Evgueni Kirillov
email: evgueni.kirillov@univ-rennes1.fr

Konstantin Rodygin
email: k.rodygin@spbu.ru

Valentine Ananikov

Recent advances in applications of vinyl ether monomers for precise synthesis of custom-tailored polymers

Keywords: vinyl ethers, poly(vinyl ethers), polymerization mechanism, stereocontrol in polymerization

Recent advances in applications of vinyl ether monomers

for precise synthesis of custom-tailored polymers E. Kirillov, K. Rodygin, Valentine P.

Introduction

Amazing achievements in vinyl ether (VE) chemistry, from the first example of VE polymerization [START_REF] Wislicenus | Ueber Vinyläthyläther[END_REF] to a plethora of recent research [START_REF] Sugihara | HCl•Et2O-Catalyzed Metal-Free RAFT Cationic Polymerization: One-Pot Transformation from Metal-Free Living Cationic Polymerization to RAFT Radical Polymerization1[END_REF][3][START_REF] Uchiyama | Cationic RAFT Polymerization Using ppm Concentrations of Organic Acid[END_REF][START_REF] Kottisch | Cationic Polymerization of Vinyl Ethers Controlled by Visible Light[END_REF][START_REF] Ciftci | Living Cationic Polymerization of Vinyl Ethers through a Photoinduced Radical Oxidation/Addition/Deactivation Sequence[END_REF][START_REF] Kottisch | Photocontrolled Interconversion of Cationic and Radical Polymerizations[END_REF][START_REF] Michaudel | Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers[END_REF][START_REF] Chen | A Versatile Ligand Platform for Palladium-and Nickel-Catalyzed Ethylene Copolymerization with Polar Monomers[END_REF][START_REF] Peterson | Electrochemically Controlled Cationic Polymerization of Vinyl Ethers[END_REF], facilitated enormous progress in VEs synthesis and in design of advanced polymer materials. The fundamentally important point in this subject is the remarkable versatility of VE structures that resonates with the tunability of the corresponding polymeric systems. To exemplify, industrially manufactured iso-butyl vinyl ether (IBVE) has being ubiquitously utilized as monomer in a diverse range of research projects [START_REF] Miyamoto | Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system[END_REF][START_REF] Aoshima | Living cationic polymerization of vinyl monomers by organoaluminum halides. 3. Living polymerization of isobutyl vinyl ether by ethyldichloroaluminum in the presence of ester additives[END_REF][START_REF] Kamigaito | Living cationic polymerization of isobutyl vinyl ether by hydrogen chloride/Lewis acid initiating systems in the presence of salts: in-situ direct NMR analysis of the growing species[END_REF][START_REF] Yamada | Atom Transfer Radical Polymerization of Poly(vinyl ether) Macromonomers[END_REF][START_REF] Sugihara | Thermosensitive Random Copolymers of Hydrophilic and Hydrophobic Monomers Obtained by Living Cationic Copolymerization[END_REF][START_REF] Shibata | Quantitative Synthesis of Star-Shaped Poly(vinyl ether)s with a Narrow Molecular Weight Distribution by Living Cationic Polymerization[END_REF][START_REF] Kanazawa | Major Progress in Catalysts for Living Cationic Polymerization of Isobutyl Vinyl Ether: Effectiveness of a Variety of Conventional Metal Halides[END_REF]. At the same time, many VEs and poly(VE)s are now ton-scale products of the chemical industry [START_REF] Reyntjens | New Materials from poly(vinyl ethers)[END_REF].

A new wave of interest to VEs was raised by sustainability demands. For instance, VEs can be obtained from various sources of carbon using calcium-carbide-based technologies from fossil carbon (coal, etc.), regenerated carbon (wastes and end-of-life sources), biomass (including biomass processing wastes), as well as from carbon dioxide in a future perspective (via conversion of carbon dioxide to carbon) [START_REF] Rodygin | Calcium-Based Sustainable Chemical Technologies for Total Carbon Recycling[END_REF]. Potential ability to access VEs from virtually any carbon source including renewable biomass [START_REF] Rodygin | Calcium carbide looping system for acetaldehyde manufacturing from virtually any carbon source[END_REF] identified these monomers as highly promising candidates to design bio-based and sustainable plastics.

The classical chain-growth polymerization protocols provide major advantages over the step-growth (e.g., polycondensation) processes in accessing functional polymers with enhanced control over the organization of repeating units. This supremacy, once declared [START_REF] Odian | Principles of Polymerization[END_REF], has been substantiated by regular upgrades of the chain-growth polymerization methods and conditions.

The regular protocols of VE polymerization include (Scheme 1): i) living cationic polymerization;

ii) reversible addition-fragmentation chain transfer (RAFT) and related polymerization reactions; iii) free radical polymerization. Another option is copolymerization of VEs by using coordination/insertion catalysts or combined composition-and topology-controlled polymerization techniques.

O R Vinyl ether

Homopolymerization Copolymerization

Poly(VEs) VEs-co-polymers i) "living" polymerization ii) RAFT-polymerization iii) free-radical polymerization i) coordination/insertion ii) combined iii) topology-controlled Scheme 1. Methods of VE polymerization.

For a long time, methods of cationic polymerization of VEs lacked control over the molecular weight distributions and also resulted in the poorly controlled nature of the chainend groups in poly(VE)s. The failure was due to excessive activity of the carbenium end groups.

A solution came with the discovery of long-lived cationic species [START_REF] Higashimura | Possible Formation of Living Polymers of p-Methoxystyrene by Iodine[END_REF][START_REF] Higashimura | Synthesis of p-Methoxystyrene-Isobutyl Vinyl Ether Block Copolymers by Living Cationic Polymerization with Iodine[END_REF] and their application in polymerization of VEs [START_REF] Ohtori | Studies on the Nature of Propagating Species in Cationic Polymerization of Isobutyl Vinyl Ether by Iodine[END_REF] and other vinylic monomers [START_REF] Miyamoto | Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system[END_REF][START_REF] Miyamoto | Synthesis of monodisperse living poly(vinyl ethers) and block copolymers by the hydrogen iodide/iodine initiating system[END_REF]. Thus, the new approach allowed suppressing the reactivity of the chain-ends thus significantly reducing the rates of chain termination without inhibiting the initiation and propagation processes. The proposed methodology allowed syntheses of well-defined products [START_REF] Cho | Cationic polymerization of isobutyl vinyl ether: livingness enhancement by dialkyl sulfides[END_REF][START_REF] Sawamoto | Modern cationic vinyl polymerization[END_REF], including stimuli-responsive polymers [START_REF]Synthesis of Stimuli-Responsive Polymers by Living Polymerization: Poly(N-Isopropylacrylamide) and Poly(Vinyl Ether)s[END_REF] and cyclic polymers [START_REF] Pasini | Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures[END_REF]. Controlled polymerization requires a catalyst (initiator) to promote chain initiation and propagation; the properties of this agent can largely define efficiency of the overall process, which provides chemists with reliable means for its control.

Besides chemical initiation, a controlled release of initiating cations can be triggered by external light or electrical stimuli [START_REF] Corrigan | Photocatalysis in organic and polymer synthesis[END_REF][START_REF] Chen | Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications[END_REF].

The RAFT approach in both cationic and radical polymerization reactions was found suitable for poly(VE) synthesis [START_REF] Mckenzie | Beyond Traditional RAFT: Alternative Activation of Thiocarbonylthio Compounds for Controlled Polymerization[END_REF]. Initiation with thiocarbonylthio or xanthate compounds in combination with a Lewis acid (LA) and a base resulted in well-defined synthetic polymers and was advantageous in terms of simplicity of the experimental procedure. Polymerization of VEs by free-radical protocols, once considered impossible, is now well established; we will highlight the recent achievements in this area as well.

The use of VEs as monomers expands rapidly, and many reviews have been focused on the mechanistic studies of VE polymerization [START_REF] Kanazawa | Recent Progress in Living Cationic Polymerization of Vinyl Ethers[END_REF][START_REF] Aoshima | A Renaissance in Living Cationic Polymerization[END_REF][START_REF] Ouchi | Transition Metal-Catalyzed Living Radical Polymerization: Toward Perfection in Catalysis and Precision Polymer Synthesis[END_REF][START_REF] Ouchi | Ring-expansion cationic polymerization of vinyl ethers[END_REF]. In this review, we do not intend surveying the mechanistic details, rather we give a scope of the most recent and relevant examples of VE involvement in different polymerization protocols. We also highlight advanced procedures for the preparation of VE monomers underscoring their potential as building blocks for sustainable and eco-friendly syntheses.

Advanced green syntheses of vinyl ether monomers

Reactions that lead to vinyl ethers can be divided into four groups: i) of laboratory interest only; ii) affording vinyl ethers as side products; iii) used in industry to produce vinyl ether monomers; iv) potentially upgradable for reasonable production of vinyl ether monomers.

