Doubly thermo-responsive ABC triblock copolymer nanoparticles prepared through dispersion RAFT polymerization

Abstract

Doubly thermo-responsive triblock copolymer nanoparticles of poly(N-isopropylacrylamide)-block-poly[N,N-(dimethylamino) ethyl methacrylate]-block-polystyrene (PNIPAM-b-PDMAEMA-b-PS) and PDMAEMA-b-PNIPAM-b-PS containing two thermo-responsive blocks of poly(N-isopropylacrylamide) (PNIPAM) and poly[N,N-(dimethylamino) ethyl methacrylate] (PDMAEMA) are prepared by a macro-RAFT agent mediated dispersion polymerization through a polymerization-induced self-assembly. The RAFT polymerization undergoes an initial slow homogeneous polymerization and then a fast heterogeneous one. During the dispersion RAFT polymerization, the molecular weight of the synthesized triblock copolymer increases linearly with the monomer conversion, and the average diameter of the in situ synthesized triblock copolymer nanoparticles increases with the triblock copolymer molecular weight. The triblock copolymer nanoparticles exhibit two separate lower critical solution temperatures (LCST), corresponding to the PNIPAM block and the PDMAEMA block in water, and this two-step thermo-responsive behavior is evidenced by combined techniques, including turbidity analysis, variable temperature ¹H NMR analysis, DLS analysis and TEM observations. It is found that the first LCST corresponding to the PNIPAM block and the second LCST corresponding to the PDMAEMA block tethered on the polystyrene core of the triblock copolymer nanoparticles are much higher than those of the reference homopolymers, and the reason for this is ascribed to the steric repulsion and the strong interaction between the PNIPAM and PDMAEMA blocks. Besides, the difference in the thermo-responsive behavior of the triblock copolymer nanoparticles of PNIPAM-b-PDMAEMA-b-PS and PDMAEMA-b-PNIPAM-b-PS ascribed to the different block order is demonstrated.