Photosensitized isomerization of olefin with benzophenone-conjugated amphiphilic graft copolymers

Two kinds of amphiphilic graft copolymers, PAA-g-P(MMA-co-BP) and P(AA-co-BP)-g-PMMA, consisting of polar poly(acrylic acid) (PAA), less polar poly(methyl methacrylate) (PMMA), and benzophenone (BP) photosensitizing units, have been synthesized. These polymers, when used as a photosensitizer for isomerization of trans-β-methylstyrene, show a sensitization activity controlled by solvents. In benzene and chloroform, P(AA-co-BP)-g-PMMA forms a micelle structure consisting of aggregated P(AA-co-BP) core with dissolved PMMA units at the outer sphere. This suppresses a triplet energy transfer (TET) from the excited state BP units to olefin and, hence, shows lower sensitization activity than a bulk sensitizer (4-methoxybenzophenone: MBP). In methanol, both polymers form a weak aggregate of less polar PMMA units containing dissolved PAA units. The internal cavity of the aggregate is less polar and stabilizes the excited state BP units. This accelerates a TET from the excited state BP units to olefin and shows higher sensitization activity than MBP.