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 TpTiCl3 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 °C. However, both Ind and Cp ligands provide a linear dependence of Mn 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 °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. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6039–6047, 2005 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). On the basis of a combination of steric and electronic properties, the Ti ligands rank as Cp ≥ Ind ≫ Cp* ≫ Tp, with an optimum for epoxide/CpTiCl3/Zn = 1/2/4 at 70–90 °C.