Aqueous photopolymerization with visible‐light photoinitiators: Acrylamide polymerization photoinitiated with a phenoxazine dye/amine system

We studied the photoinduced electron‐transfer polymerization of acrylamide with, as a visible‐light initiator, the heterocycle‐N‐oxide resazurin in the presence of triethanolamine. The irradiation of resazurin produces resorufin, which also absorbs in the visible region. Both phenoxazine dyes in the presence of the amine are efficient photoinitiators of acrylamide polymerization in an aqueous medium. The polymerization rates were measured at several amine concentrations. These values increase with the amine concentration, reaching a maximum value; further amine addition slightly decreases the polymerization rate. Time‐resolved photolysis studies of the dyes were carried out under the polymerization conditions. The quenching of the dye excited states by the amine was analyzed with static‐fluorescence and laser‐flash photolysis. These data were used for fitting curves of the polymerization rate versus the amine concentration, and it was concluded that the interaction of triplet excited dyes with the amine leads to acrylamide polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4074–4082, 2001     

Influence of different initiators on methyl methacrylate polymerization,studied by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to measure the decomposition rates of four commercially used initiators, 2,2′-azobis(isobutyronitrile) 2,2′-azobis(2,4-dimethylvaleronitrile), dilauroyl peroxide and bis(4-t-butylcyclohexyl)peroxydicarbonate, in dynamic mode, while the courses of methyl methacrylate polymerization with the listed initiators at 65, 75 and 85°C were measured isothermally. From the DSC curves, the polymerization enthalpies, the overall reaction rate constants and the activation energies for the initial steady-state polymerization were calculated. It was found that the polymerization enthalpy and the kinetic parameters depended on the type of the initiator. An initiator with a shorter decomposition half-lifetime shifted the onset of the gel effect to a higher conversion, intensified it and decreased the average molar mass of the polymer.     

High polymerization of methyl methacrylate with organonickel/MMAO systems

A series of nickel complexes, including Ni(acac)₂, (C₅H₅)Ni(η³‐allyl), and [NiMe₄Li₂(THF)₂]₂, that were activated with modified methylaluminoxane (MMAO) exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) but showed no catalytic activity for the polymerization of ethylene and 1‐olefins. The resulting polymers exhibited rather broad molecular weight distributions and low syndiotacticities. In contrast to these initiators, the metallocene complexes (C₅H₅)₂Ni, (C₅Me₅)₂Ni, (Ind)₂Ni, and (Me₃SiC₅H₄)₂Ni provided narrower molecular weight distributions at 60 °C when these initiator were activated with MMAO. Half‐metallocene complexes such as (C₅H₅)NiCl(PPh₃), (C₅Me₅)NiCl(PPh₃), and (Ind)NiCl(PPh₃) produced poly(methyl methacrylate) (PMMA) with much narrower molecular weight distributions when the polymerization was carried out at 0 °C. Ni[1,3‐(CF₃)₂‐acac]₂ generated PMMA with high syndiotacticity. The NiR(acac)(PPh₃) complexes (R = Me or Et) revealed high selectivity in the polymerization of isoprene that produced 1,2‐/3,4‐polymer at 0 °C exclusively, whereas the polymerization at 60 °C resulted in the formation of cis‐1,4‐rich polymers. The polymerization of ethylene with Ni(1,3‐tBu₂‐acac)₂ and Ni[1,3‐(CF₃)₂‐acac]₂ generated oligo‐ethylene with moderate catalytic activity, whereas the reaction of ethylene with Ni(acac)₂/MMAO produced high molecular weight polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4764–4775, 2000     

Polymerization photoinitiated by carbonyl compounds. XI. Styrene polymerization photoinitiated by cyclododecanones

To establish their potential as source of biradicals to initiate free-radical polymerization, 2-methyl- and 2,2,12-trimethylcyclododecanones were photolyzed in the presence of styrene. The initiation efficiency of both ketones is low—0.03. The molecular weight of the obtained polymer is ca. 25% higher than that obtained employing photoinitiators that produce monoradicals. This difference is explained in terms of a mixed polymerization mechanism comprising mono- and biradicals. © 1996 John Wiley & Sons, Inc.     

New azo-peroxidic initiators in the radical polymerization of styrene and methyl methacrylate

Two new azo-perester compounds, di-tert-butyl-6,6′-azobis-(6-cyanoperoxyheptanoate) (6,6-di-tBu) and di-tert-amyl-6,6′-azobis-(6-cyanoperoxyheptanoate) (6,6-di-tAm), synthesized on the basis of 6,6′-azobis-(6-cyanoheptanoic acid) (ACHpA), were investigated for their use in the radical polymerization of styrene (S) and methyl methacrylate (MMA). Their characteristics are given, including chemical (IR spectra), thermal (DSC) and kinetic, i.e., thermal decomposition studied by volumetric and gas chromatographic methods. The rate constants and activation energies of the decomposition of both the azo and perester bonds were determined. The new azo-peresters were utilized to initiate the radical solution polymerizations of S and MMA at 60 °C. The kinetic parameters of the processes, i.e., polymerization rate and overall rate constant, were determined. Subsequently, the polymerization products were characterized by IR and DSC. It was found that the perester groups were present in the obtained polymers, and hence, the polymers are “active” for further polymerization.     

