Photoinduced Electron Transfer-reversible Addition-fragmentation Chain Transfer Polymerization Research Articles

Phenanthroline Derived N-Doped Carbon Dots as Robust Metal-Free Photocatalysts for PET-RAFT Polymerization and Polymerization-Induced Self-Assembly.

Metal-free organic photocatalysts for photo-mediated reversible deactivation radical polymerization (photo-RDRP) are witnessed to make increasing advancement in the precise synthesis of polymers. However, challenges still exist in the development of high-efficiency and environmentally sustainable carbon dots (CDs)-based organocatalysts. Herein, N-doped CDs derived from phenanthroline derivative (Aphen) are prepared as metal-free photocatalysts for photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The introduction of phenanthroline structure enhances the excited state lifetime of CDs and expands the conjugated length of their internal structure to enable the light-absorption to reach green light region, thereby enhancing photocatalytic activity. The as-designed CDs exhibit unprecedented photocatalytic capacity in photopolymerization even in large-volume reaction (100mL) with high monomer conversion and narrow polymer dispersity (Mw/Mn < 1.20) under green light. The photocatalytic system is compatible with PET-RAFT polymerization of numerous monomers and the production of high molecular weight polyacrylate (Mn >250000) with exquisite spatiotemporal control. Above results confirm the potential of CDs as photocatalyst, which has not been achieved with other CDs catalysts used in photo-RDRP. In addition, the construction of fluorescent polymer nanoparticles using CDs as both photocatalyst and phosphor through photoinitiated polymerization-induced self-assembly (Photo-PISA) technology is successfully demonstrated for the first time.

Emulsion-Directed Synthesis of Poly-Porphyrin Nanoparticles as Heterogeneous Photocatalysts for PET-RAFT Polymerization.

Heterogeneous photocatalysts have attracted extensive attention in photo-induced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization due to their remarkable advantages such as easy preparation, tunable photoelectric properties, and recyclability. In this study, zinc (II) 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (ZnTAPP)-based poly-porphyrin nanoparticles (PTAPP-Zn) are constructed by an emulsion-directed approach. It is investigated as a heterogeneous photocatalyst for PET-RAFT polymerization of various methacrylate monomers under visible light exposure, and the reactions show refined polymerization control with high monomer conversions. Furthermore, it is demonstrated that the PTAPP-Zn nanoparticles with the larger pore size enhance photocatalytic activity in PET-RAFT polymerization. In addition, the capabilities of oxygen tolerance and temporal control are demonstrated and PTAPP-Zn particles can be easily recycled and reused without an obvious decrease in catalytic efficiency.

Recognition of “Oxygen-/Water-Fueled” PET-RAFT Protocol Matched to Covalent Organic Frameworks

The reactive oxygen species (ROS)-mediated photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) process is an attractive tool to enhance the oxygen tolerance of radical polymerization systems. In this paper, we investigate the relationship between the covalent organic framework (COF) structure and polymerization performance based on the O2•–/H2O system by modulating the COF at atomic and molecular levels. We combine density functional theory (DFT) calculations and experiments to clarify the “oxygen-/water-fueled” PET-RAFT polymerization mechanism and identify the key steps of the protocol. Based on the modulation of the TD-1 COF template structure, the relatively efficient charge–hole separation efficiency allows the matching of TD-2 COF and TD-3 COF with the O2•–/H2O system to achieve high-quality RAFT polymerization, providing insights into the rational design and development of catalytic systems with better performance.

Synthesis of zwitterionic copolymer containing sulfobetaine and catechol for antibacterial application

• A catechol zwitterionic copolymer was handily prepared by PET-RAFT polymerization. • Polymer grafted surfaces were readily fabricated via solution deposition method. • Incorporation of catechol rendered copolymer excellent antibacterial potency. Catechol-modified zwitterionic polymers have been successfully applied for functional modification of surfaces. However, catechol derivatives synthesized by atom transfer radical polymerization (ATRP) involve complicated procedures and harsh conditions. In this study, a random copolymer P(SBMA- co -DA) (PSD) was prepared from sulfobetaine methacrylate (SBMA) and N-3,4-dihydroxybenzenethyl methacrylamide (DA) by photo-induced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization technique. Staphylococcus aureus adhesion assay showed that PSD possessed similarly powerful antibacterial performance at high concentration as compared to dopamine-modified poloxamer 407, while PSBMA (PSB) exhibited no obvious antibacterial potency. We believe that the facile method will be beneficial to synthesis of catechol-containing polymers for different applications.

