Yuhong Ma

Ammonium-Functionalized Hollow Polymer Particles As a pH-Responsive Adsorbent for Selective Removal of Acid Dye

In this work, a novel type of ammonium-functionalized hollow polymer particles (HPP-NH3(+)) with a high density of ammonium groups in the shell has been specially designed and synthesized. Benefiting from both the high surface area and from the high density of positively charged ammonium groups, the as-prepared HPP-NH3(+) can serve as a selective adsorbent for the removal of negatively charged acid dye (e.g., methyl blue a-MB). The equilibrium adsorption data of a-MB on the HPP-NH3(+) were evaluated using Freundlich and Langmuir isotherm models, and Langmuir isotherm exhibited a better fit with a maximum adsorption capacity of 406 mg/g. Most importantly, because of the presence of dual functional groups (ammonium and carboxyl groups), the HPP-NH3(+) showed a significant pH-dependent equilibrium adsorption capacity, which increased dramatically from 59 mg/g to 449 mg/g as the solution pH decreased from 9 to 2. This uniqueness makes the dye-adsorbed HPP-NH3(+) can be facilely regenerated under mild condition (in weak alkaline solution, pH 10) to recover both a-MB and the HPP-NH3(+), whereas the recovery of conventional adsorbents is commonly performed under particularly severe conditions. The regenerated HPP-NH3(+) can be reused for dye removal and the dye removal efficiency remained above 98% even after five adsorption-desorption cycles. Because of its high adsorption capacity, pH-sensitivity, easy regeneration, and good reusability, the HPP-NH3(+) has great potential for the application in the field of water treatment, controlled drug release, and pH-responsive delivery.

Preparing of monodisperse and cation-charged polystyrene particles stabilized with polymerizable quarternary ammonium by dispersion polymerization in a methanol–water medium

A novel dispersion polymerization system, with a methanol/water (MeOH/H2O) mixture as reaction medium and a polymerizable dimethylaminomethacrylate methyl chloride (DMC) as stabilizer was developed. By monitoring the polymerization evolution and observing the morphological changes of the polystyrene (PS) particles by SEM, it was found that this system had the following unique features: (1) a much lower amount of DMC (0.025 mass% based on styrene as opposed to 5 mass% for a routine system) was required to prepare monodisperse and stable PS particles; (2) the rate of polymerization was fast and the conversion was very high; (3) the monodisperse particles with average diameters of approximately 200-1600 nm could be directly obtained. These features were explained by a synergistic interaction between water and the quarternary ammonium cations. Combined with XPS, ion-exchange/conductometric titration, FTIR and 1H NMR analysis, a plausible polymerization mechanism through which the particles were stabilized by the PS-PDMC copolymer formed in situ was proposed.

A mild strategy to encapsulate enzyme into hydrogel layer grafted on polymeric substrate.

Although the hydrogel network has been widely investigated as a carrier for enzyme immobilization, to in situ encapsulate enzymes into a hydrogel network in an efficient, practical, and active way is still one of the great challenges in the field of biochemical engineering. Here, we report a new protocol to address this issue by encapsulating enzyme into poly(ethylene glycol) (PEG) hydrogel network grafted on polymeric substrates. In our strategy, isopropyl thioxanthone semipinacol (ITXSP) dormant groups were first planted onto the surface of a plastic matrix with low density polyethylene (LDPE) film as a model by a UV-induced abstracting hydrogen-coupling reaction. As a proof of concept, lipase, which could catalyze esterification of glucose with palmitic acid, then was in situ net-immobilized into a PEG-based hydrogel network layer through a visible light-induced surface controlled/living graft cross-linking polymerization. This strategy demonstrates the following novel significant merits: (1) in comparison with the UV irradiation or high temperature, the visible light and room temperature used provide a friendly condition to maintain activity of enzyme during immobilization; (2) the uniqueness of controlled/living cross-linking polymerization not only makes it easy to form a uniform PEG hydrogel network, which is a benefit to avoid the leakage of net-immobilizing enzyme, but also to tune the net-thickness or capacity to accommodate enzyme; and (3) as compared to systems of nanoparticles and porous matrixes, the flexible/robust end-products of the surface net-immobilizing enzyme with polymer film are more suitable to be applied in a bioreactor due to their features of easier separation and reuse. We confirmed that this catalytic film could retain almost all of its initial activity after seven batches of 24 h esterifications. The proposed strategy provides an extremely simple, effective, and flexible method for enzyme immobilization.

