Cationic Polymerization Of Epoxides Research Articles

Photoinitiation Mechanisms of Novel Phenothiazine‐Based Oxime and Oxime Esters Acting as Visible Light Sensitive Type I and Multicomponent Photoinitiators

AbstractIn this work, three new photoinitiators, based on the phenothiazine scaffold as a chromophore and potentially bearing the oxime ester functionality as an initiating group are designed and synthesized for the free radical polymerization of acrylates, the cationic polymerization of epoxides, and the formation of interpenetrated polymer networks upon irradiation with a light emitting diode emitting at 405 nm. These phenothiazine‐based oxime and oxime esters revealed impressive photoinitiation ability manifested by excellent polymerization rates and high final reactive function conversions. Significantly, they can be used as both; one‐component (Type I) and two‐component photoinitiating systems. Photoinitiation mechanisms through which reactive species are produced are investigated by means of different complementary techniques including real‐time Fourier transform infrared spectroscopy, UV–visible absorption spectroscopy, electron spin resonance spectroscopy, fluorescence (steady state and time resolved), cyclic voltammetry, and molecular modeling calculations. Thermal initiation behavior of the different oxime esters is also studied by using differential scanning calorimetry, highlighting their dual thermal/photochemical initiation ability. Finally, 3D printed objects are successfully fabricated by conducting both direct laser writing and 3D printing experiments.

The new LED-Sensitive photoinitiators of Polymerization: Copper complexes in free radical and cationic photoinitiating systems and application in 3D printing

A ligand (A1) and four copper complexes (Cu1, Cu2, Cu3 and Cu4) were examined as photosensitizers in various photoinitiating systems (PISs). Interestingly, excellent photochemical reactivities were determined with the different compounds. Especially, notable photoinitiation abilities were determined upon exposure to LED@405 nm during the free radical polymerization of acrylates and the cationic polymerization of epoxides when combined with an iodonium salt and an amine. With regards to their remarkable reactivities, the different photoinitiating systems were applied to 3D printing experiments. By combining steady state photolysis and electron spin resonance spin trapping, the chemical mechanism supporting the polymerization processes could be fully detailed and proved. Among all PISs studied, the three-component PISs furnished the best monomer conversions during the free radical polymerization of trimethylolpropane triacrylate (TMPTA) and the cationic polymerization of (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) monomers. The conversion process was monitored by real-time Fourier transform infrared spectroscopy (RT-FTIR). Three-component PISs based on A1, Cu1 and Cu2 showed better performances than those based on Cu3, Cu4 and the different two-component PIS (comprising only the iodonium salt or the amine) in FRP experiments. Based on the outstanding photoinitiation abilities of A1 and Cu1 with both TMPTA and EPOX, smooth and regular 3D patterns could be obtained by direct laser write experiments by generating interpenetrating polymer networks (IPN) with the blend of these two monomers.

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.

Terthiophene Derivative-Based Photoinitiating Systems for Free Radical and Cationic Polymerization under Blue LEDs

Terthiophene derivatives were prepared and utilized as photoinitiators of photopolymerization reactions upon exposure to LEDs (410 and 445 nm). The photochemical mechanisms of the terthiophene-based photoinitiating systems were investigated and confirmed by steady-state photolysis, fluorescence quenching, and electron paramagnetic resonance spin-trapping techniques. The photoinitiation ability of terthiophene derivatives in combination with the iodonium salt and optionally with a tertiary amine in the free radical polymerization of multifunctional acrylates and cationic polymerization of epoxides under mild light irradiation was investigated. Exceptionally, a sodium salt form of a terthiophene derivative presented exceeding water solubility and admirable efficiency in rapid hydrogel formation from the 2-hydroxy ethyl acrylate and water blend. The storage stability of the effective photoinitiating systems in the formulations was also studied under moderate conditions.

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications.

