Cyclic acetals as the second co-initiators in three-component photoinitiating systems

Cyclic acetals were used as a second co-initiator in three-component photoinitiating systems. The mixtures of cyanine dye borate ion salt and cyclic acetals were used to initiate the visible light polymerization of triacrylate monomer (TMPTA). The kinetics of polymerization was recorded by differential scanning calorimetry (DSC). The results showed that the addition of cyclic acetals increases the rate of polymerization (R _p).     

High speed three-component photoinitiating systems composed of cyanine dyes borate salt and heteroaromatic thiols

Three heteroaromatic thiols (2-mercaptobenzothiazole, 2-mercaptobenzoxazole and 2-mercaptobenzimidazole) were investigated in regard to their abilities as a co-initiator in free radical polymerization induced by cyanine dye/borate salt photoinitiating system. The kinetic results shown that the new three-component photoinitiating systems are more effective than two-component ones. The mechanism of photochemical and chemical processes occurring during the irradiation of the three-component initiators was proposed on the basis of the laser flash photolysis. The addition to the cyanine dye borate salt of a second co-initiator (thiol) results in the generation besides butyl radical the second free radical which can start the polymerization chain reaction.     

Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions

Although there have been many reports on photoinitiating systems adapted to visible lights for radical photopolymerization, the challenge for the design and development of photoinitiating systems for cationic photopolymerization or concomitant radical/cationic photopolymerization (for interpenetrating polymer network IPN synthesis) with visible lights still remains open. Particularly, the recent development of cheap and easily accessible LEDs operating upon soft visible light irradiations has opened new fields for polymer synthesis. Since 2011, many novel photoinitiating systems based on organic and organometallic compounds with excellent visible light absorption have emerged and exhibited outstanding photoinitiating abilities especially for cationic photopolymerization. In this review, recent progress (mainly from 2011 to early 2014) in applications of photoinitiators and sensitive photoinitiating systems under visible lights are reported. In addition, their relative efficiencies in the photopolymerization of different monomers are exemplified and discussed.     

New photoinitiating systems and photoactive polymers and their applications

Two new types of photoinitiating systems based on transition metal carbonyls [mainly Mn₂(CO)₁₀ and Re₂(CO)₁₀] and free from abstractable halogen atoms are described. The first consists of a low concentration (typically ～0.1 mol 1^?1) of a fluoro-olefin, together with the metal carbonyl. Quantum yield data for photoinitiation by a number of these systems are presented; Re₂(CO)₁₀ is generally more active than Mn₂(CO)₁₀ and quantum yields near unity are obtainable. Polymers prepared with the aid of these initiators carry terminal groups in which the metal atom is covalently bound to the polymer chain through a fluoro-olefin unit. In the second type no halogen is present. The additive employed with the metal carbonyl is (a) acetylene or an acetylene derivative (e.g. acetylene dicarboxylic acid); or (b) an olefinic derivative carrying electron-attracting groups (e.g. diethyl fumarate). Mechanistically these photoinitiators resemble those based on fluoro-olefins; high quantum yields of initiation may be obtained and terminal groups containing covalently bound metal atoms are present in the polymers. Polymers prepared with either type of photoinitiator behave as macromolecular thermal initiators at higher temperatures (e.g. 100°C) and furnish a convenient route to block copolymers. A third type of photoinitiator, based on a vanadium (V) chelate, is also discussed. Chelates such as V^V OQ₂or, where Q represents 8-quinolyloxo, have been shown to photolyse by scission of (OR). If R is a side chain attached to a polymer molecule, photolysis yields a macroradical capable of initiating grafting reactions. The behaviour and applications of these photoactive polymers are described.     

Cyclic acetal as coinitiator for bimolecular photoinitiating systems

Cyclic acetals were used to replace the conventional amines in bimolecular photoinitiating systems. The mixtures of benzophenone derivatives and cyclic acetals were used to initiate the UV photopolymerization of 1,6-hexanedioldiacrylate (HDDA). Camphorquinone (CQ)/1,3-benzodioxole (BDO) combinations were used to initiate the visible light photopolymerization of 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (BisGMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 wt%) for dental application. The kinetics was recorded by real-time infrared spectroscopy (RTIR). Ethyl 4-N,N-dimethylaminobenzoate (EDMAB) was used as control in the same photocuring condition. The results showed that the addition of cyclic acetals greatly increased the rate of polymerization (R_p) and final double bond (DC) of HDDA. Combination of p-chlorobenzophenone (CBP)/BDO had the highest initiating reactivity. BDO also showed an effective coinitiator for camphorquinone-based initiator system. Comparing with EDMAB, CBP/BDO and CQ/BDO indicated comparable initiating reactivity. Moreover, the natural component characteristics of BDO made it a promising alternative to commercial amine in biomolecular photoinitiating system.     

