalyst in which carbonyls adsorbed at the edge of metallic Pt nanoparticles of similar sizes reacted with oxygen from ceria. The TOF reported by these authors (2 x 10 -2 s -1 calculated at 30 °C) was actually similar to ours (1.5 x 10 -2 s -1 ). These authors did not consider the possibility of the presence of oxidized Pt species. The TOF measured here (1.5 x 10 -2 s -1 ) is still significantly lower than that reported by Wang and co-workers [16] (22 x 10 -2 s -1 ) on a catalyst presenting only PtOx species and no metallic Pt. It is possible that such highly active species were present to some extent in our sample and especially account for the higher conversion observed at the beginning of the period I, III and VII (Figure 3.A). At the present time, it is not possible for us to conclude on the origin of the oxygen reacting with the Pt 0 -CO species, but it is likely coming from both ceria and PtOx species.

The mechanism most consistent with our data is thus a combination of those reported earlier on purely metallic or oxidic Pt and is depicted in Figure 8. We have clearly demonstrated that the carbonyls present at the surface of metallic Pt atoms can account for all the CO2 formed under reaction conditions (Table 1). We have also shown that about half of the Pt was present as oxidized PtOx under our conditions (see periods IV in Figure 3.A and S4 and Figure 3.C) and that the activity clearly declined with the reduction of this PtOx phase into metallic Pt (Figure 4). Adsorbed carbonyls on Pt 0 can likely react with both oxygen from ceria and oxygen from the PtOx phase to produce CO2. We have also observed that the oxidation state of the Pt nanoparticles depended on the sequence of treatments (Figure 5), such as the order of reactant introduction.

Conclusion

In this work, we carefully analyze the active Pt species for room temperature CO oxidation of nanosized Pt on ceria using operando DRIFTS and NAP-XPS techniques. We can clearly identify the presence of both metallic and oxidized Pt during the reaction, suggesting a mechanism involving the two species. It is possible that the fraction of Pt remaining oxidized in the presence of CO could be the smallest nanoparticles (clusters) present, which should be less prone to get reduced because of strongest interaction with the support, and these might contribute to the catalytic performance by supplying oxygen to the reduced platinum particles. It is not possible for us to conclude on this matter that would require the preparation of a set of very narrow and stable Pt particle size distributions. Our observation underlines the versatility of Pt and ceria-based catalysts and rationalizes the difficulty in comparing data and mechanisms proposed by various groups, as catalyst structure is strongly history-dependent. More work is needed to determine the exact structure of the metallic and oxidic Pt phases, which are likely subnanometric labile domains strongly interacting with ceria that will require operando techniques such as EXAFS to capture their evolving nature. The co-existence of oxidic and metallic Pt domains has been proposed by Jacob and co-workers in the case of electrochemical systems and extended Pt surface DFT models. [42,43] Also for methane oxidation on Pd catalysts the presence of both, oxidized and metallic Pd seem to enhance the performance, in particular in the presence of water vapour. [44] Our work is in line with these recent findings and opens a new vision in the active nature of Pt catalysts where both metallic and oxidic Pt phases can coexist.

Figure 1 .

 1 Figure 1. (A) CO oxidation of 5 % Pt_CeO2 after ex-situ reduction (40 %H2/He, 300°C, 2h, atmospheric pressure) at 30°C under dry (right axis) and in the presence of humidity (left axis). Feed composition: 0.2 % CO + 2 % O2 in He, in the wet case 0.6 % humidity is added. The measurement is done in a DRIFTS cell. (B) XRD pattern of the as-synthesized and spent catalyst.

Figure 2 .

 2 Figure 2. Representative HAADF-STEM images of the Pt/CeO2 catalyst and the corresponding distribution of Pt particle sizes before (A-C) and after CO oxidation under wet conditions (D-E).

Figure 3 .

 3 Figure 3. (A) CO conversion (full circles) and total carbonyl (= linear and bridged CO) DRIFTS signal (open triangles) measured during a typical CO oxidation experiment carried out over Pt/CeO2 at 30 °C under 0.2 % CO + 0.6 % water + 2 % O2/He in a DRIFTS cell. Reaction products were analysed using an FT-IR gas cell. CO was removed from the feed during the periods noted II and VI. O2 was removed from the feed during period noted IV. (B) Typical DRIFTS spectra collected during the first exposure to the full feed (period I) and (C) during the O2-free period IV. The sample spectrum collected under the CO-free feed before CO introduction was used as background for the whole experiment. The gas-phase spectrum of CO was subtracted.

Figure 4 .

 4 Figure 4. CO conversion plotted against the band area of Pt 0 -bound (A) L-CO and (B) B-CO corresponding to periods I, III, V and VII as indicated in Figure 3.A.

Figure 5 .

 5 Figure 5. (A) NAP-XPS Pt 4f core levels measured over the Pt/CeO2 sample at room temperature successively exposed to: 1.3 mbar of O2 (bottom left), 5 mbar of O2 + CO (top left), 1.3 mbar of CO (bottom right) and 5 mbar of CO + O2 (top right). (B) Corresponding distribution of Pt oxidation states as a function of the gas composition. The gas flow rates of CO and O2 used were 0.4 ml min -1 and 0.85 ml min -1 , ratio pCO/pO2 = 0.47. The total pressure was 5 mbar. Note that no water vapor was introduced into the NAP-XPS chamber. Peak positions and shapes slightly evolved because of inhomogeneous charging effects.

Figure 6 .

 6 Figure 6. (A) Arrhenius plot and logarithmic plot of the reaction rate versus (B)CO pressure and (C) O2 pressure used to determine the apparent activation energy and the reaction orders. Data were obtained at pseudo-steady-state at conversion lower than 20 % using the DRIFTS cell. T = 30-45 °C. The CO and O2 feed composition was slightly varied around that of the typical feed (0.2 % CO + 0.6 % water + 2 % O2/He) at atmospheric pressure according to the pressure reported in the graph.

Figure 7 .

 7 Figure 7. (A) DRIFTS spectra collected over the Pt/CeO2 at 30 °C under 0.6 % water + 2 % O2/He at various times following the removal of CO. These data correspond to the region II in Figure 3.A. The band intensity of the linear L-CO and bridged B-CO species was then plotted as a function of time in (B) normal or (C) semi-logarithmic coordinates.

Figure 8 .

 8 Figure 8. Schematic representation of the phases involved in CO oxidation at low temperature on Pt/CeO2: (Left) partially oxidized Pt nanoparticles enable reaction between CO adsorbed on metallic Pt sites and oxygen from PtOx species. (Right) Fully reduced Pt enables reaction between CO adsorbed on Pt 0 and oxygen supplied by the ceria support. Atom color code: Pt 0 (black), oxidized Pt (grey), O (red). All Pt 0 sites are covered by CO at room temperature, only one CO is represented, for the sake of clarity. The actual particle shape is not known and may change with feed composition.

Table 1 .

 1 Parameters associated with carbonyl species bound to metallic Pt during CO oxidation at 30 °C. The decomposition rate constants are derived from Figure7. The rates of CO2 formation is calculated using the conversion values just before removing CO from the feed at the end of period I (see Figure3.A)..

	Decomposition rate constant k(L-CO)	4.94 ± 0.2 10 -2	s -1
	Decomposition rate constant k(B-CO)	4.76 ± 0.2 10 -2	s -1
	Total Pt concentration	251	µmol/gcata
	Total carbonyl concentration	125	µmol/gcata
	Carbonyl decomposition rate	6.1 ± 3	µmol/(s
			gcata)
	Rate of CO2 production per gcatalyst	4.0 ± 0.4	µmol/(s
			gcata)
	Rate of CO2 production per gPt	79	µmol/s/gPt
	TOF (mol CO2 / (molPt s))	1.5 10 -2	s -1

Acknowledgements

The authors acknowledge the CERIC-ERIC Consortium for the access to experimental facilities and financial support (grant number 20187010). RG and HK thank the FUI for funding the project "AIRCLEAN" under the AAP 22. MV and BS would like to thank the Czech Science Foundation for financial support under project no. 20-13573S.


Valentin Puchelle

Haiqin Du

Nicolas Illy
email: nicolas.illy@sorbonne-universite.fr

Philippe Guégan
email: philippe.guegan@sorbonne-universite.fr

Polymerization of epoxide monomers promoted by tBuP 4 phosphazene base: A comparative study of kinetic behavior

published or not. The documents may come     Polymerization of epoxide monomers promoted by t BuP 4 phosphazene base: a comparative study of kinetic behavior

Introduction

Polyethers exhibit specific characteristics, such as low glass transition temperatures and hydrophilicity. Thus, they find applications in a wide range of fields ranging from biomedical science to solar energy harvesting or energy storage in battery. [1][2][3] Polyethers are commonly synthesized by AROP of epoxide monomers initiated by either alkali metal derivatives such as hydrides or alkoxides combined with metal counter ions.