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved—including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones’ moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

In-silico based development of photoinitiators for 3D printing and composites: Search on the coumarin scaffold

In this work, the in-silico rational design of new photoinitiators by molecular modeling for specific wavelength (here 405 nm) and specific applications (3D printing, composites) is reported. A large number of (keto)coumarin derivatives were investigated by molecular modeling and their synthesis and more detailed photochemical investigations are based on obtaining structures having both excellent predicted light absorption properties @405 nm and high excited state energy levels (to ensure high photochemical reactivity). More particularly, four new families of coumarins were designed (4 of the 19 proposed coumarins were never synthesized (N2,M6,T1,T6)): the first family is based on Nitrocoumarins (N1-N6), the second one on Methoxybenzene-based coumarins and Ethoxycoumarins (M1-M6), the third one on Thiophene-based coumarins (T1-T6) and the last family studied concerns Alkyne-based coumarin (A1). The purpose of this work concerns the study of the photoinitiating ability of these compounds in different monomers for different polymerization processes (free radical, cationic) using FTIR technique. The different compounds reported in this work are very efficient to initiate the free radical polymerization of (meth)acrylates but also the cationic polymerization of epoxides upon mild irradiation conditions using a Light Emitting Diode (LED) at 405 nm as visible light source. Nitrocoumarins were identified as the best candidates for photoinitiation among the different families of coumarins investigated in this work. More precisely, nitrocoumarins are characterized by very good polymerization profiles, great final reactive function conversions (FC) and also high rates of polymerization (Rp). The electrochemical and photochemical properties of the different compounds were also studied to get a deeper insight into the photochemical mechanisms supporting the initiation process. A full picture of the involved photochemical mechanisms is provided. Thanks to the astounding polymerization initiating ability of these coumarins, their use in 3D printing applications can be worthwhile. Remarkably, using these compounds, the preparation of photocomposites was possible even in difficult light penetration conditions resulting from the presence of fibers inside the resins.

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.

Development of new high‐performance visible light photoinitiators based on carbazole scaffold and their applications in 3d printing and photocomposite synthesis

ABSTRACTIn this article, new compounds based on the carbazole scaffold (DMs = DM1 and DM2, constituted by a carbazole unit connected on positions 3 and 6 to a two 4,4′‐dimethoxydiphenylamine groups and differing by the substituent present on the nitrogen heteroatom of the carbazole core) were synthesized and proposed as high‐performance visible light photoinitiators/photosensitizers for both the free‐radical polymerization of methacrylates and the cationic polymerization of epoxides upon visible light exposure using LED@405 nm. Remarkably, DM2 leads to higher final conversions than DM1. In order to study the photophysical and photochemical properties of the carbazole derivatives, different parameters were taken into account such as the light absorption, the steady‐state photolysis, and the fluorescence spectroscopy. Using different techniques such as fluorescence quenching, redox behavior, and cyclic voltammetry, we are able to discuss the photosensitization/photoinitiation reactions providing a full coherent picture of the involved chemical mechanisms. The photosensitization of the carbazole derivatives occurred predominantly via singlet excited states at the rate of the diffusion limit. Upon exposure to laser diode at 405 nm, DMs show high performance in initiating systems for 3D resins. Remarkably, DM2 can also be used in photocomposite synthesis using light‐emitting diode conveyor. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2081–2092

Acridone derivatives as high performance visible light photoinitiators for cationic and radical photosensitive resins for 3D printing technology and for low migration photopolymer property

This paper is devoted to study two acridone derivatives (A-2DPA and A-2PTz). These acridone compounds are synthesized and proposed as high performance visible light photoinitiators, in the presence of an iodonium salt or/and an amine (NPG or EDB), for both the free radical polymerization of (meth)acrylates and the cationic polymerization of epoxides upon irradiation with LED@405 nm. Excellent polymerization initiating abilities are found, and high final monomer conversions are obtained. A full picture of the involved photochemical mechanisms is provided. This paper is also concerned with the use of these acridone-based systems in new 3D printing resins. Finally, the usage of the new acridone derivatives for photopolymers with low migration properties is particularly outlined.

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.

