Mechanistic investigations of titanium-mediated syndiospecific styrene polymerizations

Catalysts derived from half-metallocene complexes such as CpTiCl ₃ activated with methylaluminoxane polymerize styrene to syndiotactic polystyrene (sPS), a semi-crystalline, high temperature engineering thermoplastic exhibiting a high melting temperature T_m = 270°C and a high glass transition temperature T_g =100°C. The low level of toughness exhibited by this polymeric material and its brittleness at room temperature have prevented widespread use of sPS. Attempts to improve the mechanical properties of sPS by copolymerization with ethylene have been unsuccessful to date and yield mixtures of polyethylene, syndiotactic polystyrene, and random ethylene-styrene copolymers. This observation has led to hypotheses that different oxidation states are responsible for ethylene polymerization and styrene homopolymerization. In order to address the role of oxidation state on catalytic activity, syndiospecific styrene polymerizations were conducted using catalysts derived from the monomeric, well-defined Ti(IV) complex Cp*Ti(CH ₂Ph)₃ and the Ti(III) complex Cp*Ti(C₃H₅ )₂. While catalysts derived from the Ti(IV) precursors are shown to be inactive for sPS production in the absence of light, catalysts derived from Ti(III) precursors are highly active catalysts for sPS synthesis. Propylene/styrene copolymerization studies demonstrate that catalysts derived from Ti(III) precursors exhibit high chemoselectivity for the homopolymerization of styrene, while catalysts derived from Ti(IV) complexes are competent for propylene/styrene co-oligomerization.; Studies of the coordination chemistry of titanium complexes containing ligands derived from N,N-dialkylhydroxylamines and stable nitroxide radicals are presented with an emphasis on their abilities to serve as pre-catalysts for olefin polymerization. Structural characterization of (η2-TEMPO)TiCl ₃, [η2-(CH₃CH₂)₂NO] ₂Ti(CH₂Ph)₂, and Cp′TiCl ₂(η1-TEMPO) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl; Cp′ = Cp and Cp*) shows that the binding mode of hydroxylaminato ligands to titanium depends sensitively on the steric requirements of the ligand as well as the ancillary ligation at titanium. The alkylation chemistry of these complexes is studied along with their abilities to serve as pre-catalysts for α-olefin homopolymerizations and copolymerizations. Catalysts derived from Cp*TiMe₂(TEMPO) efficiently copolymerize ethylene and 1-hexene to yield copolymers with high 1-hexene contents. Comparisons of structural and electronic features and the ethylene/1-hexene copolymerization behavior of Cp*TiMe₂(TEMPO) and Cp*Ti(CH₂Ph)₃ with the constrained geometry catalyst [MeSi₂(η5-Me ₄Cp)(η1-N-tBu)]TiMe₂ provide insights into factors governing comonomer incorporation by mono-Cp titanium complexes.