Kinetic and NMR studies of the anionic polymerization of methyl methacrylate using tetraphenylphosphonium and other phosphorus-containing, metal-free counterions indicated an equilibrium between an active ion pair and a corresponding ylide as a dormant species. We present density functional theory (DFT) calculations of the energy difference between the ion pair and the ylide for the ester enolate of methyl isobutyrate with various counterions which support this view. It is possible to establish a correlation based on the calculated energy difference between the ylide and the ion pair and the apparent activity of the chain end. The ylide formed after the first polymerization step is extremely stable compared to the ion pair, whereas after addition of one molecule of methyl methacrylate, the ylide of the dimer is destabilized through steric interactions and the polymerization can proceed much faster. This explains the observation of a pronounced induction period when using the ester enolate of methyl isobutyrate as an initiator.

Bis(triphenylphosphoranilydene)ammonium (PNP+) triphenylmethanide (Ph3C–) is a new metal‐free initiator for the living polymerization of methyl methacrylate (MMA). The kinetics of the polymerization strongly depend on the metal counterion of the initiator precursor. When the initiator is made from the metathesis reaction of Ph3CK and PNPCl, the polymerization follows first‐order kinetics up to 0°C with half‐lives below 0.1 s. The propagation rate constants are much higher than those obtained with tetraphenylphosphonium (TPP+) cations, indicating a smaller fraction of dormant ylides. When the initiator is synthesized from Ph3CLi, polymerization proceeds much slower and molecular weight distributions of the obtained polymers are broadened indicating that the active species are mostly lithium enolates in this case.