Polymerization of Methyl Methacrylate Using Zirconocene Initiators: Polymerization Mechanisms and Applications
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Abstract
The polymerization of methyl methacrylate (MMA), initiated by zirconocene complexes Cp2ZrMe2 (1) and [Cp2ZrMe(THF)][BPh4] (2), provides partially syndiotactic poly(methyl methacrylate) (PMMA) in high yield and with a narrow molecular weight distribution (MWD). The kinetics of this process were studied and reveal that in this system, the rate of initiation is much slower than that of propagation. Initiation appears to involve the rate-limiting reaction of complex 2 with monomer to generate a cationic enolate complex (3). The latter compounds also initiate polymerization of MMA, albeit at a much slower rate than that observed using initiators 1 and 2. The mechanism for propagation in polymerizations initiated by complexes 1 and 2 has a rate-limiting step involving the reaction of neutral zirconium enolate species (4), produced in situ from 3 and 1, with monomer, activated by coordination to complex 2. Neutral enolate complexes 4 and complex 2 function as effective initiators of MMA polymerization in which the rate of initiation is greater than or equal to the rate of propagation. Under these conditions, PMMA can be produced with very narrow MWD and the polymerization process is living at or below 0 °C. The rate of this process is highly sensitive to the presence of impurities and thus the degree of polymerization is practically limited by initiator concentration. The polymerization of MMA, using initiators 4 and 2, in the presence of trialkylaluminum compounds was studied with a view to overcoming these limitations. Alkylaluminum compounds appear to act as chain transfer/termination reagents in these polymerizations, although the rate of this process is much slower than the rate of propagation. In particular, triisobutylaluminum can be used to purify monomer and solvent and allows production of PMMA with a narrow MWD at high conversion. The polymerization of n-butyl acrylate (BuA), initiated by complexes 4 and 2, was studied. Poly(n-butyl acrylate) (P(BuA)) can be prepared in high yield at low temperatures with a narrow MWD. This process is not living and the principal termination process involves back-biting cyclization as revealed by MALDI-TOF mass spectra of low-MW polymer. At higher temperatures, the growing chains are deactivated by this process and also by competitive α-hydrogen transfer, such that high monomer conversions are not obtained.