3D-QSAR analysis of metallocene-based catalysts used in ethylene polymerisation

Single site olefin polymerisation catalysts are suitable candidates for modelling purposes. Their well-defined structure and the almost complete elucidation of their polymerisation mechanisms, make these organometallic complexes ideal for studies based on quantitative structure-activity relationships (QSAR). Although the QSAR technique is extensively used in drug design, there are very few reports on its application to metallocene-based polymerisation catalysis. This probably has something to do with the difficulties inherent in controlling experimental conditions during the polymerisation process. In the present study, we obtained ethylene polymerisation data using a number of zirconocene catalysts under carefully controlled experimental conditions, i.e. keeping all polymerisation variables constant except catalyst structure. The catalytic activity and molecular weight of the resulting polyethylenes were experimentally determined. We then applied 3D-QSAR methodology to explain the experimental data in terms of three-dimensional (3D) field descriptors related to the structure of the metallocene catalyst. Our results provide useful correlations between experimental ethylene polymerisation activities and the steric, LUMO and local softness fields of the catalysts. The molecular weights calculated from comparative molecular field analysis (CoMFA) models including LUMO and local softness fields correlate well with the experimental ones. The predictive capacity of the models was also tested. Based on the proposed models, steric and electronic factors affecting polymerisation performance are discussed.