In situ synthesis of thermoresponsive 4-arm star block copolymer nano-assemblies by dispersion RAFT polymerization

Well-defined 4-arm star block copolymer nanospheres of [poly(N-isopropylacrylamide)-block-polystyrene]4 [(PNIPAM-b-PS)4] containing a hydrophobic core of the polystyrene (PS) block and a thermoresponsive corona of the 4-arm star poly(N-isopropylacrylamide) (PNIPAM) were prepared by dispersion RAFT polymerization employing a tetrafunctional macro-RAFT agent of tetra(trithiocarbonate)-terminated poly(N-isopropylacrylamide) [(PNIPAM-TTC)4]. The size of the 4-arm star (PNIPAM-b-PS)4 nanospheres increases with the increasing degree of polymerization (DP) of the PS block, whereas it decreases with the DP of the star PNIPAM increasing. A comparison between the 4-arm star (PNIPAM-b-PS)4 nanospheres and the linear poly(N-isopropylacrylamide)-block-polystyrene (PNIPAM-b-PS) nanospheres is made. It is found that the 4-arm star (PNIPAM-b-PS)4 nanospheres and the linear PNIPAM-b-PS nanospheres have a similar particle size when the DPs of the PS and PNIPAM blocks in the star and linear block copolymers are close to each other. Interestingly, the topology of PNIPAM is found to exert influence on its thermo-responsive phase transition, and the (PNIPAM122-b-PS110)4 4-arm star block copolymer nanospheres have a lower critical solution temperature (LCST) lower than the PNIPAM118-b-PS125 linear block copolymer nanospheres. At temperatures above LCST of the (PNIPAM122-b-PS110)4 nanospheres, the 4-arm star PNIPAM chains deposit on the PS core to form raspberry-like nanospheres, in which the dehydrated 4-arm star PNIPAM chains form dispersed microdomains on the PS core. This synthesis of (AB)4 4-arm star block copolymer nano-assemblies is believed to be efficient and is helpful to study how the topology of PNIPAM affects the thermoresponsive phase transition.