The ligand effect in Ti-mediated living radical styrene polymerizations initiated by epoxide radical ring opening.: 2.: Scorpionate and half-sandwich LTiCl3 complexes

The ligand effect and the reaction conditions for the living radical polymerization of styrene initiated by epoxide radical ring opening was investigated in a series of piano-stool, Ti(IV) scorpionate and, half-sandwich metallocenes (LTiCl3; L = Tp, Cp*, Ind and Cp, where Tp = hydrotris(pyrazol-1-ylborato), Cp* = pentamethylcyclopentadienyl, Ind = indenyl and Cp = cyclopentadienyl). The polymerization is mediated by the reversible termination of the growing chains with Ti(III) species derived from Zn reduction of parent Ti(IV) derivatives. A poor performance was observed for TpTiCl(3) because of probable over-reduction. The strong electron donating effect of Cp* accounts for a strong C-Ti chain end bond and consequently, a living-like process is observed only at T > 110 degrees C. However, both Ind and Cp ligands provide a linear dependence of M, on conversion and narrow polydispersity over a wide range of experimental conditions. Investigation of the effect of temperature and reagent ratios generates an optimum for epoxide/CpTiCl3/Zn = 1/2/4 at 70-90 degrees C. On the basis of a combination of steric and electronic properties, the ligands rank as Cp >= Ind >> Cp* >> Tp. This trend is different from coordination polymerization, and in conjunction with our previous results on Cp2TiCl2, further supports a radical mechanism. (c) 2005 Wiley Periodicals, Inc.