Polyacrylamide cryogels by photoinitiated free radical polymerization

A novel method for the preparation of poly(acrylamide) cryogels by photoinitiated polymerization of monomeric precursors was described. A series of poly(acrylamide) cryogels were easily prepared by irradiating aqueous solutions containing acrylamide and N,N′‐methylene(bis)acrylamide as monomer and cross‐linker, respectively, in the presence of 1‐[4‐(2‐hydroxyethoxy)phenyl]‐2‐hydroxy‐2‐methyl‐1‐propane‐1‐one (Irgacure 2959) as water‐soluble photoinitiator with the help of freezing–thawing procedures. Photolysis was conducted at ?13 °C isothermally through specially designed cryostat‐integrated Rayonet merry‐go‐round photoreactor. On comparing the described photochemical method with the conventional redox counter part, the polymerization is initiated, and gelation proceeds only on external stimulation by light. This way, concomitant hydrogel formation usually observed with the redox process as a result of premature polymerization during the cooling process was prevented. The obtained cryogels exhibited superfast swelling behavior. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

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.     

Novel polymeric,thio‐containing photoinitiator comprising in‐chain benzophenone and an amine coinitiator for photopolymerization

A novel thio‐containing diamine with a benzophenone structure, 4‐amino‐4′‐[4‐aminothiophenyl]benzophenone (AATBP), was synthesized. Two kinds of polymeric photoinitiators, PUPIA and PUPI, were synthesized through the polycondensation of toluene‐2,4‐diisocyanate with AATBP and/or N‐methyldiethanolamine (MDEA). A macroamine, PUPA, was also synthesized for comparison. Fourier transform infrared, ¹H NMR, and gel permeation chromatography analyses confirmed the structures of all the polymers. The ultraviolet–visible spectra of PUPIA, PUPI, and AATBP were similar, and all exhibited the maximal absorption above 325 nm. The photopolymerization of two monomers with different functionalities, poly(propylene glycol)diacrylate and trimethylolpropane triacrylate initiated by PUPIA, PUPI/MDEA, PUPI/PUPA, AATBP/MDEA, and AATBP/PUPA, was studied through differential scanning photocalorimetry. The results showed that both PUPIA and PUPI/MDEA had high photoefficiency, and their low‐molecular‐weight counterparts could hardly initiate the photopolymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 576–587, 2007     

Reactivity of oxetane monomers in photoinitiated cationic polymerization

Studies of the onium salt photoinitiated cationic ring‐opening polymerizations of various 3,3‐disubstituted oxetane monomers have been conducted with real‐time infrared spectroscopy and optical pyrometry. The polymerizations of these monomers are typified by an extended induction period that has been attributed to the presence of a long‐lived tertiary oxonium ion intermediate formed by the reaction of the initially formed secondary oxonium ion with the cyclic ether monomer. Because the extended induction period in the photopolymerization of these monomers renders oxetane monomers of limited value for many applications, methods have been sought for its minimization or elimination. Three general methods have been found effective in markedly shortening the induction period: (1) carrying out the photopolymerizations at higher temperatures, (2) copolymerizing with more reactive epoxide monomers, and (3) using free‐radical photoinitiators as synergists. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3205–3220, 2005     

Novel dialkylaminoalkyl- and dialkylaminoalcoxy-benzophenones as polymerization photoinitiators. II. Photocalorimetric study on photoinitiated polymerization of butyl and lauryl acrylates

A photocalorimetric study of butyl and lauryl acrylates (BA and LA) polymerization photoinitiated by several benzophenone derivatives is reported. Molecular weight measurements and end-chain group analysis of the samples are also described and the results are compared with those obtained by dilatometric technique. The role played by the long methylene chain in lauryl acrylate is discussed     

Polymerization photoinitiated by carbonyl compounds. X. Methyl methacrylate polymerization photoinitiated by fluorenone in the presence of triethylamine

The photoinitiation efficiency of the fluorenone/triethylamine (TEA) system in the polymerization of methylmethacrylate (MMA) has been evaluated as a function of the monomer concentration, the amine concentration, and the polarity of the reaction medium. The polymerization proceeds readily in low polarity media (benzene/monomer), but it is negligible in more polar solvents (acetonitrile/monomer). The polymerization rate increases with the amine concentration up to 0.01 M TEA. Further increase in amine concentration produces a decrease in the polymerization rate. A similar behavior was observed for the fluorenone photoreduction yield and the yield of fluorenone derived radicals. All these processes are considered to involve the excited triplet, while quenching of the excited singlet by the amine decreases the rate of these processes. However, the decrease in photoinitiation efficiency observed at high amine concentration is larger than that expected from the singlet quenching extent, as estimated from the effect of the amine on the fluorescence yield under the same experimental conditions. This discrepancy indicates that other process(es) must contribute to the protection afforded by high amine concentrations. Quenching of the charge transfer intermediate by the amine is postulated as a competitive process that could explain the above mentioned effects. © 1994 John Wiley & Sons, Inc.     

