Initiation Of Free-radical Polymerization Research Articles

Coumarin derivatives as dual photo/thermal initiators for free radical polymerization and 3D printing

In this study, five coumarin derivatives were developed and synthesized to serve as visible light photoinitiators for the free radical photopolymerization of acrylates upon near-UV or visible light.

Synthesis and applications of a Y‐inimer with two orthogonal controlled free radical polymerization initiators

AbstractWe report the synthesis of a Y‐shaped inimer that contains two orthogonal initiators for ATRP and NMP. The inimer is synthesized through a one‐pot multi‐component reaction that vastly simplifies the typically cumbersome synthesis of similar compounds. The Y‐inimer has the versatility to be homopolymerized into a backbone for A/B Janus bottlebrush synthesis or copolymerized with glycidyl methacrylate (GMA) and cross‐linked into an ultra‐thin coating for mixed A/B brush growth from planar surfaces. Importantly, the Y‐shaped nature of the inimer ensures growth of A and B brushes are consistently in a 1:1 ratio. We demonstrate the application of the Y‐inimer in the synthesis of a PMMA/PS Janus bottlebrush as well as two different mixed A/B polymer brushes, one with the ability to microphase separate, and a second mixed polyelectrolyte brush with opposite charges. The inimer is compatible with various A/B monomer systems and offers a universal approach to the “grafting‐from” polymerization of dual vinyl polymer side chains. This study provides a unique way of utilizing multi‐component reactions in polymer chemistry to access complex functional architectures.

New substituted pentazadienes as initiators of free-radical polymerization: synthesis, photochemical properties and perspectives for holographic media

The series of 4,4′-substituted 3-hydroxyethyl-pentaza-1,4-dienes were synthesized in the reaction of aromatic diazonium salts with an ethanol amine. Photolysis of pentaza-1,4-dienes was investigated by means of UV/Vis spectroscopy. Electron donating methyl and methoxy substituents were found to increase photosensitivity whereas electron withdrawing chloro-substituents cause higher photolytic stability. 3-hydroxyethyl-pentaza-1,4-dienes as a radical polymerization initiator has been successfully investigated in 10% methyl methacrylate solution in DMF. Gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) were used for obtained polymers characteristics evaluation. The highest values of PMMA Mn and Đ can be obtained in the case of chloro-substituted derivative. The holograms of a plane wavefront were recorded in media based on obtained PMMA doped by azo-dye by a 532 nm laser in a ∼1-μm-thick sample. The diffraction efficiency values for parallel polarized acting light increased in Cl→Me→OMe series.

Preparation and properties of UV cation-initiated self-crosslinking acrylate pressure-sensitive adhesive without reactive diluent

The aim of this paper is to synthesize a UV-initiated self-crosslinking epoxy acrylate pressure-sensitive adhesive (PSA) without reactive diluents. The monomers butyl acrylate, 2-ethylhexyl acrylate, and glycidyl methacrylate (GMA) and functional monomers acrylic acid (AA) and hydroxyethyl methacrylate were used as raw materials. Dodecyl mercaptan was used as a chain-transfer agent, and benzoyl peroxide (BPO) was used as an initiator for free radical polymerization in the synthesis of an acrylate prepolymer containing epoxy groups. When a cationic UV initiator was added, the system showed the characteristics of a nonreactive diluent and emitted a faint odor. A self-crosslinking epoxy acrylate PSA was obtained by coating at room temperature and UV light initiation. The effects of AA and GMA dosages, photoinitiator dosage, and curing time on the product's properties were investigated. Viscosity was characterized with a rotary viscometer, whereas the structure, glass transition temperature, and thermal decomposition were characterized through Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis, respectively. Results showed that the PSAs containing 3.5 % AA, 12.5 % GMA, and 2 % photoinitiator 6976 and subjected to 40 s of curing time had the best overall performance, with a loop tack of 11.1 N/25 mm, 180° peel adhesion of 14.5 N/25 mm, dynamic shear strength of 9.2 N/12 mm × 12 mm, and good heat resistance at 80 °C for 4 h without adhesive loss after peeling. A novel self-crosslinking PSA glue was successfully prepared by introducing epoxy groups into the molecular chains of prepolymers and controlling the viscosity of the prepolymers with a chain transfer agent. These procedures effectively remediate the shortcomings of current acrylate PSA glue, which contain active diluents, produce irritating odor, and undergo oxygen inhibition polymerization, offering great prospects for industrial applications.

