γ‐radiation of p‐substituted polystyrenes

The electronic environment about the main chain atoms in polystyrenes has been varied systematically by introducing electronegative or electropositive substituents (H 3 C, CH 3 O, Br, Cl, NC, and O 2 N groups) in the p ‐position of the ring. Energy yields for γ‐initiated H 2 formation and crosslinking, both a measure of the radiation sensitivity of the backbone, were determined above and below T g from ( 1 ) mass spectrometric examination of the decomposition gases and ( 2 ) molecular weight changes. For most of the polymers, it is concluded that the backbones of p ‐substituted polystyrenes are more radiation stable than that of polystyrene, and that main chain macroradicals formed early in the reaction are responsible for the major changes observed, i.e., hydrogen evolution and crosslinking. The role of the p ‐substituent on the latter effects is interpreted qualitatively in terms of the contribution by the substituent to the resonance stability of these intermediate polymer radicals. In the case of poly‐ p ‐bromo‐ and poly‐ p ‐chlorostyrene, the inordinately high sensitivity to crosslinking and the absence of halogen atoms in the decomposition gases suggest the importance of a chain reaction in the radiation chemistry of these molecules.