A Porphyrinic Donor-Acceptor Conjugated Porous Polymer as Highly Efficient Photocatalyst for PET-RAFT Polymerization.

Heterogeneous catalysts offer a highly desirable platform for exploring environmental-benign transformation systems, yet, they typically suffer from significant loss of catalytic efficiency compared with their homogeneous counterparts. Here, the facile synthesis of a porphyrinic conjugated porous polymer incorporated with imidazolium bromide moieties by taking advantage of the Debus-Radziszewski reaction is reported. Owing to the unique donor-acceptor structure, this heterogeneous and metal-free photocatalyst exhibits much improved catalytic activity compared with its small molecular analogs in photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, producing polymers with narrow distribution (Đ = 1.06-1.18) and high degree of chain-end fidelity. Moreover, the heterogeneous catalyst can be easily separated at the end of polymerization by centrifugation and recycled for five independent PET-RAFT polymerizations without obvious decreases in catalytic efficiency.

Poly(carboxybetaine methacrylate) grafted on PVA hydrogel via a novel surface modification method under near-infrared light for enhancement of antifouling properties

A novel near-infrared (NIR) light induced photoinduced electron transfer-reversible addition - fragmentation chain-transfer (PET-RAFT) polymerization was developed and carboxybetaine methacrylate (CBMA) polymer brush was constructed on the PVA hydrogel via this one-spot method to enhance the antifouling properties. PVA-g-pCBMA hydrogel displayed higher hydrophilicity, while the thermal properties decreased due to the lower decomposition properties of pCBMA and the partly destruction of crystal structure inside the PVA hydrogel. The antifouling properties of PVA-g-pCBMA hydrogel was improved significantly because the surface hydration and steric repulsion effect of pCBMA polymer brush. Importantly, PVA-g-pCBMA hydrogel PVA-g-pCBMA hydrogel could be considered as no cytotoxic effect or reduction of proliferation to primary human eyelid fibroblast cells following the guidelines stated by ISO 10993-5:2009 (direct contact test). Considering the excellent penetration ability of NIR, AlPc catalyzed PET-RAFT polymerization provided a new modification approach for various biological or medical materials on industrial scale.

Porphyrin-based donor-acceptor COFs as efficient and reusable photocatalysts for PET-RAFT polymerization under broad spectrum excitation.

Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λmax from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.

PET-RAFT copolymerization of vinyl acetate and acrylic acid

Photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization of vinyl acetate (VAc) and acrylic acid (AA) was successfully realized in methanol at room temperature under blue LED (420 nm, 6 mW/cm2) irradiation with xanthate as RAFT agent and fac-[Ir(ppy)3] as the photocatalyst. The well-defined copolymers of poly(VAc-co-AA) with controlled molecular weights (Mn,GPC) and polydispersity index (PDI) were successfully obtained. The polymerization revealed the behaviors of controlled/living polymerization, which was verified by the observed linear relationship of ln([M]0/[M]t) with respect to irradiation time and that of the experimental molecular weights (Mn,GPC) with respect to monomer conversion. GPC curves showed a single peak and shifted to high molecular weights with increasing the irradiation time. The reactivity ratios of VAc and AA were 0.043 and 1.181, respectively in this system. The effect of the amount of fac-[Ir(ppy)3] on PET-RAFT polymerization was investigated. The monomer conversion increased with increasing the amount of fac-[Ir(ppy)3] and the PDI decreased with the increase in the amount of fac-[Ir(ppy)3]. The temporal controllable PET-RAFT polymerization of vinyl acetate (VAc) and acrylic acid (AA) was achieved. The structure of synthesized poly(VAc-co-AA) was characterized by nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC), respectively. The extended experiments of poly(VAc-co-AA) with fresh AA was conducted under the irradiation of blue LED, and the results demonstrated that this extension polymerization displayed controlled polymerization behavior. The mechanism was explained in this study.

