Radical Polymerization System Research Articles

Fast and efficient synthesis of polymers driven by solar radiation. New insights on dye/dendrimer photoinitiating systems

The performance of a series of visible-light driven photoinitiating systems (Vis-PIs) for radical polymerization was evaluated. The Vis-PIs formulations consisted of aqueous solutions containing xanthene dyes as sensitizers, while polyamido-amine (PAMAM) dendrimers were tested as alternative co-initiators of lower toxicity than the traditional amines. Acrylamide and HEMA were used as probe monomers and the respective polymers were characterized by FTIR, DSC and viscosimmetry. In order to elucidate the mechanism of photopolymerization, the triplet excited-states and semirreduced forms of the dyes were characterized by transient spectroscopy. Photophysical parameters as intersystem crossing and radical quantum yields were also determined for each dye/dendrimer couple. All Vis-PIs operated successfully under solar irradiation, achieving high monomer conversions after short exposure times. Interestingly, formulations with partially halogenated dyes showed the highest efficiency, which correlates inversely with the affinity and the electron transfer capability between the reactants. This study demonstrates the usefulness of dye/dendrimer combinations to operate as efficient aqueous Vis-PIs under an inexpensive, unlimited and natural energy source such as sunlight.

The role of the Terminal Benzene Derivatives in Triphenylamine‐Based Oxime Esters for Free Radical Photopolymerization

AbstractIn this study, we investigate the effect of terminal benzene derivatives in triphenylamine‐based oxime esters on their photoreactivity. Five novel oxime esters (Miko series) that containing different benzene derivatives as terminal groups, were synthesized. The electronic properties of the substituents on the benzene ring range from electron‐donating to electron‐withdrawing. Specifically, a methoxy benzene for Miko‐MOB, tert‐butyl benezene for Miko‐t‐Bu, chloro benzene for Miko‐CB, trifluoromethyl benzene for Miko‐TFM, and nitro benzene for Miko‐NB. Additionally, a compound with benzene only as the terminal substituent (TP‐1) was selected for comparision. All the new compounds exhibited higher molar extinction coefficients than TP‐1. Additionally, the steady‐state photolysis and electron spin resonance (ESR) properties would decrease from the electron‐donating substitutes to electron‐withdrawing substitutes. Finally, these oxime esters are utilized in Type I photoinitiating systems for the free radical polymerization of trimethylolpropane triacrylate (TMPTA) under UV or LED@405 nm light irradiation conditions. Among all formulations, Miko‐MOB demonstrated the highest double bond conversion efficiency under both UV and LED@405 nm light irradiation. Thus, terminal benzene derivatives in oxime esters play an important role in tuning the optical and photochemical reaction performance, offering significant value for the design strategy of such photoinitiators.

Ultrafast Sunlight-Induced Polymerization: Unveiling 2-Phenylnaphtho[2,3-d]Thiazole-4,9-dione as a Unique Scaffold for High-Speed and Precision 3D Printing.

A series of 15 dyes based on the 2-phenylnaphtho[2,3-d]thiazole-4,9-dione scaffold and 1 compound based on the 2,3-diphenyl-1,2,3,4-tetrahydrobenzo[g]quinoxaline-5,10-dione scaffold are studied as photoinitiators. These compounds are used in two- and three-component high-performance photoinitiating systems for the free radical polymerization of trimethylolpropane triacrylate (TMPTA) and polyethylene glycol diacrylate (PEGDA) under sunlight. Remarkably, the conversion of TMPTA can reach ≈60% within 20s, while PEGDA attains a 96% conversion within 90s. To delve into the intricate chemical mechanisms governing the polymerization, an array of analytical techniques is employed. Specifically, UV-vis absorption and fluorescence spectroscopy, steady-state photolysis, stability experiments, fluorescence quenching experiments, cyclic voltammetry, and electron spin resonance spin trapping (ESR-ST) experiments, collectively contribute to a comprehensive understanding of the photochemical mechanisms. Photoinitiation capacities of these systems are determined using real-time Fourier transformed infrared spectroscopy (RT-FTIR). Of particular interest is the revelation that, owing to the superior initiation ability of these dyes, high-resolution 3D patterns can be manufactured by direct laser write (DLW) technology and 3D printing. This underscores the efficient initiation of free radical polymerization processes by the newly developed dyes under both artificial and natural light sources, presenting an avenue for energy-saving, and environmentally friendly polymerization conditions.