MOF/polymer hybrids through in situ free radical polymerization in metal-organic frameworks.

We use the free radical polymerization initiator 4,4'-azobis(cyanovaleric acid) coordinated to the open metal sites of metal-organic frameworks (MOFs) to give rise to highly uniform MOF/polymer hybrids. We demonstrate this strategy on two robust zirconium MOFs (NU-1000 and MOF-808), which are the most effective catalysts for degradation of chemical warfare nerve agents. The resulting hybrid materials maintain their hydrolytic catalytic activity and have substantially improved adhesion to polypropylene and activated carbon textile fibers, yielding highly robust MOF/polymer/textile hybrid systems. These composites are suitable for the green production of active protective clothing and filters capable of detoxifying organophosphorus warfare agents.

Stereocomplexation of Poly(Lactic acid) and Chemical Crosslinking of Ethylene Glycol Dimethacrylate (EGDMA) Double-Crosslinked Temperature/pH Dual Responsive Hydrogels.

Physical crosslinking and chemical crosslinking were used to further improve the mechanical properties and stability of the gel. A temperature/pH dual sensitive and double-crosslinked gel was prepared by the stereo-complex of HEMA-PLLA20 and HEMA-PDLA20 as a physical crosslinking agent, ethylene glycol dimethacrylate (EGDMA) as a chemical crosslinking agent, and azodiisobutyronitrile (AIBN) as an initiator for free radical polymerization. This paper focused on the performance comparison of chemical crosslinked gel, a physical crosslinked gel, and a dual crosslinked gel. The water absorption, temperature, and pH sensitivity of the three hydrogels were studied by a scanning electron microscope (SEM) and swelling performance research. We used a thermal analysis system (TGA) and dynamic viscoelastic spectrometer to study thermal properties and mechanical properties of these gels. Lastly, the in vitro drug release behavior of double-crosslinked hydrogel loaded with doxorubicin under different conditions was studied. The results show that the double-crosslinked and temperature/pH dual responsive hydrogels has great mechanical properties and good stability.

Thermal Safety Performance Evaluation for Typical Free Radical Polymerization Initiator of Tert‐butyl Peroxypivalate

AbstractTert‐butyl peroxypivalate is a typical free radical polymerization initiator due to the active O−O peroxide group, and it is a thermal instability material. To evaluate the thermal safety of tert‐butyl peroxypivalate, Dewar test, Differential Scanning Calorimetry (DSC) and Accelerating Rate Calorimeter (ARC) were used to study the self‐accelerating decomposition temperature (SADT), thermal performance parameters and kinetic parameters. The results of DSC reveal that the onset decomposition temperature of the initiator was 76.30 °C. The apparent activation energy (Ea) was 91.86 kJ mol−1 by Kissinger method. In ARC test, the onset decomposition temperature was 40.64 °C. The adiabatic temperature of the reaction was 86.68 °C, and the apparent activation energy was 120.14 kJ mol−1. The results of Dewar test show that the SADT of the initiator was 38 °C. The simulated SADT value by thermal analysis software (AKTS) was about 35 °C which was basically consistent with the results of Dewar test.

Merging New Organoborane Chemistry with Living Radical Polymerization

In this issue of Cell Reports Physical Science, Pan and co-workers synthesize alkylcatecholboranes that can be oxidized by oxygen to drive the release of carbon-centered radicals for initiating reversible addition-fragmentation chain transfer (RAFT) polymerization. These organoboranes can slowly decompose to radicals that enable RAFT polymerization with exceptional control yielding ultra-high molecular weight (UHMW) polymers.

Towards Visible LED Illumination: ZnO‐ZnS Nanocomposite Particles

AbstractNowadays photo semiconductors such as ZnO are one of the driving forces of humanity towards economically and ecological safe life. Such semiconductors are used in solar applications and for light emission but also multiple investigations using the material as photocatalysts (water‐splitting), or as photo initiators were published. The combination of the inorganic material with an organic matrix leads to possible interesting hybrid materials. One drawback for ZnO and other photo semiconductors is the short UV wavelength (in the UV−range) which is necessary to photoactivation. We will present the possibility of modified ZnS‐ZnO nanocomposites being reactive towards visible light for example 406 nm LED illumination. Therefore, the particles are used as initiators for free radical polymerization. The simply to handle experimental preparation procedure of the sulphur containing ≈10 nm particles is described in this work for the first time.

