Facile synthesis of dumbbell-shaped dendritic-linear-dendritic triblock copolymer via reversible addition-fragmentation chain transfer polymerization

We report the first instance of facile synthesis of dumbbell-shaped dendritic-linear-dendritic triblock copolymer, [G-3]-PNIPAM-[G-3], consisting of third generation poly(benzyl ether) monodendrons ([G-3]) and linear poly(N-isopropylacrylamide) (PNIPAM), via reversible addition-fragmentation chain transfer (RAFT) polymerization. The key step was the preparation of novel [G-3]-based RAFT agent, [G-3]CH2SCSSCH2 center dot[G-3] (1), from third-generation dendritic poly(benzyl ether) bromide, [G-3]-CH2Br. Due to the bulky nature of [G-3]-CH2Br, its transformation into trithiocarbonate 1 cannot go to completion, a mixture containing similar to 80 mol % of 1 and 20 mol % [G-3]-CH2Br was obtained. Dumbbell-shaped [G-3]-PNIPAM(310)-[G-3] triblock copolymer was then successfully obtained by the RAFT polymerization of N-isopropylacylamide (NIPAM) using 1 as the mediating agent, and trace amount of unreacted [G3]-CH2Br was conveniently removed during purification by precipitating the polymer into diethyl ether. The dendritic-linear-dendritic triblock structure was further confirmed by aminolysis, and fully characterized by gel permeation chromatography (GPC) and H-1-NMR. The amphiphilic dumbbell-shaped triblock copolymer contains a thermoresponsive PNIPAM middle block, in aqueous solution it self-assembles into spherical nanoparticles with the core consisting of hydrophobic [G-3] dendritic block and stabilized by the PNIPAM central block, forming loops surrounding the insoluble core. The micellar properties of [G-3]-PNIPAM(310)-[G-3] were then fully characterized. (c) 2006 Wiley Periodicals, Inc.