Xiaowu Jiang

Recent Progress on Transition Metal Catalyst Separation and Recycling in ATRP

Atom transfer radical polymerization (ATRP) is a versatile and robust tool to synthesize a wide spectrum of monomers with various designable structures. However, it usually needs large amounts of transition metal as the catalyst to mediate the equilibrium between the dormant and propagating species. Unfortunately, the catalyst residue may contaminate or color the resultant polymers, which limits its application, especially in biomedical and electronic materials. How to efficiently and economically remove or reduce the catalyst residue from its products is a challenging and encouraging task. Herein, recent advances in catalyst separation and recycling are highlighted with a focus on (1) highly active ppm level transition metal or metal free catalyzed ATRP; (2) post-purification method; (3) various soluble, insoluble, immobilized/soluble, and reversible supported catalyst systems; and (4) liquid-liquid biphasic catalyzed systems, especially thermo-regulated catalysis systems.

An atom transfer radical polymerization system: catalyzed by an iron catalyst in PEG-400

A green and highly efficient AGET ATRP (activators generated by electron transfer for atom transfer radical polymerization) system was constructed in the absence of any additional ligands, using FeCl3·6H2O as a catalyst, and methyl methacrylate as a model monomer in polyethylene glycol 400 (PEG-400). The effects of various factors, such as the type of ATRP initiator, the molecular weight of PEG and the reducing agent type, polymerization temperature as well as solvent, on the polymerization were investigated. Polymerization kinetics demonstrated that the polymerization was a controlled/“living” process with molecular weight increasing linearly with conversion while maintaining a low molecular weight distribution. The living feature was further confirmed by chain extension experiments.

Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

Iron catalysts are attractive catalysts for atom transfer radical polymerization (ATRP) owing to their abundancy, low toxicity and good biocompatibility. However, the recycling of iron catalysts is still a great challenge although the recycling of copper catalysts has achieved success. In this work, we develop a facile and highly efficient separation and recycling strategy for an iron catalyst combining thermoregulated phase separable catalysis (TPSC) and initiators for continuous activator regeneration for atom transfer radical polymerization (ICAR ATRP) in a PEG-200/p-xylene biphasic system. Herein, FeCl3·6H2O was used as the catalyst, tetrabutylammonium bromide (TBABr) as the ligand, 2,2′-azobisisobutyronitrile (AIBN) as the reducing agent, ethyl-2-bromo-2-phenyl acetate (EBPA) as the initiator, and methyl methacrylate (MMA) as the model monomer. The PEG-200/p-xylene biphasic system formed a homogeneous polymerization solution at 70 °C, and the iron catalyst could be easily separated in situ just by a simple standing and decantation process, and was therefore recycled for the next polymerization when the polymerization temperature decreased to room temperature. In this novel polymerization system, well-defined PMMA with controlled molecular weights and narrow molecular weight distributions could be easily obtained, and the iron catalyst could be recycled in situ 10 times without any significant loss of the catalyst activity. In addition, this novel strategy was also extended to other hydrophobic monomers such as styrene, methyl acrylate and tert-butyl acrylate, indicating a versatile method for iron catalysis, separation and recycling.

Cu(II)‐Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate via a Strategy of Thermo‐Regulated Phase‐Separable Catalysis in a Liquid/Liquid Biphasic System: Homogeneous Catalysis, Facile Heterogeneous Separation, and Recycling

A strategy of thermo-regulated phase-separable catalysis (TPSC) is applied to the Cu(II)-mediated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in a p-xylene/PEG-200 biphasic system. Initiators for continuous activator regeneration ATRP (ICAR ATRP) are used to establish the TPSC-based ICAR ATRP system using water-soluble TPMA as a ligand, EBPA as an initiator, CuBr2 as a catalyst, and AIBN as a reducing agent. By heating to 70 °C, unlimited miscibility of both solvents is achieved and the polymerization can be carried out under homogeneous conditions; then on cooling to 25 °C, the mixture separates into two phases again. As a result, the catalyst complex remains in the PEG-200 phase while the obtained polymers stay in the p-xylene phase. The catalyst can therefore be removed from the resultant polymers by easily separating the two different layers and can be reused again. It is important that well-defined PMMA with a controlled molecular weight and narrow molecular weight distribution could be obtained using this TPSC-based ICAR ATRP system.

