No abstract

The photochemistry of poly(n-butyl acrylate) (PnBA) has been studied at 77 and 298 K. The principal photoproducts of Norrish type I and II processes have been identified and their quantum yields have been measured. The chemistry of photogenerated radicals has been monitored via ESR spectroscopy. Both Norrish type I and II products are observed at room temperature on excitation at 253.7 nm. Hydrogen abstraction takes place from backbone methylene groups in samples excited at 77 K, while tertiary hydrogen abstraction from the main chain becomes the dominant radical termination process at or above 210 K, which is the glass transition temperature of PnBA. Excitation at 313 nm in the presence of air causes photooxidation but little or no photolytic bond cleavage.

Kinetics of the reactions of the principal radical species, the tertiary alkyl and peroxy radicals, generated on photooxidation of poly(n-butyl acrylate) (PnBA) were studied at room temperature under different oxygen pressures. A simplified mechanism of photooxidation, similar to that proposed earlier (Liang, R. H.; Tsay, F.-D.; Gupta, A. Macromolecules 1982,15,974), was used to interpret the data. The kinetic rate parameters as well as the radical concentrations developed under steady-state illumination conditions were estimated by a least-squares fit to the observed data by using kinetic equations based on such a mechanism. It was found that at least two different types of tertiary alkyl radicals (i.e., radicals of different reactivities) were being formed during photooxidation of PnBA.

a tilt is introduced, models C and D become nonequivalent with respect to powder averaging, and an additional tilt angle is needed to define the plane of the benzene ring. In view of the experimental spectral distortions (cf. Experimental Section), we did not attempt further simulations to pursue this point. Thus, qualitative analysis seems to favor either model C or D, in agreement with ref 49.We note in passing that the g" state for the y-position has slightly lower conformational energy than the t state (cf. Table I), thereby favoring the parallel orientation of the para axis with respect to the helix axis.