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

Solution copolymerizations of ethylene and 1-hexene were performed using the dichloride (HfCl 2 ) and dimethyl (HfMe 2 ) analogues of the bis( n -propylcyclopentadienyl)hafnium precatalyst. Tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C 6 F 5 ) 4 ] − [Me 2 NHPh] + ) (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/HfCl 2 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 HfMe 2 /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.