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.
To investigate the effect of tensile stress on the photochemical degradation efficiencies of polymers, a modified PVC polymer with Cp(CO)3Mo-Mo(CO)3Cp (Cp = eta5-C5H5) units along the backbone was synthesized. The polymer is photochemically reactive because the Mo-Mo bonds are photolyzed with visible light and the resulting radicals are captured with Cl atoms from along the polymer backbone. Of most importance from a mechanistic standpoint, the photochemical degradation reaction occurs in the absence of oxygen, which eliminates the kinetically complicating effect of rate-limiting oxygen diffusion. Tensile stress initially caused the quantum yield of polymer degradation to increase, but, after a certain point, additional stress caused a decrease in the quantum yield. This dependence of quantum efficiency on stress is consistent with a hypothesis in which stress affects the ability of the photochemically generated radicals to recombine. At low to moderate stress, the effect of stress is to increase the separation of the radicals (by recoil), thus decreasing their recombination probability. As the stress increases, however, segments of different chains align, which induces a higher degree of orientation and crystallinity in the polymer, which in turn makes diffusion more difficult. The efficiency of degradation is predicted to decrease accordingly because of decreased radical and/or trap mobility in the ordered polymer. Infrared and X-ray data are presented, showing that the degree of orientation and crystallinity in the polymer does indeed increase with increasing stress.
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