Abilities of 2‐arylmethyloxy‐6‐cyanonaphthalene derivatives to initiate radical and cationic photopolymerizations

An investigation was undertaken to shed much light on the bond‐cleavage mode (heterolysis vs homolysis) of bichromophoric photoinitiators in styrene (St) and 1,2‐epoxycyclohexane (EpC) and then to develop a highly efficient hybrid‐type photoinitiator. Excited‐state 2‐(9‐anthrylmethyloxy)‐6‐cyanonaphthalene in toluene and 1,2‐dimethoxyethane, which were used instead of St and EpC, respectively, underwent both homolytic Ar? OCH₂ bond cleavage and heterolytic ArO? CH₂ bond cleavage to give the corresponding radical‐pair and ion‐pair intermediates. The charge‐separated state characteristic of the naphthol chromophore in the singlet excited state was found to play a pivotal role in this heterolytic bond‐cleavage mode. An inspection of the radical (St) and cationic (EpC) photopolymerization behavior observed in the presence of some 2‐arylmethyloxy‐6‐cyanonaphthalenes led us to conclude that the aforementioned 9‐anthrylmethyloxy‐substituted naphthalene derivative is a candidate for a functional hybrid‐type photoinitiator containing no halogen and/or metal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 25–31, 2006     

Evaluation of initiator systems for controlled and sequentially curable free‐radical/cationic hybrid photopolymerizations

Free‐radical/cationic hybrid photopolymerizations of acrylates and epoxides were initiated using a three‐component initiator system comprised of camphorquinone as the photosensitizer, an amine as the electron donor, and a diaryliodonium salt. Thermodynamic considerations revealed that the oxidation potential of the electron donor must be less than 1.34 V relative to SCE for electron transfer with the photoexcited camphorquinone to take place. This electron transfer leads to the production of the active centers for the hybrid polymerization (two radicals and a cation). Further investigation revealed that only a subset of electron donors that meet the oxidation potential requirement resulted in polymerization of the epoxide monomer; therefore, a second requirement for the electron donor (pK_b higher than 8) was established. Experiments performed using a combination of electron donors revealed that the onset of the hybrid system's cationic polymerization can be advanced or delayed by controlling the concentration and composition of the electron donor(s). These studies demonstrate that a single three‐component initiator system can be used to initiate and chemically control the sequential curing properties of a free‐radical/cationic hybrid photopolymerization and is a viable alternative to separate photoinitiators for each type of polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1747–1756, 2005     

Hybrid free radical/cationic frontal photopolymerizations

The irradiation of hybrid photopolymer systems consisting of a free radically polymerizable multifunctional acrylate monomer and a cationically polymerizable epoxide or oxetane monomer was conducted under conditions where only the free radical polymerization takes place. This results in the formation of a free‐standing polyacrylate network film containing quiescent oxonium ions along with the unreacted cyclic ether monomer. The subsequent application of a point source of heat to the film ignites a cationic ring‐opening frontal polymerization that emanates from that site and propagates to all portions of the irradiated sample. This article examines the impact of various molecular structural and experimental parameters on these novel hybrid frontal polymerizations that produce interpenetrating network polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4331–4340, 2007     

Germanes as efficient coinitiators in radical and cationic photopolymerizations

In this article, germanes are presented as new coinitiators for both radical and cationic photopolymerization processes. For the free radical polymerization process, the newly proposed structures are characterized by efficiencies similar or better than the reference amine. Germyl radicals are also found to be excellent initiators for free radical promoted cationic photopolymerization (FRPCP) of an epoxy resin. The associated reactivity is even better under air. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3042–3047, 2008     

Supramolecular diaryliodonium salt‐crown ether complexes as cationic photoinitiators