The first synthesis of vinyl ether was reported by Wislicenus in 1878 [START_REF] Wislicenus | Ueber Vinyläthyläther[END_REF]. The method was based on addition of sodium metal to hot monochloracetal (Scheme 2) since it was essentially an elimination reaction in the presence of a strong base. Improved by subsequent studies [START_REF] Wohl | Über die Darstellung der aromatischen Alkohole und ihrer Acetate[END_REF][START_REF] Braun | Halogen-alkylierte aromatische Amine, V.: Darstellung von Aryl-vinyläthern[END_REF][START_REF] Lauer | The Thermal Decomposition of Phenyl Vinyl Ether[END_REF], this versatile synthesis allowed obtaining a wide range of vinyl derivatives. The two-stage methodology yields a halogenated derivative (or an alcohol) at the first stage, which is followed by elimination under basic conditions (Scheme 3, part A). Although being efficient in a lab, the approach is unsuitable for industrial applications.

Another strategy developed for the generation of vinyl functionality is based on the reaction of primary aliphatic alcohols with vinyl acetate in the presence of a mercury catalyst at A fundamental breakthrough in the chemistry of vinyl ethers was made in 1888 by Favorskii who discovered nucleophilic addition of alcohols to acetylenes in the presence of bases [START_REF] Favorskii | The phenomena of isomerization in the series of hydrocarbons C n H 2n-2 (in Russian)[END_REF].

Allyl bromide, synthesized by elimination of hydrogen bromide from propylene bromide, was absorbed by cold absolute ethanol with dry potassium hydroxide added in advance. The vessel was sealed and heated at 170-180 ° C for 12 h; the ether was subsequently purified by distillation.

Nowadays, vinyl ethers are commonly synthesized in laboratory practice to be applied as synthons and precursors. An interesting iridium-catalyzed dehydrogenation with ethylene as a hydrogen acceptor affords unsaturated vinyl ethers from non-functionalized ethers, mixed or simple (e.g. diethyl ether) [START_REF] Lyons | Iridium Pincer-Catalyzed Dehydrogenation of Ethers Featuring Ethylene as the Hydrogen Acceptor[END_REF]. Iridium pincer complexes iPr4 PC(sp 3 )P-Ir(H)(Cl) and ( iPr4 PCP)-Ir(H)(Cl) catalyze dehydrogenation-reduction reactions in the presence of a base under a pressure of ethylene equal to the ambient atmospheric pressure (Scheme 4). Another organometallic route involves spontaneous decomposition of planar platinum complexes [PtCl(N-N)(η 1 -CH 2 CH 2 OR)] by a pseudo-Wacker-type mechanism [START_REF] Benedetti | Alkyl-vinyl-ethers from alcoholic substrates and the Zeise's salt, via square planar [PtCl(N-N)(η 1 -CH 2 CH 2 OR)] complexes[END_REF]. The complexes are stable in the solid state at room temperature; however, upon dissolution in CH 2 Cl 2 , CHCl 3 or benzene they slowly decompose to give methyl vinyl ether and a Pt-containing precipitate. The described method is convenient for preparation of challenging vinyl ethers in a lab due to the simplicity of both decomposition and isolation procedures (Scheme 5). A solid-phase reaction of polystyrene-supported β-phenylselenoethanol or 2phenlsulfonylethanol with phenols under the Mitsunobu reaction conditions with subsequent oxidation-elimination affords a scope of vinyl ethers in 84-95% yields [START_REF] Yu | Solid-Phase Organic Synthesis of Aryl Vinyl Ethers Using Sulfone-Linking Strategy[END_REF][START_REF] Zhang | Solid-phase synthesis of aryl vinyl ethers based on polystyrenesupported β-phenylselenoethanol[END_REF]. Despite the multistep protocol (Scheme 6), the procedure is universal for a wide range of products. All of these methods, including the workup under high pressure in an autoclave, are readily reproducible in laboratory settings. However, nucleophilic addition of alcohols to acetylene in the presence of a base (KOH, CsOH, KF or CsF) remains the only industrially relevant method.

Acetylene can be directly manufactured from oil or fossil gas; alternatively, it is accessible through carbide-based technologies [START_REF] Pässler | Acetylene[END_REF]. Acetylene manufacturing from calcium carbide emerged at the end of XIX century [START_REF] Rodygin | Calcium Carbide: A Unique Reagent for Organic Synthesis and Nanotechnology[END_REF]. This approach significantly enhanced the industrial relevance of vinyl derivatives by providing efficient route to vinyl ethers as starting materials [START_REF] Reppe | Polyvinylpyrrolidon[END_REF].

Due to their low toxicity [START_REF] Smyth | Range-Finding Toxicity Data: List VI[END_REF] and efficient manufacturing, vinyl ethers have been extensively used in research and industry without any significant amendments to the concept of their synthesis [START_REF] Ledovskaya | Methods for the synthesis of O-, S-and N-vinyl derivatives[END_REF][START_REF] Winternheimer | Methods for Vinyl Ether Synthesis[END_REF]. A number of modifications have been introduced, including variation of eliminating agents [START_REF] Hamdi | Acetylene-free synthesis of vinyloxy pyridine and quinoline[END_REF], upgrade of the catalysts and conditions for trans-vinylation processes [START_REF] Alaaeddine | An efficient method to synthesize vinyl ethers (VEs) that bear various halogenated or functional groups and their radical copolymerization with chlorotrifluoroethylene (CTFE) to yield functional poly(VE-alt-CTFE) alternated copolymers[END_REF][START_REF] Finnveden | Novel sustainable synthesis of vinyl ether ester building blocks, directly from carboxylic acids and the corresponding hydroxyl vinyl ether, and their photopolymerization[END_REF][START_REF] Kalita | Biobased poly(vinyl ether)s derived from soybean oil, linseed oil, and camelina oil: Synthesis, characterization, and properties of crosslinked networks and surface coatings[END_REF][START_REF] Yuan | Synthesis and photo-curing behaviors of silicon-containing (vinyl ether)-(allyl ether) hybrid monomers[END_REF], adjustment of the solvents and expansion of the substrate scope for the addition technique [START_REF] Trofimov | Vinylation of cellulose in superbase catalytic systems: towards new biodegradable polymer materials[END_REF][START_REF] Steinborn | Hochaktive Katalysatorsysteme für die Vinylethersynthese aus Acetylen und Alkoholen-Kaliumalkoholat/Kronenether in Kohlenwasserstoffen[END_REF]. The increasing demand for vinyl monomers requires a sustainable and scalable manufacturing methodology, preferably based on renewables. These requirements are met by the syntheses of vinyl functionalities starting from calcium carbide [START_REF] Rodygin | Calcium-Based Sustainable Chemical Technologies for Total Carbon Recycling[END_REF]. Utilization of calcium carbide instead of bulk gaseous acetylene is actively promoted in recent studies [START_REF] Ledovskaya | Efficient labeling of organic molecules using 13C elemental carbon: universal access to 13C2-labeled synthetic building blocks, polymers and pharmaceuticals[END_REF][START_REF] Teong | Direct vinylation of natural alcohols and derivatives with calcium carbide[END_REF][START_REF] Rodygin | Synthesis of vinyl thioethers and bis-thioethenes from calcium carbide and disulfides[END_REF][START_REF] Turberg | Altering Copper-Catalyzed A3 Couplings by Mechanochemistry: One-Pot Synthesis of 1,4-Diamino-2-butynes from Aldehydes, Amines, and Calcium Carbide[END_REF][START_REF] Hosseini | Synthesis of Exclusively 4-Substituted β-Lactams through the Kinugasa Reaction Utilizing Calcium Carbide[END_REF][START_REF] Gao | Synthesis of Aromatic Terminal Allenes and Aliphatic Terminal Alkynes from Hydrazones Using Calcium Carbide as an Acetylene Source[END_REF]. In addition to operational safety, this approach provides a sustainable route to vinyl derivatives. The carbide methodology allows to vinylate both aliphatic and benzylic alcohols [START_REF] Matake | Synthesis of vinyl ethers of alcohols using calcium carbide under superbasic catalytic conditions (KOH/DMSO)[END_REF], polyols and phenols [START_REF] Teong | Direct vinylation of natural alcohols and derivatives with calcium carbide[END_REF], aryl ethers and epoxides [START_REF] Teong | Vinylation of Aryl Ether (Lignin β-O-4 Linkage) and Epoxides with Calcium Carbide through C-O Bond Cleavage[END_REF], various multifunctional substrates [START_REF] Werner | A solid acetylene reagent with enhanced reactivity: fluoride-mediated functionalization of alcohols and phenols[END_REF] and carbohydrates [START_REF] Rodygin | A Green and Sustainable Route to Carbohydrate Vinyl Ethers for Accessing Bioinspired Materials with a Unique Microspherical Morphology[END_REF], with alkalis (KOH, CsOH) or salts (KF, CsF) as bases and water as a hydrolyzing agent. The use of deuterated water leads to deuterated vinyl ethers [START_REF] Ledovskaya | Direct Synthesis of Deuterium-Labeled O-, S-, N-Vinyl Derivatives from Calcium Carbide[END_REF]. The reaction proceeds at 100 °C or higher for 3-5 hours in air-tight ampoules to prevent the release of gaseous acetylene from the mixture. Acetylene gas is generated in situ and dissolved in DMSO; subsequent addition of an alcohol leads to the corresponding vinyl ether. As the external pressure is rather small, there is no need to use an autoclave. The substrate scope for this process is very wide (Scheme 10).

