The role of diphenyl iodonium salt (DPI) in three‐component photoinitiator systems containing methylene blue (MB) and an electron donor

Three‐component initiators generally include a light‐absorbing photosensitizer, an electron donor that is often an amine, and the third component, which is usually an iodonium salt. To characterize the role of diphenyl iodonium chloride (DPI) in three‐component photoinitiator systems containing methylene blue (MB) as the photosensitizer, a systematic series of electron donors was used. The Rhem–Weller equation was used to verify the thermodynamic feasibility for photo‐induced electron transfer from the electron donors to the MB. Comparison of the photopolymerization rates of each two‐component initiator system (containing the photosensitizer and amine) to those of the corresponding three‐component system (with the addition of (DPI) allowed fundamental information regarding the role of the DPI to be obtained. It was concluded that the DPI enhances the photopolymerization kinetics in two ways: (1) it consumes an inactive MB neutral radical and produces an active phenyl radical, thereby regenerating the original methylene blue, and (2) it reduces the recombination reaction of the MB neutral radical and amine radical/cation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5863–5871, 2004     

Kinetic study of H‐atom transfer in imidazoline‐, imidazolidine‐, and pyrrolidine‐based alkoxyamines: Consequences for nitroxide‐mediated polymerization

The H‐atom transfer reaction was studied for a series of imidazoline, imidazolidine, and pyrrolidine‐based alkoxyamines containing either isobutyrate‐2‐yl or 1‐phenylethyl alkyl fragments. The C O bond homolysis rate constants and activation energies were determined by ¹H NMR product analysis as a function of temperature. Inter‐ and intramolecular H‐atom transfer reactions were distinguished by examination of alkoxyamine thermolysis products in the absence and in the presence of a radical scavenger (thiophenol or deuterated styrene). A correlation between the structure of the nitroxyl fragment of alkoxyamine and the H‐transfer reaction was found. The high steric demands of the substituents on the nitroxyl part of the isobutyrate‐2‐yl alkoxyamine decrease both types of reaction. For alkoxyamines containing the 1‐phenylethoxyamines, neither inter‐ nor intramolecular H‐atom transfer was observed. Controlled polymerization of methylmethacrylate initiated with imidazoline‐based alkoxyamine was observed, although the polymer obtained was not living. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6579–6595, 2009     

Nitroxide‐mediated living free‐radical polymerization of styrene: A systematic study of the variation of the alkoxyamine concentration

The effect of the variation of the alkoxyamine concentration on the conversion and polydispersity of the nitroxide‐mediated living free‐radical polymerization of styrene is discussed. Four different alkoxyamines ( 1 – 4 ) have been used for these studies. For an alkoxyamine with a small equilibrium rate constant (K), such as styryl–TEMPO 2 , the conversion is governed by the autopolymerization of styrene. For efficient alkoxyamines 1 , 3 , and 4 , the conversion at high alkoxyamine concentrations is higher than the conversion obtained by autopolymerization. At high alkoxyamine concentrations, the conversions vary to a small extent for all the alkoxyamines studied. As long as the conversion remains high, the polydispersity index is small. In addition, simulations of polymerizations with a program for modeling nonlinear dynamics are discussed. Polymerizations with efficient alkoxyamines at high alkoxyamine concentrations are well described by the kinetic scheme applied. K for alkoxyamines 1 and 4 has been estimated with the simulations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3342–3351, 2004     

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     

Synthesis and properties of a new photostable polymer: Perylene-3,4,9,10-tetracarboxylic acid-bis-(N,N′-dodecylpolyimide)

The synthesis of perylene-3,4,9,10-tetracarboxylic acid-bis(N,N′-dodecyl-polyimide) (poly PTDI) and polyamic acid has been achieved from perylene dianhydride and dodecylamine. It was found that this new polyimide has very high thermal photostability. Poly PTDI decomposes at 475°C and shows weak fluorescence compared with the monomer PTDI possibly due to aggregration in the polymer. Its solubility in some of the common organic solvents, acetonitrile, dimethyl formamide, etc., makes spectroscopic and photochemical studies and applications possible in the liquid phase. The emission characteristics are similar to mono PTDI. Poly PTDI, a luminescent electron acceptor polymer, is a new reliable probe for a photosensitizer at energy transfer and election transfer photochemical reactions. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2137–2142, 1997     

