Non‐ideality in vinyl polymerisation, 2. Free‐radical polymerisation of styrene with azoisobutyronitrile

Polymerisation of styrene in benzene initiated by azoisobutyronitrile (AIBN) was carried out at 60°C covering a wide range of concentrations of both monomer and initiator. It is found that the monomer concentration has only little effect on the characteristic constant of primary radical termination, kprt/(kikp). This is explained on the basis of the fact that the secondary dissociation of the initial primary radicals occurs very quickly (ca. 10−11 s), thus allowing no chance of competition for the initiation process. This is in sharp contrast to the secondary dissociation of the benzoyloxyl radical which has a much longer life‐time (ca. 10−9 s). However, it is found that the polymerisation constant 2fkkdk /kt is dependent on monomer concentration and increases with the latter. The characteristic constant has also been determined at 40, 50, 70, and 80°C and the energy of activation corresponding to the initiation process obtained therefrom is about 66kJ/mol. This is appreciably higher than the value generally assumed (Ei≈Ep = 32,6kJ/mol) and could not be explained on the basis of experimental error. Ei is moreover very near to the value obtained for benzoyl peroxide initiated polymerisation of styrene determined earlier. Thus it seems that the initiation and propagation processes involve different activation energies and not the same as has hitherto been assumed.