Free Radical Polymerization Of Acrylates Research Articles

Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes.

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

Carbazole-based compounds as photoinitiators for free radical and cationic polymerization upon near visible light illumination.

Six new carbazole based compounds (Ca1-Ca6) are synthesized and proposed as high performance photoinitiators with iodonium salt (iod) and/or an amine (EDB) for both the free radical polymerization (FRP) of acrylates and the cationic polymerization (CP) of epoxides upon near UV and visible light exposure using light emitting diodes (LEDs) @385 nm and @405 nm. Excellent polymerization initiating abilities are found and high final reactive function conversions are acquired. A full picture of the involved photochemical mechanisms is given.

Strategy for rational control of stereosequence in free radical polymerization of acrylates

ABSTRACTRadical polymerization of cyclic analogs of acrylates, (S)‐ and (R)‐2‐isopropyl‐5‐methylene‐1,3‐dioxolan‐4‐ones (1S and 1R), successfully afforded a functional polymer having the tacticity continuously controlled from 29% to ∼100% of meso triad (mm) over a wide range of temperature only by changing the molar ratio of 1S/1R in feed. Plot of the number fractions of the triad versus diad of poly(1) was in good agreement with the Bernoulli statistics. In the polymerization in chiral solvents having analogs structure of the monomers, the tacticity and specific rotation of the resulting polymer were specifically varied depending on the structure and concentration of the solvents. Model propagation reaction at dimeric radical calculated with density functional theory reproduced a methodical induction of the chirality to the main chain from the branched chiral monomeric unit, which supports the experimental expectations. It is remarkable that the ceiling temperature of 1 is tremendously high, for example, 193 °C in [1 (ee = 72.6%)] = 0.05 mol/L, and the isospecific polymerization is maintained even at such a high temperature, which enabled the control of polymer tacticity over a wide range of temperature. The mechanism of the stereosequence in radical polymerization was discussed experimentally and theoretically. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 184–193

Azahelicenes as visible light photoinitiators for cationic and radical polymerization: Preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins

ABSTRACTIn this article, novel azahelicenes (AZs) were synthesized and proposed as high performance visible light photoinitiators for both the free radical polymerization of acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using Light Emitting Diodes (LEDs) @405, @455, and @470 nm. Excellent polymerization initiating abilities are found and high final conversions were obtained. Remarkably, an exceptional long lifetime photoluminescence property of the polymer films was observed when synthesized in presence of AZs. A full picture of the involved chemical mechanisms is given. AZs being high performance photoinitiators, their use in new cationic LED 3D printing resins will be also presented, that is, the cationic process upon LED projector @405 nm can be useful to reduce the shrinkage usually observed for radical polymerization. LED projector printing is very interesting compared to laser writing as this technology projects the profile of an entire layer of a 3D object at one time. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 55, 1189–1199

Iron‐Based Metal‐Organic Frameworks (MOF) as Photocatalysts for Radical and Cationic Polymerizations under Near UV and Visible LEDs (385–405 nm)

Photoinitiating systems consisting of a highly porous iron(III)‐based metal‐organic framework (MOF), an iodonium salt, and N‐vinylcarbazole are developed to initiate the free radical promoted cationic polymerization of epoxides under air and the free radical polymerization of acrylates in laminate upon exposure to near UV (385 nm) or visible (405 nm) light‐emitting diodes. These systems present a satisfactory initiating ability. Among the five tested iron carboxylate MOFs exhibiting different compositions and topologies, the flexible microporous iron(III) terephthalate MIL53 is particularly interesting, leading to a final conversion of 58% for the ring‐opening polymerization of epoxides. This provides the possibility of designing MOF/polymer composite materials with enhanced mechanical properties through the in situ incorporation of the MOF structure during the photopolymerization process. image

Polymeric Photoinitiators: A New Search toward High Performance Visible Light Photoinitiating Systems

A series of conjugated polymers and oligomers previously used in organic photovoltaics and organic light‐emitting diodes (OLEDs) have been examined for the first time as photoinitiators of polymerization. They address the present and future challenges facing photopolymerization: requirement of high extinction molar coefficients, absorption over the whole visible spectrum, ability to easily withdraw or release electrons. Interestingly, these compounds initiate the cationic polymerization of epoxides or the free radical polymerization of acrylates upon exposure of the photocurable formulations to a LED emitting at 405 nm. Some of them can potentially operate under blue, green, or red laser diodes.image

Iron Complexes in Visible‐Light‐Sensitive Photoredox Catalysis: Effect of Ligands on Their Photoinitiation Efficiencies