High-Throughput Preparation of Antibacterial Polymers from Natural Product Derivatives via the Hantzsch Reaction.

SummaryThe Hantzsch and free-radical polymerization reactions were combined in a one-pot high-throughput (HTP) system to simultaneously prepare 30 unique polymers in parallel. Six aldehydes derived from natural products were used as the starting materials to rapidly prepare the library of 30 poly(1,4-dihydropyridines). From this library, HTP evaluation methods led to the identification of an antibacterial polymer. Mechanistic studies revealed that the dihydropyridine group in the polymer side-chain structure plays an important role in resisting bacterial attachment to the polymer surface, thus leading to the antibacterial function of this polymer. This research demonstrates the value of multicomponent reactions (MCRs) in interdisciplinary fields by discovering functional polymers for possible practical applications. It also provides insights to further developing new functional polymers using MCRs and HTP methods with important implications in organic chemistry, polymer chemistry, and materials science.

Energy Landscapes in Photochemical Dissociation of Small Peroxides.

Organic peroxides are known to have important roles in many chemical and biochemical processes such as intermediates in the oxidation of various hydrocarbons, as initiators of free-radical polymerization and cross-linking agents, etc. Consequently, the study of the organic peroxides and their radicals are of fundamental interest and importance. Although several reaction pathways after dissociation of organic peroxides have been successfully identified using time-resolved optical absorption spectroscopy, interpretation of the data can be complicated due to spectral overlap of parent molecules, intermediates, and products. Therefore, a reliable theoretical framework is necessary in case of complex or less studied systems. In this study, we investigated the plausible thermal dissociation pathways of diethyl peroxide, ditert butyl peroxide, and dicumyl peroxide by density functional theory with M06-2X hybrid functional and compared its results to coupled cluster single double and perturbative triple, CCSD(T), level energies. Our results indicate that methyl radical elimination is the main dissociation mechanism for all of the studied peroxides after O-O bond cleavage which has been also observed in experiments. The resulting relative energies of the M06-2X functional were found to have reasonable accuracy in comparison with the CCSD(T) method. We also show that time-dependent density functional theory (TD-DFT) with the M06-2X functional provides a suitable guide for interpretation of time-resolved optical absorption spectra of peroxides. The experimental transient absorption spectra of dicumyl peroxide are interpreted using the theoretically predicted pathways and transient radical species. Both results agree within experimental resolution and accuracy. We propose that the traditionally assigned visible absorption is not due to the cumuloxyl radical and the photodissociation of dicumyl peroxide involves other pathways with extremely short-lived radicals.

Near-infrared light-responsive hydrogels via peroxide-decorated MXene-initiated polymerization.

Two-dimensional MXene Ti3C2T x nanosheets with peroxide decoration (p-Ti3C2T x ) are synthesized by a sonication-assisted MILD etching method. The obtained MXenes can generate hydroxyl radical species and act as an initiator for free-radical polymerization of a series of acrylic monomers without the use of light illumination or co-initiators. The monomers analyzed include acrylamide, N-isopropylacrylamide (NIPAM), N,N-dimethylacrylamide, methyl methacrylate, and hydroxyethyl methacrylate. By simply mixing N-isopropylacrylamide monomers and p-Ti3C2T x nanosheets under deoxygenated conditions, PNIPAM-based nanocomposite hydrogels are synthesized using a high concentration of the monomer. The nanocomposite hydrogels have a photothermal conversion efficiency of 34.7% and photothermal stability superior to that of pristine Ti3C2T x . Taking advantage of the thermal responsive behavior of PNIPAM, the nanocomposite hydrogels are successfully exploited as remotely near-infrared light controlled "smart" windows, fluidic valves and photodetectors.

Thermal stability, mechanical properties, impact strength, and uniaxial extensional rheology of reactive blends of PS and SBS polymers

Polystyrene (PS) has low impact strength and also shows poor strain hardening. In this work, poly(styrene–butadiene–styrene) triblock copolymer (SBS) was solution- and melt-blended with PS in the presence of a free-radical polymerization initiator, dicumyl peroxide (DCP), to enhance PS’s thermal stability, mechanical properties, impact resistance, and strain hardening. The solution-blended PS/SBS containing 0.1% DCP annealed at 180 °C showed strong strain hardening, but the melt-blended PS/SBS annealed at the same temperature and time showed poor strain hardening. The change in the blending temperature, DCP concentration, and PS-to-SBS ratio had minimal effect on the strain hardening of melt-blended PS/SBS. The tensile strength increased with an increase in the concentration of DCP up to 0.1%, and beyond that level, the tensile strength started decreasing. The impact resistance considerably improved with an increase in the SBS loading in the PS matrix, and the enhancement was more than double of the impact resistance shown by the neat PS.

