Photoinitiators-hydroperoxides

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Another way of photoinitiation is the anthracene photosensitized formation of hydroperoxides in the butadiene portion of the polymer (44). Vinyl monomers react preferably with the pendent double bonds of ABS (39). 

Hydroperoxides are potent photoinitiators and their decomposition into inactive products is an important method of stabilization in polyolefins for it prevents generation of radicals by reactions such as (4), (5), (14), and (15). Decomposition of hydroperoxides can be achieved by a reaction with phosphite esters or nickel chelates or by a catalytic action by a range of compounds including dithiocarbamates and mercaptoben-zothiazoles. Decomposition of hydroperoxides formed during processing is important for the stabilization ofPVC. 

Thus, analysis of literary data allows to come to a conclusion about presence of PA luminescence, related to the product of keto-imide structure, which is formed and consumed in chain processes, performing here the function of photoinitiator. Unlike traditional schemes of photoinitiation intermediate hydroperoxides does not play great role at photooxidative PA destruction. 

Two interesting conclusions can be drawn. First, there has been much controversy in the literature about the relative importance of the various possible mechanisms for initiation (ketone groups, hydroperoxide, catalyst residues, singlet oxygen), but the controversy is practically irrelevant if initiation does not much influence the course, rate or extent of photooxidation. Second, in terms of stabilization, minute amounts of photoinitiation will lead to practical failure so that the purification of polymers to minimize the initiating residue is not a practical alternative for stabilization. Both points underline the value of this approach to the understanding of the complex photooxidation process. 

Since pure PP does not incorporate chromophoric groups, it is clear that photoinitiation of radical degradation processes must involve chromophoric impurities. There has been a great deal of discussion of this in the past and hydroperoxides or carbonyl structures formed by oxidation of the parent polymer and transition metal residues from the polymerization catalyst seem to be the most likely candidates. It is not appropriate to discuss this aspect in the present paper, suffice it to say that the association of methane with photoinitiation, but not thermal initiation, suggests that photoinitiation involves C-CH3 bond scission to form chain side radicals in contrast to thermal initiation which involves scission of the C-C bond in the main chains. 

Photoinitiator/Peroxide or Hydroperoxide-Based Systems In these systems where useful effects occur [li], oxygen-centered radicals are generated the mechanism is not clear. In the same way, in the 1-benzoyl-cyclohexanol HAP/benzophenone BP system under air, the radicals that are formed after the a-cleavage of HAP consume oxygen and further allow the generation of a hydroperoxide ROOH whose decomposition is sensitized by BP, and so on. 

Poly(2,6-dimethyl-l,4-phenylene oxide) (PPO).— The photo-oxidation of this polymer has attracted some interest. Wandelt has found that the photoinitiated oxidation of PPO depends upon the mobility of one more unit in the polymer chain. A marked increase in the rate of photo-oxidation of the polymer occurs in the temperature range 45—60 °C, which corresponds with the j -relaxation phenomena. Chain mobility markedly controls the diffusion of oxygen. From detailed analysis of the products of PPO photo-oxidation the following reaction schemes have been proposed to account for hydroperoxide photolysis (Scheme 22) and quinone photoreaction with aromatic aldehydes to give aromatic esters, for example (16) (Scheme 23). The three chromophores are formed during... 

Bolland and Gee [16] reported the first detailed kinetic investigation of the oxidation of cyclohexene in 1946 using photoinitiation with the hydroperoxide at several concentrations of oxygen and cyclohexene, mostly at 10°C. They showed that the rate of oxygen consumption corresponded very closely to the rate of formation of hydroperoxide with a rate law... 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

Some nickel compounds, such as nickel dibutyl dithiocarbamate (NiDBC) and nickel acetophenone dioxime (NiOx), have been regarded as quenchers up to now. But neither of these two acts as quencher for carbonyl or singlet oxygen. Rather, they rapidly convert hydroperoxides into non-free-radical products at high temperatures, and thus remove the primary photoinitiators. These reactions are catalytic in the case of NiDBC, but stoichiometric in the case of NiOx. 

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]. 

Fouassier et al.f reported a four component photoinitiating system that consists of a photosensitizer, Rose Bengal, ferrocenium salt, an amine and a hydroperoxide, such as cumene hydroperoxide ... 

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 ... 

Figure 1 Polymer hydroperoxidation during processing and further photoinitiation by the hydroperoxides and the derived carbonyl compounds.

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