Applicability of hemicyanine phenyltrialkylborate salts as free‐radical photoinitiators in the visible‐light polymerization of acrylate

The photoinitiation ability of photoredox pairs composed of a hemicyanine dye cation and different borate anions for the radical polymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate was investigated. In such a system, the excited dye chromophore is reduced by different tetraorganylborate anions. Upon irradiation at 488 nm, reductive carbon–boron bond cleavage occurs, producing reactive radicals, which start the chain reaction. The efficiency of bond‐breaking processes was found to be dependent on the nature of both the acceptors and the donors. The experimental results show that the photoinitiating ability of the tested photoredox pairs were controlled by both the driving force of the electron‐transfer process between the electron donor and the electron acceptor and the reactivity of the free radical that resulted from the secondary reactions occurring after the photoinduced electron‐transfer process. Using the nanosecond flash photolysis method, we studied the spectral and kinetic characteristics of the triplet state of cyanine dye and determined the rate constants of the triplet quenching by phenyltrialkylborate salts. The results obtained show that the tetramethylammonium phenyl‐tri‐n‐butylborate (TB7) has a faster electron‐transfer rate than the tetramethylammonium n‐butyltriphenylborate (TB2) salt, which bore only one butyl group attached to the boron. The relative initiator efficiency of the triphenylbutylborate salts, as compared to the corresponding phenyltrialkylborate salts with a common chromophore, was determined. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of the reaction conditions on the photopolymerization of methyl methacrylate by diethyl dithiocarbamato‐(1,2)‐propane diol

The photopolymerization of methyl methacrylate (MMA) through the use of diethyl dithiocarbamato‐(1,2)‐propane diol (DCPD) was studied. The photoinitiator was synthesized from 3‐chloro‐1,2‐propane diol and sodium diethyl dithiocarbamate in a solvent mixture of acetone and anhydrous ethanol. The photopolymerization was carried out in a Heber multilamp photochemical reactor (COMPACT‐LP‐MP88) (Heber Scientific, Chennai, India) at 254 nm. The effects of the reaction conditions on the polymerization of MMA were studied. The conversion and molecular weight increased with an increase in the monomer concentration and reaction time. However, for the DCPD‐to‐MMA molar ratio, a critical value was found for maximum conversion. The results suggested the living radical nature of the photoinitiator, which was further investigated by the preparation of a block copolymer with styrene. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2320–2328, 2005     

Photopolymerization characteristics of bisbenzo[1,3]dioxol‐5‐ylmethanone as an initiator

Bisbenzo[1,3]dioxol‐5‐ylmethanone (BBDOM), a type of hydrogen‐abstraction photoinitiator, exhibited redshifted maximal absorption in comparison with benzophenone (BP) according to ultraviolet–visible absorption spectroscopy. The kinetics of photopolymerization of the photoinitiator in different systems was studied with real‐time infrared spectroscopy. The concentration of BBDOM, the components of the initiator, and the functionality of the monomer had great effects on the kinetics of photopolymerization. The results show that BBDOM is a more effective photoinitiator than BP. BBDOM consists of cyclic acetals that are widely distributed in nature, and with BBDOM, the use of large numbers of amines can be avoided in the system without the requirement of an additional hydrogen donor. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Asymmetric cyanine dyes as fluorescence probes and visible‐light photoinitiators of free‐radical polymerization processes

This article introduces asymmetric cyanine dyes employed as visible‐light photoinitiators of vinyl monomer polymerization and as fluorescence probes monitoring the progress of polymerization. A degree of polymer cure from the measurement of the changes in the probe emission intensity and position shifts during the thermally initiated polymerization of monoacrylate was obtained. A distinct increase in the intensity of the probe fluorescence was observed during polymerization when the degree of monomer conversion was gradually increasing. This effect was accompanied by a blue‐shift of the probe emission maxima. The second part of this work is focused on the possibility of an application of the tested dyes, in combination with borate anions, as photoinitiating systems. The kinetics of polymerization of trimethylolpropane triacrylate, with cyanine borates as photoinitiators, was studied by a microcalorimetric method. Asymmetric cyanine borates were found to be effective photoinitiators, and both the initiator and coinitiator concentration as well as the light intensity strongly affected the progress of photopolymerization, leading, for example, to an increase in the polymerization rate. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 207–217, 2006     