Synthesis and characterization of novel photoactive polymer poly(vinyl alcohol)‐graft‐poly(vinyl naphthalene)

A copolymer of poly(vinyl naphthalene) grafted onto poly(vinyl alcohol) has been synthesized with nitroxide‐mediated controlled radical polymerization. By separating the processes of the generation of grafting sites and polymerization, we can avoid the formation of the homopolymer. Because of its architecture, the polymer is soluble in water, despite the high content of hydrophobic groups. The naphthalene chromophores tend to aggregate, forming hydrophobic microdomains in an aqueous solution. Those aggregates exist in a very constrained environment that leads to extraordinarily large redshifts of both the absorption and emission of the polymer. The polymer acts as an efficient photosensitizer in photoinduced electron transfer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2675–2683, 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     

New 7‐membered diazepanone alkoxyamines for nitroxide‐mediated radical polymerization

The synthesis of new 7‐membered diazepanone alkoxyamines [2,2,7,7‐tetramethyl‐1‐(1‐phenyl‐ethoxy)‐[1,4]diazepan‐5‐one ( 3 ) and 2,7‐diethyl‐2,3,7‐trimethyl‐1‐(1‐phenyl‐ethoxy)‐[1,4]diazepan‐5‐one ( 8 )] through the Beckmann rearrangement of piperidin‐4‐one alkoxyamines was developed. Both 3 and 8 were evaluated as initiators and regulators for the nitroxide‐mediated radical polymerization of styrene and n‐butyl acrylate. 8 , a sterically highly hindered alkoxyamine readily available as a crystalline solid, allowed the fast and controlled polymerization and preparation of polymers with low polydispersity indices (1.2–1.4) up to a degree of polymerization of about 100. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3332–3341, 2004     

Cationic photopolymerization of tetrahydrofuran: A mechanistic study on the use of a sulfonium salt–phenothiazine initiation system

The kinetics aspects of the photoinitiated polymerization of tetrahydrofuran with triphenylsulfonium hexafluoroarsenate and sensitized by phenothiazine was studied under visible‐light irradiation. A photosensitizer mechanism was proposed involving electron transfer from phenothiazine to the triphenylsulfonium salt, forming a cation radical, the true precursor of the polymerization. An initial complex in the ground state between the phenothiazine and the triphenylsulfonium was formed prior to the excitation, and this complex actually activated the photopolymerization rate. The polymerization conversion presented an S‐shaped curve, suggesting an induction period caused by the existence of two pathways responsible for the beginning of the polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 46–55, 2001     

Alternative initiators for bimolecular photoinitiating systems

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the unwanted effect of back electron transfer. Therefore, a PI system consisting of benzaldoxime esters and 4‐methyl benzophenone was investigated by photo‐differential scanning calorimetry experiments. Surprisingly, reactivities are comparable to that of classical monomolecular Type I PIs like Darocur 1173, probably because of an energy transfer from the triplet state of the benzophenone‐based sensitizer to the oxime ester initiator. Furthermore, sufficient thermal stability was proven by TGA and storage stability experiments of a resin formulation under elevated temperature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

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     

Synthesis of a series of SG1 2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxyl] based alkoxyamines,SG1‐CH(Me)CO2R,and measurement of the homolysis rate constants of the C?ON bond

During nitroxide‐mediated polymerization, the polymerization time decreases with an increasing rate constant of the cleavage of the NO? C bond of dormant alkoxyamines. Thus, knowledge of the factors influencing this cleavage is of considerable interest. We have prepared a series of SG1 2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxyl] based alkoxyamines [SG1‐CH(Me)CO₂R] with various R groups (alkyl or aryl) and measured the homolysis rate constants (k_d). k_d decreases with the bulkiness and increases with the polarity of the R group. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3504–3515, 2004     