Visible-light induced controlled radical polymerization of methacrylates with Cu(dap)2Cl as a photoredox catalyst

With ethyl-α-bromophenylacetate (EBPA) as an initiator and Cu(dap)2Cl (dap = 2,9-bis(p-anisyl)-1,10-phenanthroline) as a photoredox catalyst, controlled radical polymerizations of poly(ethylene glycol)methyl ether methacrylate (PEGMA) and methyl methacrylate (MMA) are demonstrated under LED light lamp irradiation (4500 μW cm−2@420 nm). The catalysis cycles proceed in the presence of N,N-dimethylaniline (DMA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN), which could serve as reductants to regenerate a Cu(I) complex from the oxidized Cu(II) complex. In addition, Me6TREN plays another important role as an efficient ligand for the copper-based photopolymerization of methacrylate monomers. Good linear evolution of molecular weight (Mn) with monomer conversion is observed under the optimized conditions. Specifically, with [PEGMA] : [EBPA] : [Cu(dap)2Cl] : [Me6TREN] = 31 : 1 : 0.015 : (0.15–0.45), poly(PEGMA) (PPEGMA) with polydispersity indexes (PDI) as low as 1.15 are obtained. To further verify the living nature of this system, block copolymers of PPEGMA-b-PMMA with high molecular weights and narrow molecular weight distributions (Mn,GPC = 59 200 g mol−1, PDI = 1.28 and Mn,GPC = 93 700 g mol−1, PDI = 1.44, respectively) are prepared using PPEGMA-Br (Mn,GPC = 11 600 g mol−1; PDI = 1.13) as a macroinitiator. The polymers produced with Cu(dap)2Cl/Me6TREN as a catalyst and the PMMA obtained with Cu(dap)2Cl/DMA as a catalyst are colorless which is different from the heterogeneously catalyzed ATRP for its notorious Cu metal residue.

PEG molecular net-cloth grafted on polymeric substrates and its bio-merits.

Polymer brushes and hydrogels are sensitive to the environment, which can cause uncontrolled variations on their performance. Herein, for the first time, we report a non-swelling “PEG molecular net-cloth” on a solid surface, fabricated using a novel “visible light induced surface controlled graft cross-linking polymerization” (VSCGCP) technique. Via this method, we show that 1) the 3D-network structure of the net-cloth can be precisely modulated and its thickness controlled; 2) the PEG net-cloth has excellent resistance to non-specific protein adsorption and cell adhesion; 3) the mild polymerization conditions (i.e. visible light and room temperature) provided an ideal tool for in situ encapsulation of delicate biomolecules such as enzymes; 4) the successive grafting of reactive three-dimensional patterns on the PEG net-cloth enables the creation of protein microarrays with high signal to noise ratio. Importantly, this strategy is applicable to any C-H containing surface, and can be easily tailored for a broad range of applications.

Direct one-pot synthesis of chemically anisotropic particles with tunable morphology, dimensions, and surface roughness.

Previously, synthesis of anisotropic particles by seeded polymerizations has involved multiple process steps. In conventional one-pot dispersion polymerization (Dis.P) with a cross-linker added, only spherical particles are produced due to rapid and high cross-linking. In this Article, a straightforward one-pot preparation of monodisperse anisotropic particles with tunable morphology, dimensions, surface roughness, and asymmetrically distributed functional groups is described. With a cross-linker of divinylbenzene (DVB, 8%), ethylene glycol dimethacrylate (EGDMA, 6%), or dimethacryloyloxybenzophenone (DMABP, 5%) added at 40 min, shortly after the end of nucleation stage in Dis.P of styrene (St) in methanol and water (6/4, vol), the swollen growing particles are inhomogeneously cross-linked at first. Then, at low gel contents of 59%, 49%, and 69%, corresponding to the cases using DVB, EGDMA, and DMABP, respectively, the growing particle phase separates and snowman- or dumbbell-like particles are generated. Thermodynamic and kinetic analyses reveal that moderate cross-linking and sufficient swelling of growing particles determine the formation and growth of anisotropic particles during polymerization. Morphology, surface roughness, sizes, and cross-linking degrees of each domain of final particles are tuned continuously by varying start addition time and contents of cross-linkers. The snowman-like particles fabricated with DVB have a gradient cross-linking and asymmetrical distribution of pendant vinyl groups from their body to head. The dumbbell-like particles prepared using DMABP have only one domain cross-linked; i.e., only one domain contains photosensitive benzophenone (BP) groups. With addition of glycidyl methacrylate (GMA) or propargyl methacrylate (PMA) together with DVB or EGDMA, epoxy or alkynyl groups are asymmetrically incorporated. With the aid of these functional groups, carboxyl, amino, or thiol groups and PEG (200) are attached by thiol-ene (yne) click and photocoupling reactions.