Polymer-chlorella cells conjugating with aggregation-induced functionality switch towards hydrogen evolution

The manipulation of physicochemical and biological properties of the encapsulated cells by cell surface engineering technique is now attracting more and more researchers’ interest. In this study, we showed a mild and highly efficient way to build a type of polymer-living cell conjugates based on photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization (PET-RAFT). The polymerization could proceed at room temperature under the blue light initiating and showed very less influence on the activity of the living cells. Significantly, the constructed positively charged polymer-living cell conjugates could aggregate with the negatively charged native Chlorella pyrenoidosa (CP) cell to form a microscale aggregate which then allowed a functionality switching of the encapsulated CP cells from the normal photosynthetic O2 towards H2 production with an average rate of 0.26 μmol H2h−1 (mg chlorophyll)−1 for 42 h. It is then anticipated that the method we show could contribute an effective way for generating different functionalized living cell-polymer hybrid structures on demand, and the demonstrated aggregation-induced functionality switching of the CP cells is also anticipated to inspire more applications of the cell-based materials towards biology green energy.

Light and magnetism dual-gated photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization

A novel raspberry-like γ-Fe2O3@carbon dot (CD) nanocatalyst was prepared and applied for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization. The nanocatalyst was found to be an efficient photocatalyst in visible light-regulated PET-RAFT polymerization owing to the oxidative quenching mechanism between the photoexcited γ-Fe2O3@CDs and the RAFT agent in the PET process. Notably, polymerization can be reversibly ceased in the absence of light or under an external magnetic field. The superparamagnetic nature and high saturation magnetization value (∼30.4 emu g−1) of the nanocatalyst contribute to convenient recycling of the nanocatalyst after polymerization. The PET-RAFT polymerization with the nanocatalyst before and after recycling was investigated, which displayed all the characteristics of controlled/living polymerization systems.

Semiconductor Quantum Dots Are Efficient and Recyclable Photocatalysts for Aqueous PET-RAFT Polymerization.

This Letter describes the use of CdSe quantum dots (QDs) as photocatalysts for photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of a series of aqueous acrylamides and acrylates. The high colloidal solubility and photostability of these QDs allowed polymerization to occur with high efficiency (>90% conversion in 2.5 h), low dispersity (PDI < 1.1), and ultralow catalyst loading (<0.5 ppm). The use of protein concentrators enabled the removal of the photocatalyst from the polymer and monomer with tolerable metal contamination (8.41 ug/g). These isolated QDs could be recycled for four separate polymerizations without a significant decrease in efficiency. By changing the pore size of the protein concentrators, the QDs and polymer could be separated from the remaining monomer, allowing for the synthesis of block copolymers using a single batch of QDs with minimal purification steps and demonstrating the fidelity of chain ends.

Structural Engineering of Graphitic Carbon Nitrides for Enhanced Metal-Free PET-RAFT Polymerizations in Heterogeneous and Homogeneous Systems.

Developing visible-light-regulated controlled/living radical polymerization techniques for the synthesis of polymers with a predictable molecular weight, spatial and temporal control, and well-defined end-group functionality is being pursued by the macromolecular community worldwide. In this study, a new metal-free photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization system was developed for controlled macromolecular synthesis in both heterogeneous and homogeneous systems by structural engineering of graphitic carbon nitrides (g-C3N4) to improve the textural, optical, and electronic properties. A heteroatom-mediated synthesis enabled the preparation of g-C3N4 with improved structural properties and increased absorption in the visible light region. Enhanced PET-RAFT polymerization of vinyl monomers with low dispersity (Đ < 1.2), temporal control, and high chain-end fidelity was achieved under mild blue light irradiation (λmax = 465 nm, 3 mW/cm2). Moreover, we demonstrate, for the first time, that the g-C3N4-catalyzed RAFT polymerization could be realized in a homogeneous system after structural evolution of bulk g-C3N4 into soluble nanosheets with enhanced photocatalytic efficiency up to high monomer conversion. This study provides new insights into the structure–performance relationship of g-C3N4 for photoregulated PET-RAFT polymerization under visible light. Moreover, the development of a homogeneous g-C3N4-catalyzed photosynthesis system should broaden the application scope of these fascinating photocatalysts while benefiting synthetic upscaling by continuous flow and/or microfluidic reactors.

PET-RAFT Polymerization Catalyzed by Small Organic Molecule under Green Light Irradiation.