Investigation of accelerated aging of lignin-containing polymer materials

This paper presents the results of an accelerated aging test of biocomposites containing kraft lignin, where the resistance of the materials against humidity and light exposure was investigated. Low molecular weight lignin, modified with methacrylic anhydride (LWL-Met), was copolymerized with two commercial monomers: styrene (St) and methyl methacrylate (MMA). The biocomposites were obtained by a bulk polymerization method using α,α′-azoiso-bis-butyronitrile (AIBN) as a free radical polymerization initiator. The Shore D hardness of the obtained materials was determined before and after aging test. The changes in the chemical structures of polymers, as the result of aging were analyzed by using the attenuated total reflection Fourier transform infrared (ATR/FT-IR) spectroscopy method. The thermal behavior and stability of the obtained materials were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The surface topography was determined using the optical topography method to evaluate the changes on the surface of synthesized materials resulted from accelerated aging. Application of modified lignin as a biocomponent in the polymerization process and its influence on the properties of the obtained materials before and after the accelerated aging test are discussed.

Photochemical, Thermal Free Radical, and Cationic Polymerizations Promoted by Charge Transfer Complexes: Simple Strategy for the Fabrication of Thick Composites

A novel strategy for the preparation of extremely thick composites by photochemical and/or thermal free radical polymerization (FRP) and/or cationic polymerization (CP) under low-energy consumption visible light emitted diodes (LEDs) is described. The approach is based on the use of charge transfer complexes (CTCs) formed between N-aromatic amines (donors) and iodonium salts (Iod, acceptors). Remarkably, it is demonstrated for the first time that these CTCs can be used as thermal initiators for free radical polymerization (FRP) initiation with performances comparable to dibenzoyl peroxide-based systems. The decomposition temperature of these CTC thermal initiators can be modulated through the selection of the amine, providing much safer conditions for synthesis, storage, and handling than that of the classical thermal initiators. A good stability in resin is also noted.

Photocatalytic decarboxylation of diacids for the initiation of free radical polymerization

Photopolymerization, which is one of the most attractive polymerization methods, has been recently studied for the development of new photoinitiators. Herein, we use a binary mixture of titanium dioxide (TiO2) nanoparticles and carboxylic diacid as a novel photoinitiator to initiate the free radical polymerization of vinyl acetate (VAc). The polymerization of VAc is achieved both in aqueous medium and bulk. The initiation mechanism of TiO2/diacids is studied via nuclear magnetic resonance (NMR) spectroscopy using 13C labeled diacids as probing molecules. Further, a universal reaction mechanism is established, where the polymerization of VAc is initiated by the HOOC-R˙ radical, which is generated from the photocatalytic decarboxylation of the diacid. The polymerization kinetics results indicate that the polymerization rate is strongly dependant on the diacid structure. Compared to the use of diacids with an odd number of carbons, it is found that using diacids with an even number of carbons results in the polymerization rate reaching the maximum value faster.

Nontoxic hydrophilic polymeric nanocomposites containing silver nanoparticles with strong antimicrobial activity.

New nontoxic hydrophilic nanocomposites containing metallic silver nanoparticles (AgNPs) in a polymer matrix were synthesized by the chemical reduction of silver ions in an aqueous medium. A new nontoxic water soluble copolymer of 1-vinyl-1,2,4-triazole and N-vinylpyrrolidone synthesized by free radical-initiated polymerization was used as a stabilizing agent. Transmission electron microscopy, scanning electron microscopy, ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, atomic absorption, and thermogravimetric analysis were used to characterize polymeric AgNPs nanocomposites. The results showed that the diameter of the synthesized AgNPs ranged from 2 to 6 nm. The toxicity of the initial copolymer of 1-vinyl-1,2,4-triazole and N-vinylpyrrolidone and its nanocomposite with AgNPs was found to be more than 5,000 mg/kg. The synthesized AgNP polymeric nanocomposite showed significant antimicrobial activity against different strains of Gram-negative and -positive bacteria. The minimum inhibitory concentrations suppressing the growth of the microorganisms ranged from 0.5 to 8 µg/mL and the minimum bactericidal concentrations ranged from 0.5 to 16 µg/mL. The fabricated AgNP nanocomposites are promising materials for the design of novel nontoxic hydrophilic antiseptics and antimicrobial components for medical purposes.

