Cyclic vinyl ethers ring-opening polymerization

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

The similarity in the reactivities of free ions and corresponding ion pairs derived from the same cyclic monomer is more intriguing. Whereas ion pair reactivities are about 10 times smaller than corresponding free ion values in the anionic polymerization of vinyl monomers [39], and probably of the same relative proportions in cationic systems, the difference in cationic ring opening polymerizations is considerably less. For polymerization of THF in methylene chloride the factor is only 7, and for polymerization of 3,3-dimethylthietan, 40 in methylene chloride and 1 in nitrobenzene. Because the overall reactivity in cyclic monomer reactions is lower than for olefinic polymerizations, it might be expected that difference between free ion and ion pair reactivities, within one system, would also be less. However, this does not seem to be the whole answer. Plesch [44] has pointed out that in the polymerization of cyclic ethers and thietans (and presumably, therefore, other cyclic monomers)... 

Ring-Opening Polymerization of Cyclic Vinyl Ethers... 

Living cationic ring-opening polymerization (CROP) techniques represent important methods for the polymerization of a wide variety of heterocyclic monomers, such as cyclic ethers, cyclic amines, and cyclic imino ethers [7, 84-87]. The main differences between carbocationic polymerization of vinyl monomers and CROP of heterocyclic monomers arise from the nucleophilic heteroatoms... 

Aliphatic polysulfides with two or more carbon atoms per monomeric unit are accessible through ring opening polymerization of cyclic sulfides or by the addition of thiol groups onto vinyl groups. In these cases, the anionic polymerization of cyclic sulHdes differs substantially from that of cyclic ethers. The ethyl anion attacks the carbon atom in cyclic ethers. But in the ethyl lithium initiated polymerization of propylene sulfide, a lithium ethane thiolate is first formed, and its anion then starts the polymerization of propylene sulfide ... 

Scheme 10 Mechanism of ring-opening polymerization of cyclic vinyl ethers (23). Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Po/ymer2008, 49,1079-1131." ...

Percec has extended the use of cationic procedures to the synthesis of vinyl ether copolymers based on constitutional isomers, analogous to the copolysiloxanes shown in Fig. 7.16 (Section 7.4.2). Other work, by Percec and Hahn, will prove useful in the controlled synthesis of siloxane backbones (Fig. 7.23). Cationic, ring-opening polymerization of cyclic tetrameric siloxanes, in the presence of an end capper (hexamethyldisiloxane) and initiated using triflic acid, can be used to prepare homo- (Y = n = 0) or copolysiloxanes whose DP and compositions are determined by the ratio of end-capper to monomer in the feedstock. These polymers are starting materials for siloxane comb LCPs (Section 7.5.5). 

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