Photochemical and thermal isomerizations of azobenzene-containing amphiphilic diblock copolymers in aqueous micellar aggregates and in film

Homopolymers of azobenzene methacrylates (azo) and their diblock copolymers with 2-(dimethylamino)ethyl metharylate (DMAEMA) were synthesized via atom transfer radical polymerization (ATRP). Polymers of cyano- and butoxy-substituted azobenzenes exhibited liquid crystalline textures. Water-soluble diblock copolymers could be obtained with a long DMAEMA block and a short azo block. These diblock copolymers self-assembled into polymeric micelles with core-shell structure in aqueous solutions. The rates of trans-cis photoisomerization were almost the same for the DMAEMA copolymers of 6-[4-phenylazo]phenoxy]hexyl methacrylate (PPHM), p(DMAEMA₁₇₂-b- PPHM₉), and 6-[4-(4-cyanophenylazo)phenoxy]hexyl methacrylate (CPHM), p(DMAEMA₁₇₂-b-CPHM₇), whereas slightly slower rates were observed for the copolymer of 6-[4-(4-butoxyphenylazo)phenoxy]hexyl methacrylate (BPHM), p(DMAEMA₁₇₂-b-BPHM₇), most likely due to steric hindrance presented by the bulky butoxy group. The rates in aqueous micellar solutions were only marginally faster than those in films for all the three diblock copolymers. In contrast, marked rate differences for the thermal cis-trans isomerization of p(DMAEMA₁₇₂-b-CPHM₇) were observed in film and in solution due to the donor-acceptor effect of CPHM. The observation of a sizable rate difference in different environment for p(DMAEMA₁₇₂-b-CPHM₇) suggests that a rotational mechanism might be operative for these water-soluble amphiphilic diblock copolymers.