**1. Introduction**

Photopolymers can change their physical and/or chemical properties through the absorption of light [1]. Although many studies on photopolymers have been reported, a few studies show photoinduced conversion to monomers under mild conditions. If a portion of the polymer is converted completely into monomers under light irradiation, the average molecular weight decreases, and this process has potential application to sophisticated device processing, including stereolithography without solvents. Such depolymerization under mild conditions is expected to yield recyclable (and, thus, environmentally friendly or "green") polymers. However, most of the depolymerizable polymers reported require X‐ray or electron‐beam irradiation to trigger the depolymerization process [2, 3]. Only a few reports describe use

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of low‐energy irradiation for the depolymerization [4–10]. The unzipping degradation of a polymer main chain containing heteroatoms by X‐ray or electron‐beam irradiation has been achieved. For a poly(1‐butene sulfone), only partial depolymerization occurs, and olefins are present in the gas produced by the irradiation [11, 12].

Degradation of a poly(olefin sulfone) doped with a photosensitizer has been investigated [13]. The degradation of poly(1‐butene sulfone) doped with pyridine N‐oxide was induced by irradiation with 300‐nm light. In this system, however, a crosslinking reaction occurs in addition to degradation of the main chain. The number of photoinduced breaks in the main chain was 10–12. High‐efficiency depolymerization to constituent monomers was achieved for poly(olefin sulfone)s containing photobase generators using low‐energy light irradiation.
