Hydroperoxides photolysis

By contrast, no similar accelerating action of TMP derivatives on hydroperoxide photolysis at RT has been observed either by us or other workers. ... 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

The most Importeuit Initiation process involved in the early stages of the photo-oxidation of polyethylene is shown to be hydroperoxide photolysis eissociated with the decay of virylidene groups. This is followed by carbonyl photolysis occurring primarily by the Norrish type II process. 

It seems unlikely then that carbonyl (ketone and aldehyde) formed by thermal breakdown of hydroperoxides are important sensitisers for photo-oxidation of LDPE in normally processed polymers. The evidence is consistent with the theory that all.ylic hydroperoxide derived from vinylidene is the important photo-initiator initially present under these conditions. Vinylidene disappears as a concomitant of hydroperoxide photolysis, initiating photo-oxidation in a manner analogous to its function in thermal oxidation. 

Hydroperoxide photolysis by sunlight in the presence of air is considered to be the major source of free radicals and backbone scission during the photodegradation of polypropylene [37]. 

Table V. Apparent Quantum Yields for tert-Butyl Hydroperoxide Photolysis in the Presence of Methyl tert-Butyl Ketone 0.31M Hydroperoxide in Hexane at 36°C, A — 322 nm...

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... 

This catalysis is normally a disadvantage but can be used to accelerate degradation. Further, hydroperoxides are very sensitive to photolysis at solar wavelengths. The extinction coefficients are very low, but the quantum efficiency for scission is close to 1. Hydroperoxide photolysis can be sensitized by pigments. 

Although there is little doubt that all can be important in particular circumstances, it is generally agreed that hydroperoxides are normally the key to the initial photoinstability in hydrocarbon polymers. Hydroperoxides are present in commercial PP at levels of around 10 - 10 mol/kg and have molar extinction coefficients at 310 nm of around 0.4 l/(mol cm). Carbonyl groups are typically present at around 10 mol/kg and have molar extinction coefficients at 310 nm of around 10 l/(mol cm) (112). However, the quantum efficiency for radical generation in hydroperoxide photolysis is 1, whereas it is of the order of 0.05 for carbonyl groups, partly because radicals formed by Norrish I reaction recombine rather easily and partly because of the preference for Norrish II chemistry, which does not give radicals. 

Hydroperoxides have very low extinction coefficients in the solar UV range, much lower than carbonyl groups. However, the photolysis of a hydroperoxide leads to scission of the 0-0 bond with a quantum efficiency of at least unity and sometimes greater. Thus hydroperoxide photolysis is an efficient initiator of photo-oxidation. 

During each photodegradation (in vacuum or air) small amounts of several gaseous and liquid low molecular weight compounds are formed and they can only be detected by chromatographic or mass spectrometry methods (cf. section 10.6). Table 2.1 shows, for example, products formed from polypropylene hydroperoxide photolysis. 

The hydroperoxide photolysis and chain scission are the most important initiating processes occurring in the photo-oxidation of polyolefins. 

Hydroperoxides represent, as it is generally accepted, ones of the most reactive species in the polymer photodegradation [41]. They can be induced during the processing of polymer, as well as during the subsequent exposure of polymers to light and heat in the presence of air. The present transitional metals accelerate the decomposition by radicalic process of the accumulation of hydroperoxides. In the case of polyolefin, the hydroperoxides photolysis leads to the formation of some carbonyl compounds and/or alcoxy radicals. 

The atmospheric lifetime of tert-butyl hydroperoxide with respect to reaction with OH radicals is of the order of few days. Similar to other hydroperoxides, photolysis could also have an important contribution to the atmospheric removal of tert-butyl hydroperoxide. The reaction of OH with tert-butyl hydroperoxide is expected to proceed through H-atom abstraction from the —OOH group to give (CH3)3C02 radicals which will react with NO, other peroxy radicals, and ozone as suggested in the previous sections for methyl and ethyl peroxy radicals. 

---