Polymerization induced by ionizing radiation at low temperatures. I. Evidence for the simultaneous existence of ionic and free‐radical mechanisms in the polymerization of styrene and 2,4‐dimethylstyrene

Abstract

Abstract By means of initiation with ionizing radiation, styrene and 2,4‐dimethylstyrene have been polymerized in the crystalline solid state, in the pure liquid state, and in solution at different temperatures (−80°C. to 25°C.). In solution polymerization, it has been found that for the same type of initiation, other conditions being constant, the mechanism by which these monomers polymerize is temperature‐dependent. At lower temperatures a cationic mechanism predominates; at higher temperatures a free‐radical mechanism is in the ascendency. At intermediate temperatures, both cationic and freeradical polymerizations take place. Furthermore, these intermediate temperatures are different for different monomers, being higher in a monomer whose electronic structure is more favorable for cationic polymerization. The difference in mechanism at different temperatures has been proved by: ( 1 ) the dose rate dependencies, both of the polymerization rates and of the molecular weights of the resulting polymers; ( 2 ) the activation energies for the polymerization; and ( 3 ) the relative reactivities in the polymerizations of styrene and substituted styrenes.