Diaryliodonium salts spontaneously form crystalline 1:1 supramolecular complexes at room temperature in good to excellent yields with 18‐crown‐6 ether and its cyclohexano‐ and benzo‐substituted analogs. The complexes were characterized using IR, UV, MS, ¹H, and ¹³C‐NMR spectroscopy and by single crystal X‐ray crystallography. The analytical data obtained were consistent with a structure in which the positively charged iodine atom of diaryliodonium cation is positioned above and over the center of the crown ether ring with the positively charged iodine atom coordinated to the crown ether oxygen atoms. The diaryliodonium salt‐crown ether complexes are photosensitive and were used to carry out the photoinitiated cationic polymerizations of a number of mono‐ and difunctional monomers. During irradiation with UV light, the supramolecular complexes undergo photolysis with the generation of a Brønsted acid and with the concomitant release of the crown ether. When used as photoinitiators, the crown ether that is released markedly influences the kinetics of the subsequent cationic polymerization of the monomer. Further studies demonstrated that the photolysis of diaryliodonium salt‐crown ether supramolecular complexes can be photosensitized using typical‐electron transfer photosensitizers. Free radical‐promoted photosensitization using typical unimolecular free radical photoinitiators such as 2,2‐dimethoxy‐2‐phenylacetophenone also takes place readily. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synergistic effects in hybrid free radical/cationic photopolymerizations

Multifunctional alkyl glycidyl ether and oxetane monomers are usually deemed to be poorly reactive and are consequently of limited use for high speed photocuring applications. However, these monomers can be made to undergo exceedingly rapid exothermic photopolymerization when combined with a multifunctional acrylate monomer and a corresponding free radical photoinitiator. Under optimum UV irradiation conditions, these hybrid photopolymerizations take place rapidly and substantially without an induction period. A mechanism was proposed on the basis of thermal acceleration of the cationic ring‐opening polymerizations induced by the fast exothermic free radical acrylate photopolymerization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3759–3769, 2007     

Kinetic pathway investigations of three‐component photoinitiator systems for visible‐light activated free radical polymerizations

Three‐component photoinitiator systems generally include a light‐absorbing photosensitizer (PS), an electron donor, and an electron acceptor. To investigate the key factors involved with visible‐light activated free radical polymerizations involving three‐component photoinitiators and 2‐hydroxyethyl methacrylate, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with either rose bengal or fluorescein as the PS. The Rehm–Weller equation was used to verify the thermodynamic feasibility for the photo‐induced electron transfer reaction. It was concluded that key kinetic factors for efficient visible‐light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step for consuming dye‐based radical and regenerating the original PS (dye). Using the thermodynamic feasibility and kinetic data, we suggest three different kinetic mechanisms, which are (i) photo‐reducible series mechanism, (ii) photo‐oxidizable series mechanism, and (iii) parallel‐series mechanism. Because the photo‐oxidizable series mechanisms most efficiently allow the key kinetic factors, this kinetic pathway showed the highest conversion and rate of polymerization. The kinetic data measured by near‐IR and photo‐differential scanning calorimeter verified that the photo‐oxidizable series mechanism provides the most efficient kinetic pathway in the visible‐light activated free radical polymerizations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 887–898, 2009     

H‐transfer reaction during decomposition of N‐(2‐methylpropyl)‐ N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (SG1)‐based alkoxyamines

Thermal decomposition of four tertiary N‐(2‐methylpropyl)‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (SG1)‐based alkoxyamines (SG1‐C(Me)₂‐C(O)‐OR, R = Me, tBu, Et, H) has been studied at different experimental conditions using ¹H and ³¹P NMR spectroscopies. This experiment represents the initiating step of methyl methacrylate polymerization. It has been shown that H‐transfer reaction occurs during the decomposition of three alkoxyamines in highly degassed solution, whereas no products of H‐transfer are detected during decomposition of SG1‐MAMA alkoxyamine. The value of the rate constant of H‐transfer for alkoxyamines 1 (SG1‐C(Me)₂‐C(O)‐OMe) and 2 ( SG1‐C(Me)₂‐C(O)‐OtBu) has been estimated as 1.7 × 10³ M^?1s^?1. The high influence of oxygen on decomposition mechanism is found. In particular, in poorly degassed solutions, nearly quantitative formation of oxidation product has been observed, whereas at residual pressure of 10^?5 mbar, the main products originate from H‐atom transfer reaction. The acidity of the reaction medium affects the decomposition mechanism suppressing the H‐atom transfer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Bulk free‐radical photopolymerizations of 1‐vinyl‐2‐pyrrolidinone and its derivatives

