Atom Transfer Reactions: Implications for Polymerization Catalysis

Stanford University, Stanford CA

Investigators

Abstract

Dr. Robert M. Waymouth, Chemistry Department, Stanford University, is supported by the Inorganic, Bioinorganic, and Organometallic Chemistry Program of the Chemistry Division to develop the chemistry of well-defined titanium complexes in the Ti(III) oxidation state, to investigate atom transfer reactions that mediate the reversible interconversion of Ti(III) and Ti(IV)in non-polar media, and to develop new catalytic systems based on titanium complexes that change oxidation state during catalysis. These studies are directed at catalysts for the syndiospecific polymerization of styrene as well as the development of new catalyst systems for the production of block copolymers of styrene and non-conjugated olefins. Mechanistic studies of syndiospecific styrene polymerization will focus on the reactivity of well defined Ti(III) precursors for styrene polymerization. Styrene and ethylene homopolymerization experiments and styrene/ethylene copolymerization experiments will be carried out to test the competence of both Ti(III) and Ti(IV) complexes for styrene and ethylene polymerization activity. The coordination chemistry of titanium complexes with stable nitroxides such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) will be carried out as a strategy for reversibly interconverting Ti(III) and Ti(IV) in non-polar solvents. The synthesis of Ti-TEMPO complexes and the bond energies of the Ti-nitroxide bonds will be investigated by thermolysis and nitroxide exchange equilibria. The kinetics and thermodynamics of atom-transfer equilibria between titanium complexes and other metal halides will be investigated to provide fundamental information on metal-halogen bond energetics. These studies will guide the choice of appropriate atom-transfer redox partners to mediate the interconversion of Ti(III) and Ti(IV) complexes. Polyolefins and polystyrenes are two important classes of polymers. This research seeks to develop a catalyst system that can produce a polymer that contains both polyolefin and polystyrene fragments. The properties of such "block co-polymers" can be precisely engineered if the composition and the size of the "blocks" of each individual component can be controlled. In this project, new titanium catalysts that will polymerize styrene when the titanium has a 3+ charge and olefins when it bears a 4+ charge will be developed. By altering the metal charge the composition of the block copolymer can be controlled.