Hydroperoxide groups polyolefins

Hydroperoxide groups react with NO to give only nitrates as the dominant products, with only traces (< 5%) of nitrite in both oxidized polyolefins and in concentrated solutions of model hydroperoxides (-OOH levels from iodometry -ONO and -ON02 levels by IR). As reported by Shelton and Kopczewski we have confirmed that both nitrate and nitrite result from NO reaction with dilute hydroperoxide solutions (24). Rather than the NO-induced 0-0 scission proposed by these authors, our evidence points to hydrogen abstraction by NO (reaction 4). (A similar scheme may explain nitrite formation from alcohols.) Both e.s.r. and FTIR evidence is... 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

In consequence of the intramolecular chain transfer in the oxidation of polyolefins, more hydroperoxide groups than one is formed at one macromolecular skele-... 

Polyolefin products undergo photodegradation by the absorption of UV light. This is due to the unstable product formation of polyolefins during fabrication process, due to the presence of impurities like carbonyl or hydroperoxide groups [30, 31]. 

It should be noted that reaction 6 does not lead to polymer chain scission ie, there is no molar mass reduction here, in contrast to reaction 5 where there is a molar mass reduction. It should also be noted that the formation and reaction of hydroperoxide groups attached to polymer molecules (reactions 3 and 4) are much slower, ie, rate determining, than the other processes shown. It will be apparent from the free-radical products formed in reactions 1 through 4 that this whole peroxidation procedure is a branching chain reaction. Additive chemistry is required to provide polyolefins with any sort of prolonged service fife, and such chemistry is well known and entirely effective. 

A Review of Methods for Detecting and Characterizing Hydroperoxide Groups m Oxidized Polyolefins, Polymer Plastics Technology and Engineering, 34 (1945), p. 97... 

In the case of the polyolefins, random chain scission is initially the dominant process. This is shown typically for polypropylene in Scheme 2. However some low molar mass oxidation products are formed via vicinal hydroperoxides in both PP and PE [20]. The alkoxyl radicals formed by decomposition of the hydroperoxides contain weak carbon-carbon bonds in the a positions to the hydroperoxide groups, which lead to the formation of low molecular weight aldehydes and alcohols that rapidly oxidise further to carboxylic acids. These are biodegradable species, similar to products formed by hydrolysis of aliphatic polyesters and, as in the case of cis-PI, they are rapidly bioassimilated to give cell biomass (see below). 

The mechanism of autoxidation of solid polyolefins RH has many common features with hydrocarbon oxidation polymers are oxidized by the chain route the chain develops as alternation of acts of R- with O2 and RO with RH in the absence of an inhibitor and at a sufficiently high [O2] in polymer, chain oxidation occurs with autoacceleration because the hydroperoxide groups that formed are a source of initiation. However, there are several substantial distinctions. As already mentioned in Chapter 6, reactions resulting in polymer destruction play an important role. In initiated oxidation the main source of destruction is reactions of peroxide radicals, whereas in autoxidation the contribution of alkoxy radicals, which decompose in the reaction of the type... 

The individual processes shown above (reactions 3.1 to 3.9) do not by any means represent all the relevant chemistiy that occurs upon the exposure of unstabiUzed polyolefins to heat and/or UV light in the environment. What these equations do emphasize, however, is the involvement of the hydroperoxide groups in the molar mass reduction phenomenon. It must also be emphasized that the formation of these groups is rate-determining in the heat or UV-Iight initiated oxidative degradation. The only major difference between these two types of initiation is that the intermediate ketones that are formed are UV-... 

A series of reactions with gases have been selected for the rapid quantification of many of the major products from the oxidation of polyolefins. Infrared spectroscopy is used to measure absorptions after gas treatments. The gases used and the groups quantified include phosgene to convert alcohols and hydroperoxides to chloroformates, diazomethane to convert acids and peracids to their respective methyl esters, sulfur tetrafluoride to convert acids to acid fluorides and nitric oxide to convert alcohols and hydroperoxides to nitrites and nitrates respectively. 

NO Reactions. The most informative derivitization reaction of oxidized polyolefins that we have found for product identification is that with NO. The details of NO reactions with alcohols and hydroperoxides to give nitrites and nitrates respectively have been reported previously, and only the salient features are discussed here (23). The IR absorption bands of primary, secondary and tertiary nitrites and nitrates are shown in Table I. After NO treatment, y-oxidized LLDPE shows a sharp sym.-nitrate stretch at 1276 cm-1 and an antisym. stretch at 1631 cm-1 (Fig. 1), consistent with the IR spectra of model secondary nitrates. Only a small secondary or primary nitrite peak was formed at 778 cm-1. NO treatment of y-oxidized LLDPE which had been treated by iodometry (all -OOH converted to -OH) showed strong secondary nitrite absorptions, but only traces of primary nitrite, from primary alcohol groups (distinctive 1657 cm-1 absorption). However, primary products were more prominent in LLDPE after photo-oxidation. 

Polymers with mesogenic groups directly attached to backbone, properties, 97,98/ Polyolefin hydroperoxides and alcohols, IR bands of nitrates and nitrites, 384/ Polyolefins... 

Reviews appeared on the following subjects Analysis of lipid hydroperoxides , the difficulties encountered for hydroperoxide analysis in a plasma matrix , post-column derivatization after GLC of lipid hydroperoxides and methods for detection and characterization of hydroperoxy groups in oxidized polyolefins . 

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