Alkyl vinyl ethers carbocationic

Quasi-living Carbocationic Polymerization of Alkyl Vinyl Ethers and Block Copolymer Synthesis... 

The model is based on the proposition that all carbocationic polymerizations of olefins, alkyl vinyl ethers, etc., involve a spectrum of species with differing ionicitieS connected by equilibria formally expressed by the Winstein ionicity spectrum 1] the Winstein spectrum starts with a covalent species and progresses through increasingly polarized and ionized species, to fully ionized solvated ( free ) ion pairs ... 

Several other functionalities were also introduced to poly(alkyl vinyl ethers) by living carbocationic polymerization (Table 6). 

Chain end functionalization of reactive carbocationic monomers, like isobutyl vinyl ether, can occur using ionic nucleophilic quenching reagents, i.e., methanol, alkyl lithium, etc. (6). 

Chain Transfer. Chain transfer is the most important chain-breaking reaction in carbocationic polymerization. It can be considered as a subcase of termination, when this latter is accompanied by reinitiation. In chain-transfer reactions the active center is transferred to a new site to imreacted monomer or initiator, the preformed polymer, or the solvent. This can happen by /3-proton elimination from the growing chain, or Friedel-Crafts alkylation of aromatic rings. /8-Hydrogen atoms of the propagating carbenium ions are quite acidic owing to the delocalization of the positive charge. In isobutylene, vinyl ether, and styrene... 

The living cationic polymerizations discussed above are invariably based on the nucleophilic iodide counteranion (activation of the carbon-iodine terminal bond Eq. 3). It is expected, however, that similar living processes are equally possible with other counteranions that can exert, as the iodide anion does, a suitably strong nucleophilic interaction with the growing carbocation. We have in fact found the phosphate anions to meet this requirement (10). Similarly to hydrogen iodide, monoacidic phosphate esters [H0P(0)R 2 R alkyl, alkoxyl, etc.] like diphenyl phosphate ( ) form a stable adduct 5) with a vinyl ether (Eq. 5). Zinc chloride or iodide then activates the phosphate bond in 5 by increasing its polarization (as in 6), and living cationic polymerization proceeds via an intermediate (7) where the carbocationic site is stabilized by a phosphate anion coupled with the zinc halide activator. 

Narasaka found that optically enriched oxabicydic substrate 277 bearing a vinyl sulfide moiety reacts with a silyl enol ether or ketene silyl acetal in the presence of a Lewis acid to afford the protected cyclohexenols 278a and 278b, Eq. 175 [18]. The reaction was proposed to occur via a ring-opening and alkylation sequence which is equivalent to overall nucleophilic substitution with retention of configuration. Presumably, the nucleophile attacked the carbocationic intermediate from the exo face, because the methylene-OTIPS substituent was blocking the endo side. 

---