Polymerization of methyl methacrylate by metallocene imido complexes and tris(pentafluorophenyl)alane

The polymerization of methyl methaerylate (MMA) was investigated with tris(pentafluorophenyl)alane [Al(C6F5)(3)] and four metallocene imido complexes that varied in the complex symmetry/chirality, metal, and R group in the =NR moiety, as well as a zirconocene enolate preformed from the imido zirconocene and MMA. This study examined four aspects of MMA polymerization: the effects of the metallocene imido complex structure on the polymerization activity and polymer tacticity, the degree of polymerization control, the elementary reactions of the imido complex with Al(C6F5)(3) and MMA, and the polymerization kinetics and mechanism. There was no effect of the imido complex symmetry/chirality on the polymerization stereochemistry; the polymerization followed Bernoullian statistics, producing syndiotactic poly(methyl methacrylate)s with moderate (similar to70% [rr]) to high (similar to91% [rr]) syndiotacticity, depending on the polymerization temperature. Polymerization control was demonstrated by the number-average molecular weight, which increased linearly with an increase in the monomer conversion to 100%, and the relatively small and insensitive polydispersity indices (from 1.21 to 1.17) to conversion. The reactions of the zirconocene imido complex with Al(C6F5), and MMA produced the parent base-free imido complex and the [2 + 4] cycloaddition product (i.e., zirconocene enolate), respectively; the latter product reacted with Al(C6F5), to generate the active zirconocenium enolaluminate. The MMA polymerization with the metallocene imido complex and the alane proceeded via intermolecular Michael addition of the enolaluminate to the alane-activated MMA involved in the propagation step. (C) 2003 Wiley Periodicals, Inc.