Design of silica-tethered metal complexes for polymerization catalysis

Metal complex catalysts that are covalently tethered to oxide supports have been utilized for many years as hybrid molecular/heterogeneous catalysts. In this work, recent results from our laboratory in the area of silica-tethered metal complex polymerization catalysts are reviewed. The critical parameters for catalyst design when catalyst recovery and recycle are important are discussed in the context of silica-immobilized CuBr complexes for atom transfer radical polymerization (ATRP) as well as silica-tethered Zn-beta-diiminate (BDI) complexes for lactide or epoxide/CO2 polymerization. Additionally, a new strategy for the preparation of site-isolated metal complex catalysts on a silica surface is illustrated through the design of tethered constrained geometry catalysts (CGCs) for ethylene polymerization. The novel synthetic protocol is demonstrated to result in catalysts that appear to be more well-defined and that have improved catalytic properties. Major challenges and future directions in the field of oxide-tethered metal complex catalysis are discussed.