Side group scission

The evolution of carbon monoxide, carbon dioxide and methane must be ascribed to side-group scission according to reactions (35) and (39). Methane formation is due to hydrogen abstraction by methyl radicals but, since the yield of methane is lower than the yield of carbon monoxide plus carbon dioxide, only a fraction of the methyl radicals produced are able to escape the cage. Most of them recombine with macroradicals according to reactions (38) and (41). Chain scission occurs according to... 

If the parent cation is unstable, it can decompose into smaller fragments at 77°K before electron—cation recombination. Such a mechanism has been proposed for the formation of volatile products resulting from side-group scission in polymethylmethacrylate, viz. 

Volatile product evolution has been observed in the photolysis of many vinyl polymers as a consequence of side-group scission. The nature of these volatile products is therefore related to the side group. Carbon dioxide, carbon monoxide and methyl formate are produced from polymethylmethacrylate [12], carbon dioxide, carbon monoxide, methane and acetic acid in the photolysis of poly vinylacetate [13], and hydrogen chloride in the photolysis of polyvinylchloride [14]. 

Side group scission of poly (vinyl acetate) produces a conjugated polyene and acetic acid. 

The question of which degradation mechanism a particular polymer will be subjected to — random scission, side group scission, monomer reversion, or a combination of these — is simplified by considering the nature of thermal degradation as a free radical process. All of the degradation products shown, as well as minor constituents, and deviations to the simplified rules are consistent with the following general statements ... 

Aromatic Polyester (thermoplastic) Light absorption by ester functionality, followed by main chain or side group scission. Norrish 1 and II reactions. [110-113]... 

The side-group scission does not account for more than 3% of main chain reactions. 

Both side-chain and main-chain scission products are observed when polyacrylates are irradiated with gamma radiation (60). The nature of the alkyl side group affects the observed ratio of these two processes (61,62). 

Benzene rings in both the skeleton structure and on the side groups can be subjected to substitution reactions. Such reactions do not normally cause great changes in the fundamental nature of the polymer, for example they seldom lead to chain scission or cross-linking. 

Microstmcmral changes on irradiation have been observed by IR and UV spectroscopy. Changes in absorption bands due to vinyhdene double bonds [356,357], substituted double bonds, and ethyl and methyl groups give a measure of modifications in the presence of radiation. The ratio of the double bonds (located mainly at the end of a polymer chain) and scission is reported by some investigators [356-358] and found to be independent of temperature and dose. This is beheved to be due to the reaction of the methyl radical side group with hydrogen atoms on the backbone of the parent chain. 

The hyperfine splitting constants (20 and 7 gauss) found for this radical agree well with the splitting constants found for the allylic radical (III) in polyethylene. The sextet spectrum observed at —196°C. is thought to arise from radicals of the structure XVI and/or XVIII, which could be formed by C—H bond scission in the side chain and by abstraction of ethyl side groups, respectively. 

Chain-stripping the reactive substituents or side groups on the polymer chain are eliminated, leaving an unsaturated chain. This polyene then undergoes further reaction, including (3-scission, aromatization and coke formation. (Polyvinylchloride, polyvinyl fluoride, polyacrylonitrile)... 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

Side group reactions are common during pyrolysis and they may take place before chain scission. The presence of water and carbon dioxide as main pyrolysis products in numerous pyrolytic processes can be explained by this type of reaction. The reaction can have either an elimination mechanism or, as indicated in Section 2.5 for the decarboxylation of aromatic acids, it can have a substitution mechanism. Two other examples of side group reactions were given previously in Section 2.2, namely the water elimination during the pyrolysis of cellulose and ethanol elimination during the pyrolysis of ethyl cellulose. The elimination of water from the side chain of a peptide (as shown in Section 2.5) also falls in this type of reaction. Side eliminations are common for many linear polymers. However, because these reactions generate smaller molecules but do not affect the chain of the polymeric materials, they are usually continued with chain scission reactions. 

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid ... 

The side group reactions with water elimination take place at lower temperatures of about 350° C, while chain scissions are predominant at higher temperatures. The Ei water elimination reaction can be written as follows (see Section 2.1) ... 

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