4-6

However, since the beginning of 2010's, organocatalyzed polymerization has gain interest and recent developments have been applied to AROP. [7][8][9][10] Organocatalysts allow in particular to enhance the propagation rates and to improve the selectivity of the polymerization toward competing reactions. Among the large variety of commerciallyavailable organocatalysts, the extremely strong, sterically hindered, nonionic phosphazene bases are of particular interest due to their ability to strongly increase the polymerization rates and to access new macromolecular architectures. 11 Tetrameric phosphazene base tBuP 4 is able to deprotonate very low acidic molecules (XH), generating extremely reactive anions associated with very large, highly stable, soft phosphazenium cations. The range of chemical moieties used as initiators in combination with tBuP 4 has been extended to carboxylic acids, 12 thiols, 13 CH acidic compounds, 14 or nitrogen containing functions such as primary, 15 secondary amides [16][17][18] or carbamates. 19 In particular, Möller and coworkers were the first to demonstrate the ability of these superbases to initiate the AROP of ethylene oxide using alcohol initiator. 20 Alcohol moiety is probably one of the most commonly found chemical functional group. Thus, tBuP 4alcohol initiating systems have then been extended to several oxirane monomers, such as 1,2-epoxypropane (PO), 1,2epoxybutane (BO), ethoxyethyl glycidyl ether (EEGE), benzyl glycidyl ether (BnGE), tert-butyl glycidyl ether (tBuGE) and glycidylamine derivatives. 2, 21-23 To the best of our knowledge, tBuP 4 is the only organic base that has been successfully used with alcohols as initiating systems for the polymerization of epoxides. Polymerization did not occurred with milder organic bases such as tBuP 1 , tBuP 2 , TBD, DBU, TiBP or MTBD.

of experimental conditions used in the literature. A rational study involving most common monomers under standardized reaction conditions is lacking. In addition, to the best of our knowledge, the nature of the propagating species involved in the presence of phosphazenium as counter ion has not been elucidated.

In this work, we aim to address these gaps in fundamental knowledge, and to provide a rational study involving the most common epoxide monomers. The AROP of 6 different epoxides, namely PO, BO, AGE, EEGE, BnGE and tBuGE, were carried out under identical reaction conditions with benzyl alcohol-tBuP 4 as initiating system. Kinetic studies have been performed, and the propagation rate constants have been calculated. The reactivities of epoxides have been compared and the influence of the chemical nature, the electron density and the steric hindrance of the lateral substituents have been highlighted. The nature of the phosphazenium alkoxide propagating species has been investigated using BO as model monomer. In the last part, kinetic studies were performed under the same conditions but in the presence of iBu 3 Al Lewis acid, under activated monomer mechanism conditions. A monomer reactivity scale has been established and compared with the previous one.

Experimental

Materials and methods. Benzyl alcohol (anhydrous, 99.8%, Sigma-Aldrich), phosphazene base tBuP 4 solution (0.8 mol.L -1 in hexane, Sigma-Aldrich), triisobutyl aluminum (iBu 3 Al) solution (1 mol.L -1 in hexane), 2-methyltetrahydrofuran (MeTHF) (98%, Sigma-Aldrich) were used as received. 1,2epoxybutane (>99%, Sigma-Aldrich), 1,2-epoxypropane, (≥99.5, Sigma-Aldrich), tert-butyl glycidyl ether (99%, Sigma-Aldrich), allyl glycidyl ether (≥99%, Sigma-Aldrich) were distilled over CaH 2 (93%, 0-2 mm grain size, Acros Organic) two times prior to use. Benzyl glycidyl ether (99%, Sigma-Aldrich) was dried over activated molecular sieves (3 Å).

Tetrahydrofuran was dried with an MBRAUN MB SPS-800 solvent purification system under nitrogen. Ethoxyethyl glycidyl ether was synthesized from glycidol and ethyl vinyl ether following the procedure reported by Fitton and al. [START_REF] Fitton | Synthesis-Stuttgart[END_REF] and distilled twice over CaH 2 prior to use.

Instrumentation. Procedures. All polymerization were carried out at 25°C, under nitrogen in a glass reactor fitted with a Rotaflo and equipped with a stirring bar and closed with a septum. The following typical procedure was applied. In a glove box, the phosphazene base tBuP 4 solution (242 µl, 0.194 mmol) was introduced into the polymerization reactor. Then hexane was evaporated using high vacuum from a vacuum line (≈ 10 -6 hPa).

Reaction vessel was transferred back into the glovebox and dried THF was added (4.15 ml) followed by benzyl alcohol (20 µl, 0.193 mmol). Finally, the monomer BO was added (840 µl, 9.66 mmol). After closure of the reactor, the reaction was stirred at 25°C and left to react for the required period of time.

For polymerization carried out in the presence of iBu 3 Al, the Lewis acid was added after tBuP 4 and just before the monomer. It may be noted that the addition of iBu 3 Al provoked a temporary rise of the temperature inside the reaction vessel above 25°C. Kinetic studies were followed by removing aliquots at various reaction times, conversion and molar masses were determined by 1 H NMR and by SEC, respectively (Figures S1-S11). The reaction was quenched by addition of 0.1 ml of acid acetic 10%. The polymer was dissolved in chloroform and purified by passing through aluminum oxide and by removing the solvent under vacuum at 50°C to give a colorless viscous liquid. Yield ≥ 80%. Kinetic experiments were run at least twice to confirm the behavior of each monomer towards the initiating system considered.

Results and discussion

Please do not adjust margins Please do not adjust margins Ring-opening polymerizations of alkyl-substituted epoxides (PO, BO) and glycidyl ether monomers (tBuGE, EEGE, AGE, BnGE) were first performed under N 2 atmosphere with benzyl alcohol:tBuP 4 as initiating system (Scheme 1). These epoxide monomers are among the most common epoxide monomers used to prepare well-defined polyethers. 2 The monomer concentration was fixed to [M] 0 = 2 mol.L -1 and the [tBuP 4 ] 0 /[initiator] 0 ratio was fixed to 1/1. A reaction temperature of 25°C was chosen and is expected to ensure reaction completion in a reasonable amount of time.

Preliminary experiments dealing with the nature of the propagating species were conducted with BO, a quite safe monomer providing the usual characteristics of substituted epoxide.

Investigation of the nature of the propagating species

First, the chemical nature identification of the propagating centers was carried out. Various [BO] 0 /[Initiator] 0 ratios were used, and the [tBuP 4 ] 0 /[initiator] 0 ratio set to 1 (Table 1).

Initiator concentration [I] 0 ranged from 7.5 mmol.L -1 to 60 mmol.L -1 for a fixed monomer concentration [M] 0 = 2.0 mol.L -1

. Kinetic data could be fitted according to a first-order equation. For all [M] 0 /[I] 0 ratios, the reactions were first order with respect to the monomer over the entire conversion range (Figure 1), which is in good agreement with previously reported results for AROP of BO initiated by low acidic molecules-tBuP 4 initiating systems. 16, 22 Furthermore, the poly(BO) molar mass increases linearly with the monomer conversion up to final yield. Experimental molar masses M n are in good agreement with the theoretical values and dispersity Ð remains narrow over the entire conversion range. These results are illustrated in Figure S13, which corresponds to experimental data obtained for [M] 0 /[I] 0 ratio = 50/1 (Table 1 run 4), but the conclusion applies equally well to the five [M] 0 /[I] 0 ratios. The above kinetic results argue in favor of a living polymerization mechanism occurring without transfer or termination reactions under the investigated conditions. Kinetic results obtained for all the [M] 0 /[I] 0 ratios are superimposed on a pseudo first order plot (Figure 1). A linear dependence regarding ln([M] 0 /[M]) versus time can be observed for each ratio. From each curve, a value of propagating constant k p, app was determined using the slope (k p [I] 0 ) of the linear regression. Anionic polymerization in polar organic solvent proceeds via a Scheme 1 tBuP 4 catalyzed AROP of oxirane monomers using BnOH:tBuP 4 as initiating system. b Slope of the curves in Figure 3. ). Please do not adjust margins Please do not adjust margins multiple state mechanism involving contact ions, solvent separated ion pairs and free anions. [START_REF] Schmitt | [END_REF] Szwarc and Schulz have demonstrated that the overall polymerization rate constant, k p , is function of the concentration of active species (c * ) according to the equation ( 1)

Run

ARTICLE

where k ± is the rate constant of the ion pairs (contact and solvent separated ion pairs), k -is the rate constant of free ions and K D is the overall dissociation constant between the total concentration of ion pairs and free ions.

28-30

(1)

In order to determine the contribution of the different propagating species, we plotted k p, app = f([I] -0.5

). Figure 2 shows that k p, app is constant with respect to the reciprocal square root concentration of benzyl alcohol initiator and has an average value of 0.0419 ± 0.0026 L.mol . This result suggests that the propagating active species are exclusively ion pairs. For the polymerization of BO in non-polar toluene, the k p, app values reported by Dentzer et al. 16 and by Misaka et al. 21 using secondary amide-tBuP 4 and alcohol -tBuP 4 as initiated systems are equal to 0.0336 L.mol -1

.min -1 and 0.0480

L.mol -1 .min -1
, respectively. Propagation rate constants have very similar values in THF and in toluene, suggesting comparable interactions between HtBuP 4 + counter-ions and alkoxides in both solvents and that ion pairs are mainly contact ion pairs.