Copper(0)‐mediated living radical polymerization of acrylonitrile: SET‐LRP or AGET‐ATRP

Cu(0)‐mediated living radical polymerization was first extended to acrylonitrile (AN) to synthesize polyacrylonitrile with a high molecular weight and a low polydispersity index. This was achieved by using Cu(0)/hexamethylated tris(2‐aminoethyl)amine (Me₆‐TREN) as the catalyst, 2‐bromopropionitrile as the initiator, and dimethyl sulfoxide (DMSO) as the solvent. The reaction was performed under mild reaction conditions at ambient temperature and thus biradical termination reaction was low. The rapid and extensive disproportionation of Cu(I)Br/Me₆‐TREN in DMSO/AN supports a mechanism consistent with a single electron transfer‐living radical polymerization (SET‐LRP) rather than activators generated by electron transfer atom transfer radical polymerization (AGET ATRP). ¹H NMR analysis and chain extension experiment confirm the high chain‐end functionality of the resultant polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Decomposition of model alkoxyamines in simple and polymerizing systems. I. 2,2,6,6‐tetramethylpiperidinyl‐ N‐oxyl‐based compounds

The thermal decomposition of five alkoxyamines labeled TEMPO–R, where TEMPO was 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl and R was cumyl (Cum), 2‐tert‐butoxy‐carbonyl‐2‐propyl (PEst), phenylethyl (PhEt), 1‐tert‐butoxy‐carbonylethyl (EEst), or 1‐methoxycarbonyl‐3‐methyl‐3‐phenylbutyl (Acrylate‐Cum), was studied with ¹H NMR in the absence and presence of styrene and methyl methacrylate. The major products were alkenes and the hydroxylamine 1‐hydroxy‐2,2,6,6‐tetramethyl‐ piperidine (TEMPOH), and in monomer‐containing solutions, unimeric and polymeric alkoxyamines and alkenes were also found. Furthermore, the reactions between TEMPO and the radicals EEst and PEst were studied with chemically induced dynamic nuclear polarization. In comparison with coupling, TEMPO reacted with the radicals Cum, PEst, PhEt, and EEst and their unimeric styrene adducts by disproportionation to alkenes and TEMPOH only to a minor extent (0.6–3%) but with the radical adducts to methyl methacrylate to a considerable degree (≥20%). Parallel to the radical cleavage, TEMPO–EEst (but not the other alkoxyamines or TEMPO–Acrylate‐Cum) underwent substantial nonradical decay. The consequences for TEMPO‐mediated living radical polymerizations are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3604–3621, 2001     

Factors influencing the C?ON bond strength of the alkoxyamines in the styrene–acrylonitrile–TEMPO copolymerization system

Homolytic bond dissociation energy (BDE) of the (C? ON) bond for several N‐alkoxyamines derived from 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) and the corresponding (C? H) bonds were determined from quantum‐mechanical calculations including the B3‐LYP6‐31G(d), B3‐LYP/6‐311++G(2df,p), UB3‐LYP/6‐311+G(3df,2p), and integrated IMOMO (G3:ROMP2/6‐31G(d)) method. The investigated N‐alkoxyamines were considered as models for dormant forms of propagating chains in the radical copolymerization process of styrene with acrylonitrile in the presence of TEMPO according to the terminal and penultimate model. The substituent effect on BDE was investigated. Radical stabilization energies (RSE) for radicals created from homolysis of the investigated N‐alkoxyamines were calculated according to Rüchardt's method. Polar, steric, and stabilization effects on C? ON alkoxyamine bond homolysis were studied. A dramatically weakened C? ON bond in the alkoxyamine‐containing two consecutive styrene units in the propagating chain was ascribed to geometric parameters characterizing energetically unfavorable conformation of the substituents. These phenomena can be regarded as the penultimate effect in the radical living/controlled copolymerization system of styrene with acrylonitrile. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1165–1177, 2008     

