POLYMERIZATION OF METHYL-METHACRYLATE USING ZIRCONOCENE INITIATORS - POLYMERIZATION MECHANISMS AND APPLICATIONS

The polymerization of methyl methacrylate (MMA), initiated by zirconoc ene complexes Cp(2)ZrMe(2) (1) and [Cp(2)ZrMe(THF)][BPh(4)] (2), provi des partially syndiotactic poly(methyl methacrylate) (PMMA) in high yi eld and with a narrow molecular weight distribution (MWD). The kinetic s of this process were studied and reveal that in this system, the rat e of initiation is much slower than that of propagation. Initiation ap pears 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 p olymerizations initiated by complexes 1 and 2 has a rate-limiting step involving the reaction of neutral zirconium enolate species (4), prod uced in situ from 3 and 1, with monomer, activated by coordination to complex 2. Neutral enolate complexes 4 and complex 2 function as effec tive initiators of MMA polymerization in which the rate of initiation is greater than or equal to the rate of propagation. Under these condi tions, PMMA can be produced with very narrow MWD and the polymerizatio n process is living at or below 0 degrees 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. T he polymerization of MMA, using initiators 4 and 2, in the presence of trialkylaluminum compounds was studied with a view to overcoming thes e limitations. Alkylaluminum compounds appear to act as chain transfer /termination reagents in these polymerizations, although the rate of t his process is much slower than the rate of propagation. In particular , triisobutylaluminum can be used to purify monomer and solvent and al lows production of PMMA with a narrow MWD at high conversion. The poly merization of n-butyl acrylate (BuA), initiated by complexes 4 and 2, was studied. Poly(n-butyl acrylate) (P(BuA)) can be prepared in high y ield at low temperatures with a narrow MWD. This process is not living and the principal termination process involves back-biting cyclizatio n as revealed by MALDI-TOF mass spectra of low-MW polymer. At higher t emperatures, the growing chains are deactivated by this process and al so by competitive alpha-hydrogen transfer, such that high monomer conv ersions are not obtained.