Influence of the epoxide substituent on the propagation rates

Kinetic experiments were carried out with 6 epoxide monomers in THF at 25°C using benzyl alcohol:tBuP 4 as initiating system ( H NMR spectra are in excellent agreement with the theoretical ones. We can thus conclude that the initiation is quantitative and that each benzyl alcohol molecule generates one macromolecule. In addition, the M n of polyPO, polyBO, polyAGE, and polyEEGE determined by SEC in THF are also in very good agreement with the theoretical number-average molar masses, suggesting the absence or a negligible amount of transfer reactions. A discrepancy can be noticed in the case of polyether bearing bulky hydrophobic lateral substituents like polytBuGE and of polyBnGE. However, the dispersity remains low for each sample. A first explanation could be that SEC in THF using PMMA standards significantly underestimated the molar masses for these polyethers. In all cases, the reactions were first order with respect to the monomer over the entire conversion range, and the degrees of polymerization increased linearly with conversion over the same conversion range as shown in Figures S12-S17. The above kinetic results, coupled to the very narrow molar mass distributions (Ð<1.1), provide experimental evidences that support a living and controlled polymerization mechanism (no transfer or termination reactions) and a fast initiation step under the investigated experimental conditions and bearing in mind the relatively low targeted molar masses. The first order kinetic graphs of the AROP of the six epoxide monomers are superimposed in Figure 3, 2 distinctive groups having propagation rate constants with similar orders of magnitude are clearly visible: (i) BnGE, AGE and EEGE are the most reactive monomers having propagation rate constants ranging from 2.23 to 4.12 x 10 -1 L.mol We previously demonstrated that only one propagating specie exists in the case of the polymerization of BO, and we suggest a similar trend for tBuGE, PO. However, polymerizations of BnGE, AGE, EEGE provide much faster kinetics, that seem to follow a similar behavior. The reactivity differences between the two sets of monomers could be explained by the interactions between the alkoxide propagating center and the counter-ion phosphazenium HtBuP 4 + . It is known that the anionic ring-opening polymerization mechanism is based on a tri-molecular transition state valid for ionic pairs propagating species. 33 This state of transition is composed of the propagating center, the counter cation and a monomer unit. The phosphazene base is acting like a Lewis acid and interacts with the following nucleophilic species: the oxanionic propagating center and the monomer unit to be incorporated in the chain as shown in Figure 4. The coordination between alkoxide and HtBuP 4 + stabilizes the transition state and initiates the epoxide cleavage. 34 The substituents present on the monomer are known to play an important role in the transition state 35 which is confirmed in our case. The methyl and ethyl groups of PO and BO do not interact with the phosphazenium counter-ion in the transition state (Figure 4). On the contrary, in the case of BnGE, EEGE and AGE, multiple interactions are possible between the counter-ion HtBuP 4 + and the oxygen atoms in β-position of the lateral group of the last propagating chain monomer unit and of the monomer being incorporated (Figure 4). These additional interactions tend to weakened the main alkoxide -HtBuP 4 + interaction and thus to increase the nucleophilicity of the alkoxide propagating center and thus, the propagation rate. A similar effect has already been observed for the sodium phenolate initiated AROP of methyl glycidyl ether and thiomethyl glycidyl ether compared to the one of propylene oxide. 35 More recently Lynd and coll.

published a DFT calculation study reporting that for the AROP Please do not adjust margins Please do not adjust margins initiated by potassium alkoxide, the energetic transition state barrier was lower in the case of the addition of one AGE unit than one EO unit. 36 It can be noted that tBuGE has a propagation rate constant similar to those of PO and BO despite the presence of an oxygen atom in β position of its lateral substituent. We assume that in this case, the steric hindrance due to the bulky tert-butyl group prevents the coordination effect with the bulky phosphazenium counterion.

Polymerization in the presence of a Lewis acid

In this part, the influence of the addition of a Lewis acid in the reaction mixture will be investigated. To the best of our knowledge, no kinetic studies have ever been reported for the AROP of epoxide using a phosphazene base/ iBu 3 Al as polymerization promoter. Polymerizations were carried out at the same monomer concentration in MeTHF at 25°C with alcohol:tBuP 4 :iBu 3 Al (1:1:2) as catalytic system (Table 3). The solvent was switched from THF to 2-methyltetrahydrofuran (MeTHF), to avoid competitive complexation of the aluminum compound by THF. 37 As previously reported, 38 a ratio of [iBu 3 Al]/[benzyl alcohol] > 1 is required: one equivalent of aluminum derivative is consumed by the low-reactive propagating center, the excess activates the monomer and allows for the propagation. As expected compared to the polymerization without iBu 3 Al, the addition of a Lewis acid leads to faster kinetics. However for these moderate targeted polymerization degrees, a loss of polymerization control suggested by the increase of the dispersity (1.35<Ð<1.60) are concomitantly observed (Figure S18). All the samples were analyzed by MALDI TOF: only the expected populations could be detected (Supporting Information Figures S25-S30) suggesting living polymerization mechanisms without transfer reactions.

In order to determine the monomer order, monomer conversion and ln([M] 0 /[M]) were plotted as function of time (Figures S19-S24). All the polymerizations proceed at high speed: less than 30 minutes were actually needed to reach full conversion. The polymerization of PO, BO and tBuGE were zero-order with respect to the monomer. These results are similar to the ones obtained by Carlotti and coll. who have reported that the polymerizations of PO with alkoxide/iBu 3 Al, For AGE, EEGE and BnGE, kinetic data could not be fitted according to a zero order equation or a first order equation. Carlotti suggests that an induction period is observed at the early stage, and could be explain by the aggregation of complexes between low molar mass propagating macromolecules, phosphazenium counter-ion and iBu 3 Al. After the insertion of few additional monomer units the complex becomes fully soluble and the polymerization rate increased. At high monomer conversion, a reactivity decrease is observed due to competitive complexation of iBu 3 Al by the polyether chain. 41 Attempts to increase of the number of aluminum equivalents with respect to the benzyl alcohol did not led to significant improvement of the linearity of the kinetics. In order to scale the monomer reactivity, the plots of monomer conversion versus time have been superimposed in Figure 5. The presence of iBu 3 Al strongly modify the monomer reactivity scale: k p,PO > k p,BO > k p,EEGE ≈ k p,AGE > k p,BnGE ≈ k p,tBuGE .

Trialkylaluminum deactivates the alkoxide propagating center but in the meantime activates the monomer through its coordination with the oxygen atom of the oxirane ring, which is increasing the electrophilicity of the α-methylene carbon. These opposing effects result in an overall improved reactivity of all the monomers, demonstrating the key role of monomer Please do not adjust margins Please do not adjust margins complexation on the polymerization kinetics. The new reactivity scale can therefore be explained by the strength of the monomer coordination by iBu 3 Al (Figure 6). For PO and BO, iBu 3 Al can only be complexed by the oxygen atom of the epoxides ring resulting in strong monomer activation, high reactivity and fast polymerization. The activation of glycidyl ether monomers, such as EEGE and AGE, is reduced by the presence of additional oxygen atoms in the lateral substituent, which are also interacting with the Lewis acid and thus, slightly decreasing the expected electrophilicity of the α-methylene carbon, then their propagation rates. Similar reactivity differences have already been reported for the copolymerization of ethylene oxide (EO) with glycidyl ether monomers (EEGE and glycidyl methyl ether (GME)) under conventional oxyanionic or monomer-activated mechanisms. 42, 43 Lastly, the lower reactivity of tBuGE and BnGE can be explain by the steric hindrance induced by their bulky side chain which is added to the complexation of the Lewis acid by the oxygen atoms of their side-chains. This hypothesis is supported by the fact that activated-monomer AROP mechanism is already proceeding through bulky aluminum complexes, 44 and by the bulkiness of tBuP 4 , 11 which contributes to re-enforced the cumbersome environment around the propagating center.

Conclusions

Kinetics of the AROP of epoxyde monomers initiated by alcohol / tBuP 4 have been investigated. The propagating species have been identified to be exclusively ions pairs and the polymerization of epoxide monomers occurred via a living controlled polymerization process. A reactivity scale has been established for a library of 6 of the most commonly used epoxides monomers: 2 alkyl-substituted epoxides and 4 glycidyl ethers monomers: k p,BnGE > k p,AGE > k p,EEGE >> k p,tBuGE ≈ k p,PO > k p,BO . The epoxide lateral substituents have a significant influence on the monomer reactivity. Lateral substituents allowing an additional complexation of the phosphazenium couterions led to loose ion pairs and systems displaying higher reactivities. The addition of a Lewis acid in the reaction mixture, typical of monomer-activated ring-opening polymerization, led to an expected drastic increase of the kinetics but also to a full modification of the monomer reactivity scale due to differences in the strength of the monomer activation: k p,PO > k p,BO > k p,EEGE ≈ k p,AGE > k p,BnGE ≈ k p,tBuGE .

