Photoinitiators for Radical Polymerization

Norrish type II photoinitiators are bimolecular initiators. Generally an aromatic ketone is used in combination with a tertiary amine. Both aliphatic and aromatic tertiary amines can be used. A well-known example of such an initiating system is benzophenone with dimethylaminoethanol. 

All the acrylate resins described above in Section 16.8.3 can be employed in UV curing. Low molecular weight materials are mostly used since they possess a lower viscosity. In many UV applications thin layers are employed, which makes the resin viscosity an important processing parameter. 

However, the urethane acrylates, which generally possess higher viscosities, also have higher polymerization rates. 

---

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

A Study of the benzoyl radicals obtained by irradiation of the ketones (6-11) has shown that the a-cleavage results from the excited triplet state. " endo and cxu-(2-Hydroxy-[2.2.2]bicyclo-5-en-l-yl)-phenylmethanones have been synthesized and studied as potential photoinitiators for radical polymerization. The photoinitiators (12) have been investigated in some detail. ... 

Mono- and bisacylphosphine oxides 4 and 5 (Chart 7) are efficient photoinitiators for radical polymerizations. Their phofochemistry was studied by H-, C-, and P-CIDNP spectroscopy. The primary step is the breaking of the bond between the carbonyl group and phosphorous in a triplet state for the bisacylphosphine oxides, a stepwise cleavage occurs. 

The use of N-substituted maleimides as photoinitiators for radical polymerization, has gained attention as an alternative material to ketone based photoinitiators.Sensitization of maleimides to the triplet state markedly increases polymerization efficiencies. The maleimide amine system produces two radicals, one centered on the amine and the other on the maleimide. Both... 

Current reviews on photoinitiators for radical polymerization are available. " In general, initiator radicals are photogenerated by intramolecular bond cleavage or intermolecular H abstraction from an H donor. These photoinitiators may be classified as PI or PI2, since unimolecular or bimolecular reactions are involved respectively. 

This problem prompted the development of second-generation photoinitiators for radical polymerization, which do not possess benzylic H atoms, such as a,a-dimethoxy-a-phenylacetophenone (1), a,a-diethoxyacetophenone (2), and the a-hydroxyacetophenone derivatives (3) and (4). These photoinitiators undergo photocleavage, in an analogous manner to benzoin ethers, as shown by various studies, " including free-radical trapping and laser flash photolysis. 

Benzoin and its derivatives are the most widely used photoinitiators for radical polymerization of vinyl monomers. As depicted in reaction (8), they undergo a-cleavage to produce benzoyl and a-substituted benzyl radicals upon photolysis. 

Photoinitiation is an excellent method for studying the pre- and posteffects of free radical polymerization, and from the ratio of the specific rate constant (kx) in non-steady-state conditions, together with steady-state kinetics, the absolute values of propagation (kp) and termination (k,) rate constants for radical polymerization can be obtained. 

If direct homolysis occurs in the case of tetraphenylphosphonium tetrafloroborate, triphenylphosphonium ylide was expected to function as a photoinitiator of radical polymerization because of its similar structure. Therefore, another milestone was reached by Kondo and colleagues [50] who investigated the use of triphenylphosphonium ethoxycarbonylmethylide (TPPY) (Scheme 22) as an effective photoinitiator for the polym-... 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Polysilanes as Photoinitiators. Since polysilanes photolyze to give silyl radicals,(21) as explained in the section on photochemistry above, they can be used as photoinitiators for radical processes such as polymerization.38 As can be seen from the information in Table II, this process is a fairly general one a variety of... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

Photoinitiation of radical polymerization has long been known. Recently, a group of photoinitiators for cationic polymerization hase been discovered and developed by Crivello et al.1J. They include diaryliodonium (7),2) triarylsulfonium (2), 3 5) dialkylphenacylsulfonium (J), 6) and dialkyl-4-hydroxyphenylsulfonium salts (4) 7). 

Photocurable coatings are widely used for metal, plastics wood and paper. Photoinitiated free-radical polymerization, however, can only be applied to vinyl monomers. The studies of Crivello have broadened the scope of monomers. In addition, photoinitiated cationic polymerization is not sensitive toward oxygen (air). Photoinitiated free-radical polymerization sometimes requires working in inert atmosphere in order to avoid the inhibition through oxygen1). 