Substrate

Substrate

Methods for controlled synthesis of poly(vinyl ether)s

Physicochemical and thermomechanical properties of polymers significantly depend on their size (molecular weight, polydispersity) and macromolecular structure (architecture, topology, regio-and stereochemistry). A number of recent studies are specifically aimed at achieving a better control over polymerization of vinyl ether monomers. The intrinsic "living" character of cationic polymerization provides exclusive opportunity of precise control over the length and structure of monomer sequences. This opportunity has been intensively used for the construction of macromolecules with predefined structures and also for the production of polymers with functional chain-ends for subsequent assembly and/or use as building units.

Successful implementation of the new initiation and chain-transfer protocols combining cationic polymerization with free-radical processes (e.g. the RAFT-like and degenerative chain transfer polymerizations) is complemented by advanced means to control initiation and propagation by applying external stimuli (light, electrical, etc.).

Living cationic polymerization

Early examples of IBVE cationic polymerization with ionic salts [Ph 3 C] + [X] - [START_REF] Kunitake | The Effect of Counteranions in the Polymerization of Isobutyl Vinyl Ether Initiated by Triphenylmethyl Cation[END_REF] and hydrogen iodine/iodine [START_REF] Miyamoto | Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system[END_REF] as initiators was reported in 1971 and 1984 respectively. Among different types of initiating systems identified later on, the two-component combinations of initiator Sn (x = 4)) combined with a base (ethyl acetate, 1,4-dioxane, THF) and pyrrole as a chaintransfer agent [START_REF] Kanazawa | Chain Multiplying Controlled Cationic Polymerization of Isobutyl Vinyl Ether Using Pyrrole: Increment of Propagating Chains by Efficient "Initiator-Like[END_REF]. In this case, the adventitious H 2 O was identified as a proton donor. In this study, the initiating systems with Si, Ge, Ga, Zn, In and Bi chloride salts were inactive. The other systems were found active but allowing low degree of control in terms of PDI of the polymers (1.20-2.82).

Different substituted triazines reacting with a Lewis acid (TiCl ) and PDI of 1.06-1.19 bearing functional chain-ends were obtained.

Another initiating system containing IBEA/Et 1.5 AlCl 1.5 /1,4-dioxane was used for the highly controlled polymerization of CEVE, and its copolymerization with IBVE as well, to produce diand tri-block copolymers [START_REF] Yoshimitsu | Well-Defined Polymeric Ionic Liquids with an Upper Critical Solution Temperature in Water[END_REF]. Treatment of the resulted polymers with different alkylimidazolium bases allowed preparation of corresponding uniform polymeric ionic liquids of narrow polydispersity (PDI 1.06-1.22) (Scheme 12). The same initiating systems were applied for the synthesis of fluorinated poly(VE)s from the parent fluorine-containing monomers [START_REF] Shimomoto | Fluorine-containing vinyl ether polymers: Living cationic polymerization in fluorinated solvents as new media and unique solubility characteristics in organic solvents[END_REF][START_REF] Shimomoto | Synthesis of fluorinecontaining star-shaped poly(vinyl ether)s via arm-linking reactions in living cationic polymerization[END_REF]. The use of fluorinated solvents (hydrofluorocarbons, chlorocarbons, ethers and aromatics) appeared to be mandatory to provide solubility of the resulting materials. Under the same conditions, fluorine-containing and fluorine-free star-shaped crosslinked polymers were produced upon addition of difunctional DVEs as cross-linking agents. Temperature-dependent sol-gel transitions of these reticulated polymers were also investigated. The use of IBEA/coinitiator/ethyl acetate (coinitiator = TiCl 4 , SnCl 4 , Et 1.5 AlCl 1.5 ) in the presence of proton trap (2,6-di-tert-butyl-pyridine) for polymerization of IBVE in ionic liquids was reported by the same authors [START_REF] Yoshimitsu | Cationic polymerization of vinyl ethers with alkyl or ionic side groups in ionic liquids[END_REF] and others [START_REF] Wu | Cationic polymerization of isobutyl vinyl ether in an imidazole-based ionic liquid: characteristics and mechanism[END_REF] as well. IBEA-like initiators with diverse functional groups were successfully employed in controlled syntheses (PDI 1.05-1.14) of end-functionalized thermosensitive poly(MOVE)s featuring a range of temperature-induced phase-separation behaviors [START_REF] Shimomoto | Precise synthesis of end-functionalized thermosensitive poly(vinyl ether)s by living cationic polymerization[END_REF].

O Cl O Cl n N N R 1 R 2 O n N N R 1 R 2 + Cl O n N N R 1
A combination of [START_REF] Winternheimer | Methods for Vinyl Ether Synthesis[END_REF]fullerene- [START_REF] Motoyanagi | A novel [60]fullerene-appended initiator for living cationic polymerization and its application to the synthesis of [60]fullerene-end-capped poly(vinyl ether)s[END_REF][START_REF] Motoyanagi | Self-assembly behavior of amphiphilic C 60 -end-capped poly(vinyl ether)s in water and dissociation of the aggregates by the complexing of the C 60 moieties with externally added ϓ-cyclodextrins[END_REF] or thiophene- [START_REF] Ishikawa | Synthesis of Brush-shaped π-Conjugated Polymers Based on Welldefined Thiophene-end-capped Poly(vinyl ether)s[END_REF] substituted triflate esters (Scheme 13) with EtAlCl 2 and 1,4-dioxane for living polymerization of IBVE, MOVE and methoxy ethoxy ethyl vinyl ether (MEEVE) was applied to prepare the corresponding unsaturated groupend-capped polymers. Self-assembly behavior of C 60 -poly(MOVE) in aqueous solution and in the presence of -cyclodextrine was also studied. Highly-controlled polymerization of IBVE was achieved by utilizing an unusual initiating system HCl/LA/base (base = 1,4-dioxane or ethyl acetate) [START_REF] Kanazawa | Biologically synthesized or bioinspired process-derived iron oxides as catalysts for living cationic polymerization of a vinyl ether[END_REF]. In this case, the chain-growth process was initiated at the Lewis acidic (LA) centers of Fe Graphite oxide was introduced as a heterogeneous catalyst for cationic polymerization of BVE under solvent-free conditions (T pol = 22 °C) affording poly(BVE)-carbon composites featuring diverse molecular weight distributions (M n 700-8,100 g•mol 1 , PDI 1.45-13.88, depending on the monomer/initiator ratio used) and high decomposition temperature (T dec = 354 °C) [START_REF] Dreyer | Graphite Oxide as an Olefin Polymerization Carbocatalyst: Applications in Electrochemical Double Layer Capacitors[END_REF]. Graphene oxide was found to initiate cationic polymerization of IBVE in the same manner [START_REF] Nagata | Grafting of polymers onto graphene oxide by cationic and anionic polymerization initiated by the surface-initiating groups[END_REF].

The halogen bonding-mediated cationic polymerization of IBVE involved initiating systems on the basis Cl-substituted cationogens and imidazolium salts (Scheme 17) [START_REF] Takagi | Halogen-Bonding-Mediated and Controlled Cationic Polymerization of Isobutyl Vinyl Ether: Expanding the Catalytic Scope of 2-Iodoimidazolium Salts[END_REF].

