Structure-property transition-state model for the copolymerization of ethene and 1-hexene with experimental and theoretical applications to novel disilylene-bridged zirconocenes

Ethene homopolymerization and copolymerization with 1-hexene were performed with three new tetramethyldisilylene-bridged zirconocene catalysts with 2-indenyl ligand (A), 2-tetrahydroindenyl ligand (B), and tetramethyl-cyclopentadienyl ligand (C) and with methylaluminoxane as a cocatalyst. Catalysts A and B showed substantial comonomer incorporation, resulting in a copolymer melting temperature more than 20° lower than that of the corresponding homopolymer. In contrast, catalyst C produced a copolymer with a low 1-hexene content and a high melting temperature. The reduction in the molecular weight with 1-hexene addition also correlated well with the comonomer incorporation. For all three catalysts, the homopolymer and copolymer unsaturations indicated frequent chain termination after 1-hexene insertion and a high degree of chain-end isomerization during the homopolymerization of ethene. The chain transfer to Al in the cocatalyst also appeared to be important. The comonomer response could be correlated with the structural properties of the catalyst, as derived from quantum chemical calculations. A linear model, calibrated against recent experiments with unbridged (MenC5H5−n)2ZrCl2 catalysts, suggested that the low comonomer incorporation obtained with catalyst C was caused partly by a narrow opening angle between the aromatic ligands and partly by steric hindrance in the transition state of comonomer insertion. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1622–1631, 2003