Click cyclization of linear triblock copolymers at block junctions under high concentration due to end-block shielding

Linear triblock copolymers of poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) (PEG) with two azide groups at both block junctions (PNIPAM-N3-PEG-N3-PNIPAM) are click reacted with dipropargyl oxalylate under high polymer concentration (250 g/L). Benefiting from rapid feature of alkyne-azide click reaction and spatial shielding of PNIPAM end blocks, PEG center block of PNIPAM-N3-PEG-N3-PNIPAM remains separated although PNIPAM end blocks keep in contact under this high concentration. Therefore, PNIPAM-N3-PEG-N3-PNIPAM undergoes self-cyclization at block junctions to form tadpole-shaped architecture while N3-PEG-N3 without PNIPAM end blocks inter-connects linearly. The influence of block lengths of PEG and PNIPAM on the unusual cyclization under high polymer concentration is studied.