Abstract
The advent of single-site, so called metallocene catalysts is revolutionizing the field of coordination polymerization of ethylene and α-olefins. Molecular design of the organometallic catalyst precursors has enabled unprecedented control of the various structural elements of the polymer (i.e. molecular weight, dispersity, incorporation of side chains, tacticity) that determine its physical properties. However, despite the commercial importance of chromium based polymerization catalysts, and in contrast to group 4 (Ti, Zr, Hf) chemistry, little is known about the relevant organometallic chemistry of chromium, and the development of novel organochromium catalysts is only beginning. Part of the problem is the paramagnetism of relevant chromium alkyls in those oxidation states commonly thought to be catalytically active (i.e. CrII, CrIII). The limited utility of NMR spectroscopy for the characterization of paramagnetic organometallics has generally discouraged the exploration of their chemistry and thus hindered the elucidation of the structural requirements for catalytic activity. Fundamental issues like formal oxidation state, coordination number, and chemical nature of the ligands of active chromium catalysts are still relatively ill defined. Seeking to shed light upon these matters, some time ago a broadly based investigation of the organometallic chemistry of paramagnetic chromium alkyls was initiated, with a particular emphasis upon polymerization catalysis. This chapter summarizes recent results from laboratory concerning homogeneous olefin polymerization catalysts containing chromium.
Published Version
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