Fei Huo

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

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.

Seeded dispersion RAFT polymerization and synthesis of well-defined ABA triblock copolymer flower-like nanoparticles

A new kind of heterogeneous RAFT polymerization named ‘seeded dispersion RAFT polymerization’ is proposed for the preparation of well-defined triblock copolymer flower-like nanoparticles. Concentrated seed-nanoparticles of the amphiphilic polystyrene-block-poly(dimethylacrylamide) diblock copolymer including the RAFT agent Z-group at the terminal end of the solvophilic poly(dimethylacrylamide) block are prepared by diblock copolymer micellization in a ternary solvent mixture containing styrene monomer. Well-controlled seeded dispersion RAFT polymerization in the presence of the seed-nanoparticles is achieved, which is indicated by the linear increase in molecular weight with monomer conversion and the narrow molecular weight distribution of the synthesized polystyrene-block-poly(dimethylacrylamide)-block-polystyrene (PS-b-PDMA-b-PS) triblock copolymers. Compared with a soluble macro-RAFT agent mediated dispersion polymerization, the seeded dispersion RAFT polymerization is faster and either a very short induction time or no induction time is observed. The seeded dispersion RAFT polymerization affords well-defined flower-like nanoparticles of the PS-b-PDMA-b-PS triblock copolymer in the solvent which is selective for the central PDMA block. The effect of the polymerization degree of the outer PS blocks and the central PDMA block on the size and morphology of the flower-like nanoparticles is investigated, it is concluded that the size of the flower-like nanoparticles decreases with the central PDMA block length but increases with the outer PS block length.

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 a doubly thermo-responsive schizophrenic diblock copolymer based on poly[N-(4-vinylbenzyl)-N,N-diethylamine] and its temperature-sensitive flip-flop micellization

Synthesis of a doubly thermo-responsive schizophrenic diblock copolymer, poly(tert-butyl methacrylate)-block-poly[N-(4-vinylbenzyl)-N,N-diethylamine] (PtBMA-b-PVEA), by reversible addition–fragmentation chain transfer (RAFT) polymerization and its temperature-sensitive flip-flop micellization are discussed. By employing 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid as the RAFT agent, the schizophrenic PtBMA-b-PVEA diblock copolymers with different block lengths were prepared. The poly(tert-butyl methacrylate) (PtBMA) block exhibits insoluble-to-soluble phase transition at the upper critical solution temperature (UCST) and the poly[N-(4-vinylbenzyl)-N,N-diethylamine] (PVEA) block exhibits soluble-to-insoluble phase transition at the lower critical solution temperature (LCST) in methanol, respectively. At temperatures below the UCST of the PtBMA block, PtBMA@PVEA micelles containing a PtBMA core and a PVEA corona are formed in methanol. At temperatures above the UCST of the PtBMA block while below the LCST of the PVEA block, PtBMA-b-PVEA is molecularly soluble in methanol. At temperatures above the LCST of the PVEA block, inverse PVEA@PtBMA micelles containing a PVEA core and a PtBMA corona are formed. The polymerization degree of the PtBMA block or the PVEA block affecting the UCST/LCST of the schizophrenic diblock copolymer and the size of the PtBMA@PVEA or PVEA@PtBMA micelles is investigated.

Modification of block copolymer vesicles: what will happen when AB diblock copolymer is block-extended to an ABC triblock terpolymer?

Well-defined vesicles of poly(ethylene glycol)-block-polystyrene (PEG-b-PS) were prepared through the macro-RAFT agent mediated dispersion polymerization, and the modification of the PEG-b-PS vesicles by the extension of a third solvophilic poly(4-vinylpyridine) block through seeded RAFT polymerization was investigated. It was found, during the seeded RAFT polymerization, that the PEG-b-PS diblock copolymer was extended to the poly(ethylene glycol)-block-polystyrene-block-poly(4-vinylpyridine) (PEG-b-PS-b-P4VP) triblock terpolymer, and the PEG-b-PS vesicles were converted into the membrane-compartmentalized PEG-b-PS-b-P4VP vesicles (MCVs). In the MCVs, the poly(4-vinylpyridine) chains were found to be uniformly distributed in the inner side of the membrane, whereas on the outer side of the membrane several poly(4-vinylpyridine) chains were converged together and were segregated by the neighbouring poly(ethylene glycol) chains. The proposed seeded RAFT polymerization may be a promising method of vesicle modification, and the MCVs with segregated membrane structure are deemed to be a new morphology of the block copolymer nano-assemblies.