| Abstract
| - A photochemically degradable polyurethane with Mo−Mo bonds along its backbone (I)was synthesized by reacting the isocyanate-capped poly(ethylene glycol) prepolymer Hypol2000 with the Mo−Mo-bond-containing diol (η5-C5H4CH2CH2OH)2Mo2(CO)6. A copolymer (II)that contained poly(ethylene glycol) in addition to Hypol 2000 and (η5-C5H4CH2CH2OH)2Mo2(CO)6 was also synthesized. The backbones of both polymers can be photochemically cleaved,because the Mo−Mo bonds homolyze when irradiated with visible light. A net photochemicalreaction only occurs in the presence of a radical trap, such as oxygen in the air; no netphotochemical reaction occurs in the absence of radical trap. X-ray photoelectron spectroscopyshows that the Mo oxidizes during the photochemical reaction in air, initially to Mo(V) andthen to Mo(VI). Both products are likely oxide species. Infrared spectroscopy confirms therelease of CO when the polymers are irradiated. A polyurethane copolymer (III) thatphotochemically degrades in the absence of oxygen was synthesized by reacting (η5-C5H4CH2CH2OH)2Mo2(CO)6 and 1,4-butanediol with tolylene 2,4-diisocyanate terminated poly(propylene glycol) and 1-(chloromethyl)-2,4-diisocyanatobenzene. The Cl atoms are readilyabstracted by the photochemically generated Mo radicals, and this polymer readily degradeswhen irradiated in the absence of air. Despite the relatively low concentration of Cl in thispolymer, the quantum yield for photodegradation is remarkably high (0.35). Other polymerswith much higher concentrations of Cl trap are less reactive, and it is concluded that Tg isan important parameter in determining the efficiency of photochemical degradation. It isproposed that because chain motion occurs above Tg, the radical trapping reaction is morefacile, resulting in more net reaction.
- Polymer I photochemically degrades in air to form Mo(V) and Mo(VI) species, CO, and short organic chains. Copolymers similar to I that incorporate monomers with C−Cl bonds into the backbone efficiently photodegrade in the absence of air because the Cl atoms are readily abstracted by the photochemically generated Mo radicals.
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