Photochemically reactive polymers. Identification of the products formed in the photochemical degradation of polyurethanes that contain (C ₅H ₄R)(CO) ₃Mo-Mo(CO) ₃(C ₅H ₄R) units along their backbones

A photochemically degradable polyurethane with Mo-Mo bonds along its backbone (I) was synthesized by reacting the isocyanate-capped poly(ethylene glycol) prepolymer Hypol 2000 with the Mo - Mo-bond-containing diol (η ⁵-C ₅H ₄CH ₂CH ₂OH) ₂Mo ₂(CO) ₆. A copolymer (II) that contained poly(ethylene glycol) in addition to Hypol 2000 and (η ⁵-C ₅H ₄CH ₂CH ₂OH) ₂Mo ₂-(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 photochemical reaction only occurs in the presence of a radical trap, such as oxygen in the air; no net photochemical reaction occurs in the absence of radical trap. X-ray photoelectron spectroscopy shows that the Mo oxidizes during the photochemical reaction in air, initially to Mo(V) and then to Mo(VI). Both products are likely oxide species. Infrared spectroscopy confirms the release of CO when the polymers are irradiated. A polyurethane copolymer (III) that photochemically degrades in the absence of oxygen was synthesized by reacting (η ⁵-C ₅H ₄-CH ₂CH ₂OH) ₂Mo ₂(CO) ₆ and 1,4-butanediol with tolylene 2,4-diisocyanate terminated polypropylene glycol) and 1-(chloromethyl)-2,4-diisocyanatobenzene. The Cl atoms are readily abstracted by the photochemically generated Mo radicals, and this polymer readily degrades when irradiated in the absence of air. Despite the relatively low concentration of Cl in this polymer, the quantum yield for photodegradation is remarkably high (0.35). Other polymers with much higher concentrations of Cl trap are less reactive, and it is concluded that T _g is an important parameter in determining the efficiency of photochemical degradation. It is proposed that because chain motion occurs above T _g, the radical trapping reaction is more facile, resulting in more net reaction.