An investigation of the free‐radical bulk photopolymerization of 1‐vinyl‐2‐pyrrolidinone (NVP) with an NVP‐based crosslinker, 1,6‐(bis‐3‐vinyl‐2‐pyrrolidinonyl)hexane (BNVP), and an NVP‐based comonomer, 3‐hexyl‐1‐vinyl‐2‐pyrrolidinone (VHP), was carried out. The enthalpies of polymerization were determined for NVP and VHP to be 30.8 and 35.7 kJ/mol, respectively. The rates of polymerization were determined for NVP/VHP and NVP/BNVP systems at various temperatures. These photopolymerization studies revealed that the overall rates of polymerization of these 3‐alkylated‐2‐pyrrolidinone derivatives increased with substitution onto the pyrrolidinone ring. A series of pyrrolidinone‐based additives in bulk NVP were used in model photopolymerizations of NVP for the evaluation of plasticizer effects. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 694–706, 2002; DOI 10.1002/pola.10142     

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

The 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     

Synthesis and evaluation as a visible‐light polymerization photoinitiator of a new dye‐linked bis(trichloromethyl)‐1,3,5‐triazine

A dye‐linked initiator consisting of a merocyanine dye, which has an absorption maximum at 460 nm, and a substituted bis(trichloromethyl)‐1,3,5‐triazine initiator was prepared in order to achieve an efficient photopolymerization in a visible‐light region. The spectroscopic studies clearly showed that the dye‐linked initiator exhibit a marked increase in the efficiency of fluorescence quenching than a simple mixture of the dye/initiator. These results are reasonably explained in terms of the efficiency of electron transfer between the dye and the initiator. The relative photoinitiating efficiency of dye‐linked initiators in photopolymerization of acrylate monomers was evaluated and the results clearly indicated that the dye‐linked photoinitiator exhibited a marked increase in the photoinitiating efficiency of photopolymerization of acrylates compared to a simple mixture of the dye/initiator in photopolymer coatings particularly at a lower concentration of the initiator. This was explained in terms of the active quenching sphere of the dye/initiator system. Superior photosensitivity in the linked compound at a lower concentration indicates that this would be particularly useful as a visible‐light photoinitiator in holographic‐recording photopolymers. Copyright © 2004 John Wiley & Sons, Ltd.     

Photoinitiated cationic polymerization of limonene 1,2‐oxide and α‐pinene oxide

Limonene 1,2‐oxide (LMO) and α‐pinene oxide (α‐PO) are two high reactivity biorenewable monomers that undergo facile photoinitiated cationic ring‐opening polymerizations using both diaryliodonium salt and triarylsufonium salt photoinitiators. Comparative studies showed that α‐PO is more reactive than LMO, and this is because it undergoes a simultaneous double ring‐opening reaction involving both the epoxide group and the cyclobutane ring. It was also observed that α‐PO also undergoes more undesirable side reactions than LMO. The greatest utility of these two monomers is projected to be as reactive diluents in crosslinking photocopolymerizations with multifunctional epoxide and oxetane monomers. Prototype copolymerization studies with several difunctional monomers showed that LMO and α‐PO were effective in increasing the reaction rates and shortening the induction periods of photopolymerizations of these monomers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis of poly(tetrahydrofuran)‐b‐polystyrene block copolymers from dual initiators for cationic ring‐opening polymerization and atom transfer radical polymerization

