SynopsisIn the vacuum photolysis of polystyrene films at 28°C. under 2537 A. radirttion the only gaseous product is hydrogen. At the same time the polymer becomes insoluble and discolors. The intensity exponent of the reaction is unity, the quantum yield of hydrogen is 4.3 X the overall energy of activation is 2.9 kcal./mole, and the rate of photolysis is independent of molecular weight. No hydrogen is produced when 3650 A. radiation is used. These observations can be interpreted in terms of a simple reaction scheme in which the primary effect of the 2537 A. radiation is to liberate hydrogen atoms. The subsequent reaction of these and the polystyryl radicals simultaneously formed in a number of competing steps results in the appearance of hydrogen, the occurrence of crosslinking, and the production of carbon-carbon unsaturation in the main chain which ultimately leads to conjugation and color. These theories are s u p ported by infrared and ultraviolet spectral measurements and by the observation that the rate of photolysis is accelerated by nitrogen.

SynopsisThe long-wave (A > 3000 A) photo-oxidation of polystyrene in solution at 25°C has been studied osmometrically. Two types of chain scission have been observed: a purely photo process which occurs completely independently of oxygen and which is attributed to fission of photolabile groups in the polymer, and another process associated with random photolyses of the products of oxidation Scavenger experiments with I31I2 have shown that approximately two iodine atoms are incorporated per chain scission when photolysis is carried out in solution (benzene, hexafluorobenzene, methylene chloride) under high vacuum conditions in the presence of 1%.No iodine incorporation or chain scission occurs when ionically prepared polystyrenes are treated similarly. The nature of the photolabile bond has been discussed, and there is some evidence for a peroxidic linkage arising from oxygen copolymerization in the chains. It is suggested that fission of the photolabile groups contributes to the initiation of the long-wave photooxidation of the polymer.

SynopsisIn the photooxidation of polystyrene film a t 28°C. under 2537 A. radiation, water and carbon dioxide are the only significant volatile products. The intensity exponent of the reaction is unity under 600 mm. O2 but zero under 20 mm., the quantum yield for oxygen absorption is 8.73 x 10-2, the overall energy of activation is 6.0 kcal./mole, and the rate ia independent of molecular weight. In addition, the rate is directly proportional to oxygen pressure, whether or not the total pressure is made up to 600 mm.with nitrogen. Investigation of the effect of film thickness shows that neither the nonuniform distribution of initiating radiation in the film nor the diffusion of oxygen influences the reaction rate abnormally. The free radical inhibitor 2,6-di-tert-butyl-4-methylphenol has no influence on the reaction, but the ultraviolet absorber 2-hydroxy4methoxylbenzophenone accelerates oxygen absorption, having a maximum effect a t approximately 3% concentration. It appears that the effect of ultraviolet absorbers is to protect the bulk of the polymer at the expense of the surface layer. Under 3650 A. radiation the oxygen absorption is much slower and the reaction exhibits an induction period. From these results it appears that while the initially formed radicals react with oxygen to form ROO-and HOO. radicals, the next step in the Bolland hydroperoxidation mechanism, ROz -+ RH -+ ROOH + R., is effectively suppressed by the lack of mobility of the long chain radicals within the solid polymer. The kinetic behavior described above can be accounted for qualitatively in terms of three competing processes, namely: (a) direct recombination of the primary radicals, R. and He, (b) reaction of these primary radicals with oxygen, and ( c ) diffusion of the hydrogen atoms from the site of their formation.

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.

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