LIVING CATIONIC POLYMERIZATION OF VINYL ETHERS BY ELECTROPHILE LEWIS ACID INITIATING SYSTEMS .7. LIVING CATIONIC POLYMERIZATION OF ISOBUTYL VINYL ETHER BY TRIMETHYLSILYL HALIDE ZINC HALIDE INITIATING SYSTEMS IN THE PRESENCE OF PARA-METHOXYBENZALDEHYDE - EFFECTS OF HALIDE ANIONS AND ZINC HALIDES

Trimethylsilyl halides (Me3SiY), in conjunction with zinc halides (ZnX2) (Y and X: I, Br, Cl), were employed to investigate the living cationic polymerization of isobutyl vinyl ether (IBVE) in toluene at -15-degrees-C in the presence of p-methoxybenzaldehyde; with the aldehyde and IBVE monomer, Me3SiY yields an initiating species [Me3Si-O-CHC6H4(OMe)-CH2CH(OiBu)-Y] that triggers the IBVE polymerization via the activation of its carbon-halogen bond (C-Y) by ZnX2 into C-delta+ ... Y-delta- ... ZnX2. Living polymerizations occurred with the silyl iodide and bromide irrespective of the type of ZnX2, either when Y = X (Me3SiI/ZnI2 and Me3SiBr/ZnBr2) or when Y not-equal X (Me3SiI/ZnBr2, Me3SiI/ZnCl2, and Me3SiBr/ZnI2). With these five initiating systems, the number-average molecular weights (M(n)BAR) of the polymers increased in proportion to monomer conversion, and the molecular weight distributions (MWDs) of the polymers were narrow (M(w)BAR/M(n)BAR = 1.1-1.2). The Me3SiCl-based systems (Me3SiCl/ZnCl2 and Me3SiCl/ZnI2), in contrast, failed to give perfectly living polymerizations; the M(n)BAR indeed increased with conversion, but the MWDs of the polymers were broader (M(w)BAR/M(n)BAR = 1.3-1.5). Thus, the living nature of the polymerizations with Me3SiY/ZnX2 is primarily determined by the halogen Y in Me3SiY, which generates the terminal carbon-halogen bond (C-Y) that is activated by ZnX2 for the propagation via a species C-delta+ ... Y-delta- ... ZnX2. For Y-, not only the iodide but the bromide anion also is suited for living cationic polymerization. The virtual absence of the effects of X in ZnX2 implies that the halogen exchange between ZnX2 and Y from Me3SiY at the growing end (C-delta+ ... Y-delta- ... ZnX2 reversible C-delta+ ... X-delta- ... ZnXY) is absent or negligible.