Anionic polymerization of alkyl (meth)acrylates using metal-free initiators: Effect of ion pairing on initiation equilibria

Metal-free anionic polymerizations of alkyl (meth)acrylates using tetrabuty lammonium salts of diethylphenylmalonate, fluorene, and 9-ethylfluorene as initiators were performed in THF at 30 degrees C. A poor control of molecul ar weights, inconsistent initiator efficiencies, and broad or bimodal molec ular weight distributions were obtained. The effect of counterion nature wa s studied from the polymerization of methyl methacrylate using the 1,1-diph enylhexyl anion with tetrabutylammonium, tetramethyldiethylguanidinium, and lithium as counterions under otherwise identical conditions. Metal-free in itiators resulted in incomplete initiation which is attributed to the fact that the initiation is an equilibrium reaction. In conjunction with possibl e side reactions such as Hofmann elimination and transfer reactions, this l eads to broad and bimodal molecular weight distributions of the resulting p olymers.