A study of halocarbon promoter influence on catalyst reactivity and polymer Mn in vanadium-based ethylene polymerizations

Several halocarbon containing compounds were tested as promoters for a vanadium-based ethylene polymerization catalyst. Some of the halocarbon compounds, which were not anticipated to act as promoters, were found to be good promoters for the catalyst system tested. In addition to influencing the catalyst reactivity, the halocarbon promoters were found to affect the polyethylene number average molecular weight. In most cases, the polymer melt index (MI), high load MI, MI ratio (MIR), weight average molecular weight, and molecular weight distribution were not affected by the choice of promoter. Semi-empirical AMI and ab initio 3-21G(*) molecular modeling studies were undertaken in an attempt to explain promoter performance. Polymer number average molecular weight (Mn) was found to decrease as the highest occupied molecular orbital (HOMO) energy level of the promoter increased. For promoters that contained only one carbon atom, catalyst reactivity may reach a peak at about 3.2 eV for the lowest unoccupied molecular orbital (LUMO). However, when all of the promoters, which exist in different conformations, were included in the reactivity study, no correlation between catalyst reactivity and promoter attributes could be found. (C) 1999 Elsevier Science B.V. All rights reserved.