The group-transfer polymn. (GTP) of methacrylonitrile [126-98-7] in MeCN, initiated by 2-(trimethylsilyl)isobutyronitrile [50638-76-1] in the presence of Et4NCN [13435-20-6] or Bu4NF [429-41-4] as coinitiator, was accompanied by an anionic polymn. initiated by the coinitiator alone so that polymers [25067-61-2] with bimodal mol. wt. distributions were formed. In contrast, with tris(dimethylamino)sulfonium difluorotrimethylsilicate [59218-87-0] as coinitiator a pure GTP was obsd., leading to polymers with unimodal mol. wt. distribution. In all systems, severe side reactions reduced conversions and mol. wts.