Exploring the triplet state properties of thio-benzothioxanthene imides with applications in TTA-upconversion and photopolymerization.

Thio-benzothioxanthene imide (BTXI) exhibits long excited state lifetime (τT = 17.7 μs) and high ISC efficiency (ΦΔ = 97%). For the first time, BTXI derivatives were used as photosensitizers for triplet-triplet annihilation upconversion, achieving the highest efficiency of 13.8%. In addition, thio-BTXI derivatives were used as photoinitiators for photopolymerization, resulting in a series of green light-activated radical polymerization systems.

A General Strategy To Access Alternating Styrene/Substituted Styrene Copolymers by Using a Traceless Controlling Group.

We herein report a synthetic strategy for alternating copolymers of styrene and substituted styrenes by utilizing α-styryl boronate pinacol ester (StBpin) as the co-monomer through radical alternating copolymerization followed by protodeboronation. The excellent alternating polymerization behavior of the StBpin co-monomer in such a radical polymerization system is considered to be attributed to the steric hindrance and radical stabilization exerted by the Bpin group. This strategy is effective with a wide range of substituted styrene co-monomers regardless of the electronic nature of the substituents, and the protodeboronation of the alternating Bpin-containing polymers is highly efficient without polymer backbone alternation. RAFT living polymerization was also compatible with this approach. Thus, this strategy provides a way to build-up alternating copolymers consisting of similar styrene-type co-monomers, which has been inaccessible by conventional synthetic methods.

A General Strategy To Access Alternating Styrene/Substituted Styrene Copolymers by Using a Traceless Controlling Group

AbstractWe herein report a synthetic strategy for alternating copolymers of styrene and substituted styrenes by utilizing α‐styryl boronate pinacol ester (StBpin) as the co‐monomer through radical alternating copolymerization followed by protodeboronation. The excellent alternating polymerization behavior of the StBpin co‐monomer in such a radical polymerization system is considered to be attributed to the steric hindrance and radical stabilization exerted by the Bpin group. This strategy is effective with a wide range of substituted styrene co‐monomers regardless of the electronic nature of the substituents, and the protodeboronation of the alternating Bpin‐containing polymers is highly efficient without polymer backbone alternation. RAFT living polymerization was also compatible with this approach. Thus, this strategy provides a way to build‐up alternating copolymers consisting of similar styrene‐type co‐monomers, which has been inaccessible by conventional synthetic methods.

Computer-aided design of initiator utilization strategies for vinyl chloride suspension polymerization with constant reaction rate

From the viewpoint of polymer reaction engineering (PRE), polymerization with constant reaction rate is beneficial for reducing manual operations and promoting production efficiency. Using vinyl chloride (VC) suspension polymerization as a model system, manipulating the radical concentration by radical concentration-dependent optimization (RCO)-based initiators mixing (IM) strategy or continuous initiator dosing (CiD) strategy enables the polymerization with constant reaction rate. Case studies have been done to illustrate how the RCO-based IM strategy works for binary initiator combination screening and how to conduct VC suspension polymerizations with different reaction rates using RCO-based IM strategy. Importantly, decomposition kinetic parameters estimated for virtual initiators can be used for guiding the design of the capable initiators. Moreover, in the single initiator systems, the applicability of the RCO-based CiD strategy has been demonstrated by conducting VC suspension polymerization with different reaction rates and different temperatures. Optimal dosing rate profiles of the initiator can be provided for the cases demonstrated. It is expected that both RCO-based IM and CiD strategies can be extended to other free radical polymerization systems.

Latest Advances in Highly Efficient Dye-Based Photoinitiating Systems for Radical Polymerization.

Light-activated polymerization is one of the most important and powerful strategies for fabrication of various types of advanced polymer materials. Because of many advantages, such as economy, efficiency, energy saving and being environmentally friendly, etc., photopolymerization is commonly used in different fields of science and technology. Generally, the initiation of polymerization reactions requires not only light energy but also the presence of a suitable photoinitiator (PI) in the photocurable composition. In recent years, dye-based photoinitiating systems have revolutionized and conquered the global market of innovative PIs. Since then, numerous photoinitiators for radical polymerization containing different organic dyes as light absorbers have been proposed. However, despite the large number of initiators designed, this topic is still relevant today. The interest towards dye-based photoinitiating systems continues to gain in importance, which is related to the need for new initiators capable of effectively initiating chain reactions under mild conditions. In this paper we present the most important information about photoinitiated radical polymerization. We describe the main directions for the application of this technique in various areas. Attention is mainly focused on the review of high-performance radical photoinitiators containing different sensitizers. Moreover, we present our latest achievements in the field of modern dye-based photoinitiating systems for the radical polymerization of acrylates.

