Cationic polymerization Initiators properties

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

Since soluble multifunctional initiators are more readily available in cationic polymerization than in the anionic counterpart, ABA type linear triblock copolymers have been almost exclusively prepared using difunctional initiation followed by sequential monomer addition. The preparation and properties of ABA type block copolymer thermoplastic elastomers (TPEs), where the middle segment is PIB, have been reviewed recently [47]. 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

Each product exhibits different properties of the crystalline phase. Another example is the enantioelective cationic polymerization of racemic cis-1 --methylpropyl-l-propenyl ether initiated by (— )-menthoxyaluminium dichloride in toluene at 351 K. The polymer is optically active ( + ) and the remaining monomer can be transformed to a polymer with negative optical activity by BF3OEt2 [100]. 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

After this introductory review on the cationic polymerization of epoxides by latent initiators an overview of our work in recent years on the effect of moisture and different polyether diols on the photochemicaUy and thermally induced polymerization of different epoxides will be given. The polymerization behavior, as well as the final properties of the cured polymers, is covered. 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

The preparation and properties of (9-oxo-9//-fluoren-2-yl)phenyliodonium hexafluoroantimonate (14) as a new photoinitiator for the cationic polymerization of epoxides have been reported [37]. Compound 14 was prepared by the reaction of (diacetoxyiodo)benzene with fluorenone followed by treatment with sodium hexafluoroantimonate (Scheme 7.3). Photoinitiator 14 has the advantage of intramolecular photosensitization and it is a more effective initiator than the conventional iodonium salts [37]. 

The discovery of living cationic polymerization has provided methods and technology for the synthesis of useful block copolymers, especially those based on elastomeric polyisobutylene (Kennedy and Puskas, 2004). It is noteworthy that isobutylene can only be polymerized by a cationic mechanism. One of the most useful thermoplastic elastomers prepared by cationic polymerization is the polystyrene-f -polyisobutylene-(>-polystyrene (SIBS) triblock copolymer. This polymer imbibed with anti-inflammatory dmgs was one of the first polymers used to coat metal stents as a treatment for blocked arteries (Sipos et al., 2005). The SIBS polymers possess an oxidatively stable, elastomeric polyisobutylene center block and exhibit the critical enabling properties for this application including processing, vascular compatibility, and biostability (Faust, 2012). As illustrated below, SIBS polymers can be prepared by sequential monomer addition using a difunctional initiator with titanium tetrachloride in a mixed solvent (methylene chloride/methylcyclohexane) at low temperature (-70 to -90°C) in the presence of a proton trap (2,6-dt-f-butylpyridine). To prevent formation of coupled products formed by intermolecular alkylation, the polymerization is terminated prior to complete consumption of styrene. These SIBS polymers exhibit tensile properties essentially the same as those of... 

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