LIVING CARBOCATIONIC POLYMERIZATION .38. ON THE NATURE OF THE ACTIVE SPECIES IN ISOBUTYLENE AND VINYL ETHER POLYMERIZATION

The nature of the species that gives rise to living carbocationic polymerization (LP + Pzn) of isobutylene (IB) coinitiated by Friedel-Crafts acids (FCA) in the presence of electron pair donors (ED) has been investigated. It is proposed that the key species responsible for mediating LC+Pzn of IB are FCA.ED complexes, formed by Lewis acid-Lewis base interaction, where FCA = BCI3 and TiCl4, and ED = ethyl acetate, tetrahydrofuran, etc. Free EDs are inhibitors of IB polymerization. These findings are in sharp contrast to those of LC+Pzn of vinyl ethers cointiated by EtAlCl2 in which excess (i.e., free) EDs are required for the living polymerizations to occur. Comparison of IB polymerizations induced by the 2-chloro-2,4,4-trimethylpentane/BCl3 and 2-acetyl-2,4,4-trimethylpentane/BCl3 systems shows that these reactions do not proceed by the same intermediate (i.e., by tert-chlorides) and that the mechanims of BCl3- and TiCl4-coinitiated living polymerizations are different. Experiments with the proton trap 2,6-di-tert-butyl-pyridine indicate that proton scavenging by FCA.ED complexes may be responsible for the narrow molecular weight distribution (MWD) products obtained in BCl3-coinitiated LC+Pzn of isobutylene. These findings also prove that R(i) (rate of initiation) > R(p) (rate of propagation), the relation required for narrow MWD in living polymerizations.