Novel Copper-Based Catalytic Systems for Atom Transfer Radical Polymerization of Acrylonitrile

Atom Transfer Radical Polymerization (ATRP) is an effective catalytic process leading to well-defined polymers with desired properties. This process based on reversible deactivation of propagating chains has a lower rate in comparison with conventional radical polymerization, especially in the case of obtaining polymers with high molecular weights. Thus, the increase of the rate of this process with preserving control over molecular weight distribution is a challenging task. In this work, novel catalytic systems for Activators Generated by Electron Transfer (AGET) ATRP of acrylonitrile based on copper (II) bromide complexes were proposed. It was found that Rochelle salt (potassium sodium tartrate tetrahydrate) may be used as a reducing agent for regeneration of copper-based catalysts to increase the rate of the process. The provided UV-vis spectroscopy experiments have confirmed the reducing ability of tartaric anion. It was found that the use tandem catalytic system based on two copper complexes with different ligands also increases the polymerization rate. The performed experiments allowed us to develop a catalytic system for rapidly obtaining polyacrylonitrile polymers with desired molecular weights exceeding 100 kDa.

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.

High potential of new dyeing photoinitiators for fast curing of (meth)acrylate compositions under low intensity UV–Vis light

The present work describes novel two-component photoinitiating systems for radical polymerization of (meth)acrylate-based compositions.

Overcoming the penetration–saturation trade-off in binder jet additive manufacturing via rapid in situ curing

Binder jetting (BJ) enables the production of green parts for structurally complex ceramic, metal, or composite parts. However, the existence of a penetration–saturation trade-off (PSTO) in conventional BJ 3D printing hinders its development. Since an increase in saturation is often accompanied by an increase in penetration distance of binder (P), it is difficult to produce high strength and high precision green parts; this limitation is not conducive to the development of new material systems. To overcome this limitation, a methacrylate-based, thermally activated free radical polymerization system was used in this study as a binder to produce green parts under in situ curing conditions, created by increasing the temperature of the powder bed. The penetration of the powder bed by the binder is controlled by regulating its curing status during printing. The proposed rapid in situ curing printing process overcomes the PSTO associated with conventional BJ and can inform the optimization of binder formulation, equipment design, and binder jet printing. • Acrylate-based binder cures by thermally activated free radical polymerization. • The binder cures rapidly in a heated powder bed, limiting penetration distance. • In situ cured green parts demonstrate high dimensional accuracy and strength.

Development of Electrically Conductive Thermosetting Resin Composites through Optimizing the Thermal Doping of Polyaniline and Radical Polymerization Temperature

This work developed an electrically conductive thermosetting resin composite that transitioned from a liquid to solid without using solvents in response to an increase in temperature. This material has applications as a matrix for carbon fiber reinforced plastics. The composite comprised polyaniline (PANI) together with dodecyl benzene sulfonic acid (DBSA) as a liquid dopant in addition to a radical polymerization system made of triethylene glycol dimethacrylate with a peroxide initiator. In this system, micron-sized non-conductive PANI particles combined with DBSA were dispersed in the form of conductive nano-sized particles or on the molecular level after doping induced by a temperature increase. The thermal doping temperature was successfully lowered by decreasing the PANI particle size via bead milling. Selection of an appropriate peroxide initiator also allowed the radical polymerization temperature to be adjusted such that doping occurred prior to solidification. Optimization of the thermal doping temperature and the increased radical polymerization temperature provided the material with a high electrical conductivity of 1.45 S/cm.

