N‐methylpicolinium derivatives as the coinitiators in photoinitiating systems for vinyl monomers polymerization

Five N‐methylpicolinium derivatives were investigated to test their abilities to function as second coinitiators in free radical photopolymerization initiated by N,N′‐diethylcarbocyanine—n‐butyltriphenylborate photoredox pair ( P19B2 ). As it is shown by the kinetic data, an addition of picolinium derivatives into P19B2 photoinitiating system visibly increases the efficiency of photoinitiation. The results suggest that the rates of photoinitiation depend on the rate of the picolyl radicals formation. The redox potentials of tested N‐methylpicolinium derivatives were measured and the calculation of free energy change for the possible electron transfer reactions between all components of the system (both stable and transient individuals) was performed. The results suggest that cyanine dyes are able to start a specific chain of an electron transfer reactions involving different coinitiators (borate salt and N‐alkylpicolinium derivatives), giving as a result one photon—two‐radicals photochemical response. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 576–588, 2009     

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     

Synthesis and characterization of thiophene‐substituted N‐phenyl maleimide polymers by photoinduced radical polymerization

Two types of novel functionalized N‐[4‐(4′‐hydroxyphenyloxycarbonyl)phenyl]maleimide and N‐(4‐{[2‐(3‐thienyl)acetyl]oxyphenyl}oxycarbonylphenyl)maleimide (MIThi) were synthesized starting from 4‐maleimido benzoic acid. Photoinduced radical homopolymerization of MIThi and its copolymerization with styrene were performed at room temperature to give linear polymers containing pendant thienyl moieties using ω,ω‐dimethoxy‐ω‐phenylacetophenone as an initiator. Copolymers' compositions and the equilibrium constant (K) for electron donor–acceptor complex formation suggest an alternating nature of the copolymerization. The monomer reactivity ratios and Alfrey–Price Q,e values were also determined. The thermal behavior of the new synthesized monomers and polymers was investigated by differential scanning calorimetry and thermogravimetric analysis. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 995–1004, 2002     

New Squaraine‐based two‐component initiation systems for UV‐blue light induced radical polymerization: Kinetic and time‐resolved laser spectroscopy studies

In this article, the ability of two‐component photoinitiator systems for efficient polymerization of 2‐ethyl‐2‐(hydroxymethyl)?1,3‐propanediol triacrylate was presented. The photophysics and photochemistry of squaraine dyes in the presence of an electron donor as well as an electron acceptor was investigated, and it was found that the photosensitizer in an excited state might act as an electron acceptor or an electron donor. The excited states of squaraines may be quenched by tetramethylammonium n‐butyltriphenylborate ( B2 ), diphenyliodonium chloride ( I1 ), and N‐methoxy‐4‐phenylpyridinium tetrafluoroborate ( NO ). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 471–484     

The influence of N‐vinyl pyrrolidone on polymerization kinetics and thermo‐mechanical properties of crosslinked acrylate polymers

Polymerization of multifunctional acrylate monomers generates crosslinked polymers that are noted for their mechanical strength, thermal stability, and chemical resistance. A common reactive diluent to photopolymerizable formulations is N‐vinyl pyrrolidone (NVP), which is known to reduce the inhibition of free radical photopolymerization by atmospheric oxygen. In this work, the copolymerization behavior of NVP was examined in acrylate monomers with two to five functional groups. At concentrations as low as 2 wt %, NVP increases the polymerization rate in copolymerization with multifunctional acrylate monomer. The relative rate enhancement associated with adding NVP increases dramatically as the number of acrylate double bonds changes from two to five. The influence of NVP on polymerization kinetics is related to synergistic cross‐propagation between NVP and acrylate monomer, which becomes increasingly favorable with diffusion limitations. This synergy extends bimolecular termination into higher double bond conversion through reaction diffusion controlled termination. Copolymerizing concentrations of 5–30 DB% NVP with diacrylate or pentaacrylate monomer also increases Young's modulus and the glass transition temperature (T_g) in comparison to neat acrylate polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4062–4073, 2007     