AbstractThe novel role of metal‐based complexes as photoinitiator catalysts fits well into the concept of green chemistry, as it realizes the activation of polymer synthesis processes by visible light that are abundant in the solar light and allows the marked reduction of photoinitiator amount in the systems. In the present paper, a series of iron complexes (FeC_x) with various ligands have been proposed as new photoinitiator catalysts to initiate the cationic polymerization of epoxides or the free radical polymerization of acrylates upon a near‐UV or visible‐light LED exposure. The ligands play an important role on the light absorption properties and the photoinitiation ability of the iron complexes. In combination with one or two additives, FeC_x are capable to efficiently generate radicals, cations, and radical cations through an oxidative or a reductive path. Two of the newly developed FeC_x‐based photoinitiating systems exhibited comparable photoinitiation efficiency with the commercial Type I photoinitiator bis(2,4,6‐trimethylbenzoyl)‐phenylphosphineoxide (BAPO). Owing to the photocatalytic effect, remarkable photoinitiation efficiencies have been achieved by using very low concentration of iron complexes (0.02 wt %) in the systems. The involved photochemical mechanisms have been studied using electron spin resonance spin trapping, steady state photolysis, cyclic voltammetry, and laser flash photolysis techniques.

D-π-A-type aryl dialkylsulfonium salts as one-component versatile photoinitiators under UV/visible LEDs irradiation

Two D-π-A-type aryl dialkylsulfonium salts with substituted-fluorene as a conjugated system were developed as one-component, versatile photoinitiators under soft irradiation conditions (365–450 nm Light-emitting diodes). The mechanisms for photochemical generation of reactive species (cations and radicals) by the photoinitiators were studied by UV–Vis spectroscopy, theoretical calculations, fluorescence spectroscopy and cyclic voltammetry techniques. The high quantum yields for acid generation (ΦH+ = ∼0.43 to 0.63) of the compounds enable the cationic polymerizations of epoxides and vinyl ether to proceed effectively. Meanwhile, the photocleavage products of sulfoniums (i.e., radicals) initiate the free radical polymerization of acrylate. Such versatile properties allowed them to serve as effective photoinitiators for the synthesis of interpenetrated polymer networks and thiol-ene polymerization without any co-initiators or additives. Based on their beneficial features, these photoinitiators hold great promise for use in photocuring applications.

Visible‐light‐sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions

ABSTRACTThe development of iron complexes for the photoredox catalysis is a huge challenge. Indeed, Iron complexes can be ideal candidates due to their potential visible light absorption and redox properties but also because they are less toxic, inexpensive and environmentally friendly compared to other catalysts. In the present paper, a series of novel iron complexes have been synthesized and utilized to initiate the free radical promoted cationic polymerization of epoxides or the free radical polymerization of acrylates through photoredox catalysis processes upon exposure to near UV (385 nm) or visible violet (405 nm) light emitting diodes (LEDs). When combined with an iodonium salt and N‐vinylcarbazole, the iron complex‐based photoinitiating systems are able to generate radicals, cations, and radical cations. The initiation efficiency is investigated through real‐time Fourier transform infrared spectroscopy and a satisfactory initiating ability is found. The mechanisms for the generation of the reactive initiating species through photoredox catalysis are studied by different methods (steady state photolysis, cyclic voltammetry and electron spin resonance spin trapping techniques) and discussed in detail. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2247–2253

2,2,2-trifluoroacetophenone-based D-π-A type photoinitiators for radical and cationic photopolymerizations under near-UV and visible LEDs

Two D-π-A-type 2,2,2-trifluoroacetophenone derivatives, namely, 4′-(4-( N,N-diphenyl)amino-phenyl)-phenyl-2,2,2-trifluoroacetophenone (PI-Ben) and 4′-(4-(7-(N,N-diphenylamino)-9,9-dimethyl-9H-fluoren-2-yl)-phenyl-2,2,2-trifluoroacetophenone (PI-Flu), are developed as high-performance photoinitiators combined with an amine or an iodonium salt for both the free-radical polymerization of acrylates and the cationic polymerization of epoxides and vinyl ether upon exposure to near-UV and visible light-emitting diodes (LEDs; e.g., 365, 385, 405, and 450 nm). The photochemical mechanisms are investigated by UV-Vis spectra, molecular-orbital calculations, fluorescence, cyclic voltammetry, photolysis, and electron-spin-resonance spin-trapping techniques. Compared with 2,2,2-trifluoroacetophenone, both photoinitiators exhibit larger redshift of the absorption spectra and higher molar-extinction coefficients. PI-Ben and PI-Flu themselves can produce free radicals to initiate the polymerization of acrylate without any added hydrogen donor. These novel D-π-A type trifluoroacetophenone-based photoinitiating systems exhibit good efficiencies (acrylate conversion = 48%–66%; epoxide conversion = 85%–95%; LEDs at 365–450 nm exposure) even in low-concentration initiators (0.5%, w/w) and very low curing light intensities (1–2 mW cm−2). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1945–1954

