Photochemistry of ring‐substituted polystyrenes. II. Photolyses of poly(p‐fluoro‐, p‐chloro‐, and p‐bromostyrene)s

The photodegradation of thin films of p‐fluoro (PPFS), p‐chloro (PPCS), and p‐bromo (PPBS) styrenes brought about by exposure to 254‐nm radiation under high vacuum was studied. Mass spectroscopic measurements indicated that hydrogen and hydrogen halides were the only gaseous products because yields of H2 and HF from poly(p‐fluorostyrene) were much smaller than the corresponding yields of chloro‐ and bromo‐substituted polymers. UV and visible spectra of degraded films indicated the presence of unsaturated species, for the initial rates of formation were comparable in PPFS and PS but considerably greater in PPCS and PPBS. Solubility and molecular weight data indicated simultaneous crosslinking and chain scission; both PPCS and PPBS showed an inordinately high susceptibility to crosslinking. These observations can be rationalized in terms of the energetics of abstraction reactions by H and halogen atoms and in terms of scission of the Ph–Br and Ph–Cl bonds which lead to the participation of radicals in the para position in crosslinking. Some qualitative correspondence between the Hammett parameters of the p‐substituents and rates of H2 formation in the substituted polymers was observed. Quantum yields of gaseous product formation and probabilities of crosslinking and chain scission were also determined for the three polymers. Mechanisms of the various reactions are discussed.