Table 1 .

 1 1,2-epoxybutane polymerization, initiated by benzyl alcohol:tBuP 4 (1:1) at 25°C in THF; [M] 0 = 2.0 mol.

Figure 1 .

 1 Figure 1. First order plot for the polymerization of 1,2epoxybutane initiated by benzyl alcohol in decreasing concentration of initiator; in THF at 25°C. (BnOH:tBuP 4 (1:1) as initiating system at [M] 0 = 2 mol.L -1

Journal Name 4 |

 4 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

1 ; 1 .Figure 2 .

 112 Figure 2. Dependence of the propagation rate constant k p, app towards the reciprocal square root concentration of benzyl alcohol for BO polymerization in THF at 25°C.

Figure 3 .

 3 Figure 3. First-order-plot in the polymerization of BnGE, AGE, EEGE, tBuGE, PO, BO at 25°C in THF. Monomer/Initiator = 50/1, [Monomer] = 2.0 mol.L -1 .Figure 4. Transition-state structure scheme for the monomer addition regarding the substituent on the epoxide.

Figure 4 .

 4 Figure 3. First-order-plot in the polymerization of BnGE, AGE, EEGE, tBuGE, PO, BO at 25°C in THF. Monomer/Initiator = 50/1, [Monomer] = 2.0 mol.L -1 .Figure 4. Transition-state structure scheme for the monomer addition regarding the substituent on the epoxide.

38 onium salt/iBu 3

 383 Al,39 and tetraalkylammonium salt/iBu 3 Al as initiating systems40 are following a zero order kinetics according to the equation Rp = k p [I] 0 .([iBu 3 Al] 0 -[I] 0 ) where k p is the propagation rate constant and [I] 0 the initiator concentration.

Figure 5 . 1 , 1 ,

 511 Figure 5. Conversion versus time plots for the polymerization of alkylene oxide monomers BO and PO and for the protected glycidol monomers AGE, EEGE, tBuGE, BnGE. (MeTHF, 25°C, [M] 0 = 2 mol.L -1 , [benzyl alcohol] = 3,7 x 10 -2 mol.L -1 , Benzyl alcohol:tBuP 4 :iBu 3 Al (1:1:2)).

Figure 6 .

 6 Figure 6. Complexation of triisobutylaluminum by various epoxide monomer structures.

Table 2

 2

and Figures

S12-S17

). Poly(propylene oxide) (polyPO, Table

2

run 1), poly(butylene oxide) (polyBO, Table

2

run 2), poly(tert-butyl glycidyl ether) (polytBuGE, Table

2

run 3), poly(allyl glycidyl ether) (polyAGE, Table

2 run 4

), poly(ethoxyethyl glycidyl ether) (polyEEGE, Table

2 run 5

) and poly(benzyl glycidyl ether) (polyBnGE, Table

2

, run 6) were successfully synthesized with very narrow dispersities. For all runs, experimental number-average molar masses measured by end-group analysis of the 1

Table 2 .

 2 Ring-opening polymerization of epoxide monomers initiated by Benzyl Alcohol:tBuP 4 (1:1) at 25°C in THF; [M] 0 = 2 mol.L

	-1 .

k p,PO > k p,BO . When a potassium hydride suspension in THF with 18crown-6 is used as initiating system, a similar tendency was observed by Stolarzewicz et al.

31

with k p,AGE >> k p,PO > k p,BO . But, it is worth noting that the monomer reactivity scale is also strongly dependent of the initiating system as illustrated by the results obtained by Möller et al.

32

in diglyme at 120°C using potassium 3phenyl-1-propanol as initiator where k p,EEGE > k p,AGE > k p,tBuGE .

Table 3 .

 3 Monomer-activated ring-opening polymerization of epoxides initiated by benzyl alcohol:tBuP 4 :iBu 3

	Al (1:1:2) at 25°C in THF; [M] 0

This journal is © The Royal Society of Chemistry 20xx

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements H.D. acknowledges the Ph.D. scholarship offered by the China Scholarship Council (CSC).

Conflicts of interest

There are no conflicts to declare".


Valentin Puchelle

Yannick Latreyte

Mélanie Girardot

Laura Garnotel

Léa Levesque

Olivier Coutelier

Mathias Destarac

Philippe Guégan

Nicolas Illy

Functional Poly(ester-alt -sulfide)s Synthesized by Organo-Catalyzed Anionic Ring-Opening Alternating Copolymerization of Oxiranes and γ-Thiobutyrolactones

Keywords: alternating copolymerization, anionic ring-opening polymerization, γthiobutyrolactone, epoxides, polysulfide

come

INTRODUCTION

The ring-opening polymerization (ROP) of cyclic esters has been widely studied over the last decades. [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF][START_REF] Albertsson | Recent Developments in Ring Opening Polymerization of Lactones for Biomedical Applications[END_REF][START_REF] Kricheldorf | Polymerization mechanism of metal alkoxide initiated polymerizations of lactide and various lactones[END_REF][START_REF] Löfgren | Recent Advances in Ring-Opening Polymerization of Lactones and Related Compounds[END_REF][START_REF] Lecomte | Recent Developments in Ring-Opening Polymerization of Lactones[END_REF][START_REF] Lyubov | Rare-earth metal complexes as catalysts for ring-opening polymerization of cyclic esters[END_REF] It is noteworthy that several papers have reported the (co)polymerization of γbutyrolactone despite the low strain energy of its five-membered ring. [START_REF] Danko | Ring-opening polymerization of gamma-butyrolactone and its derivatives: A review[END_REF][START_REF] Song | Ring-opening (co)polymerization of γ-butyrolactone: a review[END_REF][START_REF] Duda | Oligomerization and copolymerization of γ-butyrolactone -a monomer known as unable to homopolymerize, 1. Copolymerization with ε-caprolactone[END_REF] On the contrary, the polymerization of cyclic thioesters has been the subject of little investigation due to their low polymerizability. The polymerizations of 4-, 6-and 7-membered thiolactones (respectively, βpropiothiolactone, [START_REF] Fleš | Preparation of poly-(S) (-)-[α-(p-toluenesulfonamido)-βpropiothiolactone[END_REF] δ-thiovalerolactone, [START_REF] Overberger | Anionic ring-opening polymerization of thiolactones[END_REF] and ε-thiocaprolactone [START_REF] Overberger | Anionic ring-opening polymerization of thiolactones[END_REF][START_REF] Overberger | A polythioester by ring-opening polymerization[END_REF] ) have been sporadically reported since the sixties. [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] Recently, Suzuki and coll. [START_REF] Suzuki | Thiol-Mediated Controlled Ring-Opening Polymerization of Cysteine-Derived β-Thiolactone and Unique Features of Product Polythioester[END_REF] performed the controlled ringopening polymerization of β-thiolactone derived from cysteine. 4-Hydroxyproline-derived recyclable polythioesters and DNA-mimicking polythioesters were prepared by controlled ROP of δ-thiolactones, [START_REF] Mavila | Dynamic and Responsive DNA-like Polymers[END_REF][START_REF] Yuan | 4-Hydroxyproline-Derived Sustainable Polythioesters: Controlled Ring-Opening Polymerization, Complete Recyclability, and Facile Functionalization[END_REF] and well-defined poly(ε-thiocaprolactone) were achieved by organocatalytic ROP in the presence of thiourea [START_REF] Bannin | Poly(thioester) by Organocatalytic Ring-Opening Polymerization[END_REF] and by lipase-catalyzed ROP. [START_REF] Shimokawa | Enzymatic Synthesis and Chemical Recycling of Polythiocaprolactone[END_REF][START_REF] Duchiron | Enzymatic synthesis of poly(εcaprolactone-co-ε-thiocaprolactone)[END_REF] However, to the best of our knowledge, the ring-opening homopolymerization of γ-thiobutyrolactone has never been reported. In 2011, Du Prez and coll. demonstrated that γ-thiolactones bearing allyl groups can be ring-opened by primary amines releasing a thiol-containing AB monomer, that can be subsequently polymerized by radical (photo) polymerization in a stepwise fashion. [START_REF] Espeel | One-Pot Multistep Reactions Based on Thiolactones: Extending the Realm of Thiol-Ene Chemistry in Polymer Synthesis[END_REF] And recently, the ring-opening copolymerization of substituted γ-thiolactones with trimethylene carbonate was reported. [START_REF] Langlais | Scope and limitations of ring-opening copolymerization of trimethylene carbonate with substituted γ-thiolactones[END_REF] Alternating copolymers have attracted meaningful attention from both academia and industry due to their unique properties compare to random copolymers. Alternating polymerization do open up opportunities for controlling the chain microstructure and the macroscopic properties of polymer materials. Indeed, copolymer properties are dominated not only by comonomer composition but also their sequence distribution. Perfectly alternating structure can magnify the properties of the copolymer, e.g. as separation membranes [START_REF] Zhang | Synthesis of perfectly alternating copolymers for polymers of intrinsic microporosity[END_REF] or solubilities and thermal properties, [START_REF] Nishimori | Design of a maleimide monomer to achieve precise sequence control and functionalization for an alternating copolymer with vinylphenol[END_REF] or biodegradability. [START_REF] Li | Synthesis of biodegradable polycarbonate by alternating copolymerization of carbon dioxide and propylene oxide using a cobalt/Schiff base catalyst[END_REF] In 1998, Nishikubo and coll. reported the alternating copolymerization of γ-thiobutyrolactone with oxiranes catalyzed by quaternary onium salts, affording poly(thioether-alt-ester)s with high dispersities (Ð) and low molar-mass control. [START_REF] Nishikubo | A Novel Synthesis of Poly(ester-altsulfide)s by the Ring-Opening Alternating Copolymerization of Oxiranes with γ-Thiobutyrolactone Using Quaternary Onium Salts or Crown Ether Complexes as Catalysts[END_REF] However, despite the fact that poly(thioether-alt-ester)s are very promising materials due to the presence of repeating units of both biodegradable ester groups and oxydable thioether groups in the main backbone, [START_REF] Mutlu | Sulfur Chemistry in Polymer and Materials Science[END_REF] no further investigation have been conducted using γ-thiobutyrolactone since this pioneering paper. At the same time, large libraries of functional epoxides [START_REF] Mangold | Functional PEG-based polymers with reactive groups via anionic ROP of tailor-made epoxides[END_REF] and functional γ-thiolactones [START_REF] Langlais | Straightforward Xanthate-Mediated Synthesis of Functional γ-Thiolactones and Their Application to Polymer Synthesis and Modification[END_REF][START_REF] Simonet-Davin | A Direct Xanthate-Based Route to γ-Thiolactones[END_REF] have been developed, urging the need for controlled copolymerization of such monomers and opening the way to multiple promising combinations. Very recently, phosphazene base-containing initiating systems have been shown to efficiently catalyzed the alternating copolymerizations of epoxides with either 3,4-dihydrocoumarin [START_REF] Zhang | Expanding the scope of organocatalysis for alternating copolymerization of dihydrocoumarin and styrene oxide[END_REF][START_REF] Zhang | Phosphazene-Catalyzed Alternating Copolymerization of Dihydrocoumarin and Ethylene Oxide: Weaker Is Better[END_REF] or nonstrained γ-selenobutyrolactone. [START_REF] Zhang | Metal-Free Alternating Copolymerization of Nonstrained γ-Selenobutyrolactone with Epoxides for Selenium-Rich Polyesters[END_REF] In this paper, the efficiency of alcohol-phosphazene bases as initiating systems [START_REF] Dentzer | Phosphazene-Promoted Metal-Free Ring-Opening Polymerization of 1,2-Epoxybutane Initiated by Secondary Amides[END_REF][START_REF] Hassouna | Phosphazene/triisobutylaluminum-promoted anionic ring-opening polymerization of 1,2-epoxybutane initiated by secondary carbamates[END_REF][START_REF] Rassou | Anionic ring-opening polymerization of N-glycidylphthalimide: Combination of phosphazene base and activated monomer mechanism[END_REF][START_REF] Illy | Synthesis and anionic ring-opening polymerization of crown-ether-like macrocyclic dilactones: An alternative route to PEG-containing polyesters and related networks[END_REF] for the anionic ring-opening alternating polymerization of γ-thiobutyrolactone with different oxiranes is evaluated (Figure 1). Particular attention is given to the control of molar mass, dispersity and the nature of the chain ends. The influence of the strength of the phosphazene base on the occurrence of side reactions is assessed. The possibility to extend the scope of this method to block copolymer synthesis or to substituted γ-thiolactones is presented. The resulting polymers have been fully characterized and their structure fully elucidated by NMR, SEC, and MALDI-ToF MS.