Aminothiazonaphthalic anhydride derivatives as photoinitiators for violet/blue LED‐Induced cationic and radical photopolymerizations and 3D‐Printing resins

ABSTRACTThe cations and radicals produced in aminothiazonaphthalic anhydride derivatives (ATNAs) combined with an iodonium salt, N‐vinylcarbazole, amine, or chloro triazine initiate the ring‐opening cationic polymerization of epoxides and the free radical polymerization of acrylates under LEDs at 405 or 455 nm. The photoinitiating ability of these novel photoinitiating systems is higher than that of the well‐known camphorquinone‐based systems. An example of the high reactivity of the new proposed photoinitiator is also provided in resins for 3D‐printing using a LED projector@405 nm. The chemical mechanisms are investigated by steady‐state photolysis, cyclic voltammetry, fluorescence, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1189–1196

Naphthalic anhydride derivatives: Structural effects on their initiating abilities in radical and/or cationic photopolymerizations under visible light

ABSTRACTOnly one naphthalic anhydride derivative has been reported as light sensitive photoinitiator, this prompted us to further explore the possibility to prepare a new family of photoinitiators based on this scaffold. Therefore, eight naphthalic Naphthalic anhydride derivatives (ANH1‐ANH8) have been prepared and combined with an iodonium salt (and optionally N‐vinylcarbazole) or an amine (and optionally 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine) to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources, that is, very soft halogen lamp (∼ 12 mW cm−2), laser diode at 405 nm (∼1.5 mW cm−2) or blue LED centered at 455 nm (80 mW cm−2). The ANH6 based photoinitiating systems are particularly efficient for the cationic and the radical photopolymerizations, and even better than that of the well‐known camphorquinone based systems. The photochemical mechanisms associated with the chemical structure/photopolymerization efficiency relationships are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2860–2866

Demonstration of isospecific free radical polymerization of acrylate controlled by conformation and chirality of monomer

ABSTRACTRadical polymerization of lactic acid‐based chiral and achiral methylene dioxolanones, a model for conformationally s‐cis locked acrylate, was carried out with AIBN to demonstrate an isospecific free radical polymerization controlled by chirality and conformation of monomer. Polymerization of the dioxolanones proceeded smoothly without ring opening to give a polymer with moderate molecular weight and 100% of maximum isotacticity. ESR spectrum indicated a twisted conformation of the growing poly(methylene dioxolanone) radical in contrast to an acyclic analogous radical, suggesting a restriction of the free rotation around main chain CαCβ bond of the growing radical center. Chirality as well as the polarity and bulkiness of monomer affected the polymer tacticity, and chiral alkyl substituent would afford a high isotactic polymer, in which higher the enantiomeric excess of the monomer was, higher the isotacticity of the polymer was. While, achiral or polar substituents including dibenzyl and trichloromethyl groups would afford an atactic polymer. In addition, glass transition temperature (Tg) of the resulting polymers was significantly high, ranging from 172.2 to 229.8 °C, and even for an isotactic polymer Tg was as high as 206.8 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2007–2016

Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems

Seven naphthalimide derivatives (NDP1–NDP7) with different substituents have been designed as versatile photoinitiators (PIs), and some of them when combined with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine) are expected to exhibit an enhanced efficiency to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources (i.e., the LED at 385, 395, 405, 455, or 470 nm or the polychromatic visible light from the halogen lamp). Remarkably, some studied naphthalimide derivative based photoinitiating systems (PIS) are even more efficient than the commercial type I photoinitiator bisacylphosphine oxide and the well-known camphorquinone-based systems for cationic or radical photopolymerization. A good efficiency upon a LED projector at 405 nm used in 3D printers is also found: a 3D object can be easily created through an additive process where the final object is constructed by coating down suc...

