Titanium-Mediated Living Radical Styrene Polymerizations. V. Cp2TiCl-Catalyzed Initiation by Epoxide Radical Ring Opening: Effect of Solvents and Additives

ABSTRACT: The effects of solvents, additives, ligands, and solvent in situ drying agents as well as catalyst and initiator concentrations have been investigated in the Cp2TiCl-catalyzed radical polymerization of styrene initiated by epoxide radical ring opening. On the basis of the solubilization of Cp2Ti(III)Cl and the polydispersity of the resulting polymer, the solvents rank as follows: dioxane tetrahydrofuran> diethylene glycol dimethyl ether> methoxybenzene> diphenyl ether bulk> tol-uene pyridine> dimethylformamide> 1-methyl-2-pyrrolidinone> dimethylaceta-mide> ethylene carbonate, acetonitrile, and trioxane. Alkoxide additives such as alu-minum triisopropoxide and titanium(IV) isopropoxide are involved in alkoxide ligand exchange with the epoxide-derived titanium alkoxide and lead to broad molecular weight distributions, whereas similarly to strongly coordinating solvents, ligands such as bipyridyl block the titanium active site and prevent the polymerization. By contrast, softer ligands such as triphenylphosphine improve the polymerization in less polar solvents such as toluene. Although mixed hydrides such as lithium tri-tert-butoxyaluminum hydride, sodium borohydride, and lithium aluminum hydride react with bis(cyclopentadienyl)titanium dichloride to form mixed titanium hydride species ineffective in polymerization control, simple hydrides such as lithium hydride, sodium hydride, and especially calcium hydride are particularly effective as in situ trac