|177. Weiss, H.; Steiger, S.; Jüngling, S.; Yakimansky, A.V.; Müller, A.H.E.: Quantum-Chemical Study of Structure and Activity of Chain Ends in Metal Free Anionic Polymerization of Methacrylates , Macromolecules 36, 3374 (2003)|
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.