Photochemically Degradable Polymers Containing Metal- Metal Bonds along Their Backbones: The Effect of Stress on the Rates of Photochemical Degradation

摘要

The syntheses of polymers that have metal-metal bonds along their backbones are described. The polymers are photodegradable because the metal-metal bonds homolyze when irradiated with visible light. The photochemical reactions of the polymers in solution are identical to the photochemical reactions of the discrete metal-metal bonded dimers. Typical reactions include metal-metal bond disproportionation and metal radical capture, for example, by chlorine atom abstraction from carbon tetrachloride. The polymers are also photochemically degradable in the solid state; thin films of the polymers degrade when irradiated with visible light in the presence of oxygen or in the absence of oxygen if the polymer backbone has a built-in radical trap. The origin of tensile stress-induced rate enhancements in the photodegradation of polymers was studied using the polymers with metal-metal bonds along their backbones and with built-in radical traps. By eliminating the need for external oxygen to act as a radical trap, the experimentally challenging problem of diffusion-controlled oxidation kinetics was avoided. Analysis of plots of quantum yields for degradation vs. stress reveals that stress increases the separation of the radical fragments produced by photolysis. An increased separation leads to less radical-radical recombination, which increases the efficiency of degradation. Quantitative knowledge of the factors that control polymer degradation rates will eventually allow synthesis of an ideal photodegradable polymer - one that has a tunable onset of degradation and that degrades quickly once degradation has started.