Radical cage effects in the photochemical degradation of polymers: In-cagetrapping of photochemically generated radical cage pairs in polymer model compounds

This study explored the origins of the observation that the overall quantum yields for polymer photodegradation depend on the polymer chain length. Th e (CH3(CH2)(n)C(O)NHCH2CH2Cp)(2)Mo-2(CO)(6) (n = 3, 8, 13, 18) complexes (1 -1-4-4) were synthesized and used as model complexes for the study. As is c ommon for metal-metal bonded complexes of this type, irradiation of these m olecules cleaved the metal-metal bonds and formed free radicals via the int ermediate formation of a radical cage pair. Studies on previous model compl exes showed that the quantum yields for degradation decreased as the chain length of the complex increased. The decrease in quantum efficiency was par tially attributed to an increase in the radical cage effect as the chain le ngth increased. Surprisingly, however, the overall quantum yields and cage effects for complexes 1-1-4-4 did not vary significantly with chain length. The similarity in the quantum yields and in the cage effects for these mol ecules is attributed to an internal trapping reaction of the metal radicals in the solvent cage by the H atom of the amide group. The resulting Mo ... (H)-N agostic interaction forms a six-membered ring. The trapping reaction takes place by segmental rotation of the metal-containing end of the radica l chain; the rate of this motion is independent of the chain length, and th us differences in the cage effects and the overall quantum yields will be d iminished for the four molecules. The X-ray crystal structure of the (CH3(C H2)(3)C(O)NHCH2CH2Cp)(2)Mo-2(CO)(6) molecule is also reported.