RAFT polymerization

Converting the living cationic polymerization into RAFT was firstly proposed by Kamigato et al [START_REF] Kumagai | In-Situ Direct Mechanistic Transformation from RAFT to Living Cationic Polymerization for (Meth)acrylate-Vinyl Ether Block Copolymers[END_REF]. Initiating system containing trithiocarbonate initiator, LA and a base (ethyl acetate, Since then, different metal-free initiating systems for cationic RAFT via degenerative chaintransfer have been developed. For example, binary systems combining triflic acid (TfOH) as cationogen and a RAFT agent, e.g. thiocarbonyl [START_REF] Uchiyama | Cationic RAFT polymerization using ppm concentrations of organic acid[END_REF], thioether [START_REF] Uchiyama | Thioether-Mediated Degenerative Chain-Transfer Cationic Polymerization: A Simple Metal-Free System for Living Cationic Polymerization[END_REF], phosphate or phosphinate [START_REF] Uchiyama | A phosphonium intermediate for cationic RAFT polymerization[END_REF], were used for highly controlled polymerization of vinyl ethers (IBVE, EVE, CEVE or CHVE). The resulting polymers featured high molecular weights (M n up to 10 5 g•mol 1 ) with very narrow distributions (PDI < 1.1). Another metal-free system, competent of IBVE, nitrophenyl vinyl ether or EOVE polymerization, consisted of HCl•Et 2 O adduct and thioester and provided a slightly lower degree of control (PDI of 1.20-1.28) [START_REF] Sugihara | HCl•Et2O-Catalyzed Metal-Free RAFT Cationic Polymerization: One-Pot Transformation from Metal-Free Living Cationic Polymerization to RAFT Radical Polymerization1[END_REF]. Besides, the intrinsic ability of trithiocarbonates as RAFT agents [START_REF] Mckenzie | Beyond Traditional RAFT: Alternative Activation of Thiocarbonylthio Compounds for Controlled Polymerization[END_REF] to mediate both cationic and radical polymerization mechanisms was exploited to promote in situ switch from one type of polymerization to another. The approach afforded diblock polymers consisting of a poly(VE) segment obtained by cationic polymerization (e.g. poly(IBVE) or poly(MOVE)) and another segment derived from radically polymerizable monomers (acrylates, acrylamides, vinyl acetate, styrenics, etc.) [START_REF] Kammiyada | Ring-Expansion Living Cationic Polymerization via Reversible Activation of a Hemiacetal Ester Bond[END_REF][START_REF] Kammiyada | A Study on Physical Properties of Cyclic Poly(vinyl ether)s Synthesized via Ring-Expansion Cationic Polymerization[END_REF][START_REF] Kumagai | In-Situ Direct Mechanistic Transformation from RAFT to Living Cationic Polymerization for (Meth)acrylate-Vinyl Ether Block Copolymers[END_REF][START_REF] Uchiyama | A phosphonium intermediate for cationic RAFT polymerization[END_REF][START_REF] Sugihara | Transformation of Living Cationic Polymerization of Vinyl Ethers to RAFT Polymerization Mediated by a Carboxylic RAFT Agent[END_REF][START_REF] Aoshima | Interconvertible living radical and cationic polymerization through reversible activation of dormant species with dual activity[END_REF][START_REF] Sugihara | Transformation from xanthate-type cationogen mediated metal-free RAFT cationic polymerization with "HCl•Et2O" into RAFT radical polymerization to form poly(alkyl vinyl ether)-b-polyvinyl alcohol amphiphiles[END_REF].

Novel photo-and electro-controlled methods of polymerization

The externally stimulated initiation of VE cationic polymerization has been receiving increased attention in recent years. The traditional Brønsted or Lewis acids-based initiating systems can be replaced with the systems capable of the controlled release of initiating cations under external stimuli (light, electrical, chemical, etc.) [START_REF] Yağci | Externally stimulated initiator systems for cationic polymerization[END_REF]. A number of recently introduced photoinitiating systems comprise functionalized carbazoles [START_REF] Tunc | Thioxanthone-ethylcarbazole as a soluble visible light photoinitiator for free radical and free radical promoted cationic polymerizations[END_REF][START_REF] Yilmaz | Visible light induced free radical promoted cationic polymerization using thioxanthone derivatives[END_REF], pyrenes [START_REF] Tehfe | New insights into radical and cationic polymerizations upon visible light exposure: role of novel photoinitiator systems based on the pyrene chromophore[END_REF], di-and polythiophenes [START_REF] Aydogan | Highly Conjugated Thiophene Derivatives as New Visible Light Sensitive Photoinitiators for Cationic Polymerization[END_REF] and disulfones [START_REF] Chen | Photopolymerization kinetics of α-disulfone cationic photoinitiator[END_REF], C 60fullerene [START_REF] Yilmaz | Visible Light-Induced Cationic Polymerization Using Fullerenes[END_REF], allylic ammonium or phosphonium salts [START_REF] Gupta | Novel allylic phosphonium salts in free radical accelerated cationic polymerization[END_REF][START_REF] Gupta | Novel addition-fragmentation agent in cationic photopolymerization[END_REF], acylgermanes [START_REF] Aydogan | New Photoinitiating Systems for Cationic Polymerization Acting at Near UV and Visible Range[END_REF] or different dyes [START_REF] Chen | Photopolymerization of Vinyl Ether Networks Using an Iodonium Initiator: Effect of Radiation Intensity and Iodonium Concentration[END_REF][START_REF] Xiao | Panchromatic Photopolymerizable Cationic Films Using Indoline and Squaraine Dye Based Photoinitiating Systems[END_REF], in the presence of oxidants/coinitiators AgPF 6 or [Ph 2 I] + [PF 6 ] -. Lightcontrolled catalytic polymerization of EVE (M n 10,000 g•mol 1 , PDI 1.74) was achieved with a photosensitizing BINAP complex of palladium [START_REF] Son | Synthesis and photocatalytic activity of a naphthyl-substituted photosensitizing BINAPpalladium complex[END_REF]. Supramolecular complexes of diaryliodonium salts with crown ethers were tested as initiators of cationic polymerization of triethyleneglycole divinyl ether [START_REF] Crivello | Supramolecular diaryliodonium salt-crown ether complexes as cationic photoinitiators[END_REF].

The intermediacy of radical species was specifically exploited in photogeneration of active systems for cationic polymerization. For example, a classical combination of benzyl bromide with Mn 2 (CO) A similar system involving photoinitiator combined with CTA enabled interconversion between the cationic and radical polymerization mechanisms (Scheme 23) thus promoting copolymerization of IBVE with methyl acrylate (MA) [START_REF] Kottisch | Photocontrolled Interconversion of Cationic and Radical Polymerizations[END_REF][START_REF] Peterson | On Demand Switching of Polymerization Mechanism and Monomer Selectivity with Orthogonal Stimuli[END_REF]. This new type of initiation afforded multiblock copolymers composed of poly(IBVE) and poly(MA) sequences in a highly controlled manner. Electrocontrolled VE cationic polymerization protocols are also available. A new cationic RAFT-type mechanism involved dithiocarbamate as CTA in the presence of electron-carrier or mediator (TEMPO, DDQ); by reversible transfer at the electrode, CTA is oxidized to produce carbocation [START_REF] Sang | Electro-Controlled Living Cationic Polymerization[END_REF][START_REF] Peterson | Electrochemically Controlled Cationic Polymerization of Vinyl Ethers[END_REF]. With this method, different vinyl ethers (BVE, IPVE, EVE and CEVE) were successfully polymerized in a highly controlled manner (PDI 1.11-1.24). In the absence of electroredox agent, the process was poorly controlled and afforded polymers of broad polydispersity.

Free-radical polymerization

A rare example of polymerization of vinyl ethers by a radical mechanism was reported by

Copolymerization of vinyl ethers with various monomers

Control of monomer sequence in copolymerization reactions is the holy grail of macromolecular engineering. Contributions of sequential vs. statistic copolymerization for different polymerization mechanisms (living cationic, coordination/insertion polymerization, ring-opening polymerization) have been explored by using appropriate initiating or catalytic systems.

Ethylene and alkenes. The late transition metal-based coordination/insertion catalysts favor statistical copolymers

Other combined copolymerization techniques

A straightforward access to di-and multiblock copolymers can be gained by sequential addition of different monomers to the same living polymerization system. For example, ABAtriblock copolymers of adamantyl vinyl ether with NBVE were produced by cationic polymerization with a difunctional initiating system 1,4-bis(1-acetoxy)butane/Et 1,5 AlCl 1,5 /ethyl acetate [START_REF] Imaeda | Synthesis of ABA-triblock and star-diblock copolymers with poly(2-adamantyl vinyl ether) and poly(n-butyl vinyl ether) segments: New thermoplastic elastomers composed solely of poly(vinyl ether) backbones[END_REF], while introducing cyclohexanedimethanol divinyl ether to this system afforded star-diblock copolymers featuring improved rubber elastic properties. A similar sequential protocol afforded tri-and pentablock copolymers of NBVE, CEVE and tert-butyldimethylsilyl ethylene glycol vinyl ether (SiEGVE) [START_REF] Bouchekif | Triblock and pentablock terpolymers by sequential base-assisted living cationic copolymerization of functionalized vinyl ethers[END_REF]. Diblock copolymers of IBVE and -methylstyrene were also produced by sequential approach with initiation by HBr/FeCl 3 /nBu 4 NBr [START_REF] Banerjee | Diblock Copolymers with Miscible Blocks via One-Pot Sequential Cationic Polymerization and Their Block-Length-Dependent Vesicular Aggregation[END_REF].

Copolymerization of IPVE with 1-methoxy-2-methylpropylene oxide (MOMPO) was accomplished using a specific initiating system composed of CF 3 SO 3 H and nBu 4 NI [START_REF] Kanazawa | Alkoxyoxirane, a Unique Cyclic Monomer: Controlled Cationic Homopolymerization Mediated by Long-Lived Species and Copolymerization with Vinyl Ether via Alkoxy Group Transfer[END_REF] (Scheme 28). The ring-opening copolymerization resulted in IPVE-co-MOMPO product with unimodal molecular weight distribution (M n = 6,500 g•mol 1 , PDI = 1.73) subsequently hydrolyzed under acidic conditions to a low molecular weight product (M n = 1,700 g•mol 1 , PDI = 1.7).

Noteworthy, the IPVE-IPVE junctions appeared to be resistant to acidic hydrolysis.

