AbstractExperments are reported on the group transfer polymerization of methyl methacrylate using a new bifluoride salt, bis(triphenylphosphoranylidene)ammonium hydrogendifluorideCommonly abbreviated as PPN bifluoride (PPN HF2). [((C6H5)3P)2N]+HF, as catalyst. Good molecular weight control was observed although a small deviation from the expected value was found. Side reactions of catalyst with initiator, and termination (cyclization) of the active centres at low monomer concentrations were investigated. The similarities between this method and classical anionic polymerization are discussed.