A dual initiator (4‐hydroxy‐butyl‐2‐bromoisobutyrate), that is, a molecule containing two functional groups capable of initiating two polymerizations occurring by different mechanisms, has been prepared. It has been used for the sequential two‐step synthesis of well‐defined block copolymers of polystyrene (PS) and poly(tetrahydrofuran) (PTHF) by atom transfer radical polymerization (ATRP) and cationic ring‐opening polymerization (CROP). This dual initiator contains a bromoisobutyrate group, which is an efficient initiator for the ATRP of styrene in combination with the Cu(0)/Cu(II)/N,N,N′,N″,N″‐pentamethyldiethylenetriamine catalyst system. In this way, PS with hydroxyl groups (PS‐OH) is formed. The in situ reaction of the hydroxyl groups originating from the dual initiator with trifluoromethane sulfonic anhydride gives a triflate ester initiating group for the CROP of tetrahydrofuran (THF), leading to PTHF with a tertiary bromide end group (PTHF‐Br). PS‐OH and PTHF‐Br homopolymers have been applied as macroinitiators for the CROP of THF and the ATRP of styrene, respectively. PS‐OH, used as a macroinitiator, results in a mixture of the block copolymer and remaining macroinitiator. With PTHF‐Br as a macroinitiator for the ATRP of styrene, well‐defined PTHF‐b‐PS block copolymers can be prepared. The efficiency of PS‐OH or PTHF‐Br as a macroinitiator has been investigated with matrix‐assisted laser desorption/ionization time‐of‐flight spectroscopy, gel permeation chromatography, and NMR. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3206–3217, 2003     

Dark‐cure studies of cationic photopolymerizations of epoxides: Characterization of the active center lifetime and kinetic rate constants

We have characterized the effective rate constants for termination/trapping (k_t/t) and propagation (k_p) for solvent‐free cationic photopolymerizations of phenyl glycidyl ether for conversions up to 50%. We have performed dark‐cure experiments in which active centers are produced photochemically for a specified period of time until the initiating light is shuttered off, and then the polymerization rate is monitored in the dark. This method is especially well suited for characterizing cationic polymerizations because of the long active center lifetimes. Our analysis provides profiles of the instantaneous kinetic rate constants as functions of conversion (or time). For photopolymerizations of phenyl glycidyl ether initiated with iodonium photoinitiators, k_t/t and k_p remain essentially invariant for conversions up to 50%. For the photoinitiator (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate (IPB), the values of k_t/t at 50 and 60 °C are 0.027 and 0.033 min^?1, respectively. The corresponding values of k_t/t for diaryliodonium hexafluoroantimonate (IHA) are 0.041 and 0.068 min^?1. The values of k_p at 50 °C for IPB and IHA are 0.6 and 0.4 L mol^?1 s^?1, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2064–2072, 2003     

A photo‐oxidizable kinetic pathway of three‐component photoinitiator systems containing porphrin dye (Zn‐tpp), an electron donor and diphenyl iodonium salt

A series of kinetic experiments were conducted involving visible‐light activated free radical polymerizations with three‐component photoinitiators and 2‐hydroxyethyl methacrylate (HEMA). Three‐component photoinitiator systems generally include a light‐absorbing photosensitizer (PS), an electron donor and an electron acceptor. To compare kinetic efficiency, we used thermodynamic feasibility and measured kinetic data. For this study, 5,10,15,20‐tetraphenyl‐21H,23H‐porphyrin zinc (Zn‐tpp) and camphorquinone (CQ) were used as the PSs. The Rehm‐Weller equation was used to verify the thermodynamic feasibility for the photo‐induced electron transfer reaction. Using the thermodynamic feasibility, we suggest two different kinetic mechanisms, which are (i) photo‐reducible series mechanism of CQ and (ii) photo‐oxidizable series mechanism of Zn‐tpp. Kinetic data were measured by near‐IR spectroscopy and photo‐differential scanning calorimetry based on an equivalent concentration of excited state PS. We report that the photo‐oxidizable series mechanism using Zn‐tpp produced dramatically enhanced conversions and rates of polymerizations compared with those associated with the photo‐reducible series mechanism using CQ. It was concluded from the kinetic results that the photo‐oxidizable series mechanism efficiently retards back electron transfer and the recombination reaction step. In addition, the photo‐oxidizable series mechanism provides an efficient secondary reaction step that involves consumption of the dye‐based radical and regeneration of the original PS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3131–3141, 2009     

