Factors controlling the rate of photodegradation in polymers: The effect of temperature on the photodegradation quantum yield in a PVC polymer containing metal-metal bonds in the polymer chain

The effect of temperature on the degradation quantum yield of a poly(vinyl chloride) polymer with Cp ₂Mo ₂(CO) ₆ units incorporated into its chains was studied (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′ ₂Mo ₂(CO) ₆ dispersed in PVC and Cp′ ₂Mo ₂(CO) ₆ in hexane/CCl ₄ solution (Cp′ = η ⁵-C ₅H ₄CH ₃). The quantum yields of the two control systems showed only slight increases with an increase in temperature. For the reaction in hexane/CCl ₄, this 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′ ₂Mo ₂(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 ₂Mo ₂(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.