Quanlong Li

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.

Doubly thermo-responsive nanoparticles constructed with two diblock copolymers prepared through the two macro-RAFT agents co-mediated dispersion RAFT polymerization

A new strategy to prepare doubly thermo-responsive nanoparticles constructed with the two diblock copolymers poly(N-isopropylacrylamide)-block-polystyrene (PNIPAM-b-PS) and poly[N,N-(dimethylamino) ethyl methacrylate]-block-polystyrene (PDMAEMA-b-PS) through the two macro-RAFT agents co-mediated dispersion polymerization is proposed. In this two macro-RAFT agents co-mediated dispersion polymerization, two macro-RAFT agents are simultaneously adopted, and the two in situ synthesized diblock copolymers PNIPAM-b-PS and PDMAEMA-b-PS co-assemble into nanoparticles containing a PNIPAM/PDMAEMA mixed corona and a common PS core. It is found that the molecular weight of PNIPAM-b-PS and PDMAEMA-b-PS in the mixed corona–core nanoparticles increases with the monomer conversion, and the size of the mixed corona–core nanoparticles increases with the PS block extension during the two macro-RAFT agents co-mediated dispersion polymerization. In water, the mixed corona–core nanoparticles exhibit two separate phase transition temperatures of 44 °C and 56 °C, corresponding to the PNIPAM and PDMAEMA blocks, respectively, which is confirmed by turbidity analysis, 1H NMR analysis and TEM observation. Our strategy named the two macro-RAFT agents co-mediated dispersion polymerization is believed to be a valid method to prepare multi-thermo-responsive nano-objects constructed with two or more than two thermo-responsive diblock copolymers.

A new strategy to prepare thermo-responsive multicompartment nanoparticles constructed with two diblock copolymers

Multicompartment block copolymer nanoparticles (MCBNs) constructed with two diblock copolymers of poly[N-(4-vinylbenzyl)-N,N-diethylamine)]-b-polystyrene (PVEA-b-PS) and poly[2-(dimethylamino) ethyl methacrylate]-b-polystyrene (PDMAEMA-b-PS) were prepared through the two macro-RAFT agents co-mediated dispersion polymerization. These MCBNs dispersed in water contain a 33 nm solvophobic core of the PS block, 5 nm discrete nodules of the PVEA block and a solvophilic corona of the PDMAEMA block. These MCBNs are thermo-responsive both in water and in methanol. In water, the corona-forming PDMAEMA block exhibits the soluble-to-insoluble phase transition at a temperature above the phase-transition temperature (LCST) of 70 °C to deposit on the PS core of MCBNs. In methanol, the PVEA nodules on the PS core are dissolved at a temperature below LCST of the PVEA block at 53 °C, and when the temperature increases above LCST the PVEA block reversibly deposits on the PS core to form discrete PVEA nodules on the PS core. The prepared MCBNs showing thermo-response both in water and in methanol are believed to be useful in nanotechnology.

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.