Free radical initiators photoinitiators

When free-radical initiation is used, cocatalysts, eg, phosphites (112), and uv photoinitiators such as acetophenone derivatives (113) can be used to increase the rate and conversion of the olefins to the desired mercaptans. 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

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. 10 Synthesis of a mixed brush may be accomplished by two strategies (a) A free radical initiator with low efficiency is used. This guarantees that some of the initiator is left after deposition of the first brnsh. Immersion into a second monomer yields a mixed brush where the two polymers are intercalated between each other, (b) A mixed monolayer could be used whereby A and B may be initiated independently (e.g. one is a photoinitiator and the other is a thermal initiator)...

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

Photoinitiators [INITIATORS - FREE-RADICAL INITIATORS] (Voll4) hexafluoroantamonatesas [FLUORINE COMPOUNDS, INORGANIC - ANTIMONY] (Vol 11) phosphine-based [PHOSPHINE AND ITS DERIVATIVES] (Vol 18) polymethines [POLYMETHINE DYES] (Vol 19)... 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

For applications, such hybrid systems are limited to either diaryliodonium salts or aryldiazonium photoinitiators and suitable radical initiators. Triphenylsulfonium salts, however, are not active as cocatalysts in the presence of free-radical initiators. 

Monroe, B. M., Weed, G. C., Photoinitiators for Free radical initiated Photoimaging Systems,... 

When composite resins were first introduced, their polymerization involved free radical initiation by a combination of benzoyl peroxide with tertiary amine activator, de-hvered in two different pastes, known respectively as base and catalyst. Contemporary composites are visible light-activated one-paste systems and the principal photoinitiator used is camphorquinone [70,80], It is used in conjunction with a co-initiator, typically an amine [81], This latter compound does not absorb hght, but is necessary to react with the fight-activated camphorquinone molecule to generate free radicals and hence initiate the chain polymerization. 

The initiator not only influences the rate of polymerization, however it can, under certain conditions, also influence the constitution of the polymer produced. For example, AIBN produces linear polymer at low monomer conversions, but lightly branched products at higher conversions when used to polymerize / -vinyl benzyl methyl ether. If this monomer is initiated with dibenzoyl peroxide, cross-linking occurs at high yields, and the monomer produces cross-linked polymer even at low yields if diacetyl is used as photoinitiator. What happens with these free radical initiators is that transfer to polymer occurs the polymer free radicals produced can add on monomer molecules, whereby branched products are produced, or recombination resulting in cross-linking can occur ... 

The acetylacetonate complexes of cobalt(II) and manganese(111) are efficient catalysts for the thermally intiated oxidation of tetralin, but do not influence the photoinitiated process. The reverse situation is observed for the iron(III) and cobalt(III) complexes [70a]. The thermal oxidation can be influenced by the addition of free-radical initiators like t-butyl hydroperoxide or 2,2 -azobisisobutyronitrile [70b]. 

Some initiators can function as both, thermal and photoinitiators. Such an initiator, for instance, is 2,2 -azobisisobutyronitrile. Also, Engel and coworkers [16] reported synthesis of an initiator that can function both as a thermal free radical initiator and a photoinitiator (see Sect. 3.2.4). It can be illustrated as follows ... 

Also, it was found that hexaarylbisimidazole will initiate polymerizations as a result of irradiation with visible light. The same is true of bisacylphosphine oxde. Other eompounds are ketocoumarins that are efficient photoinitiators for acrylic and methacrylic monomers in the presences of amines, phenoxy acetic acid, and alkoxy pyridinium salts. It was also shown that free-radical initiation is possible through visible light decomposition of ferrocenium salts in a three component composition, combined with either a hydroperoxide or a epoxide, and a third ingredient, a dicyanobutadiene derivative ... 

A comparison was made of the activity of xanthene dyes as free-radical polymerization photoinitiators that included succinylfluorescein and some of its halogenated derivatives as well as an ester of Eosin with an orrAo-benzoyl-a.-oxooxime group (shown below) under different experimental conditions. The polymerization studies were carried out on 2-hydroxyetiiyl methacrylate with ethylene glycol dimethacrylate as cross linkers using visible light irradiation. N,N,- dimethylaniline was the co-initiator. The polymerization rate obtained with xanthene dyes was found to be similar to or as much as 4.5 times higher (in the case of the Eosin ester) than that reached with Eosin alone. ... 

The majority of commercial methacrylic ester polymers are produced by free-radical initiators. Peroxides and azo compounds ftinction as t5ipical initiators for this type of polymerization. Other possible routes for producing methacrylic polymers with radicals include photoinitiation and radiation-induced polymerization. Both Y ray and electron-beam radiation have been employed in the production of methacrylic ester polymers (36-38). At constant temperature, there is a first-order dependence of the polymerization rate on monomer concentration and a one-half-order dependence on initiator concentration. Rate data for the polymerization of various methacrylic monomers using the azo compoimd 2,2 -azobisisobut5ironitrile [78-67-1] (AIBN) are shown in Table 8. 

In order to produce free radicals that initiate polymerization, photoinitiators absorb light of a certain frequency. Upon absorption, the photoinitiator molecule is promoted from the ground electronic state to either a singlet or triplet excited electronic state. This excited molecule then undergoes either cleavage or reaction with another molecule to produce initiating free radicals. Numerous photoinitiators have been developed to meet the needs of a variety of photopolymerization systems, as described in a number of recent papers and reviews (3-6,8-12). 

The biggest factor that accounts for the wide range of photoinitiation rates in Figs. 9 and 10, is the method of polymerization of the acrylic copolymer, namely, the type and amount of initiator and the solvent used in the polymerization. Acrylic polymers are prepared by free-radical polymerization. Typical free-radical initiators... 

UV dryers are intended for curing of coatings. UV-curable coatings consist of a blend of reactive monomers or oligomers capable of free-radical-initiated polymerization. Photoinitiators are used with UV-curable coatings and they are the source of free radicals produced on irradiation. 

Free-radical initiators trigger the cross-linking reaction. In EB-cured adhesives, the electrons act as free-radical initiators for addition polymerization. Therefore, no chemical initiator additives are needed. In UV-cured adhesives, photoinitiators, which release free radicals when exposed to UV radiation, are required to initiate addition polymerization. The most recent UV- and EB-curing systems involve cationic polymerization mechanisms. 

Free-radical initiated copolymerization can be accomplished either by conventional initiation with organic peroxides or hydroperoxides or phytochemically by using a photoinitiator in UV light. A detailed discussion of UV cured systems is outside the scope of this section therefore, only conventional initiation Is discussed here. The conventional curing of unsaturated polyester resins can proceed either by... 

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