EXPERIMENTAL SECTION

Materials. Phosphazene base tBuP 4 solution (0.8 mol.L -1 in hexane, Sigma-Aldrich), phosphazene base tBuP 2 solution (2.0 mol.L -1 in THF, Sigma-Aldrich), phosphazene base EtP 2 (>98%, Sigma-Aldrich), phosphazene base tBuP 1 (>97%, Sigma-Aldrich), CaH 2 (93%, 0-2 mm grain size, Acros Organics), benzyl alcohol (99.8% anhydrous, Sigma-Aldrich), activated aluminum oxide (neutral, Brockmann I) and acetic acid (99%, Aldrich) were used as received. 1,2-epoxybutane (BO) (>99%, Sigma-Aldrich), tert-butyl glycidyl ether (tBuGE) (99%, Sigma Aldrich), allyl glycidyl ether (AGE) (>99%, Sigma-Aldrich) and γthiobutyrolactone (TBL) (98%, Sigma-Aldrich) were distilled over CaH 2 twice prior use.

Pentyl-substituted γ-thiobutyrolactone was synthesized following the reported procedure [START_REF] Langlais | Straightforward Xanthate-Mediated Synthesis of Functional γ-Thiolactones and Their Application to Polymer Synthesis and Modification[END_REF] and dried over molecular sieves (3 Å). Poly(ethylene glycol) methyl ether (PEG750) (M n = 750 g mol -1 , Aldrich) was dried by azeotropic distillation of toluene. Ethoxyethyl glycidyl ether (EEGE) was synthesized according to Zhang et al. [START_REF] Zhang | A new strategy for synthesis of "umbrella-like" poly(ethylene glycol) with monofunctional end group for bioconjugation[END_REF] and was distilled over CaH 2 twice prior to use. Toluene and THF were dried with an MBRAUN MB SPS-800 solvent purification system under nitrogen.

Instruments. [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] H and [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR spectra were recorded in CDCl 3 or CD 3 COCD 3 using a Bruker 400 MHz NMR spectrometer. Size Exclusion Chromatography Experiment (SEC) were carried out on three PL gel Mixed-C 5µm columns (7,5  300 mm; separation limits: 0,2 to 2000 kg.mol -1 ) maintained at 40°C and sample Viscotek GPCmax delivery module and 2 modular detectors: a Viscoteck 3580 differential refractive index (RI) detector and a Shimadzu SPD20-AV diode array UV detector. THF was used as the mobile phase at a flow rate of 1 mL.min -1 , toluene was used as a flow rate marker. All polymers were injected (50 μL) at a concentration of 5 mg.mL -1 after filtration through a 0.45 μm pore-size membrane.

The OmniSEC 4.6.2 software was used for data acquisition and analysis. Number-average molar masses (M n ), weight-average molar masses (M w ) and dispersities (Đ = M w /M n ) were determined by SEC with a calibration curve based on poly(methyl methacrylate) (PMMA) standards (from Polymer Standard Services), using the RI detector. Matrix-Assisted Laser Desorption and Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS). Mass spectra were recorded by MALDI -TOF MS using dithranol as a matrix and NaI as cationizing agent using a Bruker Autoflex III smartbeam mass spectrometer, equipped with a laser that produces pulses at 337 nm or using an Applied Biosystems 4700 Proteomics Analyzer instrument. Spectra were recorded in reflector or in linear mode at an accelerating potential of 20 kV. Samples were prepared by dissolving the polymer in THF at a concentration of 2 mg.mL -1 . A 10 μL aliquot of this solution was mixed with 20 μL of matrix solution and 10 μL of NaI solution (both at 20 mg.mL -1 in THF). Poly(ethylene oxide) standards (Polymer Standards Service) of known structures, M n = 1120, 3500 and 5500 g.mol - 1 were used to calibrate the mass scale. In all cases, to determine m/z, the molar mass of the sodium cation was added. The thermal decomposition of a poly(TBL-alt-tBuGE) was evaluated by thermogravimetric analysis (TGA) using a TA Q50 apparatus. Data were collected using a 20 °C/min ramp to 600 °C. The experiment was run under a stream of nitrogen. DSC measurements were carried out using a TA DSC Q2000 calibrated with indium standard. Sample (ca 15 mg) was placed in an aluminum cell and initially cooled at -70°C.

Two heating (10 K min -1 ) -cooling (10 K min -1 ) cycles were then applied in the -70 to 200 °C temperature range.

General copolymerization procedure. The (co)polymerization was carried out according to the following typical procedure. In a glove-box, 1.0 of mL toluene, 20 μL of benzyl alcohol (0.1933 mmol), 0.685 mL of tert-butyl glycidyl ether (4.8252 mmol), and 0.420 mL of γthiobutyrolactone (4.8517 mmol) were introduced under nitrogen into a polymerization tube fitted with a Rotaflo ® . Then, 240 μL of tBuP 4 solution (0.1920 mmol) was added using a microsyringe. After closure of the reaction flask, the reaction mixture was stirred at 50 °C and left to react for the required period of time. The progress of the reaction was monitored by removing samples from the polymerization reactor through a septum at various reaction times for 1 H NMR analyses. The conversion of tBuGE was calculated by comparing the signals corresponding to the -C(CH 3 ) 3 of both monomer and polymer and the CH group of the oxirane group (Figure S2). The conversion of TBL was calculated by comparing the signals of the -CH 2 -CH 2 -CH 2 -of the monomer to the one of its ring-opened counterpart (Figure S2).