5,12-Dihydroindolo[3,2-a]carbazole: A promising scaffold for the design of visible light photoinitiators of polymerization

5,12-Dihydroindolo[3,2-a]carbazole is a polycyclic structure combining in its scaffold two carbazole moieties sharing a fused aromatic ring. Since the pioneering works reported in 2019 on this structure concerning the substitution of the two carbazoles by similar functional groups, no recent investigation has been devoted to asymmetrize the substitution of the two carbazoles. In this work, a series of 13 compounds based on the 5,12-dihydroindolo[3,2-a]carbazole scaffold is presented. By carefully controlling the reaction conditions, functional groups such as a formyl, a nitro or an acetyl group could be selectively introduced on one of the two carbazoles. By combining X-ray diffraction and 2D NMR experiments, the higher reactivity of one of the two carbazoles could be clearly evidenced, enabling to generate asymmetrically substituted structures. Thanks to this unprecedented approach on the 5,12-dihydroindolo[3,2-a]carbazole scaffold, the absorption of the resulting dyes could be shifted towards the visible range whereas the parent structure (5,12-dihexyl-6,7-diphenyl-5,12-dihydroindolo[3,2-a]carbazole) exhibits a strongly UV-centered absorption. In light of their visible light absorption properties, several dyes have been examined as photoinitiators of polymerization activable at 405 nm and under low light intensity in two-component photoinitiating systems for the free radical polymerization of acrylates or the cationic polymerization of epoxides. Chemical mechanisms supporting the different polymerization processes have been fully elucidated by combining several techniques including cyclic voltammetry, UV–visible absorption and photoluminescence spectroscopy as well as photolysis experiments.

Conjugated Cross-linked Phenothiazines as Green or Red Light Heterogeneous Photocatalysts for Copper-Catalyzed Atom Transfer Radical Polymerization.

Using the power of light to drive controlled radical polymerizations has provided significant advances in synthesis of well-defined polymers. Photoinduced atom transfer radical polymerization (ATRP) systems often employ UV light to regenerate copper activator species to mediate the polymerization. Taking full advantage of long-wavelength visible light for ATRP would require developing appropriate photocatalytic systems that engage in photoinduced electron transfer processes with the ATRP components to generate activating species. Herein, we developed conjugated microporous polymers (CMP) as heterogeneous photocatalysts to exploit the power of visible light in promoting copper-catalyzed ATRP. The photocatalyst was designed by cross-linking phenothiazine (PTZ) as a photoactive core in the presence of dimethoxybenzene as a cross-linker via the Friedel-Crafts reaction. The resulting PTZ-CMP network showed photoactivity in the visible region due to the extended conjugation throughout the network because of the aromatic groups connecting the PTZ units. Therefore, photoinduced copper-catalyzed ATRP was performed with CMPs that regenerated activator species under green or red light irradiation to start the ATRP process. This resulted in efficient polymerization of acrylate and methacrylate monomers with high conversion and well-controlled molecular weight. The heterogeneous nature of the photocatalyst enabled easy separation and efficient reusability in subsequent polymerizations.

Effective Singlet Oxygen Sensitizers Based on the Phenazine Skeleton as Efficient Light Absorbers in Dye Photoinitiating Systems for Radical Polymerization of Acrylates.

A series of dyes based on the phenazine skeleton were synthesized. They differed in the number of conjugated double bonds, the arrangement of aromatic rings (linear and/or angular system), as well as the number and position of nitrogen atoms in the molecule. These compounds were investigated as potential singlet oxygen sensitizers and visible light absorbers in dye photoinitiating systems for radical polymerization. The quantum yield of the singlet oxygen formation was determined by the comparative method based on the 1H NMR spectra recorded for the tested dyes in the presence of 2,3-diphenyl-p-dioxene before and after irradiation. The quantum yield of the triplet state formation was estimated based on the transient absorption spectra recorded using the nanosecond flash photolysis technique. The effectiveness of the dye photoinitiating system was characterized by the initial rate of trimethylolpropane triacrylate (TMPTA) polymerization. In the investigated photoinitiating systems, the sensitizer was an electron acceptor, whereas the co-initiator was an electron donor. The effectiveness of TMPTA photoinitiated polymerization clearly depended on the arrangement of aromatic rings and the number of nitrogen atoms in the modified phenazine structure as well as the quantum yield of the triplet state formation of the photosensitizer in the visible light region.

Facile Synthesis of Unimodal Polymethacrylates with Narrow Dispersity via NIR LED Light-Controlled Bromine-Iodine Transformation Reversible-Deactivation Radical Polymerization.