In principle, the photoreactions of CT s are able to offer a great number of photoinitiator systems for radical polymerization. But, so far, this subject has only received little attention, and the current knowledge relative to the photochemistry of such complexes is poor. In addition to the amine complexes mentioned above, chinoline-bromine [124-127], chinoline-chlorine [128], 2-methylpyridine-chlorine [129], pyridine-bromine [130], IV-vinylpyrrolidone-bromine [131], acridone-bro-mine [132], acridone-chlorine [133], benzophenone-S02 [134], isoquinoline-S02 [135, 136], and 2-methylquinoline-S02 [136] combinations are used for radical polymerization of AN, alkyl methacrylates, acrylic and methacrylic acid, and for... 

As outlined in Scheme 2, for cationic photopolymerization a photoinduced formation of species X+ or Lewis acids is required [162]. Such species are formed both by PET between neutral donors and acceptors (see Scheme 3), between neutral donors and positive charged acceptors (see Schemes 9 and 11), respectively, and by an indirect PET between nucleophilic radicals and onium salts or halogen compounds (see Eq. (16) and Scheme 12). Therefore, combinations of compounds, whose light-induced reactions are based on the pathways given above, are usable as photoinitiators for cationic polymerizations, too. Prerequisites for the use of cations... 

The use of polysilanes as photoinitiators of radical polymerization was one of the hrst means whereby they were incorporated within block copolymer structures [38 0], albeit in an uncontrolled fashion. However the resulting block copolymer structures were poorly defined and interest in them principally lay in their application as compatibilisers for polystyrene (PS) and polymethylphenylsilane blends PMPS. The earliest synthetic strategies for relatively well-defined copolymers based on polysilanes exploited the condensation of the chain ends of polysilanes prepared by Wurtz-type syntheses with those of a second prepolymer that was to constitute the other component block. Typically, a mixture of AB and ABA block copolymers in which the A block was polystyrene (PS) and the B block was polymethylphenylsilane (PMPS) was prepared by reaction of anionically active chains ends of polystyrene (e.g. polystyryl lithium) with Si-X (X=Br, Cl) chain ends of a,co-dihalo-polymethylphenylsilane an example of which is shown in Fig. 2 [43,44,45]. Similar strategies were subsequently used to prepare an AB/ABA copolymer mixture in which the A block was poly(methyl methacrylate) (PMMA) [46] and also a multi- block copolymer of PMPS and polyisoprene (PI) [47]. 

Detection of triplet states and photogenerated radicals and in the nanosecond timescale by their optical spectra is probably the most common method used for several decades. However, detection of radicals by their IR spectra becomes valuable in cases of low optical extinction coefficients of transients and overlay of spectra of radicals and parent compounds. In conclusion, laser flash photolysis with optical, IR, and ESR detection provides a powerful arsenal to investigate radical species produced in the photoinitiation of radical polymerization. 

During the past twenty years, development of compounds that efficiently initiate polymerization on irradiation have made possible the development of several new commercially important technologies based on these photoinitiators [1]. Their use in UV curable coatings is particularly notable. The most useful photoinitiators that have been explored to date are radical photoinitiators. Many applications today use this technology, in spite of important drawbacks [2]. The recent development of diaryliodonium, triarylsulfonium and ferrocenium salts as highly efficient photoinitiators for cationic polymerization has generated a new class of fast polymerizations. 

Although both positive and negative working photoresists based on photoinduced condensation and free-radical chemistry are well-known, cationic polymerization chemistry has received little attention for the fabrication of photoresists. The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (1-6). The facile synthesis of onium salts I-III together with their ready structural modification to manipulate... 