Photochemical behavior of p-dimethylaminobenzoylated polystyrene as polymerization photoinitiator: Synthesis of graft copolymers

The photochemical behavior of p-dimethylaminobenzoylated polystyrene (PS-MI) in benzene solution has been investigated, both in the presence and absence of methyl methacrylate (MMA). This behavior has been compared with that of the model compound, 4-isopropyl-4′-N,N-dimethylaminobenzophenone (CU-MI). PS-MI photoreduction takes place only through excimer formation due to the high local chromophore concentration, and therefore, PS-MI disappearance quantum yield is close to the previously calculated limiting value (0.02) and independent of chromophore concentration. Several parameters that characterize the polymerization process have been determined; it has been found that the obtained PMMA is photografted onto PS-MI backbone. This is in agreement with the proposed mechanism for radical generation. No homo-PMMA formation has been detected. © 1993 John Wiley & Sons, Inc.     

Iodonium‐polyoxometalate and thianthrenium‐polyoxometalate as new one‐component UV photoinitiators for radical and cationic polymerization

Four novel onium salts (onium‐polyoxometalate) have been synthesized and characterized. They contain a diphenyliodonium or a thianthrenium (TH) moiety and a polyoxomolybdate or a polyoxotungstate as new counter anions. Outstandingly, these counter anions are photochemically active and can sensitize the decomposition of the iodonium or TH moiety through an intramolecular electron transfer. The phenyl radicals generated upon UV light irradiation (Xe–Hg lamp) are very efficient to initiate the radical polymerization of acrylates. Cations are also generated for the cationic polymerization of epoxides. Remarkably, these novel iodonium and TH salts are characterized by a higher reactivity compared with that of the diphenyliodonium hexafluorophosphate and the commercial TH salt, respectively. Interpenetrating polymer networks can also be obtained under air through a concomitant cationic/radical photopolymerization of an epoxy/acrylate blend (monomer conversions > 65%). The photochemical mechanisms are studied by steady‐state photolysis, cyclic voltammetry, and electron spin resonance techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 981–989     

Synthesis and vesicular polymerization of novel counter‐ion polymerizable/crosslinkable surfactants

Two novel two‐tail surfactants, dicetyldimethylammonium 4‐vinyl benzoate (DDVB) and dicetyldimethylammonium 3,5‐divinyl benzoate (DDDB), were synthesized by neutralizing the corresponding quaternary ammonium hydroxide with the appropriate benzoic acid. As expected, these surfactants formed both homo and mixed‐vesicles, which were readily polymerized with a suitable radical photo‐initiator. The polymerization process was followed by UV–vis spectroscopy and also reconfirmed by NMR and IR spectroscopy. Polymerization of vesicles prepared from DDVB, unlike the more commonly polymerized vesicles, in which the polymerizable group forms an integral part of the surfactant, leads to the formation of a linear polyelectrolyte chain that is only electrostatically bound to the lipid bilayer. On the other hand, polymerization of DDDB vesicles leads to the formation of a crosslinked shell (or net) that encases the vesicle bilayer. Such counterion crosslinked vesicles were shown to be resistant to destabilization both by lysis as well as in the presence of a fairly high volume fraction of an organic solvent, such as ethanol. However, although the simple polymerized (linearly) vesicles, formed from DDVB, exhibit enhanced stability toward lysis when compared to their unpolymerized counterparts, they are readily destabilized in the presence of ethanol, leading to precipitation. This sharp contrast in the behavior of linearly polymerized and crosslinked systems suggests that crosslinking is essential to arrest conformational reorganization of the polyelectrolyte chains induced by a change in the solvent medium, which in turn leads to precipitation. Such counterion crosslinked vesicular systems also have an added advantage; they may retain the fluidityof the lipid bilayer while at the same time possess enhanced stability. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5271–5283, 2004     

Kinetic study of azidopentaammine cobalt(III) complex as photoinitiator and termination agent of N‐vinylpyrrolidone radical polymerization

