LIVING CATIONIC POLYMERIZATION OF ISOBUTYL VINYL ETHER BY PROTONIC ACID ZINC HALIDE INITIATING SYSTEMS - EVIDENCE FOR THE HALOGEN EXCHANGE WITH ZINC HALIDE IN THE GROWING SPECIES

Living cationic polymerization of isobutyl vinyl ether (IBVE) was achieved by protonic acid/zinc halide (HB/ZnX2; B = I, Cl, OCOCF3, X = I, Br, Cl) initiating systems. The rate decreased in the order: HI/ZnI2 > HI/ZnCl2 almost-equal-to HCl/Zn2 almost-equal-to HCl/ZnBr2 > HCl/ZnCl2 >> CF3CO2H/ZnCl2. The number-average molecular weight of the polymers was directly proportional to monomer conversion and in good agreement with the calculated value assuming that one polymer chain forms per HB molecule. The molecular weight distribution of the polymer was very narrow (M(W)BAR/M(n)BAR less-than-or-equal-to 1.1). As model reactions of these living polymerizations, a series of mixtures of the HB-IBVE adduct 1 and ZnX2 were directly analyzed by H-1 NMR spectroscopy. The two methylene protons (-OCH2-) of 1 [CH3C*H(OCH2CH(CH3)2)B], which are not chemical shift equivalent because of the chiral alpha-carbon C*, are found interchangeable in the presence of ZnX2 due to the interaction of the C-B bond with ZnX2 in the activated form 3 (C(delta+)...B(delta-)...ZnX2). When B in 1 and X in ZnX2 are both halogens, the exchange reaction between B and X occurs to give another adduct [CH3CH(OCH2CH(CH3)2)X], whereas such an exchange was absent for the adduct with B = OCOCF3. These observations by H-1 NMR were consistent with the results of the polymerizations of IBVE by the HB/ZnX2 system and showed that the activated species 3 is at least partly ionic.