Free Radical Polymerisation Studies of 4-Substituted Styrenes

Only a limited amount of work has been previously published concerning the effects of chain transfer to 2,2-azobis(2-methylpropanonitrile) (AIBN). Therefore, the present investigation was undertaken to establish whether AIBN undergoes chain transfer reactions with some 4-substituted styrenes. The monomers chosen for this study were styrene, 4-methoxystyrene, 4-methylstyrene and 4-chlorostyrene. Rates of polymerisation have been determined gravimetrically and rates of initiation were calculated from the radioactivity of the polymer, obtained by labelling AIBN with carbon -14. This is the first time that the effects of labelling AIBN in the methyl group have been compared with labelling it in the cyano group. Number-average molar masses (M,) were determined by gel permeation chromatography ’ (GPC) using polystyrene standards to calibrate the instrument. Since GPC is a relative method, some of the M, values were checked by membrane osmometry and dilute solution viscometry. The order of reaction with respect to initiator concentration for 4-methoxystyrene and 4-methylstyrene is less than 0.50. This is because both monomers are susceptible to primary radical termination (PRT). In the case of 4-chlorostyrene the order is higher than 0.50 due to its greater tendency to undergo chain transfer reactions. Both the end group analysis technique and the classical kinetic relationship (based on the degree of polymerisation) used to determine the constant for chain transfer to initiator (C,), are influenced by the degree of PRT. Hence values of C, were found to be 0.001 for 4-methoxystyrene, 0.0037 for 4-methylstyrene and 0.020 for 4-chlorostyrene. However, during the polymerisation of styrene, a value of C, could not be determined, because of the effect of PRT. An insight into the mechanisms of the initiation, chain transfer and termination reactions of the systems studied, has been obtained. Evaluation of the extent of monomer conversion can be used to obtain more information concerning the stationary state kinetics and reaction mechanisms of the four monomers studied.