Photocatalyzed polymerization using organic molecules as catalysts has attracted broad interest because of its easy operation in ambient environments and low toxicity compared with metallic catalysts. In this work, we reported that 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) can act as an efficient photoredox catalyst for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization under green light irradiation. Well-defined (co)polymers can be obtained using this technique without any additional additives like noble metals and electron donors or acceptors. The living characteristics of polymerization were verified by kinetic study and the narrow dispersity (Đ) of the produced polymer. Excellent chain-end fidelity was demonstrated through chain extension as well. In addition, this technique showed great potential for various RAFT agents and monomers including acrylates and acrylamides.

Far-Red Light-Induced Reversible Addition-Fragmentation Chain Transfer Polymerization Using a Man-Made Bacteriochlorin.

To overcome the challenge of photoregulated living radical polymerization in long-wavelength radiation, a photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization in far-red wavelength (λmax = 740 nm) is reported by using a man-made bacteriochlorin as a photocatalyst. A reduced tetraphenylporphyrin (RTPP) having a natural bacteriochlorin macrocycle ring with two reduced pyrrole rings was synthesized with strong absorption in the far-red light region (700-765 nm) and applied for the PET-RAFT polymerization as a photoredox catalyst, which offered excellent control over molecular weight and polydispersities and oxygen tolerance for the polymerization of (methyl) acrylates monomers, and exhibited attractive features of "living" radical polymerization. Benefiting from high penetration of far-red light, the polymerization was also well-controlled when the reaction vessel was covered by translucent animal tissue barriers, for example, skin.

Photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of acrylonitrile in miniemulsion

Photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of acrylonitrile (AN) in miniemulsion was reported. PET-RAFT polymerization of acrylonitrile (AN) was successfully accomplished with 4-cyanopentanoic acid dithiobenzoate (CPADB) as chain transfer agent (CTA), sodium dodecyl sulfate (SDS) as emulsifier, hexadecane (HD) as co-stabilizer and TiO2 as photocatalyst at 25 °C. The linear first-order kinetic plots were observed in miniemulsion with different amounts of SDS. Excellent temporal control was demonstrated by switching between ON/OFF states multiple times, and the prepared PAN macro-CTA was used successfully to perform the chain extension experiments, indicating high retention of chain end functionality. Furthermore, the obtained PAN was amidoximated with NH2OH·HCl. The Cd2+ was extracted with amidoxime (–C(NH2)=NOH) from aqueous solutions. The maximum adsorption of 98.6% Cd2+ with 400 mg of the adsorbent was observed at pH 6.0 and an initial Cd2+concentration of 4 mmol/L.

A versatile signal-enhanced ECL sensing platform based on molecular imprinting technique via PET-RAFT cross-linking polymerization using bifunctional ruthenium complex as both catalyst and sensing probes

Molecularly imprinted technique (MIT) has proven to be a significant tool in the analyzing area in virtue of its obvious advantages such as specific recognition, favorable stability to high temperature and higher sensitivity. Electrochemiluminescence (ECL) technology has also been receiving enormous attention as a powerful tool in sensing fields. However, sensors based on the combination of MIT and ECL technologies have seldom been reported yet. Herein, we find that Ru(bpy)32+ cannot only work as an efficient catalyst for photo-induced electron transfer–reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, but also as a sensing probe for ECL sensor. Based on this, we successfully construct ECL sensors via the combination of MIT and ECL techniques. In details, poly(methacrylic acid) (PMAA) and cross-linked PMAA were synthesized first via a well-controlled PET-RAFT polymerization using Ru(bpy)32+ as catalyst under illumination of visible light with a wavelength of 460 nm, as confirmed by 1H NMR and gel permeation chromatography (GPC). Then, negatively-charged Au nanoparticles (AuNPs) with average sizes of 20 nm were prepared and modified with Ru(bpy)32+ via electrostatic incorporation. MIPs were prepared on the surface of AuNPs using melamine (MEL) as the template via PET-RAFT controlled cross-linking polymerization. The MIPs modified AuNPs (AuNPs-MIPs) were then fixed on the surface of working electrode with Nafion to achieve a solid-state ECL sensing platform employing Ru(bpy)32+ as the ECL probes. The as-prepared sensor showed a wide detection range of 5.0 × 10–13 − 5.0 × 10−6 mol/L and a low detection limit of 1.0 × 10–13 mol/L (S/N ≥ 3) was reached in the detection of MEL. Moreover, further tests for analyzing MEL structural analogues proved that the constructed ECL sensing platform could be utilized to detect various substances via specific recognitions.

