Chain transfer reactions in propylene polymerization with zirconocene catalysts

The chemical structures of end groups of medium-low molecular weight atactic and isotactic polypropylenes (a-PP and i-PP), produced with zirconocene/methylalumoxane catalysts, have been analyzed and used to infer the chain-transfer reaction mechanisms, which are then correlated with the zirconocene ligand structure and the polymerization conditions. For the chiral, isospecific ansa-zirconocenes such as rac-[ethylenebis(1-indenyl)]ZrCl2/methylalumoxane (1/MAO) and rac-[ethylenebis(4,7-dimethyl-1-indenyl)]ZrCl2/methylalumoxane (2/MAO) catalysts, i-PP molecular weight is dependent on the regiospecificity of the catalyst, as shown by the presence of cis-2-butenyl end groups, formed by chain transfer to the monomer after a secondary propylene insertion. At low monomer concentration, chain-transfer with 2/MAO shifts from predominant transfer to the monomer after a secondary propylene insertion to beta-methyl (allyl end groups) and beta-hydrogen transfers after a primary insertion (2-propenyl, or vinylidene, end group). Ansa-bis(3-R-indenyl)ZrCl2 (ansa = CH2CH2, Me2Si, Me2C; R = Me, t-Bu, Me3Si) catalysts, which are highly regiospecific, produce polypropylenes with chain transfer via both beta-hydrogen transfer after a primary insertion and beta-methyl transfer. For example, rac-Me2C(3-t-Bu-Ind)(2)ZrCl2 (4) exhibits the highest selectivity for beta-methyl transfer so far observed in an isospecific zirconocene. As for 2/MAO, the rate of \beta-methyl transfer in 4/MAO increases by lowering [propylene].