Metallocene polymerization catalysts—two cyclic aromatic rings flanking a central metal (generally Ti, Zr, or Hf) center—have recently been optimized for the commercial production of plastics. Although heterogeneous catalysts are more widely used, the well-defined structure and ligand tunability of the metallocenes offer more rational control over the characteristics of the polymer product, particularly its stereochemistry. However, these molecular catalysts have generally been ineffective in making ultrahigh-molecular-weight polyethylene, an especially tough, resilient plastic.
Starzewski et al. have designed a zirconocene that overcomes this deficiency and yields polyethylene with chain molecular weights exceeding a million g/mol. They tuned the catalyst's electronic properties to favor continual insertion of ethylene monomers into the growing polymer chain and achieved the necessary >10,000:1 selectivity for chain growth over termination by linking the cyclic ligands around Zr through a dative bond between a phosphine on one ring and a borane on the other. — JSY
Angew. Chem. Int. Ed. 45, 10.1002/anie.200504173 (2006).