The Effects of Small amounts of Methanol on the Ionizing Radiation Induced Polymerization and Photopolymerization of Styrene.

The mechanisms whereby ionizing radiation produce the species which initiate neutral free radical and ionic polymerization are investigated. Of particular interest is the remarkable enhancement effect on radiation induced polymerization of styrene by small concentrations of methanol. The approach is to compare the effects of methanol on the steady state polymerization of styrene induced by two different initiation methods: (1) gamma ray; (2) ultraviolet light (UV). In addition to these steady state reaction studies, microsecond pulse radiolysis with 6 MeV electrons is used to study the transient aspects of styrene polymerization and especially, the effect of methanol on the intiation step. The results for photopolymerization initiated with wavelengths above 280 nm, and therefore dominated by free radical polymerization, show that methanol has no sensitizing effect. This leads to eliminate the interaction between methanol and free radical in the initiation stage of a polymerization. By means of studies with different alcohols and an analysis of the dimer and trimer yield, a proton donor mechanism is successfully established for the enhancement effect of methanol on the radiation induced polymerization of styrene. The role of methanol is explained to serve as a rapid proton donor to the anion radical converting the latter to a neutral propagating free radical. The resulting methoxide anion then neutralizes the cation radical converting it to an additional neutral propagating free radical. Trace amounts of methanol (less than 0.05 M) in system with 2-3 mM water is found to have a desensitizing effect because of a quenching influence of methanol on propagating free ionic species; this effect is not observed in the water saturated (35 mM water) system or even at higher concentrations of methanol in the system with 3 mM water. In pulse radiolysis, a quantitative agreement between steady state polymerization and pulse radiolysis is for the first time obtained in the analysis of the radiation yield of propagating radicals. This agreement gives rise to quantitative values for the extinction coefficient of the carbon-center propagating free radical and the bimolecular rate constant for the termination reaction.