Radical Cage Effects in the Photochemical Degradation of Polymers:  In-Cage Trapping 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. The (CH3(CH2) n C(O)NHCH2CH2Cp)2Mo2(CO)6 (n = 3, 8, 13, 18) complexes (1 − 1 − 4 − 4) were synthesized and used as model complexes for the study. As is...

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Bibliographic Details
Published in:Macromolecules 1999-06, Vol.32 (12), p.3898-3906
Main Authors: Male, Jonathan L, Yoon, Myungok, Glenn, Anne G, Weakley, T. J. R, Tyler, David R
Format: Article
Language:English
Subjects:
Applied sciences
Crosslinking and degradation
Exact sciences and technology
Physicochemistry of polymers
Polymers and radiations
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Summary:This study explored the origins of the observation that the overall quantum yields for polymer photodegradation depend on the polymer chain length. The (CH3(CH2) n C(O)NHCH2CH2Cp)2Mo2(CO)6 (n = 3, 8, 13, 18) complexes (1 − 1 − 4 − 4) were synthesized and used as model complexes for the study. As is common for metal−metal bonded complexes of this type, irradiation of these molecules cleaved the metal−metal bonds and formed free radicals via the intermediate formation of a radical cage pair. Studies on previous model complexes showed that the quantum yields for degradation decreased as the chain length of the complex increased. The decrease in quantum efficiency was partially attributed to an increase in the radical cage effect as the chain length 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 molecules 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 radical chain; the rate of this motion is independent of the chain length, and thus differences in the cage effects and the overall quantum yields will be diminished for the four molecules. The X-ray crystal structure of the (CH3(CH2)3C(O)NHCH2CH2Cp)2Mo2(CO)6 molecule is also reported.
ISSN:0024-9297
1520-5835
DOI:10.1021/ma990074z