Vinyl Ether Oligomers with Conjugated-Polyene and Acetal Terminals: A New Chain-Transfer Mechanism for Cationic Polymerization of Vinyl Ethers

Abstract

Cationic oligomerizations of isobutyl vinyl ether (IBVE) and 2-chloroethyl vinyl ether (CEVE) were carried out with various initiators (BF3OEt2, I2, CF3SO3H, CH3SO3H, and CH3COClO4) in benzene at 70°C, and the structure and molecular weight distribution of the products were investigated. The oligomers obtained were constituted of oligo(vinyl ether)s with a conjugated polyene (I) or an acetal (II) terminal. To account for the formation of I and II, a new chain-transfer mechanism has been proposed, which involves successive dealcoholation from the growing chain to give polyene I and a reaction of the released alcohol with the growing vinyl ether cation to yield acetal-capped oligomers II. Oxo acid initiators (CH3SO3H, CF3SO3H, and CH3COClO4) induced the dealcoholation more than BF3OEt2 or iodine, giving higher yields of I. It was also demonstrated that the polyene and acetal terminal are formed not only in the oligomerization at high temperatures but also in the polymerization at around 0°C. Thus the coloration of reaction mixtures in vinyl ether polymerization has now been interpreted by the formation of polyenes I.