N‐Vinylpyrrolidone polymerization photoinitiated at 365 and 546 nm by azidopentaammine cobalt(III) {[Co(NH₃)₅N₃]²⁺} was investigated at room temperature in an argon atmosphere. By excitation into the ligand to metal charge transfer (LMCT), the cobalt complex showed an efficient photoredox process leading to the formation of a cobalt(II) and an azide radical (N, Φ_photoredox = 0.24). The same process was found to occur by excitation into the ligand field band with a low but not negligible quantum yield (Φ_photoredox = 0.016). Two different domains were clearly present when the plot of the rate of polymerization as a function of the cobalt(III) complex was studied; for [Co(III)] < 2.0 × 10⁻⁴ M, the termination step mainly involved a mutual annihilation of growing radicals whereas an oxidative termination was present in the range of 2.0 × 10⁻⁴ M < [Co(III)] < 1.0 × 10⁻³ M. Within the former domain the rate of polymerization (R_p ) varied with the first power of the monomer concentration and with the square root of the absorbed light intensity while for the latter domain the R_p was proportional to the monomer concentration and absorbed light intensity. Further investigations using the viscosity‐average molecular weight data allowed us to corroborate the proposed polymerization mechanism. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3997–4005, 2000     

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     

Novel dialkylaminoalkyl- and dialkylaminoalcoxi-benzophenones as photoinitiators of polymerization. I. Photochemical characteristics and radical efficiencies

The behavior of the nonconjugated aminated benzophenones—4-[2′-N,N-(diethylamino)ethoxy]benzophenone (E4), 2-[2′-N,N-(diethylamino)ethoxy]-4-methoxybenzophenone (E2), and 4-N,N-dimethylaminomethylbenzophenone (DM)—as photoinitiators of MMA polymerization has been studied and the results compared with those obtained with the conjugated aminobenzophenone 4-N,N-dimethylamino-4′-isopropyl-benzophenone (CU—MI). Photoreduction behavior of these compounds in various solvents in the presence and absence of MMA has been also examined. The order of the polymerization reaction with respect to monomer and initiator concentrations has been investigated; values of initiation quantum yield (Φ_i), K_p/K^1/2_t and efficiencies of the different radicals have also been determined. Similar polymerization rates (R_p) of methyl methacrylate (MMA) were found when E4 and CU-MI were used as photoinitiators under the same range of absorbed irradiation intensity. This fact results from a compensation between the higher rate of E4 radical production (n-π* transition type) and the greater reactivity of the radicals generated from CU-MI.     

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     

N‐Bromosuccinimide as a thermal iniferter for radical polymerization

N‐Bromosuccinimide (NBS) was used as a thermal iniferter for the initiation of the bulk polymerizations of methyl methacrylate, methyl acrylate, and styrene. The polymerizations showed the characteristics of a living polymerization: both the yields and the molecular weights of the resultant polymers increased linearly as the reaction time increased. The molecular weight distributions of the polymers were 1.42–1.95 under the studied conditions. The resultant polymers could be used as macroiniferters to reinitiate the polymerization of the second monomer. The copolymers poly(methyl methacrylate)‐b‐polystyrene and polystyrene‐b‐poly(methyl methacrylate) were obtained and characterized. End‐group analysis of the resultant poly(methyl methacrylate), poly(methyl acrylate), and polystyrene confirmed that NBS behaved as a thermal iniferter. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2567–2573, 2005     

Chain transfer to solvent in the radical polymerization of N‐isopropylacrylamide

Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide‐mediated radical polymerization (NMP) of N‐isopropylacrylamide (NIPAM) in N,N‐dimethylformamide (DMF) at 120 °C. The extent of chain transfer to DMF can significantly impact the maximum attainable molecular weight in both systems. Based on a theoretical treatment, it has been shown that the same value of chain transfer to solvent constant, C_tr,S, in DMF at 120 °C (within experimental error) can account for experimental molecular weight data for both conventional radical polymerization and NMP under conditions where chain transfer to solvent is a significant end‐forming event. In NMP (and other controlled/living radical polymerization systems), chain transfer to solvent is manifested as the number‐average molecular weight (M_n) going through a maximum value with increasing monomer conversion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Phenylglycine derivatives as coinitiators for the radical photopolymerization of acidic aqueous formulations

