Structure-Property Relations for Some Highly Active ansa-Metallocene Catalysts in Olefin Polymerization - a Polymerization Study

摘要

Metallocene catalysts are characterized by their chemically well-defined active sites. The structure of the catalyst determines the microstructure of the polymer. The polymer architecture can thus be tailored by rational design of the catalyst. This requires understanding of structure-property relations of the catalysts, the establishment of which was the central issue of this work. The influence of metallocene ligand structures and process conditions on the resulting polymer microstructure was investigated. Structural variations of the C_2-symrnetric metallocenes applied in this study comprised the length and rigidity of the interannular bridge, hydrogenation of the indenyl ligand, and siloxy substitution.The emphasis of the polymerization studies was put on finding relations between the catalyst structures and their copolymerization ability, their predominant chain termination mechanisms, and their tendency to undergo chain end isomerization reactions. Appropriate siloxy substitution patterns were demonstrated to improve polymerization performance, especially at reduced methylaluminoxane concentrations, in terms of productivity, copolymerization ability, and vinyl end group selectivity. The microstructure of a polymer is determined by all kinetically competitive reactions participating in the polymerization process. A relation could be established between the catalyst structure and its preferred mode of chain termination, which was further shown to be reflected in the tendency toward chain end isomerization, thus influencing vinyl end group selectivity in ethene polymerizations and stereoselectivity in propene polymerizations. Metallocene hydrogenation was found to increase propensity toward #beta#-H elimination to the metal, which was shown to have a marked influence on the feasibility of chain end isomerization. Based on this knowledge, the influence of temperature, ethene concentration, and comonomer addition on the end group pattern and molecular weight of polyethenes, as well as the influence of temperature and propene concentration on the stereoregularity of polypropenes, could be rationalized.Stereoerror formation in propene polymerizations with novel dual-side C_1-symmetric metallocene catalysts was shown, in contrast to C_2-symmetric catalysts, not to be influenced by chain end isomerization. A mechanistic model was supported which declares omittance of chain back-skip as the source for the observed isolated stereoerrors. The amount of these stereoerrors can be regulated and determines together with the molar mass the elastic properties of the novel homopolypropene material.