Ethylene-propylene copolymerisations:: Effect of metallocene structure on termination reactions and polymer microstructure

Ethylene-propylene (EP) copolymerisations were performed with two sterically different metallocenes activated by methylaluminoxane (MAO) in an attempt to better understand the effect of catalyst structure on termination reactions and polymer microstructure. The metallocene precursors under investigation were rac-dimethylsilylbis(2-methyl-4-phenyl-1-indenyl)zirconium dichloride (1) and a more sterically hindered counterpart rac-dimethylsilylbis(2isopropyl-4-[3,5-diiiiethylphenyl]indenyl) zirconium dichloride (2). For both catalyst systems, the most common termination mechanism was chain transfer to aluminium. In addition, for polymer samples polymerised with 1/MAO, chain growth was terminated by chain transfer to Zr metal in propylene-rich polymerisations and by chain transfer to ethylene monomer in ethylene-rich polymerisations. The steric hindrance of 2 was able to suppress the chain transfer to the ethylene monomer, and chain transfer to Zr metal was also found in the ethylene-rich polymerisations. The greater steric hindrance of 2 also affected the EP copolymer microstructure: regioregularity in the propylene-rich copolymers was greater and isotacticity less with 2/MAO than with 1/MAO.