Modification of photoinitiated cationic epoxide polymerizations by sulfides

The addition of sulfides has a marked effect on the rates of onium salt induced photoinitiated cationic ring‐opening polymerizations of epoxide monomers. Various behaviors have been observed that depend on the structure of the sulfide. Dialkyl sulfides strongly inhibit the photopolymerizations of these monomers, whereas diaryl sulfides have a retarding effect on the photopolymerizations. Real‐time infrared spectroscopy and optical pyrometry have been employed as analytical methods to probe the kinetic effects of the addition of a variety of sulfides on cationic epoxide ring‐opening photopolymerizations. A mechanism is proposed that involves the formation of sulfonium salts as intermediates. The observations made in this study have important implications for cationic photopolymerizations in general and for photoinitiated cationic ring‐opening polymerizations of epoxides in particular. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2504–2519, 2005     

Studies on the polymerization of methyl methacrylate activated by azobisisobutyramidine

Kinetic studies on methyl methacrylate polymerization were carried out with watersoluble 2,2′-azobisisobutyramidine (ABA). The rate of polymerization was proportional to the square root of the initiator concentration in the solvents chloroform, methanol, and dimethyl sulfoxide (DMSO), which confirms the bimolecular nature of the termination reaction. The monomer exponent was unity in chloroform but in methanol and DMSO the rate of polymerization passed through a maximum when plotted against the monmer concentration. This behavior in methanol has been attributed to be due to the enhanced rate of production of radical with increasing proportion of methanol. The rate of decomposition of the ABA has been observed to be faster in methanol than in chloroform. The situation becomes more complicated with DMSO, which was found to reduce the value of δ = (2k_t)^1/2/k_p in methyl methacrylate polymerization. The rate of polymerization was observed to be highly dependent on the nature of the solvent, the rate increasing with increased electrophilicity of the solvent. The dependence of R_p on the solvent has been explained in the light of the stabilization of the transition state due to increased solvation of the basic amidine group of the initiator with the increased electrophilicity of the solvent.     

Mechanism of cyclic dye regeneration during eosin‐sensitized photoinitiation in the presence of polymerization inhibitors

A visible light photoinitiator, eosin, in combination with a tertiary amine coinitiator is found to initiate polymerization despite the presence of at least 1000‐fold excess dissolved oxygen, which functions as an inhibitor of radical polymerizations. Additionally, 0.4 μM eosin is able to overcome 100‐fold excess (40 μM) 2,2,6,6‐Tetramethyl‐1‐piperidinyloxy (TEMPO) inhibitor, initiating polymerization after only a 2 min inhibition period. In contrast, 40 μM Irgacure‐2959, a standard cleavage‐type initiator, is unable to overcome even an equivalent amount of inhibitor (40 μM TEMPO). Through additional comparisons of these two initiation systems, a reaction mechanism is developed which is consistent with the kinetic data and provides an explanation for eosin's relative insensitivity to oxygen, TEMPO, and other inhibitors. A cyclic mechanism is proposed in which semireduced eosin radicals react by disproportionation with radical inhibitors and radical intermediates in the inhibition process to regenerate eosin and effectively consume inhibitor. In behavior similar to that of eosin, rose bengal, fluorescein, and riboflavin are also found to initiate polymerization despite the presence of excess TEMPO, indicating that cyclic regeneration likely enhances the photoinitiation kinetics of many dye photosensitizers. Selection of such dye initiation systems constitutes a valuable strategy for alleviating inhibitory effects in radical polymerizations. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6083–6094, 2009     

Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile-1-methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ? pyrene < phenanthrene, fluorene ≈ 2-methoxynaphthalene ≈ biphenyl < anthracene < 2-methylnaphthalene < 1-methoxynaphthalene < 2,3,6-trimethylnaphthalene < 2,3-dimethylnaphthalene ≈ naphthalene < 1-methylnaphthalene < 2,6-dimethylnaphthalene < p-terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k_q for the formation of exciplex as well as the self-quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k_q were calculated from the Stern-Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k_q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2-methylnaphthalene ≈ 1-methylnaphthalene ≈ 2,3-dimethylnaphthalene < 2,6-dimethylnaphthalene < acenaphthene < p-terphenyl < 1-methoxynaphthalene. The study of the concentration effect reflecting the self-quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self-quenching efficiency of aromatic hydrocarbons: 2-methoxynaphthalene ≈ pyrene > anthracene > 1-methoxynaphthalene > fluorene > 2,6-dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6-trimethylnaphthalene > 2,3-dimethylnaphthalene > 1-methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above-mentioned sequences.     

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