Ligand effect in Ti-mediated living radical styrene polymerizations initiated by epoxide radical ring opening. III. Substituted sandwich metallocenes

The ligand effect and reaction conditions for the living radical polymerization of styrene initiated by epoxide radical ring opening were investigated in a series of substituted sandwich metallocenes: L2Ti(IV)Cl-2 [L = cyclopentadienyl (Cp), EtCp, (PrCp)-Pr-i, (BuCp)-Bu-t, pentamethylcyclopentadienyl (Cp*), or indenyl (Ind)], CP2TiX2 (X = F, Cl, Br, or CO), (BuCp2TiMe2)-Bu-t, and PcTiCl2 (PC = phthalocyanine). The polymerization was mediated by the reversible termination of growing chains with Ti(III) species derived from the Zn reduction of the Ti(IV) precursors. With the exception of Ti derivatives with Cp*, PC, F, CO, and Me ligands, which could not be reduced, all other complexes catalyzed a living radical process with a linear dependence of the number-average molecular weight on the conversion and a narrow polydispersity at 70-90 degrees C. A relatively weak dependence (RCp: R = H similar to Et similar to Pr-i tilda Bu-i > Ind) of the polymerization on the nature of the Cp substituent was observed. This was provisionally rationalized as a result of the constant balance between electronic and steric effects with increasing substituent size. By contrast, the apparent Cl > Br >> F and L2TiCl2 > LTiCl3 trend qualitatively paralleled the decrease in the Lewis acidity with a decreasing number of Cl ligands and the increased electron donation and steric effects of the metallocene ligands. Thus, sandwich metallocenes are better suited for this polymerization than Ti bisketonates, alkoxides, and half-sandwich derivatives. (c) 2005 Wiley Periodicals, Inc.