A facile and clean strategy for synthesizing unimodal polymethacrylates with narrow dispersity (Đ <1.10) is successfully developed by a near-infrared (NIR) light-emitting diode (LED) light (λmax = 740nm)-controlled in situ bromine-iodine transformation reversible-deactivation radical polymerization system without the use of NIR dyes and expensive catalysts. In this system, alkyl iodide ethyl α-iodophenylacetate (EIPA) initiator is generated in situ by the nucleophilic substitution reaction between an alkyl bromide compound ethyl α-bromophenylacetate and sodium iodide (NaI). At the same time, excessive NaI is also acted as a highly active catalyst by forming halogen bonds with terminal iodine of the polymer chains in this system to make it capable of precisely synthesizing polymethacrylates with narrow dispersities (Đ = 1.03-1.10). In addition, the strong penetration ability of NIR LED light is illustrated by the successful polymerization even through 11 pieces of A4 paper.

Fabrication of polymer electrolyte via lithium salt-induced surface-initiated radical polymerization for lithium metal batteries

The fabrication of flexible polymer electrolytes (PEs) which possess simultaneously high thermal stability, robust mechanical strength, and high ionic conductivity is very challenging for lithium (Li) batteries. Herein, cellulose-based PE is fabricated via a Li salt-induced surface-initiated radical polymerization of polyethylene glycol methyl ether acrylate (PEGA). The polymerization of PEGA is catalyzed by various Li salts, such as Li perchlorate (LiClO4), bis(trifluoromethane)sulfonimide Li (LiTFSI), Li difluoro(oxalato)borate (LiDFOB), and LiClO4/LiDFOB, which also serve as charge transfer carriers in PEs. The Li salt can be used as the catalyst to promote the cleavage of C–Br bond on the surface of Br-modified cellulose, and then initiate the polymerization of PEGA. The PE obtained from the dual salt (LiClO4/LiDFOB)-induced radical polymerization system exhibits high ionic conductivity of 3.5 × 10−5 S cm−1 at 30 °C, good thermal stability (up to 240 °C), and wide electrochemical stability window (5.0 V vs. Li/Li+). Moreover, Li/PE/Li symmetric cells show good interfacial stability between PE and Li foil and the assembled Li/PE/LiFePO4 cells possess 150 mA h g−1 at 0.1C. Li salt-induced surface-initiated polymerization provides a simple and efficient method for fabricating PE for high-performance Li metal batteries (LMBs).

Freeform Three-Dimensionally Printed Microchannels via Surface-Initiated Photopolymerization Combined with Sacrificial Molding.

Precise freeform microchannels within an aqueous environment have several biomedical applications but remain a challenge to fabricate. Carbohydrate glass materials have shown potential for three-dimensionally (3D) printing precise, microscale structures and are suitable as a sacrificial material to reconstruct complex channel architectures, but due to the rapid dissolution kinetics in hydrogels and the aqueous environment, protective coatings are required. Here, conformal coatings were applied to carbohydrate structures via surface-initiated photopolymerization (SIP) by incorporating a photoinitiator (PI) into freeform 3D printed isomalt structures using a custom 3D printer. Structures were then immersed into a photocurable prepolymer bath and exposed to light for reaction initiation. To achieve uniform distribution of photoinitiator molecules in 3D printed constructs, miscibility between commercial photoinitiators and isomalt was modeled using the group contribution method. A dye-based, type-two photoinitiator, Eosin Y disodium salt (EY), was selected for its miscibility with isomalt and stability under high temperature. A previously described Eosin Y (EY)/triethanolamine (TEA) radical polymerization system was used to polymerize poly(ethylene glycol) diacrylate (PEGDA). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR), surface morphology, and swelling ratio characterizations via SIP were performed. Coatings around freeform structures and solid surfaces were presented to demonstrate the capability of coating complex architectures. This coating method should facilitate the application of 3D sacrificial molding in a variety of hydrogels toward building biomimetic vascular constructs.

A New Phosphine for Efficient Free Radical Polymerization under Air.

A new phosphine is proposed as efficient coinitiator for camphorquinone (CQ)-based photoinitiating systems for the free radical polymerization of (meth)acrylates. Remarkably, this new co-initiator can exhibit two functionalities: a phosphine moiety to overcome oxygen inhibition and an iodonium salt moiety as counter cation to initiate the polymerization process. Excellent polymerization performances in the presence of CQ for the free radical polymerization of methacrylates under blue light are observed, and amine-free systems can be easily developed from the proposed structure. The photopolymerization of composites is also investigated in the presence of the new phosphine (without iodonium counter cation) and very interesting depth of cure can be obtained from the new developed photoinitiating system after only 20 s of irradiation with a blue light-emitting diode.