Free radical formation in oriented polybutadiene during mechanical deformation below Tg

The tensile behavior of oriented polybutadiene at 83°K is systematically studied as a function of strain rate and pre-test orientation. Electron spin resonance studies of radical formation are made in conjunction with the mechanical tests. Three different modes of mechanical behavior are observed (brittle, crazing and a second ductile behavior without crazing), depending on test conditions. Radical formation is observed in association with the two ductile modes of behavior. The ESR spectra obtained are attributed to a combination of allyl radicals formed by chain scission between the α-methylene groups and peroxy radicals. The relative quantity of the two radical species present is thought to be related to the ratio of cis/trans-isomerism. Stability of the radicals observed with time and with an increase in temperature is studied. Further studies are made of the quantity of environmental test gases absorbed during crazing.     

Coalescence in three-component polymer systems

The change in the ratio on incorporation of a third (noncoalescing) disperse component between the number of coalescence and dispersion events alters the polydispersion and increases the degree of dispersion of the second (basic) component in a fibre-forming polymer blend melt. Incorporation of a third highly disperse component is a method of changing the degree of dispersion in fibre-forming melts of polymer blends. __________ Translated from Khimicheskie Volokna, No. 5, pp. 24–26, September–October, 2006.     

Investigation of a system capable of photoinitiating radical polymerizations in thick pigmented media

A new system consisting of a xanthenic dye, a ferrocenium salt, a hydroperoxide, and an amine, capable of photoinitiating the radical polymerization of thick and pigmented films exhibiting a thickness of ca. 100 microns was studied. This is a one‐step process, with a working speed of 10 m min⁻¹, involving four 80‐W cm⁻¹ lamps. This is one present‐day system capable of working under industrial conditions. The photolysis investigation in solvent medium of its various components is a first approach to discuss the photochemical mechanism involved in this four‐component system. Thus, one can assume that both mechanisms could occur through charge transfer : dye/amine interaction and dye/ferrocenium salt interaction, the latter one being presumably the most efficient. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1911–1923, 2001     

New photoinitiating systems designed for polymer/inorganic hybrid nanocoatings

Conventional UV-curable formulations consist of photoinitiators, multifunctional monomers and oligomers, reactive diluents, pigments, and additives. While photoinitiators are the key components of the formulations and responsible for the absorption of light and relevant performances (e.g., cure speed, high-percent conversion, etc.), the final properties of the cured coating are governed by the other high-volume components. With recent advances in the use of nanomaterials like metals, metal oxides, and silicates in coatings, it is now possible to prepare nanocoatings with enhanced physical, chemical, and biological properties. This is due, in part, to the difference in surface area per unit of volume at the nanoscale. Nanocoatings are usually prepared by UV irradiation of formulations containing dispersed nanoparticles. However, the homogeneous dispersion of these nanoparticles is a key challenge due to their easy agglomeration arising from their high surface-free energy. It is often difficult to obtain well-dispersed formulations providing good transmission of light for a complete cure. In this article, we report several synthetic methodologies for the preparation of epoxy and (meth)acrylate-based nanocoatings containing clay or metal nanoparticles. In the former case, photolysis of intercalated photoinitiator within the layers of montmorillonite clay in the presence of monomers resulted in the in situ formation of exfoliated structures. For the preparation of metal nanocoatings, the formation of silver or gold nanoparticles and crosslinking are accomplished simultaneously by photoinduced electron transfer and polymerization processes. The nanoparticles are homogenously distributed in the network without macroscopic agglomeration. Applicability to both free radical and cationic systems is demonstrated. Moreover, a novel photochemical route for grafting from the self-assembled monolayers on gold is presented.     

Electron transfer processes in photoinitiating systems composed of hemicyanine sec-butyltriphenylborate ion pairs

Summary Borate hemicyanine salts, namely sec-butyltriphenyl styrylbenzoxazole borates are shown to be effective photoinitiators for the polymerization of vinyl monomers. Mechanism of the photoinitiation involves sec-butyl radicals formed from the heterolytic cleavage of carbon-boron bond that follows the electron transfer process. The capability of the salts to act as initiators for the polymerization of multifunctional monomer is documented.     

A modified copolymerization equation for three-component systems

A modified copolymerization equation substantiated by experimental evidence has been proposed for tricomponent systems.     

