Polymerization of ethene with zirconocene catalysts: an experimental and quantum chemical study of the influence of para-substituent in benzyl in bis{η 5-(1-benzyl)indenyl}zirconium dichlorides

Abstract

The meso- and rac-like isomers of bis{η 5-(1-benzyl)indenyl}zirconium dichloride ( 5), bis{η 5-(1- para-methoxybenzyl)indenyl}zirconium dichloride ( 6), bis{η 5-(1- para-fluoro-benzyl)indenyl}zirconium dichloride ( 7) and bis{η 5-(1-phenylethyl)indenyl}zirconium dichloride ( 8) were synthesized and isolated. Solid-state structures of meso- and rac-like 5 were determined by X-ray structure analysis. Polymerization properties of the methylaluminoxane (MAO) activated diastereomers of complexes 5– 8 were studied in ethene polymerizations under different monomer concentrations. The rac-like isomer of 1-phenylethyl-substituted 8/MAO showed significantly higher activity than the 1-benzyl substituted analogs 5– 7/MAO. In addition, rac- 8/MAO behaves like a single center catalyst producing polyethene with narrow molar mass distribution (1.8–1.9), while diastereomers of 5– 7/MAO produce polymers with molar mass distributions varying from 2.7 up to 10.3. The rac and meso-like isomers of 5– 7/MAO have different response on the monomer concentration. Quantum chemical calculations suggest a strong interaction between the benzyl substituent and the electron deficient zirconium center. The phenyl metal coordination energies depend on the electronic properties of the para-substituent. In 8/MAO, due to the ethyl spacer, the coordination does not have a significant role and therefore much higher activity and single center polymerization behavior is observed.