Incorporation of the ring-opened MOMPO unit into IPVE chain by alkoxy-group transfer was confirmed by 1 H, 13 C, and 2 D NMR spectroscopic studies. Cyclic acetals were also copolymerized by ring opening reactions with CEVE and IBVE; the SnCl 4 -promoted process yielded multiblock-like copolymers [START_REF] Shirouchi | Controlled Cationic Copolymerization of Vinyl Monomers and Cyclic Acetals via Concurrent Vinyl-Addition and Ring-Opening Mechanisms[END_REF]. Propagating species were provided by both VE-and cyclic acetal-derived end groups. The distribution of comonomers within the chains was defined by the nature of cyclic acetal and VE. The use of acetals as initiators in living cationic polymerization afforded block and graft copolymers containing poly(p-methylstyrene) and poly(VE) segments [START_REF] Yokoyama | Synthesis of block or graft copolymers containing poly(styrene derivative) segments by living cationic polymerization using acetal moieties as latent initiating sites[END_REF]. IBVE and pmethylstyrene, conventionally incompatible with each other, were successfully copolymerized in the presence of TiCl 4 /SnCl 4 initiating system and macroinitiators having acetal moieties.

Linear macroinitiators with acetal moieties at chain termini provided block copolymers (Scheme Concurrent cationic vinyl-addition, ring-opening, and carbonyl-addition mechanisms were described for terpolymerization of IPVE, oxetane, and methyl ethyl ketone (MEK) [START_REF] Kanazawa | Cationic Terpolymerization of Vinyl Ethers, Oxetane, and Ketones via Concurrent Vinyl-Addition, Ring-Opening, and Carbonyl-Addition Mechanisms: Multiblock Polymer Synthesis and Mechanistic Investigation[END_REF]. Given each of the monomers is capable of providing propagating chain ends, formation of statistical multiblock polymers was the result of this polymerization reaction. During the reaction, the non-homopolymerizable ketone monomer reacted with the oxetane-derived oxonium ion giving rise to carbocation capable of reacting with VE only (Scheme 31). Vinyl-addition polymerization of VE and simultaneous ring-opening polymerization of εcaprolactone under hafnium-mediated control afforded graft copolymers [START_REF] Higuchi | Concurrent Cationic Vinyl-Addition and Coordination Ring-Opening Copolymerization via Orthogonal Propagation and Transient Merging at the Propagating Chain End[END_REF][START_REF] Higuchi | Design of Graft Architectures via Simultaneous Kinetic Control of Cationic Vinyl-Addition Polymerization of Vinyl Ethers, Coordination Ring-Opening Polymerization of Cyclic Esters, and Merging at the Propagating Chain End[END_REF].

Copolymerization of the conventionally incompatible monomers occurred by virtue of orthogonal propagating reactions and transient merging, yielding the poly(VE) main chain and several poly(caprolactone) side chains. The growing alkoxide ends attached to the Hf-metal center promoted both cationic vinyl-addition and ring-opening polymerizations. Exchange reactions between the terminal alkoxy groups of polyester chains and the VE-derived alkoxy groups, combined with the repetitive mode of propagation, provided graft copolymer composed of a poly(VE) chain with several inserted polyester chains (Scheme 32). An approach based on successive living cationic polymerization of MOVE, EOVE or EOEOVE, followed by ring-opening polymerization of L-lactide, enabled precision synthesis of block copolymers exhibiting well-defined organization of poly(VE) and poly(lactide) blocks [START_REF] Seki | Precision Synthesis of Polylactide-Based Thermoresponsive Block Copolymers via Successive Living Cationic Polymerization of Vinyl Ether and Ring-Opening Polymerization of Lactide[END_REF].

Yagci et al applied the photoinduced Ph 3 CBr/Mn 2 (CO) 10 /[Ph 2 I] + [Br] --mediated initiation to prepare a Ph 3 C-capped poly(IBVE) macroinitiator by living cationic polymerization. Subsequent thermally promoted radical polymerization of acrylates or styrene yielded diblock copolymers [START_REF] Ciftci | Block Copolymers by Mechanistic Transformation from PROAD to Iniferter Process[END_REF].

An alkoxyamine-based TEMPO/SnBr 4 initiating system promoted cationic polymerization of IBVE producing a TEMPO-terminated macroinitiator. Used at the next step to initiate radical polymerization of styrene, this macroinitiator afforded well-defined copolymer of narrow polydispersity (PDI 1.2) [START_REF] Le | A well-defined block copolymer synthesis via living cationic polymerization and nitroxide-mediated polymerization using carboxylic acid-based alkoxyamines as a dual initiator[END_REF].

A combination of living cationic polymerization with RAFT in a three-step protocol afforded diblock brush-like copolymers of MEEVE and N-isopropylacrylamide featuring thermoresponsive properties [START_REF] Minoda | Thermoresponsive NIPAM block copolymers containing densely grafted poly(vinyl ether) brushes synthesized by a combination of living cationic polymerization and RAFT polymerization[END_REF].

Tetraflouroethylene (TFE) was for the first time copolymerized with IBVE by conventional radical and RAFT (RAFT/MADIX) methods [START_REF] Puts | Conventional and RAFT Copolymerization of Tetrafluoroethylene with Isobutyl Vinyl Ether[END_REF] using O-ethyl-S-(1-methyloxycarbonyl)ethyl xanthate as a chain transfer agent and benzoyl peroxide as an initiator. As expected, copolymers obtained by radical copolymerization were heavier (M n 4,400-11,000 g•mol 1 ) and of broader polydispersity (PDI > 2) than copolymers obtained by controlled processes (M n 1,200-2,000 g•mol 1 , PDI 1.08-1.11) (Scheme 33).

Photoorganocatalyzed reversible-deactivation radical alternating copolymerization of CTFE and vinyl ethers (EVE, BVE, IBVE, CEVE, SiBVE and SiEGVE) was recently reported [ [START_REF] Jiang | Photoorganocatalyzed Reversible-Deactivation Alternating Copolymerization of Chlorotrifluoroethylene and Vinyl Ethers under Ambient Conditions: Facile Access to Main-Chain Fluorinated Copolymers[END_REF]].

Utilization of functionalized 10-phenylphenothiazine as photoinitiator in combination with RAFT agents (xanthates) allowed synthesis of various alternated copolymers under mild conditions (LED irradiation, room temperature, 1 bar pressure) and in controlled manner (M n 2,000-45,300 g•mol 1 , PDI 1.12-1.32). Living homopolymerization of fluorinated vinyl ethers (FVE, with F 9 , F 12 and F 13 groups in the side chains) and their copolymerization with IBVE or MOVE were achieved in fluorinated solvents under regular conditions (IBEA/Et 1.5 AlCl 1.5 /1,4-dioxane, 0 °C) [START_REF] Shimomoto | Fluorinated vinyl ether homopolymers and copolymers: Living cationic polymerization and temperature-induced solubility transitions in various organic solvents including perfluoro solvents[END_REF]. The thermally reversed micellization and sol-gel transition properties of the resulting di-and tri-block copolymers were investigated.

Comb-like poly(VE) polymers with fluorinated side chains were studied to determine their macromolecular organization and interrelated physicochemical and thermal characteristics [START_REF] Ishige | Unique Difference in Transition Temperature of Two Similar Fluorinated Side Chain Polymers Forming Hexatic Smectic Phase: Poly{2-(perfluorooctyl)ethyl acrylate} and Poly{2-(perfluorooctyl)ethyl vinyl ether}[END_REF].

In a search for alternative biosourced polymers, several ribose-derived vinyloxy monomers were copolymerized with diethyl fumarate and diethyl maleate and alkoxy monomers [START_REF] Pichavant | Reactivity of Vinyl Ethers and Vinyl Ribosides in UV-Initiated Free Radical Copolymerization with Acceptor Monomers[END_REF].

Detailed kinetic investigations confirmed higher reactivity of the sugar-derived VEs as compared to that of alkyl VEs in the free radical-mediated donor-acceptor copolymerization with butadienoates.

A highly active Ir-Pd catalyst facilitated copolymerization of styrene with 2,2,2-triflouroethyl vinyl ether (TFEVE) under Xe irradiation [START_REF] Pichavant | Reactivity of Vinyl Ethers and Vinyl Ribosides in UV-Initiated Free Radical Copolymerization with Acceptor Monomers[END_REF]. The isolated copolymer was clearly a co-polymer; the reaction carried out in the dark produced a mixture of homopolymers (Scheme 34). Turning on the lamp in the course of the reaction altered the type of synthesized polymers. Functional aliphatic copolymers of cyclic ketene acetals (CKAs) with VEs were synthesized by a simple free-radical approach (Scheme 36) [START_REF] Tardy | Radical Copolymerization of Vinyl Ethers and Cyclic Ketene Acetals as a Versatile Platform to Design Functional Polyesters[END_REF]; it provided access to functionalized fluorescent probe copolymers, amphiphilic copolymers grafted with poly(ethylene glycol) side chains, antibacterial films and crosslinked bioelastomers. Interestingly, di(ethylene glycol) divinyl ether was successfully integrated into CKA chain yielding a linear copolymer instead of a crosslinked homopolymer. The isolated copolymers revealed potential for tissue engineering applications. Another promising access to linear diblock copolymers incorporating poly(vinyl ether) and poly(styryl) or poly(acryl) fragments is based on a two-step approach [START_REF] Mishima | Precision synthesis of hybrid block copolymers by organotellurium-mediated successive living radical and cationic polymerizations[END_REF][START_REF] Ouchi | Consecutive living polymerization from cationic to radical: a straightforward yet versatile methodology for the precision synthesis of "cleavable" block copolymers with a hemiacetal ester junction[END_REF]. Thus, a conventional living polymerization is switched to a different mode (cationic or radical) by introducing a chain end-capped initiating function at the first step.