Unexpected visible‐light‐induced free radical photopolymerization at low light intensity and high viscosity using a titanocene photoinitiator

Most visible‐light photoinitiators are based on electron transfer processes and are comprised of two or more components. These initiators can lose effectiveness in viscous systems because the underlying reactions are diffusion controlled. In this contribution, the visible‐light photoinitiator bis(cyclopentadienyl) bis[2,6‐difluoro‐3‐(1‐pyrryl)phenyl]titanium is characterized for polymerization of viscous systems and low light intensities. This compound absorbs visible light at wavelengths up to 550 nm, and does not rely on diffusion‐controlled electron transfer reactions because it undergoes unimolecular decomposition. In contrast to trends observed for other photoinitiators, the effectiveness of the compound is found to increase markedly with the addition of protonic acids and with increasing system viscosity. For a given concentration of initiator and acid, a remarkably low optimal light intensity for effective polymerization is observed. The origins of these surprising results are discussed in terms of the mechanism of decomposition of the photoinitiator. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Development of new heterobicationic monomethine dyes as effective photoinitiator of free radical polymerization in visible‐light region

The dye initiators consisting of N‐[3‐(p‐alkylpyridine)propyl]‐2‐[N‐(3‐bromopropyl)quinoline]‐2‐methylenebenzotiazolium diiodides as chromophores and n‐butyltriphenylborate anion as electron donor were prepared to achieve an efficient photoinitiator of free radical polymerization in a visible‐light region. The relative photoinitiating efficiencies of novel photoinitiators of vinyl monomers polymerization were evaluated. The results obtained clearly documented that the bicationic photoinitiators exhibit a marked increase in the photoinitiation ability compared to the initiators consisting of a single charged hemicyanine dye. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Novel N‐ethyl‐2‐styrylquinolinium iodides as sensitizers in the photoinitiated free‐radical polymerization of trimethylolopropane triacrylate. II

Results of kinetic studies of two‐component photoinitiator systems used in the visible‐light photoinduced polymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate are presented. Nine different styrylquinolinum dyes coupled with n‐butyltriphenylborate as a coinitiator have been used as photoinitiating systems. Reactive radicals that initiate the polymerization are formed by the well‐known mechanism of photoinduced electron transfer between dye cations acting as electron acceptors and borate anions acting as electron donors. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Syntheses of an azo‐group‐bound silica initiator and silica–polystyrene composites

In this article, we present a facile method for the synthesis of an azo‐group‐bound silica (SiO₂–azo) initiator. The azo groups were introduced onto the surface of silica (SiO₂) nanoparticles via facile condensation between 4,4′‐azobis‐4‐cyanopentanoic acid and the alkyl–hydroxyl groups ‐ immobilized on the SiO₂ nanoparticle surface under ambient conditions. The polystyrene (PS) chains were grafted onto the SiO₂ nanoparticle surface by in situ polymerization with the resulting SiO₂–azo as an initiator, and then, the SiO₂–PS composite was prepared. The syntheses and properties of the SiO₂–azo initiator and the composite were characterized by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, thermogravimetric analysis, gel permeation chromatography, and differential scanning calorimetry techniques. The results confirm that the SiO₂–azo initiator and the composite were synthesized successfully. Styrene was polymerized with the initiation of SiO₂–azo, and the resulting PS domain accounted for 48.6% of the total amount of composite. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis of cassava starch‐g‐poly(methyl methacrylate) copolymers with benzoyl peroxide as an initiator

Graft copolymers of cassava starch and methyl methacrylate (MMA) were synthesized by free‐radical polymerization with benzoyl peroxide (BPO) as an initiator in an aqueous medium at 80°C. The formation of graft copolymers was confirmed by analysis of the obtained products with Fourier transform infrared spectroscopy and scanning electron microscopy. The effects of the amount of cassava starch, the amount of MMA monomer, the amount of BPO, and the reaction time on the grafting characteristics were studied. The optimum condition for grafting were obtained when 5 g of cassava starch, 5 g of MMA, 0.1 g of BPO, and a reaction time of 3 h were used. These condition provided a graft copolymer with 25.00% add‐on, 81.40% monomer conversion, 54.30% homopoly(methyl methacrylate) formed, 45.70% grafting efficiency, 37.20% grafting ratio, and 95.54% yield. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4083–4089, 2006     