Free radical hydrosilylation of polypropylene

Hydrosilylation of terminal double bonds in polypropylene was investigated via a free radical process in the melt-phase. This process involved the following two reactions occurring in parallel: chain scission, which leads to formation of double bonds, and the addition of silyl radicals to such bonds. Fourier Transform Infrared Spectrometry (FT–IR) was used to follow silane incorporation and terminal double bond formation. The experimental trends were verified using elemental analysis obtained from energy dispersive x-ray analysis (EDX). Using a single screw extruder and a model silane compound, a 2³ full factorial experiment design was implemented with the following investigated factors: residence time, silane concentration, and peroxide concentration. Statistical analysis of the FT–IR results, showed that, for the silane incorporation, residence time, peroxide concentration, and the interaction between the silane and peroxide concentrations had significant effects. The significant effects for the forma-tion of terminal double bonds were residence time, silane concentration, and peroxide concentration. The experimental results have shown that melt-phase hydrosilyla-tion of polypropylene can be accomplished through this noncatalytic free radical method. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:439–447, 1997     

Free radical desorption in continuous emulsion polymerization

A mathematical model for continuous emulsion polymerization which accounts for the desorption of radicals from polymer particles is presented. The desorption factor has been incorporated into the Sato-Taniyama model and the resulting systems of equations solved iteratively. The effects of desorption on variables such as concentration of particles, polymerization rate, and average number of radicals per particle are illustrated. Fit of the model to experimental data is presented and discussed.     

Free radical bulk polymerization in cylindrical molds

Bulk styrene homopolymerization in cylindrical molds was studied in order to simulate the reactive injection process when the reaction kinetics was much slower than the filling process. Mathematical models were built and implemented, allowing the analysis of certain kinetic phenomena, such as the gel‐effect, and the relevance of natural convection. Based on a detailed kinetic mechanism, the polymer properties (average molecular weight and polydispersity) and monomer conversion were calculated as functions of spatial coordinates and time. The results obtained show that polymer properties and monomer conversion may be very heterogeneous inside the cylindrical mold, although insensitive to variations of the diffusion coefficient, so that defects are expected to appear as a consequence of the heterogeneity of polymer properties inside the mold. Experiments confirmed the simulation results. Simulations indicate that heterogeneities may be expected to increase when the reaction rates are one order of magnitude higher, as in acrylic acid polymerizations.     

Diketopyrrolopyrrole dyes: Structure/reactivity/efficiency relationship in photoinitiating systems upon visible lights

Several diketopyrrolopyrrole derivative based dyes (DPP), combined with an iodonium salt or an amine (and optionally an additive), are studied as photoinitiating systems for the cationic polymerization CP of epoxides or the free radical polymerization FRP of acrylates under different irradiation sources i.e. a very soft halogen lamp as well as laser diodes at 473 nm (blue light) and 532 nm (green light). The diketopyrrolopyrrole-furan derivative (FuDPP) is very efficient in CP. The structure/reactivity/efficiency relationships in this series of DPP derivatives are investigated. A comparison with a well known reference for visible light photoinitiating system (i.e. camphorquinone based photoinitiating system) is also provided showing the better efficiency of the new proposed structures. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin-trapping techniques.     

Free radical copolymerization behavior of myrcene

Summary Myrcene (7-methyl-3-methylene-1,6-octadiene) binary copolymers with styrene, methyl methacrylate or p-fluorostyrene were synthesized. Polymer compositions were measured by H-NMR and reactivity ratios were calculated using a nonlinear least squares error-in-variables method. For myrcene-styrene copolymerizations the polymer radicals have a marked preference for myrcene monomer. In the case of the myrcene-MMA copolymerization the growing polymer chains prefer to add the other monomer, i.e. the crossover reaction dominates. The same is true for the myrcene/p-fluorostyrene copolymerization, although the myrcene radical has a higher preference for myrcene monomer in this copolymerization than it does in the myrcene-MMA copolymerization.     

Free radical polymerization of p-methyl styrene

An experimental investigation is reported of the free-radical synthesis kinetics of poly(p-methyl styrene) in cyclohexane at low monomer conversion in the temperature range 50°–70°C using AIBN initiator. The p-methyl styrene monomer contained greater than 97% paraisomer. Isothermal conversion/time curves were obtained using glass ampoule reactors and gravimetry and molecular weight distribution and weight-average molecular weights were measured by s.e.c. and low angle laser light scattering photometry (LALLSP). Transfer to small molecules (monomer, cyclohexane, —) was found to be small or negligible with virtually all of the polymer chains formed by termination by combination. Predicted and measured weight-average molecular weights are in good agreement for polymer synthesized at low conversions.