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Only a limited amount of work has been previously published concerning the free radical polymerisation of 4-methoxystyrene and 4-methylstyrene initiated by 2,2'-azoisobutyronitrile (2,2'-azobis(2-methylpropiononitrile)) (AIBN). The present investigation was, therefore, undertaken to establish whether the substituents have a considerable affect on the reaction of 2-cyano-2-propyl radicals at 60°C, relative to styrene. Rates of polymerisation were determined gravimetrically and rates of initiation were calculated from the radioactivity of the polymer, obtained by labelling AIBN with carbon-14. Number average molecular weights (@, ) were determined by gel permeation chromatography (GPC). The order of reaction with respect to initiator concentration was found to be 0.50 for styrene and 0.48 for both 4-methoxystyrene and 4-methylstyrene, which was attributed to their susceptibility to primary radical termination. Polystyryl radicals were also found to undergo primary radical termination and values for the constant [kp,,/(kik,)) were determined as 1.37 x lo4 molsdm-3 for styrene, 1.47 x lo4 mol * s * dm-3 for 4-methoxystyrene and 1.30 x lo4 molsdm-3 for 4-methylstyrene. The rate of initiation for 4-methoxystyrene is greater than for styrene, which in turn is greater than for 4methylstyrene. a) Submitted by A. Scheme 1: Re-initiation of Terminated Polymer with an Unsaturated End-group Primary Radical Termination Transfer to InitiatorThe previous studies have shown that the decomposition chemistry of AIBN is complex and influences its effectiveness as an initiator. The degree of "geminate" interaction has a large effect on the rate at which AIBN initiates polymerisation6). The percentage of radicals that avoid "geminate" interactions and initiate polymerisation is a measure of the efficiency of initiation 0. Except for the hydroperoxide