The Ligand Effect in Ti-Mediated Living Radical Styrene Polymerizations Initiated by Epoxide Radical Ring Opening. 1. Alkoxide and Bisketonate Ti Complexes

ABSTRACT: Bisketonate and alkoxide Ti(III) complexes derived from Zn reduction of Ti(IV) precursors were evaluated as catalysts for the living radical polymerization (LRP) of styrene initiated by Ti-catalyzed epoxide radical ring opening and mediated by reversible termination with Ti(III). No polymerization occurred with tris(2,2,6, 6-tetramethyl-3,5-heptanedionato)titanium (III), whereas dichlorobis(2,2,6,6-tetra-methyl-3,5-heptanedionato)titanium (IV) affords only a free radical polymerization. Preliminary living features were displayed by (iPrO)2TiCl2. Investigations of the effect of epoxide/Ti/Zn ratios, temperature, and nature of the epoxide demonstrated that (iPrO)3TiCl provides a linear dependence of Mn on conversion over a wide range of conditions with an optimum for [Sty]/[epoxide group]/[Ti]/[Zn] 50/1/2/4 at 90 8C. However, the polydispersity could not be reduced below 1.4–1.5, with an initiator effi-ciency of 0.15. These results were rationalized in terms of a combination of decreased Ti oxophilicity and ligand exchange. The lowered oxophilicity decreases the initiation rate and broadens Mw/Mn. The fast alkoxide exchange promotes a weak dependence of the polymerization on reaction conditions and generates macromolecular Ti species with reduced ability to mediate LRP. Thus, while monofunctional epoxides provide homogeneous polymerizations and narrower Mw/Mn, difunctional initiators may lead to gel formation at high conversion. Nonetheless, all polymerizations were light gra