Perylene as a visible light photoredox catalyst for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of MMA

The organic photocatalyst, perylene, was used to mediate photoinduced electron transfer (PET) reversible addition-fragmentation chain transfer polymerization (RAFT) of methyl methhacrylate (MMA) under light irradiation in N,N-dimethylformamide (DMF) at 25°C with 4-cyanopentanoic acid dithiobenzoate (CPADB) as chain transfer agent (CTA). Kinetic studies confirmed that the polymerization obeyed the first order kinetic m'odel. The production of PMMAs with a good control of molecular weights (Mn,GPC) and narrow polymer molecular weight distribution (Mw/Mn) were obtained. It is found that well-controlled PET RAFT polymerization of MMA can be manipulated even with the amount of perylene decreasing to ppm level. No polymer was obtained in the absence of light irradiation, implying that the model of PET RAFT polymerization of MMA is an ideal light “on”-“off” switchable system. Furthermore, the speed of PET RAFT polymerization of MMA was also finely tunable by the external light irradiation intensity. The resultant PMMA macro-CTA was characterized by 1H nuclear magnetic resonance spectrum (1H NMR) and gel permeation chromatography (GPC). The accessibility of the high end group fidelity was further demonstrated by chain extension experiments.

Construction of Hierarchical Polymer Brushes on Upconversion Nanoparticles via NIR-Light-Initiated RAFT Polymerization

Photoinduced reversible addition-fragmentation chain transfer (RAFT) polymerization generally adopts high-energy ultraviolet (UV) or blue light. In combination with photoredox catalyst, the excitation light wavelength was extended to the visible and even near-infrared (NIR) region for photoinduced electron transfer RAFT polymerization. In this report, we introduce for the first time a surface NIR-light-initiated RAFT polymerization on upconversion nanoparticles (UCNPs) without adding any photocatalyst and construct a functional inorganic core/polymer shell nanohybrid for application in cancer theranostics. The multilayer core-shell UCNPs (NaYF4:Yb/Tm@NaYbF4:Gd@NaNdF4:Yb@NaYF4), with surface anchorings of chain transfer agents, can serve as efficient NIR-to-UV light transducers for initiating the RAFT polymerization. A hierarchical double block copolymer brush, consisting of poly(acrylic acid) (PAA) and poly(oligo(ethylene oxide)methacrylate-co-2-(2-methoxy-ethoxy)ethyl methacrylate) (PEG for short), was grafted from the surface in sequence. The targeting arginine-glycine-aspartic (RGD) peptide was modified at the end of the copolymer through the trithiolcarbonate end group. After loading of doxorubicin, the UCNPs@PAA-b-PEG-RGD exhibited an enhanced U87MG cancer cell uptake efficiency and cytotoxicity. Besides, the unique upconversion luminescence of the nanohybrids was used for the autofluoresence-free cell imaging and labeling. Therefore, our strategy verified that UCNPs could efficiently activate RAFT polymerization by NIR photoirradiation and construct the complex nanohybrids, exhibiting prospective biomedical applications due to the low phototoxicity and deep penetration of NIR light.

Engineering live cell surfaces with functional polymers via cytocompatible controlled radical polymerization.

The capability to graft synthetic polymers onto the surfaces of live cells offers the potential to manipulate and control their phenotype and underlying cellular processes. Conventional grafting-to strategies for conjugating preformed polymers to cell surfaces are limited by low polymer grafting efficiency. Here we report an alternative grafting-from strategy for directly engineering the surfaces of live yeast and mammalian cells through cell surface-initiated controlled radical polymerization. By developing cytocompatible PET-RAFT (photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization), synthetic polymers with narrow polydispersity (Mw/Mn < 1.3) could be obtained at room temperature in 5 minutes. This polymerization strategy enables chain growth to be initiated directly from chain-transfer agents anchored on the surface of live cells using either covalent attachment or non-covalent insertion, while maintaining high cell viability. Compared with conventional grafting-to approaches, these methods significantly improve the efficiency of grafting polymer chains and enable the active manipulation of cellular phenotypes.