Factors controlling the rate of photodegradation in polymers

The onset of degradation in a photochemically degradable polymer should be reliably predictable. However, it is difficult to predict polymer lifetimes in practice because there is limited understanding of the parameters, both molecular and environmental, that control degradation rates and degradation onsets. In this study, the effect of temperature on the degradation quantum yield of a poly(vinyl chloride) polymer with Cp₂Mo ₂(CO)₆ units incorporated into its chains was investigated (Cp = cyclopentadienyl). The polymer is photochemically reactive in the absence of oxygen because the CpMo(CO)₃ radicals formed by photolysis of the Mo-Mo bonds react with C-Cl bonds to form CpMo(CO)₃Cl units. Quantum yields as a function of temperature were obtained for this polymer and for two control systems, Cp'₂Mo₂(CO)₆ dispersed in PVC and Cp'₂Mo₂(CO)₆ in hexane/CCl₄ solution (Cp' = ν⁵-C₅H₄CH₃). The quantum yields of the two control systems showed only slight increases with an increase in temperature. For the control reaction in hexane/CCl₄, the slight temperature dependence is attributed to the decrease in viscosity of the solution and the subsequent decrease in the radical-radical recombination efficiency. For the Cp'₂Mo₂(CO)₆ dispersed in PVC, the small temperature dependence is attributed to an increase in free volume as the temperature increases. In contrast to these results, the temperature dependence of the quantum yield of the PVC polymer with Cp ₂Mo₂(CO)₆ units along its chains is relatively large. It is proposed that an increase in temperature facilitates the polymer chain relaxation processes (involving recoil and rotation) following photolysis of the Mo-Mo bond. The radical-radical recombination efficiency is subsequently decreased, which leads to a net increase in chain cleavage and degradation efficiency. In support of this proposal, the apparent activation energy obtained from the temperature dependence of the quantum yield of the metal-metal bond containing PVC polymer (14.1 ± 0.3 kcal mol^-1) is consistent with secondary relaxation chain movements in polymers.