In the presence of trimethylsilyl iodide (TMSI) the group transfer polymerization (GTP) of n-butyl acrylate (n-BuA) using 1-methoxy-1-(trimethylsiloxy)-2-methyl-1-propene (MTS) as an initiator and mercuric iodide (HgI2) as a catalyst in toluene is drastically accelerated. The rate of polymerization is comparable to that of the nucleophilic catalyzed GTP with half-lives in the range of seconds to minutes. The induction 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 investigations show that the polymerization follows first-order kinetics with respect to catalyst and TMSI concentrations. The rate of polymerization strongly depends on the polarity of the reaction medium. An external 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 of magnitude in CH2Cl2/toluene mixtures. The Arrhenius plot displays a maximum at similar to-60 degrees C, indicating a negative apparent activation energy in the temperature range from +40 to -40 degrees C, and a positive one at temperatures <--70 degrees C. The kinetic results support a mechanism in which mercuric iodide and TMSI form an active complex 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.