Aromatic amines are known to give very poor performance as coinitiators for camphorquinone in the photopolymerization of acidic aqueous formulations. Differential scanning photocalorimetry investigations using N‐phenylglycine (NPG) as an alternative coinitiator proved the suitability of this derivative. Furthermore, it was demonstrated that the generally poor photoreactivity had to be assigned to the presence of water and not to the acidity of the formulation. The poor storage stability of NPG‐containing formulations was significantly improved by derivatives containing electron‐withdrawing substituents in the para position of the aromatic moiety, and the photoreactivity was kept at a very high level. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 115–125, 2006     

Iodonium sulfonates as high‐performance coinitiators and additives for CQ‐based systems: Toward aromatic amine‐free photoinitiating systems

Iodonium sulfonates are proposed here as a new class of high‐performance coinitiators for camphorquinone (CQ)‐based systems for the polymerization of methacrylates under blue light irradiation. When combined with CQ, the new proposed coinitiators present excellent polymerization performances and are excellent candidates for the replacement of tertiary aromatic amines subjected to toxicological concerns in the well‐established CQ/amine photoinitiating system (PIS). Remarkably, good bleaching properties are obtained after polymerization. The use of the new PIS for dental adhesives is also investigated. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1664–1669     

Copper and iron complexes as visible‐light‐sensitive photoinitiators of polymerization

The utilization of visible lights for the fabrication of polymeric materials is recognized as a promising and environmentally friendly approach. This process relies on the photochemical generation of reactive species (e.g., radicals, radical cations, or cations) from well‐designed photoinitiators (PIs) or photoinitiating systems (PISs) to initiate the polymerization reactions of different monomers (acrylates, methacrylates, epoxides, and vinyl ethers). In spite of the fact that metal complexes such as ruthenium‐ or iridium‐based complexes have found applications in organic and polymer synthesis, the search of other low‐cost metal‐based complexes as PISs is emerging and attracting increasing attentions. Particularly, the concept of the photoredox catalysis has appeared recently as a unique tool for polymer synthesis upon soft conditions (use of light emitting diodes and household lamp). This highlight focuses on recently designed copper and iron complexes as PI catalysts in the application of photoinduced polymerizations (radical, cationic, interpenetrated polymer networks, and thiol‐ene) or controlled radical polymerization under visible light irradiation. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2673–2684     

Styrylpyridinium borate salts as dye photoinitiators of free‐radical polymerization

Styrylpyridinium borate salts photoinitiate free‐radical polymerization. The rate of photopolymerization depends on the ΔG^o of electron transfer between a borate anion and a styrypyridinium cation. This latter value was estimated for a series of styrylpyridinium borate salts, and the relationship between the rate of polymerization and the free energy of activation gives the dependence predicted by the classical theory of electron transfer. This relation was independently observed for the two series of styrylpyridinium borate salts tested—one for the photoredox pair with an iodine atom and the second without. Styrylpyridinium borate salts were stable at ambient temperature in the formulations prepared for the photopolymerization experiments. Photopolymerization initiated by the photoredox pairs tested proceeded by the conventional mechanism in which bimolecular termination occurs by a reaction between two macroradicals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1433–1440, 2002     

Light‐induced controlled free radical polymerization of methacrylates using iron‐based photocatalyst in visible light

A novel visible light mediated catalytic system based on low cost iron complex, that is, Fe(bpy)₃(PF)₆ photocatalyst that initiates and control the free radical polymerization of methacrylates using ethyl α‐bromoisobutyrate (EBriB) as an initiator and 20 watt LED as light source is developed. The polymerization is initiated with turning the light on and immediately terminated by turning the light off. In addition, the molecular weight of polymer can be varied by changing the ratio of monomer and initiator. The merits of the present methodology lie in the use of low cost less precious, highly abundant iron‐based photocatalyst, avoidance of sacrificial donor and need of lower catalyst amount under visible light. The optimum amount of catalyst and initiator were established and successful polymerization of various methacrylates was achieved under the optimized polymerization conditions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2739–2746