Crosslinkable photoacid generators for ultrahigh loading in epoxide functionalized molecular resists

A series of ionic onium salt photoacid generators (PAGs) functionalized with epoxide or phenolic groups capable of participating in the cationic polymerization of epoxides have been synthesized and investigated for use in ultrahigh PAG loaded negative tone epoxide functionalized molecular resists. An epoxide functionalized PAG [tris(4-glycidylether-3,5-dimethylphenyl)sulfonium hexafluoroantimonate] was found to be capable of use at loadings up to 100 mol. % in an epoxide functionalized resist [1,1,2,2-tetrakis(4-(oxiran-2-ylmethoxy)phenyl)ethane (4-Ep)] without inhibiting its ability to form fully crosslinked features, and was able to resolve 50 nm features under 100 keV electron-beam lithography at PAG loadings exceeding 50 mol. %. A phenol functionalized PAG [tris(4-hydroxyphenyl)sulfonium hexafluoroantimonate] was similarly capable of use at loadings up to 50 mol. % in 4-Ep without inhibiting cross-linking, and was able to resolve both 50 and sub-50 nm features at PAG loadings exceeding 30 mol. % using 100 keV electron-beam lithography. Initial results from this study indicate that the functionalized PAGs can enable higher PAG loadings in these negative tone resists than was previously possible; however, increasing PAG loading beyond at least 30 mol. % has a negative impact on sensitivity when using 100-keV high energy e-beam exposures.

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

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation.

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

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

Spontaneous Emergence of Nonlinear Light Waves and Self-Inscribed Waveguide Microstructure during the Cationic Polymerization of Epoxides

We report spontaneous pattern formation due to modulation instability (MI) of a broad, uniform, incandescent beam as it propagates through a fluid medium undergoing cationic ring-opening polymerization of epoxide moieties and show that the dynamics of the process can be controlled through polymerization kinetics. By strong contrast, MI in the half century-old field of nonlinear light propagation has until now been described predominantly in terms of optical parameters such as coherence, intensity and wavelength. The increase in refractive index (Δn) originating from the cross-linking polymerization of biscycloaliphatic epoxy monomers pushes the system into a nonlinear regime, where normally negligible spatial noise becomes greatly amplified. The perturbed optical field stabilizes by spontaneously dividing into thousands of self-trapped filaments of light. Because each filament inscribes a permanent microscopic channel along its propagation path, the initially isotropic fluid medium solidifies into a dense...

Blue LED light-sensitive benzo pyrazolo (or imidazo) isoquinolinone derivatives in high-performance photoinitiating systems for polymerization reactions

Isoquinolinone derivatives (IQ) have been synthesized and combined with different additives (an amine, 2,4,6-tris(trichloromethyl)-1,3,5-triazine, an iodonium salt, or N-vinylcarbazole) to produce reactive species (i.e. radicals and cations) being able to initiate the radical polymerization of acrylates, the cationic polymerization of epoxides, the thiol-ene polymerization of trifunctional thiol/divinylether, and the synthesis of epoxide/acrylate interpenetrated polymer network IPN upon exposure to very soft polychromatic visible lights, blue laser diode or blue LED lights. Compared with the use of camphorquinone based systems, the novel combinations employed here ensures higher monomer conversions (∼50–60% vs. ∼15–35%) and better polymerization rates in radical polymerization. The chemical mechanisms are studied by steady-state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 567–575

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.

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.

Redox initiated cationic polymerization: Silane‐N‐aryl heteroaromatic onium salt redox couples

AbstractIn this article, a new route for the synthesis of N‐aryl heteroaromatic onium salts by the direct copper catalyzed arylation of pyridine, substituted pyridines, isoquinoline, and acridine with diaryliodonium salts is described. It was demonstrated that these N‐aryl heteroaromatic onium salts undergo facile platinum or rhodium‐catalyzed reduction by silanes bearing SiH groups. The reduction of N‐aryl heteroaromatic onium salts generates Brønsted acids. When this redox reaction was carried out in situ in the presence of an appropriate monomer, cationic polymerization was observed. Using this approach, the cationic polymerizations of epoxides, oxetanes, 1,3,5‐trioxane, styrene, and vinyl ethers were carried out. The use of optical pyrometry to monitor the redox initiated cationic polymerizations of some representative multifunctional monomers is described. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010