Studies on the anionic oligomerization of methacrylonitrile by alkali metal alkoxides

The bulk anionic oligomerization of methacrylonitrile (MAN) by RO/ROH solutions was studied at constant temperature. The disappearance of monomer and of alcohol, and the formation of the different oligomers up to n = 3 (trimers) were followed quantitatively by gas chromatography, and the change of DP of the oligomers with conversion was determined. The effects of the type of alcohol, [MAN]/[ROH] ratio, initiator concentration, and the nature of the alkali metal counterion were investigated. DP decreased in the order methanol > ethanol > isopropanol, and the rate of conversion vice versa. Increase of the [MAN]/[ROH] ratio and of initiator concentration increased DP. While the same behaviour was observed with sodium and potassium alkoxides, with the corresponding lithium alkoxides practically no oligomerization occurred even under more drastic conditions. Under the investigated conditions, the formation of the addition product, RO-MAN (n=1) was reversible, contrary to the formation of the higher oligomers. The Koq of the reversible reaction, [MAN][ROH]/[RO-MAN] was determined from reactions using excess ROH; for methanol it was 55 mole/1., and for ethanol 7·8 mole/1. Some of the rate constants of the various steps of the oligomerization reactions for methanol and ethanol were estimated taking into account the reversibility of the initiation reaction, and assuming steady state conditions in the concentration of the various anions present in the system. The rate constant of the initiation reaction was greater than the rate constants of propagation; the rate constant of propagation of M₁^- (formation of dimer anion) was greater than that of M₂^-. The rate constants of termination of the various oligomer anions by ROH were only 2-4 times greater than the corresponding propagation rate constants. The steady state concentrations of M₁^-, M₂^- and M₃^- were estimated, and in some cases were relatively high so that their sum was as much as 90 per cent of the initial concentration of the alkoxide initiator. The initiation rate constant with C₂H₅O^-/C₂H₅OH was about 4-5 times greater than that with CH₃O^-/CH₃OH, and the corresponding termination rate constants were smaller by a factor of 4-5, which is in accordance with the acidity of the alcohols.