Chengqiang Gao

Dispersion RAFT polymerization: comparison between the monofunctional and bifunctional macromolecular RAFT agents

Bifunctional macromolecular RAFT (macro-RAFT) agent mediated polymerization affords one-step synthesis of the BAB triblock copolymer, which has special morphology. However, the bifunctional macro-RAFT agent mediated polymerization under heterogeneous conditions is rarely reported. In this contribution, the dispersion RAFT polymerizations of styrene in a methanol–water mixture mediated with poly(ethylene glycol) based monofunctional and bifunctional macro-RAFT agents, which afford the AB diblock copolymer of poly(ethylene glycol)-block-polystyrene (mPEG-b-PS) and the BAB triblock copolymer of polystyrene-block-poly(ethylene glycol)-block-polystyrene (PS-b-PEG-b-PS), respectively, are compared. It is found that these two dispersion RAFT polymerizations have similar polymerization rates, and almost full monomer conversion can be achieved. The molecular weight of both the mPEG-b-PS diblock copolymer and the PS-b-PEG-b-PS triblock copolymer linearly increases with the monomer conversion, whereas the control over the polydispersity index (PDI) of the PS-b-PEG-b-PS triblock copolymer is not as good as that of the mPEG-b-PS diblock copolymer. The monofunctional macro-RAFT agent mediated dispersion polymerization affords the in situ synthesis of mPEG-b-PS colloidal nanoparticles, which can be uniformly distributed in the polymerization medium and whose size increases with the extension of the solvophobic PS block, whereas the bifunctional macro-RAFT agent mediated dispersion polymerization leads to a mixture of colloidal nanoparticles and gel-like networks of the PS-b-PEG-b-PS triblock copolymer.

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.

Macro-RAFT agent mediated dispersion copolymerization: a small amount of solvophilic co-monomer leads to a great change

The macro-RAFT agent mediated dispersion copolymerization of two monomers, in which one is hydrophobic and the other is hydrophilic, is proposed to conveniently tune the morphology of the in situ synthesized block copolymer nano-objects. The poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion copolymerization of styrene and 4-vinylpyridine (St/4VP) in alcoholic solvent affords the in situ synthesis of the diblock copolymer nano-objects of poly(ethylene glycol)-block-poly(4-vinylpyridine-co-styrene) [PEG-b-P(4VP-co-St)]. It is found that, the morphology of the PEG-b-P(4VP-co-St) diblock copolymer nano-objects can be easily tuned either by changing the polymerization degree of the random P(4VP-co-St) block or the molar ratio of the PS/P4VP segments in the random P(4VP-co-St) block. The poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion copolymerization of St/4VP is compared with the dispersion RAFT polymerization of St, and the advantage of the dispersion RAFT copolymerization in tuning the block copolymer morphology is demonstrated. Our study is believed to be a promising extension of the polymerization induced self-assembly (PISA) under dispersion RAFT polymerization.

Macro-RAFT agent mediated dispersion polymerization: the monomer concentration effect on the morphology of the in situ synthesized block copolymer nano-objects

The monomer concentration affecting the morphology of the in situ synthesized block copolymer nano-objects during the macro-RAFT agent mediated dispersion polymerization is investigated. It is found that the monomer concentration exerts a great influence on both the polymerization kinetics of the poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion polymerization and the morphology of the in situ synthesized nano-objects of the poly(ethylene glycol)-block-polystyrene (PEG-b-PS) diblock copolymer. The poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion polymerization of styrene in an alcoholic solvent at 50% high monomer concentration follows similar kinetic behaviour to homogeneous RAFT polymerization as indicated by the linear ln([M]0/[M])–time plot, and good control both on the molecular weight of the PEG-b-PS diblock copolymer and the molecular weight distribution is achieved. With the extension of the PS block, the morphology of the in situ synthesized PEG-b-PS nano-objects changes from the porous nanospheres to the bicontinuous nanospheres and finally to the entrapped vesicles, which is much different from the dispersion RAFT polymerization at low monomer concentrations. Our results demonstrate that the monomer concentration is an important parameter affecting the morphology of the in situ synthesized block copolymer 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.

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.

Asymmetrical vesicles: convenient in situ RAFT synthesis and controllable structure determination

Asymmetrical vesicles constructed with two diblock copolymers of poly(ethylene glycol)-b-polystyrene (PEG-b-PS) and poly(4-vinylpyridine)-b-polystyrene (P4VP-b-PS) were prepared through the in situ synthesis strategy of the two macro-RAFT agents co-mediated dispersion polymerization. The structure of the PEG-b-PS/P4VP-b-PS asymmetrical vesicles was found to be dependent on the degree of polymerization (DP) of the poly(ethylene glycol) (PEG) and poly(4-vinylpyridine) (P4VP) blocks. In the case of the DP of the P4VP block being smaller, slightly larger, and much larger than the DP of the PEG block, the P4VP chains were located at the inner sides of the vesicle wall, located at both the outer and inner sides of the vesicle wall, and located at the outer side of the vesicle wall, respectively. The proposed two macro-RAFT agents co-mediated dispersion polymerization affords great convenience in the synthesis of asymmetrical vesicles with a well-defined structure, and it is also very helpful to understand the correlation between the block copolymer composition and the structure of asymmetrical vesicles.

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.