Polyenyl free radical

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

Up to now we have been able to observe 13 separate absorption bands in the ultraviolet region of the spectrum between 220 and 400 m/. The shift in the Amax of polyenyl free radicals to longer wavelengths with increasing n would be expected to be linear with n, similar to the... 

On heating, migration of a hydrogen atom, alpha to a polyene group, to an alkyl free radical site would also convert a polyene to a polyenyl free radical, a transformation similar to transformation of the allyl free radical described above ... 

Free radicals have been detected in irradiated polyethylene by ultraviolet and infrared absorption [60—64]. The Am ax for allyl and polyenyl free radicals are given in Table 6 and compared with the corresponding... 

When polyethylene is irradiated at room temperature, allyl and polyenyl type free radicals are formed (2) and are much more stable than alkyl type free radicals (7, 13). Electron spin resonance (ESR) experiments have demonstrated that the allyl or polyenyl free radicals are quite stable, persisting for relatively long periods (hours or days)... 

In studying the polyenyl free radical decay at temperatures above room temperature by ultraviolet spectroscopy, the polyethylene was melted onto a quartz disc in vacuo, and, after cooling, irradiated, and subsequently annealed while firmly stuck to the quartz disc. This was... 

Oxidation reactions in polyethylene, which has a lipid-like molecular structure, also follow a similar mechanism of oxidation as lipids do in vivo[14—16]. In irradiated UHMWPE, the carbon free radicals resulting from the breakage of the C-H bonds are prevalent [17], and these radicals are alkyl, allyl, and polyenyl free radicals [18], the latto- formed at higher radiation doses [19]. Most of these free radicals recombine in the amorphous portion of the polymer [20], with the... 

In addition to ESR spectroscopy, which is a general method for detecting radicals, Dole et al. (9, 10, 11, 12) have developed a method of ultraviolet spectroscopy at low temperatures, which is specific for allylic and polyenylic radicals. Numerous papers have dealt with changes in polymers on irradiation, and all of these conclude that the reactions, in one way or another, arise from the formation of free radicals. Only a few papers describe experiments in which the radicals have been observed directly by ESR or ultraviolet spectroscopy at low temperatures. This article merely summarizes the present knowledge of the nature of radicals formed in polyolefins by irradiation in vacuum (ionizing radiation and ultraviolet light) and discusses some new trends in studying these radicals. 

A minor part of the spectra had a singlet contour which was assigned to polyenyl radicals, —(—CR CR2 ) CH—, where R2 and R2=H or CH3, and n is about 3, judging from the linewidth. Free radicals were found to be stable at —196° C but decayed sharply near the glass-transition temperature. Trans-l,4-polyisoprene irradiated with UV light gave a similar spectrum to that of the ds-form. 

ESR spectra of polycarbonate irradiated with ultraviolet light were reported by Hama et al. (120) and Lyons et al. (121). The spectrum observed after irradiation at -196° C was a singlet of A Hmsl = 16 gauss and having a g factor of 2.0045. This spectrum was attributed to three kinds of free radicals, phenoxy-type radicals, phenyl radicals and polyenyl-type radicals (120). The first two radicals are produced by the following reactions (69). 

What, specifically, are the first-formed X intermediates Their identity is not entirely clear. Nevertheless, some reasonable possibilities are polyenyl cation monoradicals, polyenyl cation diradicals, and chlorine atoms formed by the 3-scission of one or both of those species [12], In any event, it now appears that the prevention of autoacceleration by the ester thiols is likely to result, at least in part, from free-radical scavenging. Thiols, in general, are well-known to react very readily with a large variety of free radicals via equation 5, and the resultant... 

Here X - chromophore (CA or photoinitiator) Ri - acetoxyalkyl radical with high reactivity R2 - low-active polyenyl radical with free valency, conjugated with double bond AcOH - acetic acid Ki - Ke - constants of the rate of corresponding reactions. K3 = K3 y -l-Kj , where K3 t and K , - constants of the rate of the process in the darkness and under light action. 

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