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‐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 ～80 mol % of 1 and 20 mol % G‐3]‐CH₂Br was obtained. Dumbbell‐shaped G‐3]‐PNIPAM₃₁₀‐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 G‐3]‐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 ¹H‐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]‐PNIPAM₃₁₀‐G‐3] were then fully characterized. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1432–1445, 2007