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     

Controlled/living polymerization of methyl methacrylate using new sterically hindered imidazoline nitroxides prepared via intramolecular 1,3‐dipolar cycloaddition reaction

A series of imidazoline nitroxides with bulky spirocyclic moieties at the positions 2 or 5 of imidazole ring were synthesized using intramolecular 1,3‐dipolar cycloaddition in 2H‐imidazole 1‐oxides or 4H‐imidazole 3‐oxides with pent‐4‐enyl groups followed by isoxazolidine ring opening and oxidation. Capability of the nitroxides to control radical polymerization of methyl methacrylate (MMA) and styrene was investigated. For that purpose, alkoxyamines were synthesized from the aforementioned nitroxides and tert‐butyl α‐bromoisobutyrate. Homolysis rate constants of the alkoxyamines were measured and possible contributions of side reactions were quantified. Nitroxide‐mediated polymerization of styrene and MMA was studied using the alkoxyamines as initiators. MMA polymerization was found to proceed in controlled regime up to 55% of monomer conversion and the polymer obtained was able to reinitiate the polymerization of styrene. Quota of “living” chains estimated to reach 90%. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 929–943     

The synthesis of water‐soluble PHEMA via ARGET ATRP in protic media

2‐Hydroxyethyl methacrylate has been polymerized in methanol using activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP), to produce water‐soluble poly(2‐hydroxyethyl methacrylate) (PHEMA). The various parameters that determine control of the living polymerization have been explored. Using the Cu(II)/TPMA catalyst system (TPMA = tris(2‐pyridylmethyl)amine), controlled polymerization was achieved with Cu concentrations as low as 50 ppm relative to HEMA, with a [TPMA]/[Cu(II)] ratio of 5. Use of hydrazine as the reducing agent generally gave better control of polymerization than use of ascorbic acid. The polymerization conditions were tolerant of small amounts of air, and colorless polymers were easily isolated by simple precipitation and washing steps. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4084–4092, 2010     

Air‐tolerantly surface‐initiated AGET ATRP mediated by iron catalyst from silica nanoparticles

Well‐defined polymer‐nanoparticle hybrids were prepared by a newly reported method: atom transfer radical polymerization using activators generated by electron transfer (AGET ATRP) mediated by iron catalyst. The kinetics of the surface‐initiated AGET ATRP of methyl methacrylate from the silica nanoparticles, which was mediated by FeCl₃/triphenylphosphine as a catalyst complex, ascorbic acid as a reducing agent, N,N‐dimethylformamide as the solvent in the presence of a “sacrificial” (free) initiator, was studied. Both the free and grafted polymers were grown in a control manner. The chemical composition of the nanocomposites was characterized by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and ¹H nuclear magnetic resonance spectroscopy. Thermogravimetric analysis was used to estimate the content of the grafted organic compound, and transmission electron micrographs was used to observe the core‐shell structure of the hybrid nanoparticles. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2006–2015, 2010     

Synthesis and characterization of donor–bridge–acceptor alternating copolymers containing perylene diimide units and their application to photovoltaic cells

We have used Suzuki coupling to prepare a series of alternating copolymers featuring coplanar cyclopentadithiophene and hole‐transporting carbazole units. We observed quenching in the photoluminescence spectra of our polymers after incorporating pendent electron‐deficient perylene diimide ( PDI ) moieties on the side chains, indicating more efficient photoinduced electron transfer. Electrochemical measurements revealed that the PDI ‐containing copolymers displayed reasonable and sufficient offsets of the energy levels of their lowest unoccupied molecular orbitals for efficient charge dissociation. The performance of bulk heterojunction photovoltaic cells incorporating the copolymer/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester blends (1:4, w/w) was optimized when the active layer had a thickness of 70 nm. The photocurrents of the devices were enhanced as a result of the presence of the PDI moieties, thereby leading to improved power conversion efficiencies. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1298–1309, 2010