Analysis of radiation-induced ionic polymerization of iso butene

Abstract

Previously published experimental details of the radiation-induced polymerization of iso butene, together with new experimental details are reviewed critically in the light of a kinetic scheme. The essential features of this scheme are ( i ) initiation of polymerization by radiation-produced ions, ( ii ) propagation of polymer chains by a carbonium ion mechanism, ( iii ) termination of chains by a unimolecular process, ( iv ) termination of chains by a bimolecular process involving reaction with a radiation-produced inhibitor, ( v ) degradation of polymer molecules by further radiation. Various reaction constants are deduced; in particular the G value for ionic initiation is about 0.19 at —78.5 °C. In the bulk monomer at —78.5°C these reactions occur simultaneously. In a solvent for both monomer and polymer at this temperature, the initiation step is modified in that only the energy absorbed by the monomer can initiate polymerization. In a solvent which allows precipitation of polymer, while this latter condition is still fulfilled, the bimolecular termina­tion step is largely inoperative. In the presence of solid additives which are capable of increasing the polymer yield, the rate of the initiation reaction is increased and G values for ionic initiation of up to 2.9 have been deduced. Such investigations can provide information on the fate of the electron ejected by ionization. In all these cases degradation of the already formed polymer by further radiation occurs and this largely controls the average degree of polymerization; transfer reactions play little part.