Photo-induced Decrosslinking of Oxime-ester Based Covalent Adaptable Networks in Film State

Degradation techniques of networked polymeric materials are highly desired from the viewpoint of recycling, repair, and reuse of thermosets. Covalent adaptable networks enable easier degradation through bond exchange, and oxime esters attract attention for their associative bond exchange between free oximes at milder condition without a catalyst. On the other hand, oxime esters are known to show high photoreactivity when chromophore units are involved. Herein we demonstrate thermal crosslinking of copolymers bearing oxime ester groups with a molecular multifunctional oxime and their decrosslinking on UV-irradiation in film state. As monomers bearing oxime ester units, acetone O-methacryloyloxime and cyclohexanone O-methacryloyloxime were copolymerized with styrene and methyl methacrylate. On baking at 180 ℃ in the presence of truxenone trioxime, these copolymer films became insoluble in chloroform. Furthermore, the baked films became soluble on irradiation with Hg-Xe lamp. These behaviors suggested the proceeding of thermal bond exchanges of oxime ester groups in side-chains and the photolysis of the resulting oxime ester linker units which became photoreactive with truxene chromophore. The effect of comonomers and substituents in oxime esters are discussed. Also, we could find the effect of photoirradiation on the thermal exchange by stripe pattern formation. These results showed the potential of oxime ester units as exchanging ability of covalent bonds which can be controlled by photoirradiation.