Yaqing Qu

In situ synthesis of thermo-responsive ABC triblock terpolymer nano-objects by seeded RAFT polymerization

RAFT polymerization of N-isopropylacrylamide under heterogeneous conditions in the presence of diblock copolymer nano-objects of polystyrene-block-poly(N,N-dimethylacrylamide) trithiocarbonate (PS-b-PDMA-TTC) with the Z-group RAFT terminal on the outer side of the solvophilic poly(N,N-dimethylacrylamide) (PDMA) block is performed. This heterogeneous RAFT polymerization, which is named seeded RAFT polymerization, affords the in situ synthesis of the polystyrene-block-poly(N,N-dimethylacrylamide)-block-poly(N-isopropylacrylamide) (PS-b-PDMA-b-PNIPAM) triblock terpolymer nano-objects. The molecular weight of the triblock terpolymer linearly increases with the monomer conversion during the seeded RAFT polymerization. The morphology of the PS-b-PDMA-b-PNIPAM triblock terpolymer nano-objects is merely duplicated from the seed of the PS-b-PDMA-TTC diblock copolymer, which is the binary mixture of nanospheres and nanorods, when the polymerization degree (DP) of the poly(N-isopropylacrylamide) (PNIPAM) block is low or moderately large. When the DP of the PNIPAM block is relatively large, the triblock terpolymer nanospheres are formed. The size of the PS-b-PDMA-b-PNIPAM triblock terpolymer nano-objects slightly increases initially and subsequently decreases with the monomer conversion during the seeded RAFT polymerization. In water at temperature above the phase-transition temperature (PTT) of the PNIPAM block, the PNIPAM chains deposit onto the polystyrene (PS) core to form the triblock terpolymer multicompartment nano-objects containing a microphase separated solvophobic core of PS/PNIPAM and a solvophilic PDMA corona. Our findings are anticipated to be useful in preparation of concentrated ABC triblock terpolymer nano-objects.

Synthesis of multicompartment nanoparticles of a triblock terpolymer by seeded RAFT polymerization

Seeded RAFT polymerization is proposed to prepare multicompartment nanoparticles of the poly(N,N-dimethylacrylamide)-b-polystyrene-b-poly(4-vinylpyridine) (PDMA-b-PS-b-P4VP) triblock terpolymer, which contain a polystyrene (PS) core, discrete poly(4-vinylpyridine) (P4VP) microphases on the PS core, and a solvated poly(N,N-dimethylacrylamide) (PDMA) corona. Following this seeded RAFT polymerization, the seed nanoparticles of poly(N,N-dimethylacrylamide)-b-polystyrene are initially prepared through dispersion RAFT polymerization, and then the P4VP block is introduced onto the seed nanoparticles by seeded RAFT polymerization to prepare the corona–core nanoparticles of PDMA-b-PS-b-P4VP containing a PS core and a mixed corona of P4VP and PDMA. When the corona–core nanoparticles of PDMA-b-PS-b-P4VP are dispersed in water, the P4VP chains, which are segregated by the neighboring PDMA chains, deposit on the PS core to form the discrete P4VP microphases on the PS core, and they convert into multicompartment nanoparticles. It is found that the size of the P4VP microphases on the PS core increases with the polymerization degree of the P4VP block. This seeded RAFT polymerization is believed to be a valid method to prepare block copolymer multicompartment nanoparticles.

One-pot preparation of BAB triblock copolymer nano-objects through bifunctional macromolecular RAFT agent mediated dispersion polymerization

One-pot preparation of the BAB triblock copolymer nano-objects of polystyrene-block-poly(4-vinylpyridine)-block-polystyrene (PS-b-P4VP-b-PS) containing a central solvophilic A block and two outer solvophobic B blocks through dispersion RAFT polymerization is proposed. Ascribed to the bifunctional macro-RAFT agent of bis(trithiocarbonate)-terminated poly(4-vinylpyridine) (TTC-P4VP-TTC), which contains two functional RAFT moieties at each terminal of the polymer backbone, the bifunctional TTC-P4VP-TTC macro-RAFT agent mediated dispersion polymerization affords the in situ synthesis of the PS-b-P4VP-b-PS triblock copolymer nano-objects. The parameters affecting the morphology of the PS-b-P4VP-b-PS triblock copolymer nano-objects are investigated, and it is found that the morphology of the PS-b-P4VP-b-PS nano-objects undergoes a transition from nanospheres, to worms, to vesicles and finally to lacunal nanospheres with the extension of the PS block during the dispersion RAFT polymerization. The bifunctional macro-RAFT agent mediated dispersion polymerization is demonstrated to be a valid method to prepare BAB triblock copolymer nano-objects with interesting morphology.

In situ synthesis of a self-assembled AB/B blend of poly(ethylene glycol)-b-polystyrene/polystyrene by dispersion RAFT polymerization

A diblock-copolymer/homopolymer self-assembled blend of poly(ethylene glycol)-block-polystyrene/polystyrene (PEG-b-PS/PS) was synthesized through dispersion RAFT polymerization. In this dispersion RAFT polymerization reaction, a macro-RAFT agent and a small RAFT agent containing the same RAFT moiety were simultaneously used. By tuning the molar ratio of the macro-RAFT agent and the small RAFT agent fed into the dispersion RAFT polymerization mixture, PEG-b-PS/PS self-assembled blends with different morphologies could be prepared. It is found that the PEG-b-PS/PS self-assembled blend synthesized by dispersion RAFT polymerization is very different from the blend prepared via micellization of a pre-synthesized PEG-b-PS/PS mixture, and its morphology changes from vesicles, to compartmentalized vesicles and finally to porous nanospheres with a decreasing ratio of PEG-b-PS/PS in the blends. It is believed that the present dispersion RAFT polymerization technique, which simultaneously uses a macro-RAFT agent and a small RAFT agent, is a valid method to synthesize self-assembled block copolymer blends.

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.