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.