Similar results in the synthesis of poly(vinyl ether)-poly(styrene) diblock copolymers were achieved by introducing a hybrid initiator composed of OCH(Me) + X -and AIBN capable of initiating cationic and radical polymerizations, respectively, and trithiocarbonate as RAFT agent [START_REF] Ma'radzi | Synthesis of thermoresponsive block and graft copolymers via the combination of living cationic polymerization and RAFT polymerization using a vinyl ether-type RAFT agent[END_REF].

The inherent reactivity of VEs in polymerization reactions can be developed in relation to monomers incapable of homopolymerization. According to this strategy, desired fragments can be inserted into the main chain to modify the final properties of the resulting polymers. For example, straightforward homopolymerization of biomass-derived isosorbide is impossible.

However, it readily undergoes cationic copolymerization with IBVE in the presence of Lewis acids, giving up to 42 mol% of isosorbide derivative incorporated into the copolymer structure (Scheme 37) [START_REF] Kieber | Cationic copolymerization of isosorbide towards value-added poly(vinyl ethers)[END_REF]. Conventional cationic copolymerization promoted a competing process of ring-opening polymerization and rearrangement, whereas photo-driven cationic polymerization protocol with a RAFT agent facilitated chain propagation against the rearrangement. [B(C 6 F 5 ) 4 ] -catalyzed statistical cationic copolymerization of BVE and CEVE [START_REF] Zouganelis | Statistical copolymers of n-Butyl vinyl ether and 2chloroethyl vinyl ether via metallocene-mediated cationic polymerization. A scaffold for the synthesis of graft copolymers[END_REF]. Regardless of different monomers ratios used for polymerization, the reaction yielded polymers with similar characteristics (M n 10,000-14,000 g•mol 1 , PDI 1.5) (Scheme 38). The structure of the copolymers was confirmed by NMR spectroscopy. The same group reported preparation of block copolymers based on zirconocenium-promoted cationic copolymerization of perfluoro ethyl vinyl ether with 2-R-oxazolines (R = Me, Ph) and -caprolactone [START_REF] Kourti | Block copolymers based on 2-methyl-and 2-phenyl-oxazoline by metallocene-mediated cationic ring-opening polymerization: synthesis and characterization[END_REF].

Composition-and topology-controlled copolymerization of vinyl ethers

A number of well-controlled cationic polymerization techniques allowed synthesis of copolymers featuring several particular types of topological motifs [START_REF] Du | Fluoropolymer synthesis in supercritical carbon dioxide[END_REF]. Conventional living polymerization of monofunctional monomers affords a linear polymer, whereas controlled incorporation of difunctional monomers can produce a variety of topologies e.g. star-, beadedor brush-shaped polymers.

For

O O R' O R' O OR n+1 O O R' O R' O OR n+1 O O R' O R' O
Scheme 39. Synthesis of star-shaped (co)polymers.

A non-aqueous inverse suspension copolymerization approach enables formation of beaded-like poly(VE) gels with low crosslinking levels [START_REF] Greenland | Synthesis of beaded poly(vinyl ether) solid supports with unique solvent compatibility[END_REF]. A combination of CEVE or 4acetoxybutyl vinyl ether with methoxybutyl vinyl ether and a divinyl ether was induced into copolymerization promoted by a cationic initiating system (BF

Miscellaneous

Poly(VE)s with azobenzene functionalities in the repeating unit, synthesized by living cationic polymerization, were blended with poly(-caprolactone) to provide a material with unique anisotropic photomechanical response properties [START_REF] Tanaka | Anisotropic Photomechanical Response of Stretched Blend Film Made of Polycaprolactone-Polyvinyl Ether with Azobenzene Group as Side Chain[END_REF]. Cationic polymerization of a TEMPO-capped vinyl ether monomer afforded a hydrophilic polymer proposed as a cathode active material for rechargeable devices [START_REF] Koshika | An Aqueous, Electrolyte-Type, Rechargeable Device Utilizing a Hydrophilic Radical Polymer-Cathode[END_REF]. Linear poly(VE)s are considered for diverse applications including production of advanced materials for hydrogen generation [START_REF] Winther-Jensen | Conducting polymer composite materials for hydrogen generation[END_REF] and chitin nanofiber composites embedded with gold nanoparticles [240].

Stereocontrol in polymerization of vinyl ethers

From a more applied viewpoint, structure of the macromolecules is the crucial parameter that determines the properties of polymer materials. Enhancement of the stereoselectivity control will eventually allow fine tuning for a broad range of plastic materials with diverse thermal, mechanical and other physicochemical properties.

Several pioneering reports described the efforts to maintain stereocontrol of VE polymerization. Stereoselective polymerization of IBVE was demonstrated to proceed by cationic mechanism at -80-60 °C under control of BF 3 •(Et 2 O) initiating system [241]. The structure of the resulting solid thermoplastic materials was studied by X-ray diffraction analysis

showing that the polymers were isotactic by nature [START_REF] Natta | Polymères isotactiques[END_REF][START_REF] Natta | Über die kettenstruktur des kristallinen polyvinylisobutyläthers[END_REF]. Isotactic poly(IBVE)s with similar properties were obtained by coordination/insertion mechanism in a TiCl 4 /R 3 Al Zigler-Natta-type catalytic system [START_REF]Polymerization of vinyl ethers by Ziegler catalysts[END_REF]; in the presence of titanium-based LAs [START_REF] Ouchi | Stereoregulation in Cationic Polymerization by Designed Lewis Acids. 1. Highly Isotactic Poly(isobutyl vinyl ether) with Titanium-Based Lewis Acids[END_REF]; phosphoric acid derivatives coupled with SnCl 4 [START_REF] Ouchi | Stereoregulation in cationic polymerization. III. High isospecificity with the bulky phosphoric acid [(RO)2PO2H]/SnCl4 initiating systems: Design of counteranions via initiators[END_REF]; and aromatic acetals in conjunction with tin halides The tBuOH/FeSO 4 binary system [START_REF] Kanazawa | A stepping stone to stereospecific living cationic polymerization: Cationic polymerization of vinyl ethers using iron(II) sulfate[END_REF] revealed multi-site behavior in IBVE polymerization at 0 °C; the reaction afforded mixtures of isotactic and atactic poly(IBVE) fractions. Subsequent fractionation with MEK allowed isolation of the minor isotactic fraction containing up to 82% of [m] dyads.

A new cationic initiating system featuring chiral anion (Scheme 41) was optimized in order to provide a higher degree of stereocontrol in IBVE, IPVE, BVE and EVE polymerization [START_REF] Teator | Catalyst-controlled stereoselective cationic polymerization of vinyl ethers[END_REF].

The most promising products, with up to 93 % of isotactic dyads [m], were obtained upon polymerization of IBVE at 78 °C in a toluene/hexane mixture. The resulted crystalline materials featured high melting points (138-152 °C), valuable thermomechanical properties and improved adhesion to polar surfaces.

Conclusions

The review highlights recent advances in polymerization of VEs, including the means for precise structural and functional design of the polymers. The mainstream living polymerization protocols, ubiquitously applied to synthesize poly(vinyl ethers), become actively supplemented by new methods of controlled synthesis. The rapidly developing area of initiating systems and precise control eventually gained over polymerization reactions allows the involvement of unconventional VE substrates as monomers and co-monomers thus expanding the variety of polymers with desired features and properties.

The remarkable versatility of VEs makes them highly valuable monomers not only providing polymeric materials with unique properties, but also conferring clues to understanding of a complicated nature of polymerization processes. At the same time, the easy access to VE from available and sustainable raw materials (acetylene or calcium carbide) makes them highly relevant to our industrial future. Of special importance are the efforts to produce calcium carbide form biomass and biomass refining wastes, which could associate production of plastics with natural photosynthetic cycle.