Free‐radical bulk polymerization of styrene with a new trifunctional cyclic peroxide initiator

The bulk free‐radical polymerization of styrene in the presence of a new cyclic trifunctional initiator, 3,6,9‐triethyl‐3,6,9‐trimethyl‐1,4,7‐triperoxonane, was studied. Full‐conversion‐range experiments were carried out to assess the effects of the temperature and initiator concentration on the polymerization kinetics, molecular weight, and polydispersity. Gel permeation chromatography was used to measure the molecular weight and the molecular weight distribution of polystyrene. When this multifunctional initiator was used for styrene polymerization at higher temperatures, it was possible to produce polymers with higher molecular weights and narrower molecular weight polydispersity at a higher rate. This showed that the molecular weight and polydispersity were influenced by the initiator concentration and the polymerization temperature in an unusual manner. Moreover, polystyrene, obtained with trifunctional peroxide, had O? O bonds in the molecular chains and was investigated with differential scanning calorimetry and gel permeation chromatography. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1035–1042, 2004     

Synthesis and characterization of high‐impact polystyrene using a multifunctional cyclic peroxide as the initiator

This work deals with the synthesis of high‐impact polystyrene (HIPS) using a novel cyclic multifunctional initiator, the diethyl ketone triperoxide (DEKTP), analyzing its effect on the prepolymerization step and its influence over the HIPS final properties. DEKTP showed a low selectivity toward grafting reactions at any of the concentrations of initiator and temperatures evaluated, and when it was used under a temperature ramp (120–130°C, heating rate = 2°C/min), the synthesis of HIPS was possible with an adequate development of morphology and impact strength (i.e., 67 J/m) depending on DEKTP concentration. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of the structure of silane‐coupling agent on dynamic mechanical properties of dental resin‐nanocomposites

This work was aimed at the study by dynamic mechanical analysis (DMA) of dental composites consisted of a Bis‐GMA/TEGDMA (50/50 wt/wt) matrix and silica nanoparticles (Aerosil OX50) as filler, silanized with various silanes. The silanes used were 3‐[(1,3(2)‐dimethacryloyloxypropyl)‐2 (3)‐oxycarbonylamido] propyltriethoxy‐silane (UDMS), 3‐methacryloxypropyl‐trimethoxysilane (MPS), octyltrimethoxysilane (OTMS), blends of UDMS/OTMS (50/50 wt/wt), or MPS/OTMS (50/50 wt/wt). The total amount of silane was kept constant at 10% by weight fraction relative to the filler weight. The silanized nanoparticles were mixed with the dimethacrylate matrix (60% filler by weight fraction). The composites were light cured and tested by DMA for the determination of storage modulus (E′), loss modulus (E″), tangent delta (tan δ), and glass transition temperature (T_g). Measurements were performed in samples immediately after curing and samples stored in water at 37°C for 1, 7, 30, or 120 days. OTMS‐composite in which OTMS does not form covalent bond with the dimethacrylate matrix showed lower elastic modulus both in dry and wet conditions. The ability of bifunctional UDMS for crosslinking was found not to increase the elastic behavior of the composite, as it was expected, compared with that of MPS‐composite, because of the high amount of the silane used. After immersion in water the elastic modulus of OTMS‐composite remained constant, while that of the other composites increased after 1 day and then remained constant up to 120 days. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and photoinitiating activity study of polymeric photoinitiators bearing BP moiety based on hyperbranched poly(ester‐amine) via thiol‐ene click reaction

