|74. Baskaran, D.; Müller, A.H.E.: Kinetic investigation on metal-free anionic polymerization of methyl methacrylate using tetraphenylphosphonium counterion in THF, Macromolecules 30, 1869 (1997)|
The anionic polymerization of methyl methacrylate using tetraphenylphosphonium triphenylmethanide as an initiator proceeds in a living manner even at room temperature. The rate constants of propagation were measured between -20 and +20 C using a flow tube reactor. At 0 C the reaction half-lives range from 0.3 to 1 s. The polymerization follows first-order kinetics with respect to monomer conversion (with a short induction period) and shows a linear dependence of the number-average degree of polymerization on conversion with high initiator efficiencies and narrow molecular weight distributions (Mw/Mn < 1.1). The dependence of the measured rate constants on the active center concentration is consistent with the coexistence of ion pairs and free ions. The Arrhenius parameters obtained in the temperature range from -20 C to +20 C (activation energy, Ea = 28 kJ mol-1 and frequency exponent, log A = 7.9) are similar to those for the Li+ cation. The determined rate constants for the ion pairs are 1-2 orders of magnitude smaller than those expected for such a large cation, indicating a large fraction of dormant species. NMR and quantum-mechanical studies of a model compound confirm the presence of a dormant ylide in equilibrium with a small amount of enolate ion pairs.