  1-(Isobutoxy)ethyl acetate (IBEA) 1-(Methoxyethoxy)ethyl acetate (MOEA) Ethyl vinyl ether (EVE) n-Butyl vinyl ether (BVE) iso-Butyl vinyl ether (IBVE) iso-Propyl vinyl ether (IPVE) tert-Butyl vinyl ether (TBVE) Cyclohexyl vinyl ether (CHVE) 2-Cloroethyl vinyl ether (CEVE) 2-Methoxyethyl vinyl ether (MOVE) 2-ethoxyethyl vinyl ether (EOVE) (2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) Dodecyl vinyl ether (DDVE) 8-Vinyloxytricyclo[5.2.1.0]decane (TCDVE) 1,4-butanediol divinyl ether (BDVE) 1,6-Hexanediol divinyl ether (HDVE) Di(ehtylene glycol) divinyl ether (DEGDVE) Tri(ethylene glycol) divinyl ether (TEGDVE) 2-Hydroxyethyl vinyl ether (HEVE) 4-hydroxybutyl vinyl ether (HBVE) 1,4-Cyclohexanedimethanol vinyl ether (CHMVE) Methyl ethyl ketone (MEK) 1,4-cyclohexane-dimethanol divinyl ether (DVE) Diethylene glycole monovinyl ether (DEGV) Chlorotrifluoroethylene (CTFE) Chain-transfer agent (CTA) Cyanomethyl methyl(phenyl)carbamodithioate (CMPCD) 2,6-di-tert-butyl-pyridine (DTBP) Near-infrared region (NIR) Dichloromethane (DCM) Differential scanning calorimetry (DSC) Thermogravimetric analysis (TGA)

Scheme 2 .

 2 Scheme 2. The first synthesis of vinyl ether.

- 60 -Scheme 3 .

 603 Scheme 3.Common strategies for the vinyl functionality incorporation.

Scheme 5 .

 5 Scheme 5. Plausible mechanism of vinyl ether formation from [PtCl(Me 2 phen)η 1 -CH 2 CH 2 OR)].

(Scheme 11 .

 11 Scheme 11. Controlled cationic polymerization of vinyl ethers initiated by a cationogen/coinitiator combination

Scheme 12 .

 12 Scheme 12. Synthesis of polymeric ionic liquids.

Scheme 13 .

 13 Scheme 13.Examples of cationogens for cationic polymerization of vinyl ethers.

Scheme 14 .Scheme 15 .Scheme 16 .

 141516 Scheme 14.Example of cationic cyclopolymerization of divinyl ether.

Scheme 17 .Scheme 18 .

 1718 Scheme 17. Initiating systems for cationic polymerization of vinyl ethers based on organic cationogens and imidazolium derivatives.

Scheme 19 )Scheme 19 .

 1919 Scheme 19) was shown to induce the reversible termination cationic polymerization of IBVE at 20 °C in toluene affording narrowly dispersed polymers (PDI 1.18-1.49). The group also explored the influence of metal salts (AgOTf, AgPF 6 , AgSF 6 , Na + [B(C 6 H 3 (CF 3 ) 2 ) 4 ] -, etc.) on the IBVE/HCl-initiated RAFT and the related degenerative chain transfer polymerization reactions of IBVE and EVE[START_REF] Uchiyama | Diversifying Cationic RAFT Polymerization with Various Counteranions: Generation of Cationic Species from Organic Halides and Various Metal Salts[END_REF].

Scheme 20 .

 20 Scheme 20. The mechanism of vinyl halide-mediated photoinitiation-dependent cationic polymerization of vinyl and divinyl ethers.

Scheme 21 .Scheme 22 .

 2122 Scheme 21.Examples of photocatalysts that promote photocontrolled cationic polymerization.

Scheme 23 .

 23 Scheme 23. Controlled interconversion of polymerization mechanisms.

Scheme 24 .

 24 Scheme 24. Controlled radical polymerization of vinyl ethers.

Scheme 28 .

 28 Scheme 28. Copolymerization of IPVE with MOMPO. Reaction conditions: [IPVE] = 0.80 M, [MOMPO] 0 = 0.20 M, [CF 3 SO 3 H] 0 = 4.0 mM, [nBu 4 NI] 0 = 4.2 mM, in dichloromethane at -40 °C.

Scheme 29 .

 29 Scheme 29. Ring-opening copolymerization of vinyl ethers and cyclic acetals.

30 ,Scheme 30 .

 3030 Scheme 30. Block and graft copolymer preparation using macroinitiators.

Scheme 31 .

 31 Scheme 31. Terpolymerization of VE, Oxetane and MEK.

Scheme 32 .

 32 Scheme 32. The alkoxy group exchange mechanism and graft copolymer construction.

F 2

 2 Scheme 33.Copolymerization of TFE with IBVE by conventional and RAFT/MADIX approaches.

2 Scheme 34 .

 234 Scheme 34. Homopolymerization and copolymerization of styrene and TFEVE in the presence of Ir-Pd catalyst under different light conditions.

Scheme 35 .

 35 Scheme 35.A system for controllable VE (co)polymerization.

Scheme 36 .

 36 Scheme 36.Copolymerization of CKAs and VEs via free-radical mechanism.

Scheme 37 .

 37 Photodriven copolymerization

Scheme 38 .

 38 Scheme 38. Cationic copolymerization of BVE and CEVE.

Scheme 40 .

 40 Scheme 40. Chiral titanium-based precatalysts studied in VE polymerization.

Scheme 41 .

 41 Scheme 41. A chiral ion-pair used in isoselective cationic polymerization of vinyl ethers.

  study of polymerization of IBVE initiated with H 2 O/BF 3 •(Et 2 O) in aqueous media demonstrated a varying degree of control over molecular weights and distributions depending on the conditions (notably for suspensions, dispersions and emulsions)[START_REF] Radchenko | Cationic polymerization of isobutyl vinyl ether in aqueous media: physical chemistry tricks to fight against thermal runaway[END_REF].AuCl/AgPF 6 at -78 °C in DCM produced poly(BVE)s of moderate molecular weight (M n , 4,500-44,000 g•mol 1 ) and narrow to broad polydispersity (PDI, 1.41-8.28); the reaction proceeded at high rates (TOF up to 675 h -1 )[START_REF] Hashmi | Gold Catalysis: AuCl-induced Polymerization of Styrene and n-Butylvinylether[END_REF]. Another coinage metal-based initiating system AgClO 4 /C 6 H 5 CH 2 Br/base (base = Et 2 O, iPr 2 O, CPME, Me 2 S) was shown to provide a similar degree of control (PDI of 1.36-5.63) at 41 -0 °C in different solvents (CH 2 Cl 2 , Et 2 O, toluene) [91,92]. Better results were achieved with Cp 2 MMe 2 /activator (M = Zr, Hf; activator = B(C 6 F 5 ) 3 , [Ph 3 C] + [B(C 6 F 5 ) 4 ] -, [PhNMe 2 H] + [B(C 6 F 5 ) 4 ] -) in polymerization of EVE, BE and IBVE carried out at SnCl 4 , ZnCl 2 , TiCl 4 , FeCl 3 , AlCl 3 ) were used for initiation of cationic polymerization of IBVE, BVE and HBVE-TBS in CH 2 Cl 2 at 78 °C [96]. The corresponding poly(VE)s with M n of 2,900-31,500 g•mol 1

[START_REF] Uchiyama | Cationic RAFT Polymerization Using ppm Concentrations of Organic Acid[END_REF]

, SnCl 4 ) afforded corresponding N-methyleneamines to be used in situ as cationic initiators of TBVE polymerization (78 °C, CH 2 Cl 2 )

[START_REF] Leasure | Effect of temperature, solvent, Lewis acid and additives on the polymerization of tert-butyl vinyl ether using Lewis acid-induced Nmethyleneamines as cationic initiators[END_REF]

. Only a moderate degree of control was achieved in the most polymerization experiments (PDI of 1.18-3.4), including those carried out in the presence of additives (nBu 4 NCl or 1,4-dioxane). A 10 °C in CH 3 CN

[START_REF] Batagianni | Metallocene-mediated cationic polymerization of vinyl ethers: Kinetics of polymerization and synthesis and characterization of statistical copolymers[END_REF]

. The system allowed obtaining corresponding polymers with narrower PDI (1.18-2.38) and broader M n (5,600-206,000). Statistical copolymers of EVE and BE were prepared under the same experimental conditions and studied by DSC and TGA techniques.

In situ generation of cationogen CH 3 CH(OR)X from parent monomers IBVE, IPVE and TBVE with a Lewis acid-free initiating system CF 3 SO 3 H/nBu 4 NX (X = Cl, Br, I, CF 3 SO 3 ) was examined under a range of conditions (78-0 °C, 1 min-600 h, CH 2 Cl 2 )

[START_REF] Kanazawa | Design of Benign Initiator for Living Cationic Polymerization of Vinyl Ethers: Facile in Situ Generation of Vinyl Ether-Hydrogen Halide Adducts and Subsequent Controlled Polymerization without a Lewis Acid Catalyst[END_REF]

. The nature of halide anion X - in nBu 4 NX was demonstrated to influence the degree of polymerization control as I -> Br -> Cl -; the narrowest PDI distributions (1.13-1.16) were achieved in polymerizations of IPVE and TBVE with nBu 4 NI as coinitiator.

Lewis acid-free triflate ester-based systems were reported to initiate carbocationic polymerization of IBVE at room temperature in CH 2 Cl 2 providing access to polymers with a narrow range of M n (8,320-9,620 g•mol 1 ) and PDI (1.8-2.1)

[START_REF] Radchenko | Triflate esters as in-situ generated initiating system for carbocationic polymerization of vinyl ethers, isoprene, myrcene and ocimene[END_REF]

.