Iron-Mediated Homogeneous ICAR ATRP of Methyl Methacrylate under ppm Level Organometallic Catalyst Iron(III) Acetylacetonate

Atom Transfer Radical Polymerization (ATRP) is an important polymerization process in polymer synthesis. However, a typical ATRP system has some drawbacks. For example, it needs a large amount of transition metal catalyst, and it is difficult or expensive to remove the metal catalyst residue in products. In order to reduce the amount of catalyst and considering good biocompatibility and low toxicity of the iron catalyst, in this work, we developed a homogeneous polymerization system of initiators for continuous activator regeneration ATRP (ICAR ATRP) with just a ppm level of iron catalyst. Herein, we used oil-soluble iron (III) acetylacetonate (Fe(acac)3) as the organometallic catalyst, 1,1′-azobis (cyclohexanecarbonitrile) (ACHN) with longer half-life period as the thermal initiator, ethyl 2-bromophenylacetate (EBPA) as the initiator, triphenylphosphine (PPh3) as the ligand, toluene as the solvent and methyl methacrylate (MMA) as the model monomer. The factors related with the polymerization system, such as concentration of Fe(acac)3 and ACHN and polymerization kinetics, were investigated in detail at 90 °C. It was found that a polymer with an acceptable molecular weight distribution (Mw/Mn = 1.43 at 45.9% of monomer conversion) could be obtained even with 1 ppm of Fe(acac)3, making it needless to remove the residual metal in the resultant polymers, which makes such an ICAR ATRP process much more industrially attractive. The “living” features of this polymerization system were further confirmed by chain-extension experiment.

Thermoregulated phase transfer catalysis in aqueous/organic biphasic system: facile and highly efficient ATRP catalyst separation and recycling in situ using typical alkyl halide as initiator

Developing a highly efficient and facile method for catalyst separation and recycling from an ATRP system facilitates wide application of atom transfer radical polymerization (ATRP). In this work, an important development of thermoregulated phase transfer catalysis (TRPTC)-based initiators for continuous activator regeneration (ICAR) ATRP for transition metal catalyst separation and recycling in a water/p-xylene biphasic system was achieved using alkyl halide (ethyl-2-bromo-2-phenyl acetate, EBPA) as the initiator for the first time. Herein, poly(poly(ethylene glycol) methyl ether methacrylate)-supported dipicolylamine (PPEGMA-BPMA) was designed as the thermoregulated ligand, CuBr2 as the catalyst, 1,1′-azobis(cyclohexanecarbonitrile) (ACHN) as the azo-initiator and methyl methacrylate (MMA) as the model monomer, respectively. The polymerization kinetics was investigated in detail, and the “living” feature of this novel polymerization system was confirmed by chain-end analysis and chain extension experiments for the resultant PMMA. It is noted that the catalyst complex (PPEGMA-BPMA/CuBr2) existed only in an aqueous phase at room temperature, and it transferred to an organic phase and subsequently catalyzed the ICAR ATRP of MMA when the temperature increased to 90 °C. After polymerization the catalyst complex successfully transferred to the aqueous phase almost completely from the organic phase again while the resultant PMMA existed in the p-xylene phase once the temperature cooled down to room temperature. Therefore, the process described above combined the advantages of homogeneous catalysis in the organic phase and heterogeneous catalyst separation in situ in the aqueous/organic biphasic phase system by just changing the reaction temperature. Importantly, the catalyst complex in the aqueous phase could be recycled easily, and the catalyst retained high catalytic activity even after eight recycling times.

Facile synthesis of poly(vinyl acetate)-b-polystyrene copolymers mediated by an iniferter agent using a single methodology

The preparation of well-defined block copolymers from both non-conjugated and conjugated monomers is still challenging. In this work, a typical block copolymer poly(vinyl acetate) (PVAc)-b-polystyrene (PS) was successfully synthesized by using a successive iniferter strategy. Herein, PVAc was first obtained in the presence of an iniferter agent (e.g., 1-cyano-1-methylethyldiethyldithiocarbamate (MANDC), 2-(N,N-diethyldithiocarbamyl)-isobutyric acid ethyl ester (EMADC) or 2-(ethoxycarbonothioyl)sulfanyl propanoate (EXEP)). Then the resultant PVAc with high end-group functionalities could be used as an effective macroiniferter agent for the polymerization of styrene (St). The kinetic studies of the chain extension experiment with St indicated the “living” features of the polymerization system. In addition, the obtained PVAc-b-PS copolymers could be easily converted into PVA-b-PS amphiphiles by methanolysis of the poly(vinyl acetate) block. Importantly, the synthesis of the copolymers just uses a single simple and metal-free catalyzed polymerization method, which may be of great potential for industrial production.