Synthesis of pH-responsive hydrogel thin films grafted on PCL substrates for protein delivery

A new visible light induced graft polymerization method was utilized to prepare pH-sensitive hydrogel layers covalently attached to polymer substrates for drug delivery. In our strategy, isopropyl thioxanthone semi-pinacol (ITXSP) dormant groups were firstly introduced on the surface of a polycaprolactone (PCL) film by a UV-induced abstracting hydrogen-coupling reaction. Then visible light induced graft cross-linking polymerization was performed to initiate polymerization of poly(ethylene glycol) diacrylate (PEGDA) and acrylic acid (AA), resulting in the formation of a hydrogel layer. The thickness of the hydrogel film can be controlled by varying the exposure time and monomer composition. The grafted hydrogel layers showed a flat morphology and dense structure, which is different from the traditional reported porous structure. The water contact angle of the hydrogel layer exhibited a reversible change from 38° to 18° when the film was alternatively treated in buffers of pH 2.0 and 7.4, respectively. Patterned hydrogel layers were prepared as a model to determine the change in the height of the grafted hydrogel layer as a function of pH. As the pH changed from 2.0 to 7.4, the hydrogel pattern showed an increase in height due to the swelling of the hydrogel network, and the hydrogel layer formed by 0.2 wt% PEGDA and 25 wt% AA showed the most increase (30%) in height. Bovine serum albumin (BSA) and lysozyme as models of protein drugs were incorporated in the hydrogel network, and their release also showed obvious pH-sensitivity. At pH 2.0, hydrogels present a faster initial burst release due to the squeezing mechanism. Tertiary structure analysis showed that encapsulation and release did not affect the protein conformation. These findings have improved our understanding of hydrogel thin films, which may be useful as potential vehicles of therapeutic proteins in drug delivery applications.

Visible Light-Controlled Radical Polymerization of Propargyl Methacrylate Activated by a Photoredox Catalyst fac-[Ir(ppy)3]

Despite the robustness and flexibility of thermal controlled radical polymerization (CRP), it still remains a big challenge to prepare clickable polymers bearing acetenyl groups. In this paper, visible light CRP of propargyl methacrylate (PgMA) is investigated using ethyl-α-bromophenylacetate as initiator and fac-Ir(ppy)3 as a photoredox catalyst. By controlling the polymerization time, a linear increase of Mn with monomer conversion and narrow polydispersity index is achieved. Specifically, with [PgMA]/[Initiator]/[fac-Ir(ppy)3]=210:1:0.0285, the products show Mn=8300 g/mol and Mw/Mn=1.24 at 17% monomer conversion and Mn=17100 g/mol and Mw/Mn=1.52 at 50% monomer conversion. This result is much better than that of thermally activated Cu-ATRP of PgMA. When the ratio of MMA/PgMA is 5:1, the copolymerization product shows Mn=25300 g/mol and Mw/Mn=1.75 at 66% monomer conversion. The FTIR, 1H-NMR and 13C-NMR spectra of the PPgMA show no sp2-sp2 carbon-carbon bond, indicating that acetenyl groups are intact. In conclusion, this technology offers a versatile route for the preparation of (co)polymers with pendant alkynyl groups.

Facile synthesis of core-shell/hollow anisotropic particles via control of cross-linking during one-pot dispersion polymerization.

Preparation of anisotropic particles based on seed phase separation involves multiple processes, and asymmetrical structures and surfaces cannot be produced when anisotropic shapes emerge. In conventional one-pot dispersion polymerization (Dis.P) using cross-linker, only spherical particles are prepared due to rapid and high cross-linking. Herein, monodisperse snowman-like particles with core-shell/hollow structures and partially rough surface were synthesized straightforward by a modified one-pot Dis.P, in which ethylene glycol and water (6/4, vol.) were used as medium, and ammonium persulfate (APS) aqueous solution, vinyl acetate (VA) and/or acrylic acid (AA), divinylbenzene (DVB) and styrene (St) were added at 6h. The cross-linking of growing particles was confined to exterior (forming cross-linked shell), and gel contents were low, leading to phase separation. Asymmetrical morphologies, structures, sizes and surface roughness were flexibly tuned by varying amounts of APS, VA and/or AA, water and DVB, and DVB adding speed. At low APS contents or high DVB amounts, the inhomogeneous cross-linking of head enabled its phase to separate, producing elongated head. With addition of VA and AA, phase separations inside head and body were induced, generating hollow structure. Adding DVB very slowly, nonlinear growth of third compartment occurred, forming bowed head.

Direct Observations of Three Nucleation/Growth Processes of Charge-Stabilized Dispersion Polymerizations with Varying Water/Methanol Ratios

This article presents observations of three polymerization modes of a self-developed cation-charge-stabilized styrene/water/methanol dispersion polymerization system: (1) a water/methanol (20/80) system, corresponding to a typical dispersion polymerization mode where the particle nucleation occurred in the solution phase and growth in the particle phase; (2) a pure CH(3)OH system, including a first nucleation in the solution phase with growth by absorption of the small particles and polymers formed in this phase, and a secondary nucleation with growth in the particle phase, when high molecular weight copolymers appeared in the solution phase; and (3) a water/methanol (5/95) system, similar to the conventional dispersion polymerization mode during the first 90 min, with subsequent epitaxial growth. Interestingly, the metastable state of the nucleation stage, including minuscule 6-nm particles, their aggregates, and the aggregating process, was first observed experimentally. By quantitatively following the relationship of the deposited molecular weight and the nucleation/growth process in the three systems, it was proposed that the molecular weight of the deposited polymer had to reach a specific high value before they could absorb or capture monomer to form smooth/spherical nuclei or particles.