Group-transfer polymn. of Me methacrylate and Me acrylate with (1-methoxy-2-methyl-1-propenyloxy)trimethylsilane and (1-methoxy-1-propenyloxy)trimethylsilane as initiators, resp., and tris(piperidino)sulfonium bifluoride as catalyst in THF as solvent was investigated. While the former system nearly behaved like a living system, in the latter system side reaction between the active centers and the catalyst were of greater importance and limited monomer conversion in some cases. The character of the side reactions could be revealed by NMR and GC/MS measurements. No.-av. mol. wts. and polydispersities, as detd. by gel permeation chromatog., showed that tailoring of polymers with definite chain lengths and relatively narrow mol. wt. distribution was only possible in the Me methacrylate system. Reaction rates were very high, even at very low catalyst concns.