On the mechanisms of growing-chain-end isomerization and transfer reactions in propylene polymerization with isospecific, C2-symmetric zirconocene catalysts

Abstract A new mechanism is proposed to explain the occurrence of primary-growing-chain-end epimerization, and the structure and frequency of olefinic end groups arising from primary β-hydride transfer, in low molecular weight isotactic polypropylenes produced with C 2 -symmetric zirconocenes. This mechanism, based on the reversible formation of a zirconocene allyl dihydrogen complex, accounts for both loss of catalyst stereospecificity and decrease of polypropylene molecular weights observed by lowering [propylene], and accommodates a number of previous and new experimental observations. Inter alia: (i) the formation of internal vinylidene unsaturations; (ii) the increase of catalyst activity induced by H 2 addition; (iii) the observed deuterium scrambling in the [1-D]propylene and [2-D]propylene polymerizations recently reported by Brintzinger. This new mechanism is discussed in the light of the polymerization results from two MAO-activated catalysts: rac -C 2 H 4 (Ind) 2 ZrCl 2 and the novel, highly regiospecific rac -Me 2 C(3- t -Bu-Ind) 2 ZrCl 2 .