UV-violet-blue LED induced polymerizations: Specific photoinitiating systems at 365, 385, 395 and 405 nm

Two naphthalimide derivatives (DMAENs) containing tertiary amine groups have been designed and synthesized. Upon exposure to near UV and visible LEDs (365 nm–455 nm), they lead to radicals without adding a hydrogen donor and, in combination with 2,4,6-tris(trichloromethyl)-1,3,5-triazine, an iodonium salt or N-vinylcarbazole, they produce radicals and cations. Compared to the well-known camphorquinone or bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide photoinitiator, the novel DMAENs containing photoinitiating systems are characterized by a very high reactivity for both the free radical polymerization of acrylates and the cationic polymerization of epoxides. This outstanding performance paves the way for polymerization in soft conditions (e.g. upon LED irradiation). The photochemical mechanisms are studied by Molecular Orbitals MO calculations, steady state photolysis, fluorescence, cyclic voltammetry, electron spin resonance spin trapping and laser flash photolysis techniques.

Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol–ene and IPN polymerization reactions upon exposure to visible light

Chalcone derivatives are proposed as novel photoinitiators for polymerization upon exposure to visible light (even under soft irradiation conditions: blue LED bulb at 462 nm, halogen lamp, and sunlight). Remarkably, these structures are highly versatile allowing access to photoinitiating systems for (i) the free radical polymerization of acrylates, (ii) the cationic polymerization of epoxides and vinyl ethers, (iii) the synthesis of interpenetrated polymer networks (IPNs) and (iv) the thiol–ene processes. Excellent polymerization profiles are obtained. The initiation mechanisms are analyzed through ESR, steady state photolysis, cyclic voltammetry, laser flash photolysis and fluorescence experiments.

Squarylium-Triazine Dyad as a Highly Sensitive Photoradical Generator for Red Light

New dyads, based on squarylium dye and substituted-triazine, were synthesized that exhibit an intramolecular photodissociative electron-transfer reaction. The compounds were used as a red-light photoradical generator. The photochemical activity of the dyad was compared to the corresponding unlinked systems (S+T) by determining the rate constant of electron transfer. The efficiency of the radical generation from the dyad compared to the unlinked system was demonstrated by measuring the maximum rate of free radical polymerization of acrylates in film. An excellent relationship between the rate of electron transfer and the rate of polymerization was found, evidencing the interest of this new approach to efficiently produce radicals under red light.

A Density Functional Theory Study of Secondary Reactions in n‐Butyl Acrylate Free Radical Polymerization

AbstractIn this work, secondary reactions involved in the free radical polymerization of butyl acrylate are investigated using quantum chemistry. First, various backbiting reactions are studied by adopting a simplified molecular model suitable for treating long polymer chains. The predicted reaction kinetics suggest the possibility of a radical migration along the poly(butyl acrylate) (PBA) chain as a consequence of subsequent j:j + 4 hydrogen abstractions, which are characterized by a low activation energy. Moreover, branching propagation and β‐scission reactions originating from mid‐chain radicals are investigated using a complete PBA model composed of five monomer units. The reaction kinetics involving short‐branch radicals are also examined, and a novel backbiting step leading to the formation of short branches is proposed. magnified image

Design of new Type I and Type II photoinitiators possessing highly coupled pyrene–ketone moieties

New Type I and Type II photoinitiators based on 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzophenone (BP) and thioxanthone (TX) linked to a pyrene (Py) moiety (Py_DMPA, Py_BP, Py_TX) are proposed here for the free radical polymerization of acrylates and the cationic polymerization of epoxides and vinylethers under a Xe–Hg lamp, laser diode at 405 nm or a halogen lamp. Excellent polymerization profiles using Py_DMPA and Py_BP are obtained, i.e. better than those recorded with the reference compounds (DMPA and BP). Specifically, Py_DMPA and Py_BP absorb in the near UV-visible range (360–420 nm range; absorption red-shift ∼15 nm), exhibit huge e values (up to a 90 fold increase) and high photochemical reactivity. The mechanisms analyzed by electron spin resonance, steady state photolysis, fluorescence and laser flash photolysis experiments are discussed for the different initiating systems.

New chromone based photoinitiators for polymerization reactions under visible light

Novel photoinitiators derived from a chromone structure are proposed for the free radical polymerization of acrylates and the free radical promoted cationic polymerization of epoxides upon exposure to a 457 nm laser diode and even under soft irradiation conditions (blue LED bulb at 462 nm and halogen lamp). Good polymerization profiles are obtained. These original dyes exhibit a push–pull molecular character as revealed by MO calculations. The initiation mechanisms are analyzed through ESR, fluorescence and laser flash photolysis experiments.