Stability of propagating species in living cationic polymerization of isobutylene

The stability of living polyisobutylene chains (PIE) obtained by di- and monofunctional initiators in conjunction with TiCl4 coinitiator was investigated under monomer starved conditions (i.e. after 100% monomer conversion) in the absence and presence of different additives, such as N,N-dimethylacetamide (DMA), 2,6-di-tert-butylpyridine (DtBP), pyridine (Py) and 2,4-dimethylpyridine (DMPy), in CH2Cl2/hexane (40:60 v/v) mixture at -78 degrees C. Only negligible amounts of chain ends with expected double bonds were formed as verified by H-1 NMR, and all the additives, with the exception of DtBP, resulted in constant molecular weights for a period of four hours. However, chain coupling occurred in the presence of DtBP. On the basis of our experimental findings this effect is interpreted by proton abstraction in a reaction between DtBP and propagating chains leading to external double bonds which further react with active chain ends. Molecular weight distribution data indicate that there are differences among the examined nucleophilic compounds in their mode of action during living polymerization of isobutylene.