Reaction of singlet oxygen with cts-l,4-polyisoprene gives cis-l,4-poly(isoprene hydroperoxide). The kinetics and mechanism of the photolysis of this polymer-singlet oxygen adduct have been studied at 313 nm in solution in the absence of oxygen. The primary quantum yield for photolysis of the polymer hydroperoxide was 0.8. The subsequent radical-induced chain decomposition of the hydroperoxide results in a high overall quantum yield for hydroperoxide decomposition ( )• The chain propagation reaction rate constant is estimated as 9 X 104 M"1 s"1. The polymer degradation kinetics was studied and the rate constant for the ß scission of the polymeric alkoxy radical (generated from primary hydroperoxide decomposition) is estimated as 2.7 X 104 s_1. The quantum yield of polymer chain scission ( §) is of the order of 0.1. From the experimental values of and 8, the ratio 8/ can be approximated as a constant of 0.014. This agrees remarkably well with the theoretical value of 0.01 derived from the mechanism proposed in this study.
Studies of the photolysis of ketone-containing polymers such as poly(styrene-co-phenyl vinyl ketone) and poly(styrene-co-methyl isopropenyl ketone) in the presence of di-tert-butyl peroxide and tert-butyl hydroperoxide show that both the singlet and triplet -* state of the ketones are quenched by the peroxides at diffusion-controlled rates. The energy transfer results in the ketone-sensitized decomposition of the hydroperoxides and is best explained by proposing the formation of an intermediate exciplex between the excited state of the ketone and the ground state of the peroxide. In view of the simultaneous presence of keto, hydroperoxy, and peroxy groups in the photooxidation of hydrocarbons generally, and hydrocarbon polymers in particular, this phenomenon should be considered in all attempts to establish detailed mechanisms of photodegradation.
SynopsisSorption and diffusion of toluene vapor in linear polyethylene with mass-fraction crystallinity between 0.48 and 0.82 and draw ratios h up to 10 have been studied at 30°C. The sorbed concentration in the amorphous phase C. is little affected by crystallinity, indicating that the free-volume fraction is roughly the same for all isotropic samples. However, the diffusion path becomes more tortuous with increasing crystalline content, thereby leading to a sixfold drop in the zeroconcentration diffusion coefficient D,,. Drawing has more drastic effects, reducing C. and Do by factors of 4 and 60, respectively, as A increases to 10. These large changes result from the transformation of the initially spherulitic material into a fibrous structure, which is composed of aligned microfibrils with taut tie molecules lying on the outer boundaries. The effects of crystallinity and orientation on the concentration dependence of the diffusion coefficient are also discussed.
is a rapid and satisfactory method provided that a blank control is used to correct for loss of volatile material from the support during the heat treatment.
References and Notes(1) (a) University of Toronto; (b) Ecole Polytechnique; (c) McGill University.(2) R.
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