The reaction was quenched by addition with 0.2 mL of 1 mol.L -1 acetic acid (2.0 mmol). After removing residual monomers and solvent by rotary evaporation, the polymer was dissolved in ethyl acetate, stirred for 30 min in the presence of neutralized aluminum oxide and purified by passing through neutralized aluminum oxide. After removing the solvent under vacuum at 50 °C a colorless viscous liquid was obtained. Yield: 88 %. Deviations from this general procedure are summarized in Tables 1 and2. The polymers were fully characterized by 1 H and [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR spectrometry and MALDI TOF mass spectrometry. Molar masses were determined by SEC using PMMA standards and by [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] H NMR (end-group analysis) using the aromatic protons of the initiator and the CH 3 in each repeating unit. [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF]

Table 1. Experimental conditions and molecular characteristics of poly(TBL-alt-tBuGE)

samples synthesized in toluene using benzyl alcohol and different phosphazene bases as initiating systems.

Run

Base

[tBuGE] 0 /[TBL] 0 /[ROH] 0 /[Base] T (°C) Time (h) Cv tBuGE (%) Cv TBL (%) M n,th (g mol - 1 )
M n,NMR a (g mol -1 ) c PEG750 was used instead of benzyl alcohol.

M n,SEC b (g mol - 1 ) Ð b 1 tBuP 4 50/0/1
d tBuP 2 was introduced first in the polymerization tube and THF was subsequently evaporated using high vacuum from a vacuum line (≈10 -6 hPa) before introducing the solvent and all the other reactants.

e The pentyl-substituted γ-thiobutyrolactone (TBL2) was used instead of TBL.

RESULTS AND DISCUSSION

In the first part of the work, homopolymerizations (Table 1, runs 1 and 2) and copolymerization (Table 1, runs 3 to 11) of tBuGE and TBL were performed in toluene using benzyl alcohol-tBuP 4 (1-1 or 1-0.5) as initiating system at different temperatures. Targeted degrees of polymerization (assuming quantitative initiation) were deliberately set at moderate values (X n =50-100) so that chain ends could be easily evidenced by [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] H NMR and MALDI TOF MS.

As previously reported with tBuP 4 and benzyl alcohol, [START_REF] Puchelle | Polymerization of epoxide monomers promoted by tBuP4 phosphazene base: a comparative study of kinetic behavior[END_REF] the homopolymerization of tBuGE is controlled at 25°C (Table 1, run 1). On the contrary after 48 h, no homopolymerization of TBL has occurred at 90°C (Table 1, run 2). TBL conversion was determined by 1 H NMR analysis of the reaction mixture after solvent evaporation (Figure S1) and is equal to 4% demonstrating the ring-opening of TBL, possibly followed by one additional monomer unit.

Several reaction temperatures were investigated for the copolymerization (Table 1 runs 3-11).

The obtained polymers display spectroscopic data consistent with the ring-opened alternating structure, whatever the polymerization conditions. An assignment of all the signals observed in 1 H, [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C and COSY NMR could be made and is available directly on the spectra (Figure 2 and Figure S3). In the case of the tBuGE homopolymerization (Table 1 run 1), the -CH 2 -of the initiator is in alpha position of an ether group and its 1 H NMR singlet is located at 4.52 ppm (Figure S4). For the copolymerization reactions, no 1 H NMR signal is visible around 4.6

ppm and the singlet corresponding to the initiator -CH 2 -is shifted at 5.11 ppm. This value is in agreement with a CH 2 attached to an ester group. From these observations, it can be concluded that after deprotonation by the phosphazene base, the benzyl alcoholate predominantly attacked the thioester and ring-opened the TBL (Scheme 1). The first monomer unit of a polymer chain is therefore a ring-opened TBL.

The chemical shift of the 1 H NMR quintet corresponding to the -CHR-group of the oxirane monomer at 4.98 ppm and the absence of signals corresponding to polyether blocks between 3.6 and 3.8 ppm are in favor of a strictly alternating structure of the chains (Figure 2). The low intensity signal at 3.79 ppm can be attributed to the -CHR-group of the last oxirane monomer unit. In addition, from the intensity ratio of protons of the oxirane -CHR-group at 4.98 and 3.79 ppm (h and h' in Figure 2) vs TBL -CH 2 -CH 2 -CH 2 -at 1.91 ppm (e in Figure 2), the tBuGE/TBL ratio was found to be 1/1 for each run of Table 1 having total monomer conversion ranging between 35 and 100 %. Interestingly, Nishikubo et al. prepared alternating copolymers using initiating systems for which the homopolymerization of both monomers was not possible under the same conditions. [START_REF] Nishikubo | A Novel Synthesis of Poly(ester-altsulfide)s by the Ring-Opening Alternating Copolymerization of Oxiranes with γ-Thiobutyrolactone Using Quaternary Onium Salts or Crown Ether Complexes as Catalysts[END_REF] Here, using benzyl alcohol -tBuP 4 as initiating system, NMR spectra confirmed that all the copolymers were obtained by the perfect ringopening alternating copolymerization of tBuGE and TBL as shown in Scheme 1 even if the homopolymerization of tBuGE is possible under the same conditions. (Table 1, run 4).

Scheme 1. Postulated mechanism for the alternating ROP of γ-thiobutyrolactone and tertbutylglycidylether using benzyl alcohol-phosphazene base tBuP 4 as initiating system.

The effect of the reaction temperature was examined. A first copolymerization was performed at 90°C (Table 1, run 3), which is the temperature predominantly used by Nishikubo et al. with quaternary ammonium salts or crown ether complexes as initiating systems. [START_REF] Nishikubo | A Novel Synthesis of Poly(ester-altsulfide)s by the Ring-Opening Alternating Copolymerization of Oxiranes with γ-Thiobutyrolactone Using Quaternary Onium Salts or Crown Ether Complexes as Catalysts[END_REF] After 22 h reaction time, full conversions of both monomers were obtained. No polyether fragments are detectable by MALDI ToF, 1 H and [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR analyses (Figure 4 and S5). The experimental molar mass determined by SEC (Figure 3A) was lower than the theoretical one. A part of this difference could be due to difference in the hydrodynamic volume of the PMMA standards used for SEC calibration with that of the poly(ester-altsulfide). However, the extent of this difference and a broad dispersity (Figure 3A) strongly suggest the occurrence of transfer reactions. This observation is in contrast to the results obtained by Nishikubo et al. who observed at the same temperature the formation of higher molar masses than the theoretical ones due to a step-growth polymerization mechanism. [START_REF] Nishikubo | A Novel Synthesis of Poly(ester-altsulfide)s by the Ring-Opening Alternating Copolymerization of Oxiranes with γ-Thiobutyrolactone Using Quaternary Onium Salts or Crown Ether Complexes as Catalysts[END_REF] The MALDI TOF spectrum (Figure 4, Table 1 run 3) displays 4 distributions, for which it is worth noting that the molar mass difference between two peaks is strictly equal to the molar mass of one alternating repeat unit (232.11 mass unit). The four populations have been identified as follows in order of importance. The main distribution corresponds to the expected alternating polymer cationized by a sodium ion and end-terminated by a benzyloxy group at one end and by a hydroxy group at the other end. The second most important distribution can be attributed to perfectly alternating macrocycles cationized by sodium ion. It can be noted, that the presence of polymer chains initiated by enolate anions has been ruled out due to the absence of the corresponding double bond signals on the 1 H and [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR spectra (Figure S5). The macrocyclization can be explained by an intramolecular transesterification reaction occurring during the polymerization process as shown in Scheme S1. Similarly, intermolecular transesterification reactions must occur but they cannot be detected by mass spectrometry because they do not generate structural differences. The large distribution and the gap observed between theoretical and experimental molar masses measured by SEC in this reaction might be partly explained by these intermolecular transesterification reactions. The third distribution is due to alternating macrocycles containing one additional oxirane unit. We assume that the additional unit might be due to the introduction of a slight initial excess of oxirane and is occurring after the full BTL conversion. Surprisingly, the linear counterpart, namely the expected alternating structure containing one additional oxirane unit, is barely visible on the MALDI TOF spectrum, suggesting their high cyclization tendency. The last minor population corresponds to alternating polymer chains containing one additional oxirane unit, cationized by a sodium ion and end-terminated by a hydroxy group at both ends. This chain structure could be explained by the presence of moisture traces in BTL and water initiation according to Scheme S2. using benzyl alcohol-phosphazene base tBuP 4 as initiating system at 90°C (Table 1, run 3).

