GROUP-TRANSFER POLYMERIZATION OF N-BUTYL ACRYLATE WITH LEWIS-ACID CATALYSTS .2. KINETIC INVESTIGATION USING THE HGI2 ME(3)SII CATALYST SYSTEM IN TOLUENE AND METHYLENE-CHLORIDE/

In the presence of trimethylsilyl iodide (TMSI) the group transfer pol ymerization (GTP) of n-butyl acrylate (n-BuA) using 1-methoxy-1-(trime thylsiloxy)-2-methyl-1-propene (MTS) as an initiator and mercuric iodi de (HgI2) as a catalyst in toluene is drastically accelerated. The rat e of polymerization is comparable to that of the nucleophilic catalyze d GTP with half-lives in the range of seconds to minutes. The inductio n periods which characterize the GTP in the absence of TMSI completely disappear. A very slight curvature in the first-order time-conversion plots is attributed to some ''back-biting'' termination. Kinetic inve stigations show that the polymerization follows first-order kinetics w ith respect to catalyst and TMSI concentrations. The rate of polymeriz ation strongly depends on the polarity of the reaction medium. An exte rnal reaction order of 1.52 with respect to the monomer concentration is due to the higher polarity of the reaction medium at higher monomer concentrations. The rate of polymerization is increased by 2 orders o f magnitude in CH2Cl2/toluene mixtures. The Arrhenius plot displays a maximum at similar to-60 degrees C, indicating a negative apparent act ivation energy in the temperature range from +40 to -40 degrees C, and a positive one at temperatures <--70 degrees C. The kinetic results s upport a mechanism in which mercuric iodide and TMSI form an active co mplex which acts as a nucleophilic catalyst. At lower temperatures the concentration of active species increases, overriding the decrease of the rate constant of propagation. The molecular weight is controlled by the ratio [M](0)/[I](0), and the molecular weight distributions of the polymers formed are very narrow and are not effected by TMSI.