Synthesis of Amphiphilic Diblock Copolymer Using Heterobifunctional Linkers, Connected by a Photodegradable N-(2-Nitrobenzyl)imide Structure and Available for Two Different Click Chemistries

Two photodegradable heterobifunctional linkers (PHBL) connected by a N-(2-nitrobenzyl)imide structure have been developed as tools to obtain photodegradable diblock copolymers with precisely controlled molecular weight. They can undergo two different click reactions, i.e., Huisgen cyclo-addition and Michael addition. One linker has an alkyne moiety and maleimide protected as adduct with furan. Reaction of azido-terminated polystyrene (PS), poly(epsilon-caprolactone) (PCL) or poly(ethylene glycol) (PEG) with this PHBL followed by deprotection affords the corresponding maleimide-terminated polymer, which was coupled with thiol-terminated PEG or poly(N-isopropylacrylamide) (PNIPAM) to synthesize PS-b-PHBL-b-PEG, PCL-b-PHBL-b-PEG, and PEG-b-PHBL-b-PNIPAM. The other linker has maleimide and an alkyne protected by a trimethylsilyl group. This affords diblock copolymers by the reverse route, i.e., maleimide reacts with thiol-terminated PS to obtain a product by Michael addition, and then after deprotection, Huisgen cycloaddition of the generated alkyne- and azido-terminated PEG affords PEG-b-PHBL-b-PS. The resulting photosensitive diblock copolymers were characterized by UV spectroscopy in solution before and after photoirradiation. The energy needed for photolysis of the diblock copolymers in solution ranged from 12 to 17 J cm(-2), and was independent of polymer type and molecular weight.