The reaction temperature was decreased to 50°C (Table 1, run 4). For this run, the M n determined by [START_REF] Liu | Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters[END_REF] H NMR and SEC is in very good agreement with the theoretical one. In addition, the resulting polymer exhibits a lower molar mass distribution (Đ = 1.21) with significantly less low-molar-mass tailing compared to the polymer prepared at 90°C suggesting a strong decrease of the transesterification reactions (Figure 3). Under the same conditions, M n increased linearly with conversion and low dispersities (<1.22) are observed (Figure S7). These results support the control of M n at 50°C. This result is confirmed by MALDI TOF MS analysis, which displays a very major distribution corresponding to the expecting alternating structure initiated by benzyl alcoholate and a minor distribution due to moisture initiation (Figure 5). However, residual alternating macrocycles are still detectable in the low molar mass region. A detailed kinetics experiment was carried out at 40°C. As shown in Figure S8, conversions of both monomers TBL and tBuGE increased equally with time as expected for an alternating copolymerization. The fast increase of the conversions at the beginning of the polymerization is due to the heat release observed during phosphazene base addition. SEC traces gradually shift toward higher molar masses (Figure S9). However, the M n versus conversion plot shows an increasing discrepancy between theoretical and experimental molar masses with increasing conversion (Figure S10). A further temperature decrease to 35°C fully suppressed the cyclization reaction according to the MALDI TOF spectrum (Figure S11). The regioregularity of the ring-opening reactions of epoxides during the polymerization was calculated using 1 H NMR spectra. The β-cleavage of the epoxy ring of tBuGE was found to be higher than 98% at 90°C (Figure S6) and 100% for lower temperatures.

In addition, a copolymerization was performed at 25°C using tBuP 4 to deprotonate a PEG750 macroinitiator (Table 1, run 12). SEC traces of the PEG750 macroinitiator and of the resulting copolymer are shown in Figure 3C. No remaining homopolymer is visible on the chromatogram, demonstrating a 100% macroinitiator efficiency. A well-defined block copolymer with an alternating segment, PEG750-block-poly(TBL-alt-tBuGE), is obtained.

Interestingly, at a temperature of 50°C or lower, only perfectly alternating structures are detected by MALDI TOF MS without any defect. Moreover, for all the tested conditions and for partial monomer conversions, all the linear polymer chains are terminated by a hydroxy group. Taking into account the absence of thiol-terminated polymer chains, the perfectly alternating structure, the absence of tBuGE homopolymerization and the fact that the copolymerization is slower than the epoxide homopolymerization, we postulate that a fast complexation takes place between the generated phosphazenium alcoholates and a thiolactone (Scheme 1) preventing tBuGE homopolymerization. The generated complex is in equilibrium with the thiolate-terminated polymer chain. In the next step, the ring-opening of the thiolactone is immediately followed by a fast nucleophilic addition between the highly nucleophilic thiolate anion [START_REF] Heise | Characterization of imidazole-cured epoxyphenol resins[END_REF] and an oxirane ring. In this mechanism, the equilibrium is the rate-determining step, which is supported by the linear increase of monomer conversions with time (Figure S8), demonstrating a zero-order kinetics independent of monomer concentration. using benzyl alcohol-phosphazene base tBuP 4 as initiating system at 50°C (Table 1, run 4).

Tetrameric tBuP 4 H + phosphazenium alcoholates are highly reactive and have a tendency to attack ester groups. [START_REF] Zhao | Phosphazene-Promoted Metal-Free Ring-Opening Polymerization of Ethylene Oxide Initiated by Carboxylic Acid[END_REF] Alcoholates of monomeric and dimeric phosphazenium salts are expected to be less reactive with ester groups due to a reduced charge delocalization of their cations. The results obtained with monomeric (tBuP 1 , pKa, ACN = 26.9) and dimeric (tBuP 2 , pK a,ACN = 33.5; EtP 2 , pK a,ACN = 32.9) [START_REF] Boileau | Activation in anionic polymerization: Why phosphazene bases are very exciting promoters[END_REF] phosphazene bases are summarized in Table 1. The use of tBuP 1 led to a strong decrease of the polymerization kinetics at 90°C (Table 1, run 13).

This decrease of the reactivity was expected considering the less bulky HtBuP 1 + cation and shorter interionic distances with the propagating centers. [START_REF] Boileau | Activation in anionic polymerization: Why phosphazene bases are very exciting promoters[END_REF] The reactivity decrease simultaneously led to the reduction of the transfer reactions and to higher M n compared to tBuP 4 at 90°C.

In the presence of the dimeric phosphazene base tBuP 2 (Table 1, runs 14-20), an intermediate behavior between tBuP 1 and tBuP 4 was expected. However, the propagating species are far more reactive in the presence of HtBuP 2 + than HtBuP 4 + cations: 99% monomer conversion after 5 h at 25°C with tBuP 2 (Table 1, run 16) compared to only 45% conversion after 96 h with tBuP 4 (Table 1, run 11). A polymerization was carried out at -20°C for which full monomer conversions were reached after 18h (Table 1, run 14). Commercially-available tBuP 2 is diluted in THF and added as-is in the polymerization tube. But, it is known that the addition of a small amount of Lewis bases, such as ethers, has a tremendous effect on the kinetics of the propagation in apolar solvent with higher propagation rates than the original ones. [START_REF] Müller | 26 -Carbanionic Polymerization: Kinetics and Thermodynamics[END_REF] Therefore, a reaction was performed at 25°C by adding first tBuP 2 and by evaporating out the THF prior to the addition of the solvent and the other reactants (Table 1 run 17). In addition, an identical reaction was carried out with pure EtP 2 (Table 1 run 18). Kinetics remained much faster with dimeric phosphazene bases than with tBuP 4 even in the absence of THF. In order to explain the rate difference, we assume that the basicity of the phosphazene base has an impact on the equilibrium between the alcoholate-thiolactone complex and the thiolate-terminated polymer chain: the higher basicity of tBuP 4 being more favorable to an intramolecular backbiting attack of the thiolate regenerating the γ-thiolactone. The polymerization performed with tBuP 2 at -20°C or 25°C reached full conversions while maintaining a good control of the number-average molar masses (Table 1, runs 16-18 and 20).

The dispersity decreased with the reaction temperature, even though a high-molar-mass shoulder is visible on the size exclusion chromatogram for the reaction at -20°C (Figure S12).

As previously, a temperature increase led to more transfer reactions and a discrepancy between experimental and theoretical number-average molar masses is observed for polymerization performed at temperature higher than 35°C (Table 1, runs 14-15).

Reaction conditions have been identified allowing the synthesis of well-defined poly(TBLalt-tBuGE) with limited degrees of polymerization (X n < 50). In order to increase the polymer number-average molar masses, two reactions were carried out by increasing the BO boiling point is 63°C, part of the monomer evaporated and penetrated the rubber stopper.

In order to develop a family of poly(ester-alt-thioether) with various pendant groups, we decided to extend the scope of used monomers. To determine if the alternating copolymerization can be extended to substituted γ-thiobutyrolactone, preliminary experiments have been carried out with a pentyl-substituted γ-thiobutyrolactone (BTL2, Figure 1). As the copolymerization is expected to be more challenging due to the steric hindrance introduced by the pentyl group in alpha position of the sulfur atom, a first polymerization was performed at 90°C in the presence of previously determined very efficient tBuP 2 (Table 1, run 21). After 26 h, full monomer conversions were achieved. The 1 H NMR spectrum of the obtained polymer is compatible with a fully alternating polymer structure (Figure S14). The MALDI TOF spectrum is shown in Supporting Information (Figure S15). The 5 major and 3 minor populations correspond to the expected alternating repeating unit confirming the 1 H NMR result. 4 of the 5 major populations could be identified (Table S1). The main population corresponds to the expected structure. As previously at high temperature, two types of macrocycles are present in the final polymer: perfectly alternating macrocycles and alternating macrocycles with one additional oxirane unit. Thus, MALDI TOF spectrum demonstrates the presence of non-negligible transesterification reactions, which explains the low number-average molar mass determined by SEC. In a second experiment, the reaction temperature was decreased to 30°C (Table 1, run 22). Compared to the previous reaction, a higher M n and a lower dispersity were obtained, suggesting a decrease of the transesterification reactions.

In the last part of this work, the polymerization reactions were performed in the bulk with γthiobutyrolactone and four different oxirane monomers: tBuGE, AGE, EEGE and BO.

Regardless of the oxirane nature, 1 H NMR spectra are in excellent agreement with alternating copolymerization mechanisms (Figure 6, Figures S16-17 S17) imply that the ring-opening of BO also occurs in a α-scission manner when attacked by the thiolate at 60°C, however to a much lesser extent (<4%) than for the more sterically favored β-scission.

The thermal behavior of a poly(TBL-alt-tBuGE) was determined using DSC and TGA. A TGA trace for a poly(TBL-alt-tBuGE) (Table 1 run 4) is presented in Figure S18. The decomposition temperature after 20% weight loss occurred around 330°C and the decomposition temperature at maximum degradation rate (derivative thermogravimetry) was 350°C. DSC curves corresponding to a poly(TBL-alt-tBuGE) (Table 1 run 4) revealed a glass transition temperature at -32°C (Figure S19). (Table 2, run 7).

