| Group 4 metallocene initiation systems were discovered for efficient polymerization of methyl methacrylate (MMA). The initiation systems consisted of a dimethyl zirconocene (initiator) and B(C6F5) 3 (coinitiator). The molar ratio of the dimethyl zirconocene to B(C 6F5)3 was found crucial for efficient polymerization. Instead of a slow polymerization (or no polymerization) with a 1:1 molar ratio, a 2:1 molar ratio gave a 20-fold (or more) rate increase. The quantitative conversion of MMA was obtained in less than 1 h. A further increase of the molar ratio did not result in additional acceleration. Two representative initiation systems, in which Cp2ZrMe2 (1) and rac-Me2Si(IndH4)2ZrMe 2 (2) were used as the initiator respectively, were prepared. The kinetics, stereochemical control, and living characteristics of these polymerization systems were studied. System 2, in which 2 was used, produced a living polymerization: the continuation of polymerization after the addition of a second batch of monomer, a narrow molecular weight distribution (MWD) throughout the whole polymerization (M¯ w/M¯n = 1.2) and a linear increase of M¯n versus conversion in both the first and second polymerization batches were obtained. Highly isotactic ([rr] ≥ 88%) polymer was produced and an enantiomorphic site-control mechanism for the chain propagation was shown by 13C. However, System 1, in which 1 was used, showed a chain termination (or transfer) process and new active species were generated during the polymerization. Moderately syndiotactic polymer ([rr] = 55%) was obtained. The chain propagation followed neither an enantiomorphic site-control mechanism nor a chain-end control mechanism. The kinetics of these two systems were also different. For System 2, the polymerization rate was first order in the monomer concentration and second order in the initiation system; while for System 1, they were zero order and third order respectively. A mechanism is proposed to accommodate the kinetic results and the ratio effect.; In addition, preliminary experiments for polymerization of methyl acrylate, 1-hexene, ϵ-caprolactone, propylene oxide, vinyl acetate and copolymerization of MMA with ϵ-caprolactone, MMA with 1-pentene were also performed. |
