Mechanistic Aspects of the Reversibility and Irreversibility of the Anionic Polymerization of α-Methylstyrene

The anionic polymerization of α-methylstyrene is examined in terms of the reversibility and the irreversibility of its propagation. The conditions for strict reversibility of a polymerization are defined. Deviation from strict reversibility for polymerization in bulk and in non-polar solvent is discussed together with its implication for thermodynamic studies based on data obtained from the equilibrium polymerization. In the case of the bulk polymerization initiated with tert-butyllithium in presence of N,N,N’,N’- tetramethylethylenediamine (TMEDA), the reversible propagating species formed at low temperature can be transformed into irreversible propagating species at high temperature. Transfer to the monomer also takes place and the presence of short active chains and irreversible species perturbs the equilibrium position of the polymerization. For the polymerization in a non-polar solvent, the specific case of the polymerization initiated with potassium in toluene is considered. Reversible propagating species are formed. However, because the propagation is very slow, transfer to the solvent and monomer and transformation of the reversible species into irreversible ones occur before the polymerization can reach the equilibrium position predicted by thermodynamics.