Atypical Multiple Site Behavior of Hafnocene Catalysts in Ethylene/1-Hexene Copolymerization Using Trioctylaluminum and Borate

Solution copolymerizations of ethylene and 1hexene were performed using the dichloride (HfC1(2)) and dimethyl (HfMe2) analogues of the bis(n-propyl-cyclopentadienyl)hafnium precatalyst. Tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B-(C6F5)(4)](-)[Me2NHPh](+)) (B) or methylaluminoxane (MAO) was employed as activator, and tri-n-octylaluminum (TOA) was used as scavenger. Crystallization analysis fractionation (CRYSTAF) or crystallization elution fractionation (CEF) was used to measure the chemical composition distributions (CCD). Above a minimum 1-hexene threshold concentration, the CCD profiles of all copolymers were bimodal, and the areas under the peaks depended on the B/HfCl2 and B/Al ratios. Decreasing the TOA concentration reduced the weight fraction of the higher crystallinity copolymer. Copolymerizations using MAO also produced copolymers with bimodal CCDs. Copolymerizations using HfMe2/B without TOA produced copolymers with unimodal CCD. These results suggest that TOA reacts with either the catalyst or the activator, generating a new active site with a lower propensity for incorporating comonomer than the original catalyst system.