Bulk semiconductors and powders have been used as initiators for radical polymerization reactions [140-144], Recently the study has been extended to semiconductor nanoclusters [145-147]. It was found that polymerization of methyl methacrylate occurs readily using ZnO nanoclusters. Under the same experimental conditions, no polymerization occurred with bulk ZnO particles as photoinitiators [145], In a survey study, several semiconductor nanoclusters such as CdS and Ti02, in addition to ZnO, were found to be effective photoinitiators for a wide variety of polymers [146], In all cases nanoclusters are more effective than bulk semiconductor particles. A comparison of the quantum yields for polymerization of methyl methacrylate for different nanoclusters revealed that Ti02 < ZnO < CdS [146]. This trend is parallel with the reduction potential of the conduction band electron. The mechanism of polymerization is believed to be via anionic initiation, followed by a free-radical propagation step. 

Benzophenone (BP) is a photoinitiator that can be employed in a number of ways for the preparation of patterned graft polymers. BP can be directly used as an initiator for radical polymerization, either via immobilization of BP on the surface or by adding BP to the monomer solution (Figures 3.9A and 3.9B). Another possibility is to synthesize initiators modified with BP, which can then be immobilized on surfaces using UV light (Figure 3.9C). Yet another alternative is to add BP to the initiator solution for BP-mediated attachment of the initiator to the surface (Figure 3.9D). 

Yu, C., Xu, M., Svec, F., and Frechet, J. M. J., Preparation of monolithic polymers with controlled porous properties for microfluidic chip applications using photoinitiated free radical polymerization,... 

One useful property of cationic polymerization is that it is not air inhibited. In the absence of nucleophilic impurities, there are no inherent modes of termination. Thus, the polymerization may continue for long periods (hours to days) after the light is turned off, in contrast to photoinitiated free radical polymerization (24). This post-cure should be useful for laminating two opaque materials, in that irradiation of the adhesive followed by lamination should result in a well cured adhesive, although it may take 1-2 days to form maximum bond strength. 

ESR evidence supports the presence of an Fe(lll) radical intermediate. Irradiation of the iron(ll) arene compound shown in Figure I9.40 leads to slippage of the linkage to the >74 bonding mode. The resulting compound is coordinatively unsaturated (16 e ). Continued irradiation leads to the loss of the arene li d and replacement by three solvent molecules. This species has been used as a photoinitiator for cationic polymerization reactions, as shown in Figure 19.40. 

Novikova et al., reported that some pentaaza-1,4-dienes can photo initiate polymerization. Thus, l,5-Bis(4-methoxyphenyl)-3-methyl-l,4-pentazadiene, l,5-diphenyl-3-(2-hydroxyethyl)-l,4-pentaza-diene, and 1,5-diphenyl-3-methyl-l,4-pentazadiene were used as photoinitiators of radical polymerization of hexanediol diacrylate and of methyl methacrylate. Photoinitiation abilities of these compounds were compared with those of a commercial photoinitiator, Irgacure 1700. The pentaazodienes showed a high initiation capacity. Also, the activation energy of the polymerization in the presenee of the pentaazodiene compounds was lower than that for Irgacure 1700. 

Radical polymerization is induced by photoinitiators, cleaverage type, and hydrogen abstraction type. The merits of radical polymerization are its higher cure rate and availability of a variety of materials. Most acrylate and methacrylate monomers are applied for radical polymerization. Figure 4.7 shows typical polymerization of a monomer. Mono-, bi-, and multifunctional vinyl monomers are used for polymerization. Figure 4.8 shows typical initiators for photopolymerization. Benzophenone is a hydrogen abstract type initiator. 2,2-Diethoxy-l,2-diphenylethane-l-one is a cleavage type initiator. 

Other applications of polysilanes include their utility as photoinitiators for vinyl polymerization. This application is based on the phenomenon of generating silyl radicals upon photolysis. Although polysilanes are not efficient photoinitiators they have unique properties such as being not susceptible to oxygen. Many organic monomers such as acrylates, and styrene have been polymerized by this method [4]. 

Autoacceleration, where the rate of polymerization increases with conversion in isothermal conditions, is observed in both thermal- and photoinitiated free-radical polymerizations because the termination mechanisms are the same for both. As the chains grow longer, it becomes more difficult for the active centers to diffuse and imdergo bimolecular termination thus, termination frequency decreases and active centers at the chain ends can become trapped. In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67). As is discussed for chain cross-linking photopolymerizations below, more complicated kinetic treatments must then be considered, including unsteady-state kinetics. 

---