OLEFIN POLYMERIZATION AT BIS(PENTAMETHYLCYCLOPENTADIENYL)ZIRCONIUM AND HAFNIUM CENTERS - CHAIN-TRANSFER MECHANISMS

Chain transfer via beta-CH3 elimination by a homogeneous bimetallic Ziegler-Natta propylene polymerization catalyst is reported. Propylene is converted by Cp*2MCl2/MAO catalysts (Cp* = pentamethylcyclopentadienyl; M = Zr, Hf; MAO = methylalumoxane) to atactic propylene oligomers and low polymers. GC-MS and H-1 and C-13 NMR analyses of the oligomers obtained at 50-degrees-C (P(n)BAR almost-equal-to 4.5 for Zr, 3.4 for Hf) show these products to be mainly allyl- and isobutyl-terminated (1/1 ratio). The allyl/vinylidene ratio is 92/8 for Zr and 98/2 for Hf. No other unsaturated end groups could be detected. This end group structure is produced by first monomer insertion into the M-CH3 bond and then chain transfer by beta-CH3 elimination. On the contrary, Cp*2MCl2/MAO promotes 1-butene polymerization with the chain transfer being exclusively beta-H elimination and transfer to Al: no beta-ethyl elimination could be detected. The behavior of these catalysts toward propylene and 1-butene is compared with known Cp2MCl2/MAO catalysts.