A series of systems containing various thienyl chlorides RS-Cl in combination with different LAs (

  Sugihara et al. Polymerization of several vinyl ethers was triggered by azo-initiators (e.g. 2,2'-

azobis(2-methylpropionate)); the scope included IBVE, DEAVE, and hydroxy-functionalized HEVE, HBVE, CHMVE and DEGV

[START_REF] Sugihara | Direct Radical Polymerization of Vinyl Ethers: Reversible Addition-Fragmentation Chain Transfer Polymerization of Hydroxy-Functional Vinyl Ethers[END_REF][START_REF] Sugihara | Design of Hydroxy-Functionalized Thermoresponsive Copolymers: Improved Direct Radical Polymerization of Hydroxy-Functional Vinyl Ethers[END_REF][START_REF] Sugihara | Synthesis of Various Poly(2-hydroxyethyl vinyl ether)-Stabilized Latex Particles via Surfactant-Free Emulsion Polymerization in Water[END_REF]

. The presence of RAFT agent (cyanomethyl methyl(phenyl)carbamodithioate, CMPCD) conveyed living behavior to the system and enabled block-copolymerization of poly(VE) macromonomers with vinyl acetate or N-vinylpyrrolidone (PDI < 1.38). Yet, HEVE homopolymers obtained by radical polymerization were found to be poorly isotactic ([m] = 51%).

  Homopolymerization of vinyl ethers by coordination/insertion mechanisms has beenextensively studied[START_REF] Lin | Cationic polymerization of vinyl ethers and pmethoxystyrene by a benign initiating system: Silver salt/arylmethyl halide/dialkyl sulfide[END_REF][START_REF] Peterson | Electrochemically Controlled Cationic Polymerization of Vinyl Ethers[END_REF][START_REF] Chen | Cationic Polymerization and Insertion Chemistry in the Reactions of Vinyl Ethers with (α-Diimine)PdMe+ Species[END_REF][START_REF] Chen | Palladium-Catalyzed Dimerization of Vinyl Ethers to Acetals[END_REF][START_REF] Zhang | Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes[END_REF][START_REF] Kanazawa | Structure Effects of Lewis Acids on the Living Cationic Polymerization ofp-Methoxystyrene: Distinct Difference in Polymerization Behavior from Vinyl Ethers[END_REF][172][START_REF] Kikuchi | Nonradical Light-Controlled Polymerization of Styrene and Vinyl Ethers Catalyzed by an Iridium-Palladium Photocatalyst[END_REF][START_REF] Ye | Heteroaryl Backbone Strategy in Bisphosphine Monoxide Palladium-Catalyzed Ethylene Polymerization and Copolymerization with Polar Monomers[END_REF]. Many contributions over the past decade dealt with the synthesis of functional polyolefins by catalytic copolymerization of alkenes (ethylene, propylene, etc.) with polar monomers (e.g. VEs, Scheme 25).

	D P P O N R Ph Ph Pd R R	Me DMSO B(C 6 F 5 ) 4	E P O P R R Ph Ph Ar	Pd	Me Cl	F Ph Ph P R R P O Ni Ar	B(C 6 F 5 ) 4	P S O G O O OMe Pd OMe Me DMSO
	Scheme 26. Examples of catalysts used for copolymerization of vinyl ethers with alkenes.
	Copolymerization of ethylene with BVE or divinyl formal (DVF) was studied by Mecking et
	al.; the authors used a neutral palladium system G [172]. Thus, copolymerization of ethylene
	with BVE afforded PE-co-BVE copolymers with similar characteristics (M n 1,300-1,900 g•mol 1 ,
	PDI 1.3-1.7, incorporated BVE (2.1-6.9 mol%). Copolymerization of ethylene with DVF afforded
	linear polymers (M n 1,300-4,700 g•mol 1 , PDI 1.2-2.3) containing five-and six-membered cyclic
	acetal units with the total DVF content reaching 12.9 mol% (Scheme 27).
					+
					O	O		[cat]			x				y
							OR	+		R'		[cat]	O	O	n	O	O
																	R'
					R' = H, Me etc
	P P O R R Ph Ph Pd Me A	Me N	Me	SbF 6		P S O Ph Ph Pd Me O O B	Me N	Me		N R	N O C	R Pd Me	Me N	Me	R R P P O Pd N N Me D	Me N	Me	B(C 6 F 5 ) 4

x OR n Scheme 25. Copolymerization of vinyl ethers with alkenes. Nozaki et al. and Carrow et al. developed palladium catalysts (Scheme 26, A-C and D, respectively) for copolymerization of ethylene with BVE [170,175-178]. The copolymerization reactions proceeded under moderate conditions (P ethylene = 10-40 bar, T pol = 80-100 °C) affording PE-co-BVE copolymers (M n 8,000-39,000 g•mol 1 , PDI 1.6-4.1) containing 0.1-7.7 mol% of randomly incorporated VE. Comparable results, in terms of molecular weight characteristics and BVE incorporation, were reported by Chen et al. for palladium-and nickelbased catalytic systems D-F [3,9,179]. m Scheme 27. Copolymerization of ethylene with divinyl formal.

  example, Aoshima et al. succeeded in synthesis of well-defined star-shaped copolymers (Scheme 39) from monovinyl and divinyl ethers (respectively, IBVE, MOVE, EOVE, EOEOVE, CHVE, PIVE, and DVE, BVP) using cationic initiating systems composed of IBEA, LA and a base (LA = Et 1.5 AlCl 1.5 , EtAlCl 2 , SnCl 4 ; base = 1,4-dioxane) [209-212]. The resulting materials exhibited a range of stimuli-responsive behaviors (temperature, pH) depending on the nature of comonomer functionalities. Introduction of aldehydes (e.g. p-methylbenzaldehyde, isophthalaldehyde, (1R)-()-myrtenal) during the EtSO 3 H/GaCl 3 /1,4-dioxane-promoted polymerization of CEVE, IBVE and linear or cyclic enol ethers allowed sequence-controlled preparation of both linear and star-shaped polymers [213,214]. Under mildly acidic conditions, these polymers underwent degradation into lower molecular weight polymers.

		O O	R'	OR n+1
	n
	OR	OR

  3 •Et 2 O, CH 2 BrCl, 78 °C). The resulting solid gels featured improved swelling characteristics in different solvents. afforded linear poly(VE)s, which were then subjected to polymerization involving the acetylene moieties by either Cu-catalyzed oxidative coupling or a Rh-catalyzed process, with both catalytic systems promoting formation of brush-shaped polymers. The same research group reported a synthesis of hydrophilic glycerol-functionalized poly(VE)s for the use

	in preparation of monodispersed core-shell polymer particles via dispersion polymerization
	with styrene [218].
	Syntheses of biosourced vinyl ethers, derived from different plant oils (soybean, linseed,
	camelina), and their homo-and copolymerization by a cationic mechanism (IBEA/Et 1.5 AlCl 1.5 or
	SnCl 4 /1,4-dioxane) were reported by Chisholm et al. [64,219-222]. Some of the resulting
	materials were tested as alkyd-type surface coatings.
	A number of poly(VE) homo-and copolymers obtained by cationic polymerization of various
	functional group-capped VE monomers were reported by Sakaguchi et al. The products were
	probed for diverse applications including preparation of hydrophilic thermoresponsive
	polyurethanes, development of gas-selective permeable membranes and production of
	transparent polymeric materials for optical lenses [223-231].

Minoda et al. designed new monomers with both vinyl ether and phenyl acetylene functionalities

[START_REF] Motoyanagi | Synthesis of brush-shaped polymers consisting of a poly(phenylacetylene) backbone and pendant poly(vinyl ether)s via selective reaction of 2-Vinyloxyethyl 4-Ethynylbenzoate[END_REF][START_REF] Ishikawa | Stimuli-responsive brush-shaped conjugated polymers with pendant well-defined poly(vinyl ether)s[END_REF]

. Living cationic polymerization (TFA/Et 2 AlCl/1,4-dioxane) of the vinyl ether functionalities Living cationic copolymerization was used by Aoshima et al. to prepare amphiphilic copolymers of IBVE and aminoethyl vinyl ether

[START_REF] Oda | Block versus random amphiphilic copolymers as antibacterial agents[END_REF][START_REF] Oda | Aggregation of Cationic Amphiphilic Block and Random Copoly(vinyl ether)s with Antimicrobial Activity[END_REF]

; the products featured antibacterial and hemolytic activity. Linear diblock copolymers of poly(CHVE-b-MOVE) and poly(CHVE-b-EOVE), showing enhanced blood-compatibility, were proposed as candidate materials for construction of bioinert interfaces

[START_REF] Oda | Design of Blood-Compatible Interfaces with Poly(vinyl ether)s[END_REF][START_REF] Itagaki | Surface Characterization and Platelet Adhesion on Thin Hydrogel Films of Poly(vinyl ether)[END_REF][START_REF] Matsuno | Platelet adhesion on the surface of a simple poly(vinyl ether)[END_REF]

.