A series of polymeric photoinitiators (BP‐HPEAs) bearing BP moiety based on hyperbranched poly(ester‐amine) were synthesized via the thiol‐ene click reaction of 3‐(4‐benzoylphenoxy)propyl 2‐mercaptoacetate (BPPM) with acrylated HPEA. BPPM was obtained by the esterification of (4‐(3‐hydroxypropoxy) phenyl) phenyl methanone (HPPM) with mercaptoacetic acid in the presence of p‐toluene sulphonic acid as a catalyst. HPEA was prepared through Michael addition of piperazine with tri(hydroxymethyl)propane triacrylate. Their molecular structures were confirmed by the ¹H NMR, ¹³C NMR, and FTIR analysis. The UV–vis spectrum analysis results showed that BP‐HPEAs exhibit the stronger n–π* absorption at ～ 340 nm with over two times higher molar extinction coefficients than BP at the concentration of 1.00 × 10^?3M. The photoinitiating activity study showed that the maximum photopolymerization rates of 1,6‐hexanediol diacrylate initiated by BP‐HPEAs in the absence of coinitiator were obtained by two times higher than that by BP in the presence of triethylamine as a coinitiator. Moreover, the excellent miscibility of BP‐HPEAs with the commercial bisphenol A epoxy diacrylate was achieved according to the T_s/T_g ratios of over 9.0 from DMTA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Atom transfer radical polymerization of styrene with 2‐(1‐bromoethyl)‐anthraquinone as an initiator

2‐(1‐Bromoethyl)‐anthraquinone (BEAQ) was successfully used as an initiator in the atom transfer radical polymerization of styrene with CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine as the catalyst at 110°C. The polymerizations were well controlled with a linear increase in the molecular weights (M_n's) of the polymers with monomer conversion and relatively low polydispersities (1.1 < weight‐average molecular weight (M_w)/M_n < 1.5) throughout the poly merizations. The resultant polystyrene thus possessed one chromophore moiety (2‐ethyl‐anthraquinone) at the α end and one bromine atom at the ω end, both from the initiator BEAQ. The intensity of UV absorptions of the resultant polymers decreased with increasing molecular weights of the polymers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2081–2085, 2006     

Tetrahydrosalen‐stabilized,chloride‐containing rare‐earth complexes: Facile initiator precursors for the preparation of hydroxytelechelic poly(ε‐caprolactone)s

End‐capped poly(ε‐caprolactone)s (PCLs) have been prepared elsewhere by various initiators. However, hydroxytelechelic PCLs have been reported less frequently, although there are two hydroxyl end groups in one polymer chain, which allows diversified functionalization. Two tetrahydrosalen‐backboned chlorides containing rare‐earth complexes, YbLCl(DME)₂ and ErLCl(DME) {where L is 6,6′‐[ethane‐1,2‐diylbis(methylazanediyl)]bis (methylene)bis(2,4‐di‐tert‐butylphenol) and DME is dimethoxyethane}, were first synthesized in this study, and they were used as initiator precursors for a ring‐opening polymerization in the presence of NaBH₄ to afford hydroxytelechelic PCLs. The polymerization under different conditions was investigated, and a possible mechanism is proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Electron Transfer Photoinitiating Systems. The Effect of the Co‐Initiator Structure on the Photoinitiation Ability of a Photoredox Pair Containing Neutral Hemicyanine Dyes as Sensitizers

Summary: The photoinitiation ability of photoredox pairs composed of a neutral hemicyanine dye and different co‐initiators for the free radical polymerization of 2‐ethyl‐2‐(hydroxymethyl)propane‐1,3‐diol triacrylate is investigated. p‐(N,N‐dimethylaminostyryl)benzthiazole, p‐(N,N‐dimethylaminostyryl)benzoxazole, p‐(N,N‐dimethylaminostyryl)α‐naphthiazole are tested as dyes, and, as co‐initiators, an aromatic amino‐acid, an aromatic thio‐ and oxycarboxylic acid, and alkyltriphenylborate are applied as electron donors. N,N′‐dialkoxybipyridinium salts are used as ground‐state electron acceptors. The experimental results show that the photoinitiating ability of the tested photoredox pairs are controlled by both the driving force of the electron transfer process between an electron donor and an electron acceptor and the reactivity of the free radical that results from the secondary reactions occurring after the photoinduced electron transfer process. The greatest photoinitiation ability of free radical polymerization is observed when the tested dyes are applied as electron donors in their singlet excited states in combination with N,N′‐dialkoxybipyridinium salts acting as ground‐state electron acceptors.

