Kinetics of anionic polymerization of methylmetacrylate with caesium and sodium as counterions in tetrahydrofuran

Abstract

The kinetics of the anionic polymerization of methylmethacrylate in tetrahydrofuran at −75 are investigated. Cumylcaecium, α-methylstryrylcaesium and α-methylstyrylsodium were used as initiators. The results show that the polymerization proceeds practically without side reactions under these conditions; as for the anionic polymerization of styrene in polar solvents, ion pairs and anions contribute to the propagation. The rate constant of monomer addition to the ion pair has at −75 values of 60 and 80 for polymethylmethacrylsodium and polymethylmethacrylcaesium, respectively, and for the anion about 5 × 10 4 l mole −1 sec −1. The dissociation constant was measured as 3·5 × 10 −9 for polymethylmethacrylsodium and 2 × 10 −9 mole/l for the caesium compound at this temperature: the corresponding dissociation enthalpies are −0·3 and −1·3 kcal mole −1. The relatively low activation energy for monomer addition to the ion pair of polymethylmethacrylsodium of 2·3 kcal mole −1 suggests the existence of two types of ion pairs whereas the corresponding value for polymethylmethacrylcaesium of 4·5 kcal mole −1 does not allow such an interpretation. The kinetic results are compared with those of the corresponding polystyrylcompounds: the differences are explained by the fact that, in the case of the polymethylmethacryl compound, the estergroup competes with the solvent for solvation of the cation.