CONCLUSION

Benzyl alcohol -phosphazene bases are effective initiating systems for the alternating copolymerization of epoxides and γ-thiobutyrolactone. Perfectly alternating sequences were achieved and proved by 1 H, [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR and for the first time by MALDI TOF mass spectrometry. Optimized polymerization conditions were determined to allow the synthesis of well-defined poly(ester-alt-sulfide) with controlled chain ends. However, molar mass of the alternating copolymer is still limited. Indeed, reaction temperature or time increase led to the apparition of transesterification reactions resulting in macrocycle formation, molar mass limitation and higher dispersities. Different phosphazene bases were evaluated: tBuP 1 is not sufficiently active to ensure high monomer conversion in a reasonable amount of time, tBuP 4 is more active but tBuP 2 displayed the highest reactivity with very fast conversion at room temperature while maintaining well-defined copolymer structures. Different epoxide monomers were copolymerized with γ-thiobutyrolactone leading to original poly(ester-alt-sulfide)s. The possibility to synthesize well-defined polyether-block-poly(ester-alt-sulfide) was also demonstrated. In addition, the polymerizability of γ-substituted thiobutyrolactone was shown by preliminary experiments, which paves the way to a multitude of combinations between epoxides and γ-thiobutyrolactones considering the rich chemistry of both families of monomers. [START_REF] Langlais | Straightforward Xanthate-Mediated Synthesis of Functional γ-Thiolactones and Their Application to Polymer Synthesis and Modification[END_REF][START_REF] Brocas | Polyether synthesis: From activated or metal-free anionic ring-opening polymerization of epoxides to functionalization[END_REF] The properties of this attractive class of functional, (bio)degradable and ROSsensitive [START_REF] Li | Synthesis of biodegradable polycarbonate by alternating copolymerization of carbon dioxide and propylene oxide using a cobalt/Schiff base catalyst[END_REF] alternating copolymers are currently under investigation.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS publication website at DOI: 1 H and [START_REF] Fritze | Thiolactone polymerization and catalysts[END_REF] C NMR spectra of a TBL homopolymerization reaction mixture (

Figure 1 .

 1 Figure 1. γ-thiobutyrolactones and oxiranes used as monomers for the alternating

  H NMR (CDCl 3 ): 7.34 (m, 4 H, aromatic), 5.11 (s, 2 H, Ar-CH 2 -OCO-), 4.98 (quint, 13.34 H, -CHR-O-polymer backbone), 3.79 (m, 13.34 H, -CHR-OH), 3.49 (d+m,31.81, -CH 2 -O-tBu), 2.56-2.83 (m, 62.78, -S-CH 2 -CHR-O, -CH 2 -CH 2 -S-), 2.44 (t, 29.68 H, -OCO-CH 2 -), 1.91 (quint, 29.68, -CH 2 -CH 2 -CH 2 -S-), 1.16 (m, 139.58 H, -C(CH 3 ) 3 ).13 C NMR (CDCl 3 ): 172.54 (-OCO-), 128.64, 128.29, 128.27 (Ar-), 73.34 (-O-C(CH 3 ) 3 ), 72.65 (-CHR-O-), 69.88 (-CHR-OH), 66.33 (Ar-CH 2 -O-), 64.47 (-CH 2 -O-tBu terminal unit), 61.73 (-CH 2 -O-tBu), 35.71 (-S-CH 2 -CHR-OH), 33.20 (-CH 2 -CH 2 -CH 2 -S-), 32.48 (-S-CH 2 -CHR-O-), 31.92 (-CH 2 -CH 2 -CH 2 -S-), 27.58 (-C(CH 3 ) 3 ), 24.81 (-CH 2 -CH 2 -CH 2 -S-).

Figure 2 .

 2 Figure 2. (a) 1 H NMR and (b) 13 C NMR spectra of poly(TBL-alt-tBuGE) in CDCl 3 at 25°C

Figure 3 .

 3 Figure 3. A. SEC traces of poly(TBL-alt-tBuGE) initiated by benzyl alcohol in the presence

Figure 4 .

 4 Figure 4. MALDI TOF spectra (reflectron mode) of a poly(TBL-alt-tBuGE) synthesized

Figure 5 .

 5 Figure 5. MALDI TOF spectra (reflectron mode) of a poly(TBL-alt-tBuGE) synthesized

  ): -CHR-attached to an ester group and no signals corresponding to polyether blocks. For the monomers having an electronegative oxygen in beta position of the oxirane rings (tBuGE, AGE and EEGE), reaction conditions have been identified that allow the control of the number-average molar mass and the preparation of monomodal polymers with dispersities around 1.2-1.3. In the case of 1,2-epoxybutane, only low molar-mass polymers have been synthesized due to difficulties in the control of the copolymerization with slower kinetics and transfer reactions even at 25°C. The β-cleavage occurred with 100% selectivity in the case of the copolymerization of BTL with EEGE and AGE. However, the weak signals from 4.0 to 4.3 ppm in the 1 H NMR spectrum (Figure

Figure 6 .

 6 Figure 6. SEC trace (left) and 1 H NMR spectrum (right) of poly(TBL-alt-EEGE) in CDCl 3

Table 1

 1

, run 3) and B. at 50°C (Table

1, run 4); C. SEC traces of PEG750 (dashed line) and of PEG750-block-poly(TBL-alt-tBuGE) initiated by PEG750 in the presence of tBuP 4 synthesized at 25°C (Table 1, run 12).

  ([tBuGE] 0 +[BTL] 0 )/[ROH] 0 ratio. X n values of 100 and 200 were targeted in the presence of tBuP 4 at 50°C (Table1, run 7) and in the presence of tBuP 2 at 25°C (Table1, run 19), respectively. In both cases however, the increase of the reaction time favored the generation of macrocycles and thus, the obtained number-average molar masses (M n = 6900 and 9000 g.mol -1 ) are lower than the theoretical ones (M n,th = 11100 and 14000 g.mol -1 , respectively). A similar limitation of the M n due to intramolecular transesterification (backbiting) has already been reported for the alternating copolymerization of epoxides and dihydrocoumarin catalyzed by phosphazene bases.[START_REF] Hu | Ring-Opening Alternating Copolymerization of Epoxides and Dihydrocoumarin Catalyzed by a Phosphazene Superbase[END_REF]

Table 2 .

 2 Experimental conditions and molecular characteristics of poly(TBL-alt-M 1 ) samples synthesized in the bulk using benzyl alcohol and tBuP 4 as initiating system. Determined by 1 H NMR in CDCl 3 . b Determined by SEC (THF, 40 °C, PMMA standards). c

	Run	M 1	[M 1 ] 0 /[TBL] 0 /[ROH] 0 /[tBuP 4 ]	T	Time	Conv M1	Conv TBL	M n,th	M n,NMR	a	M n,SEC	b	Ð b
				(°C)	(h)	(%)	(%)	(g mol -	(g mol -1 )	(g mol -1 )
								1 )
		tBuGE	25/25/1/1	90	18	99	99	5850	4200		3050		1.87
		tBuGE	12.5/12.5/1/1	60	18	98	98	2900	2100		2250		1.44
		tBuGE	25/25/1/1	40	24	81	83	4800	4100		4050		1.22
		AGE	25/25/1/1	40	4.5	79	82	4450	4200		3900		1.28
		AGE	25/25/1/1	25	22	93	93	5100	4800		4200		1.27
		EEGE	25/25/1/1	50	48	99	99	6250	4100		3000		2.41
		EEGE	25/25/1/1	25	96	99	99	6250	6050		6200		1.35
		BO	25/25/1/1	60	19	81 c	62	2800	2100		1500		1.32

a

Table 1 ,

 1 run 2) of a tBuGE homopolymerization reaction mixture (Table1, run 1), of a copolymerization between tBuGE and TBL (Table1, run 3 and 9), of a poly(TBL-alt-BO) in CDCl 3 (Table2, run 8), of a poly(TBL2-alt-tBuGE) (Table1, run 21) and of a poly(TBL-alt-AGE) in CDCl 3 (Table2, run 5). 2D COSY-NMR spectrum of a poly(TBL-alt-tBuGE) (Table1, run 4).Relationship observed between number-average molar mass and monomer conversion for a copolymerization at 40°C and at 50°C (Table1, runs 5 and 8). MALDI TOF spectra of a poly(TBL-alt-tBuGE) (Table1, run 9) and of a poly(TBL2-alt-tBuGE) (Table1, run 21). SEC traces of poly(TBL-alt-tBuGE) (Table1, run 8 and 19-20), of poly(TBL2-alt-tBuGE) (

Table 1 ,

 1 run 21) and of poly(TBL-alt-AGE) (Table2, run 5). Mechanisms for the intramolecular transesterification explaining the formation of macrocycles and for the alternating ROP of γthiobutyrolactone and tert-butyl glycidylether initiated by moisture traces. TGA trace and DSC curve of a poly(TBL-alt-tBuGE) (Table1, run 4).

AUTHOR INFORMATION

Corresponding Author

* Nicolas Illy: Phone 33 (0)1 44275513 E-mail: nicolas.illy@sorbonne-universite.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest ACKNOWLEDGMENT The authors would like to acknowledge Gilles Clodic and Dr Lucrèce Matheron-Duprez (Mass Spectrometry facility, Institut de Biologie Paris Seine) for their help and the constructive discussions on MS analysis.