Schematic of the investigated photoinitiation systems.     

Homoleptic lanthanide amidinate complexes: A single‐component initiator for ring‐opening polymerization of trimethylene carbonate and copolymerization with ϵ‐caprolactone

The ring‐opening polymerization of trimethylene carbonate (TMC) using homoleptic lanthanide amidinate complexes [CyNC(R)NCy]₃Ln as single component initiators has been fully investigated for the first time. The substituents on amidinate ligands and center metals show great effect on the catalytic activities of these complexes, that is, Me > Ph, and La > Nd > Sm > Yb. Among them, [CyNC(Me)NCy]₃La shows the highest catalytic activity. Some features of the TMC polymerization initiated by [CyNC(Me)NCy]₃La were studied in detail. A mechanism that the polymerization occurs via acyl‐oxygen bond cleavage rather than alkyl‐oxygen bond cleavage was proposed. The copolymerization of TMC with ?‐caprolactone initiated by [CyNC(Me)NCy]₃La was also tested. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 22–28, 2006     

Preparation and characterization of hybrid thiol‐ene/epoxy UV–thermal dual‐cured systems

Hybrid thiol‐ene/epoxy coatings were prepared by combining thiol‐ene photo‐curable formulations with epoxy monomers, through a dual UV–thermal curing process. An increase in glass transition temperature and in storage modulus was observed for the hybrid thiol‐ene/epoxy coatings when compared with the pristine thiol‐ene UV‐cured system. Also, the bisphenol A moieties introduced into the hybrid networks during the dual‐curing process induced an increase in thermal stability of the cured materials. It has been demonstrated that the addition of epoxy monomer to the thiol‐ene photo‐curable system is a good strategy to follow in order to improve the final properties of thiol‐ene‐based coatings leading to a wide range of possible applications for the hybrid materials. Copyright © 2010 Society of Chemical Industry     

Viscosity effects in the photopolymerization of two‐monomer systems

Photopolymerization kinetics and viscosity behavior of five different two‐monomer systems forming hydrogen bonds and composed of mixtures of a high viscosity monomer (HVM) and a low viscosity monomer (LVM) at various molar ratios were investigated at six polymerization temperatures. The monomers used were mono‐ or dimethacrylates. Detailed viscosity measurements of the monomer mixtures showed significant negative deviations from the theoretical values (characterized by excess logarithm viscosities) indicating that interactions between the molecules of the same type (in individual monomers) are stronger than those between two molecules of different types (HVM and LVM). The photopolymerization kinetics were analyzed from the point of view of the appearance, viscosity and temperature behavior of the most reactive composition (MRC), the one showing the highest value of the maximum polymerization rate within a range of the HVM: LVM ratios. It was found that MRC appearance is determined mainly by the initial viscosity of the two‐monomer system, whereas the functionality of the monomers (and network formation) is much less important (MRC is observed even in linear systems). The initial viscosity of all the monomer mixtures showing MRC lay in the range of 0.06–2 Pa s, which is narrow compared to the range of viscosities of the monomers (approximately 10^?3–10³ Pa s). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of di(4‐hydroxyl benzophenone) sebacate and its usage as initiator in the photocrosslinking of polyethylene

A new benzophenone (BP) derivative with a long carbon chain and two chromophoric groups—di (4‐hydroxyl benzophenone) sebacate (BP‐S) used as a photoinitiator in the photocrosslinking of polyethylene (PE) has been synthesized and characterized by ultraviolet (UV) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and elemental analysis. The kinetic characteristics of BP‐S photoinitiated crosslinking of PE in the melt and its compatibility with PE resin have been examined by gel content measurement and thermal migration experiment. The results show that BP‐S is an excellent photoinitiator, which has better compatibility, less volatility, higher photoinitiating efficiency, and longer storage time than BP itself. It is promising for industrial applications of the photocrosslinking of polyolefins. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1581–1586, 2002;