High performance of the linked visible photoinitiator for free radical polymerization based on erythrosine B derivative

ABSTRACTA novel erythrosine B derivative linked with a tertiary amine used as a visible light initiator for free radical polymerization was synthesized and characterized. For comparison, the free hydrophobic erythrosine B derivative was also synthesized and its combination with the tertiary amine was used as the separated two component photoinitiator. The linked initiator exhibited higher photoinitiator efficiency in high viscosity monomer than in low viscosity one, compared with the separated counterpart. Especially, when it was combined with iodonium salt, the further promotion of the photoinitiator efficiency was observed, compared with the separated three component system. It was found that as iodonium salt together with tertiary amine was continuously added to the formula with the linked initiator, the photoinitiator efficiency was dramatically enhanced compared with the same addition to the formula with the separated counterpart, revealing the application potential of the linked initiator. And the continuous addition of either the iodium salt or the tertiary amine to the formula only leaded to the small size increase of the photoinitiator efficiency. These results suggested that there existed an interdependent relationship between iodonium salt and tertiary amine in promoting the photoinitiator efficiency of the dye/amine/iodonium salt system. For these, the corresponding synergistic mechanism was proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42361.

Two-cationic 2-methylbenzothiazole derivatives as green light absorbed sensitizers in initiation of free radical polymerization.

N-Methylpyridinium esters derivatives of 2-methylbenzothiazole hemicyanine dyes photoinitiators/photosensitizers derived from N-propyl-3-[N-2-methylbenzothiazolo]-4-pyridyno phenylacetic acid ester diiodide and N-propyl-3-[N-2-me]thylbenzothiazolo]-4-pyridino diphenylacetic acid ester diiodide were synthesized and proposed as new photoinitiators of polymerization of 2-ethyl-(2-hydroxymethyl)-1,3-propanediol triacrylate under argon laser exposure at 514 nm. These compounds exhibit a strong absorption around 520 nm. The dye/borate salt, dye/borate salt/N-methoxypyridinium salt, dye/borate salt/diphenyliodonium salt, and dye/borate salt/1,3,5-triazine derivative combinations are very efficient in initiating of radical photopolymerization of triacrylate. Excellent polymerization profiles were obtained. The effect of both sensitizer and co-initiator structure on the ability to initiate of free radical polymerization of photoinitiating systems was also presented. The mechanism was discussed for different multicomponent initiating systems. Graphical Two-cationic 2-methylbenzothiazole derivatives as green light absorbed sensitizers in initiation of free radical polymerization.

Controlling the morphology of (concentric) microtubes formed by in situ free radical polymerization

Templates are used extensively for materials synthesis applications. A material, the morphology of which can be controlled by utilizing different material deposition methods, is formed within the template followed by its removal. In this work an in situ, one-step synthesis protocol is employed to produce polystyrene and polymethacrylate microtubes within small-diameter fused silica templates. The versatility of this protocol is also shown by the ability to multiplex tube formation using a microstructured fibre template with 126 individual capillaries. The material synthesis involves functionalizing the inner wall(s) of the template followed by the in situ free radical-initiated polymerization of the pre-polymer solution that contains a radical initiator and monomer/crosslinker mixed with an appropriate porogenic solvent. Distinct morphologies result depending on the template pre-treatment, synthesis temperature, template dimension and the relative polarity of the porogenic solvent. Tubular morphology is promoted through the use of hydrophobic monomers (e.g. benzyl methacrylate or styrene), lower polymerization temperatures and a more polar porogenic solvent mixture. Subsequent polymerizations can be carried out within capillary templates already containing polymer tubes to increase tube wall thickness, or to produce concentric, mixed material tubes (e.g. two- and three-layer combinations of poly-(benzyl methacrylate-co-1,4-butanediol dimethacrylate) and poly-(styrene-co-divinylbenzene) tubes). Morphological control of poly-(benzyl methacrylate-co-1,4-butanediol dimethacrylate) structures, particularly their tubular character and thickness, will prove useful for optimizing their performance as porous-layer open-tubular chromatographic columns, small tubular conduits and high-performance layered materials.