Benzoin ether photoinitiated polymerization

As a photoinitiator for the polymerization of bulk methyl methacrylate, benzil was found to be considerably less efficient than benzoin or benzoin methyl ether (20), e,g. at photoinitiator concentrations of 10 M, the polymerization rate observed using benzoin was eight times that observed using benzil, despite the fact that the latter was found to absorb three times as much of the incident radiation (Fig. 1). The photo-initiating efficiency of benzil was improved by a factor of three on addition to the methyl methacrylate of 10% v/v tetrahydrofuran, whereas the same additive had no appreciable effect on rates of benzoin- and benzoin methyl ether-photoinitiated polymerizations direct evidence that photoinitiation by benzil proceeds by a hydrogen abstraction mechanism rather than by fragmentation. 

The chemistry involved in LfV-curable resin systems has been extensively investigated and thoroughly surveyed [88-94]. LfV-radiation polymerization, is in principle, completely analogous to the conventional addition polymerization. A photoinitiator is used in UV polymerization. Its function is the same as the free-radical initiator. A conventional initiator possesses a thermally labile bond which is cleaved to form free-radical species, but the photoinitiator has a bond which breaks upon absorption of radiant energy. Benzoin ethers, benzyldialkyl ketals, benzophenone, and acetophenone derivatives are the important LfV-photoinitiators [95-99]. 

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Benzoic ethers, dioxolane, and sulfur derivatives are photoinitiators which undergo photochemical dissociation reactions into pairs of radicals. The polymerization of vinyl (monomer) into the polymers of the polyvinyl series can be photoinitiated by an ammonium salt of a benzoin ether derivative. In this case the active species is the benzoyl radical (Figure 6.11). 

The nematic LC is the original eutectic mixture of 35% (weight %) 4 (m-octyl oxy)-4-cyanobiphenyl (80CB) and 65% 4/-(n-pentyl)-4-cyanobiphenyl (5CB). The sample is doped with perylene and 2% (mol %) of NIAC, which is acrylate monomer containing the easily reduced l,4 5,8-naphthalenediimide moiety (see Scheme 3). Finally, 0.5% (mol %) of benzoin methyl ether (BME) is added to photoinitiate polymerization of the NIAC. 

In the photopolymerization of methacrylamide by benzoin methyl ether, chain-transfer to monomer has been found to be important, and benzalde-hyde is reported to be an inefficient photoinitiator of methyl methacrylate polymerization unless benzophenone and triethylamine are present. Acetophenone has been found to sensitize the cycloaddition of maleic anhydride to 7-oxabicyclo[2.2.1]heptan-5-one-2,3-dicarboxylic anhydride, , a-hydroxy-acetophenone derivatives have been found to be non-yellowing initiators, and h.p.l.c. has been used to determine residual carbonyl photoinitiators in u.v.-cured resins. In the emulsion-polymerization of methyl methacrylate using an aromatic ketone and a continuous or intermittent laser, the former conditions were found to be similar to those under continuous u.v. irradiation. The dependence of the polymerization rate and average chain-length on the absorbance of the initiator used in the photoinitiated polymerization of vinyl monomers has been studied. Interestingly, irrespective of all conditions, maximum conversion is observed when initiator absorbance is 2.51. "... 

Polymeric systems containing side-chain benzoin moieties, recently appearing [100,102] in the patent literature, have been applied as photoinitiators to UV curing of prepolymers of a different nature. In particular, siloxane-compatible photocrosslinking initiators (PSBME) are manufactured [100] by reacting silox-anes with benzoin ether derivatives (Scheme 23) and used in UV curing of acryloxypropyl-terminated dimethylsiloxane prepolymers. 

Table 23. Photoinitiated polymerization of methyl methacrylate (MMA) in benzene solution, using polymeric photoinitiators based on benzoin methyl ether and benzoin moieties (BE) as well as their low-molecular-weight analogues [111]...

Our interest in the photochemistry of benzoin ethers was prompted by two major reasons (1) a discrepancy existed between recent mechanistic studies on the photochemistry of benzoin ethers and earlier reports on the benzoin ether photo-initiated polymerization of reactive monomers, and (2) the extensive, commercial utilization of benzoin ethers as photoinitiators in uv curable coatings and printing inks warranted further investigation of this discrepancy. 

Photoinitiation of Addition Polymerization.—Much of the research reported in this section has arisen from commercial and environmental needs for surface coatings which do not rely on polymerization in solvent media. One approach to this problem has been to initiate the addition polymerization of monomer/ oligomer/pigment formulations with u.v. light and suitable photoinitiators. The initiators most commonly used are benzophenone, Michler s ketone (1), and benzoin ethers, but the search for other efficient photoinitiators continues. 

But photoinitiated polymerizations are more frequently started by the decomposition of an intentionally added photoinitiator. Azobisisobutyro-nitrile and other azo compounds not only undergo thermal scission [see also Equation (20-11)], but produce two isobutyronitrile free radicals under the influence of light. Similar photohomolysis is also observed with peroxides and disulfides. Benzoin and its alkyl ethers fragment with especially high quantum yields ... 

A common approach to the preparation of polymeric photoinitiators is to attach photoinitiator molecules, like benzophenone, or benzoin ether, or thioxanthones to some polymerizable groups For instance, acryloyl chloride was reacted with hydroxyfliioxanthone and flien copolymerized with styrene ... 

If the rate of addition to monomer is low, primary radical termination may achieve greater importance. For example, in photoinitiation by the benzoin ether (23), both a fast initiating species (24, high fej) and a slow initiating species (25, low ky are generated (Scheme 20). The polymerization kinetics are complicated and the initiator efficiency is lowered by primary radical termination involving the dimethoxybenzyl radical (25)." " ... 

A disadvantage of benzoin ethers as photoinitiators is their apparent instability in the presence of reactive resins, resulting in premature polymerization in dark storage. This behavior has been attributed to the presence of the benzylic H atom, which may be abstracted by adventitious free-radical species, such as peroxy radicals. ... 

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. 

The net result is that the rate of polymerization is lower by about a factor of 10, when polysilanes are compared with conventional photoinitiators such as benzoin methyl ether. 

The efficiencies of benzoin derivatives in promoting the light induced hardening of styrene-containing unsaturated polyester resins have been found to vary considerably (7), benzoin alkyl ethers and a-alkylated benzoins being more reactive than benzoin itself. However, as photoinitiators for the polymerization of bulk methyl methacrylate, benzoin and benzoin methyl ether exhibit comparable efficiencies (20), and this is also indicated for polymerisation of methyl acrylate in tert-butanol (21). The relative photoinitiating efficiencies of benzoin derivatives may be influenced by several factors, some of which will depend on the nature of the monomer system and the environment in which the polymerizations are carried out. 

The reaction of glycidyl acrylate with a-(2-carboxyethyl)benzoin methyl ether has allowed one to obtain [101] the corresponding acrylic monomer which, upon copolymerization with different amounts of MMA, butyl methacrylate and 2-(V, V-dimethylamino)ethyl methacrylate, gives rise to polymeric photoinitiators, containing side-chain benzoin methylether moieties, for photocurable coatings ... 

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