Novel N‐methylbenzothiazolium salts as hardeners for epoxy and acrylate monomers

Novel N‐methylbenzothiazolium salts [N‐methyl‐2‐benzylthiobenzothiazolium, N‐methyl‐2‐(4‐nitrobenzylthio)benzothiazolium, N‐methyl‐2‐(1‐ethoxycarbonylethylthio)benzothiazolium, and N‐methyl‐2‐methylthiobenzothiazolium hexafluoroantimonates] were synthesized by the reaction of the corresponding 2‐substituted benzothiazole with dimethylsulfate, followed by anion exchange with KSbF₆. These benzothiazolium salts cationically polymerized an epoxy monomer by photoirradiation. They also polymerized an acrylate monomer via a photoradical process. The use of aromatic compounds such as 2‐ethyl‐9,10‐dimethoxyanthracene as photosensitizers was effective in enhancing the polymerization. These benzothiazolium salts also served as thermal cationic initiators. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3828–3837, 2003     

“Kick‐Starting” oxetane photopolymerizations

In the presence of small amounts of 2,2‐dialkyl‐, 2,2,3‐trialkyl‐, or 2,2,3,3‐tetraalkyl substituted epoxides such as isobutylene oxide, 1,2‐limonene oxide, and 2,2,3,3,‐tetramethyl oxirane, the photoinitiated cationic ring‐opening polymerizations of 3,3‐disubstituted oxetanes are dramatically accelerated. The acceleration affect was attributed to an increase in the rate of the initiation step of these latter monomers. Both mono‐ and disubstituted oxetane monomers are similarly accelerated by the above‐mentioned epoxides to give crosslinked network polymers. The potential for the use of such “kick‐started” systems in applications such as coatings, adhesives, printing inks, dental composites and in three‐dimensional imaging is discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2934–2946     

N-alkoxy-pyridinium and N-alkoxy-quinolinium salts as initiators for cationic photopolymerizations

The polymerization of n-butyl vinyl ether (BVE), cyclohexene oxide (CHO) and 3,4-epoxycyclohexyl(methyl)-3′,4′-epoxycyclohexane carboxylate (EEC) was initiated upon UV irradiation (λ_inc > 300 nm) of dichloromethane solutions containing N-ethoxy-2-methylpyridinium ( V ), N-ethoxy-4-phenyl-pyridinium ( VI ) or N-ethoxy-isoquinolinium hexafluorophosphate ( VII ). Whereas the bifunctional EEC was converted into an insoluble gel, BVE and CHO formed polymers of molar mass: M_w = 2 X 10⁴?2 X 10⁵ (PCHO) and M_w ≈ 2 X 10⁴ (PBVE). Protons are formed with a rather high quantum yield [ø(H+) = 0.48 on irradiating VII in dichloromethane; titration with sodium p-nitrophenolate] and it is, therefore, assumed that the polymerization is initiated by photochemically generated protons. © 1992 John Wiley & Sons, Inc.     

Photocationic and radical polymerizations of epoxides and acrylates by novel sulfonium salts

Novel sulfonium salts [methyl‐, 2‐indany‐, or 1‐ethoxycarbonylethyl methyl‐2‐naphthylsulfonium hexafluorophosphate and 2‐indany‐, 1‐ethoxycarbonylethyl‐, 2‐methyl‐2‐phenylpropyl‐, 2‐phenylpropyl‐, 2‐phenylethyl‐, 2‐(4‐methoxyphenyl)‐ethyl‐, or 3‐(4‐methoxyphenyl)‐2‐propyl methylphenylsulfonium hexafluorophosphates] were synthesized by the reaction of dimethylsulfate and the corresponding sulfides followed by anion exchange with KPF₆. These sulfonium salts could polymerize epoxy monomers at lower temperatures than previously reported for benzylsulfonium salt initiators. In particular, sulfonium salts with naphthyl groups showed higher photoactivity than already reported for di(4‐tert‐butylphenyl)iodonium and triphenylsulfonium hexafluorophosphates. These sulfonium salts showed higher activity in photoradical polymerization and photocationic polymerization. The photopolymerization was accelerated by the addition of 4‐methoxy‐1‐naphthol, N‐ethylcarbazole, 2,4‐dimethylthioxanthone, phenothiazine, and 2‐ethyl‐9,10‐dimethoxyanthracene as photosensitizers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3816–3827, 2003     

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

The 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