Photochemically degradable polymers containing metal-metal bonds along their backbones

This review describes the synthesis and photochemistry of polymers that contain metal-metal bonds along their backbones. Such polymers are photodegradable because the metal-metal bonds are photolyzed by visible light. Understanding the factors that control the rate of polymer photodegradation is important for the technological applications of these materials, and this review focuses on two such factors, namely the radical cage effect and the applied stress on the polymer. With regard to the cage effect, data are reviewed showing that the k(d)/k(c) ratio (i.e. the ratio of the rate constant for diffusive separation out of the cage to that for radical-radical recombination in the cage) is proportional to m(1/2)/r(2), where m is the mass of the radical fragment and r the radius. The effect of stress on a polymer undergoing degradation is to increase the initial separation of the radical particles following photolysis of a bond along the backbone. The increase in separation of the radicals decreases the likelihood of radical-radical recombination, leading to an increase in degradation efficiency. Quantitative knowledge of these and other factors that control polymer degradation 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. (C) 2003 Elsevier B.V. All rights reserved.