Side-group elimination

This is usually a two-stage process in which the polymer chain is first stripped of atoms or molecules attached to the backbone of the polymers, leaving an unsaturated chain. This polyene then undergoes further reactions, including scission, aromatisation, and char formation. 

It is known that, in the absence of oxygen, PE is thermally stable. The thermogravimetric analysis (TGA) of this polymer shows that decomposition of this polymer begins at about 280 °C, and when the temperature is near to 350 °C, its thermal degradation proceeds rapidly with the elimination of considerable quantities of volatile materials. The half-life temperature (i.e., that leading to 50% weight loss on heating for 40-45 

Analysis of the degradation products of PE produced under vacuum and under atmospheres of nitrogen and helium at different temperatures made it possible to establish that the higher the decomposition temperature, the lower the molecular mass 

A comparison of the decomposition processes of three PE samples (commercial PE having a molecular mass of 20,000, unbranched high molecular mass polymethylene and strongly branched PE) differing in structure and molecular mass has enabled the characterisation of substantial differences in their thermal degradation. 

Studies on the rate of thermal degradation of unbranched high molecular mass PE (polymethylene) have shown that the process follows almost ideal first-order kinetics. The rate of thermal degradation is directly proportional to the temperature of the process. The dependence of the rate of elimination of volatiles on their quantity 

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Besides the water elimination from the position 2,3 of the glucose unit as indicated above, other elimination positions such as 1,2 or 3,4 or 3,6 are possible. Because the side group elimination does not affect the DP value of the polymer, this reaction must be followed by the cleavage of the polymeric bonds to generate smaller molecules. These small molecules are the ones identified in cellulose pyrolysis products by techniques such as Py-GC/MS. 

Py-GC-MS can also be used to identify the polymer type in a plastic (Learner, 2001). The principle of the technique is based on the use of heat to volatilize and break down macromolecules into smaller components capable of being analysed using GC-MS (Table 5.6). Degradation mechanisms for polymers under pyrolysis are free radical processes initiated by bond dissociation due to the heat. The specific pathway followed by a particular polymer is dependant on the strength of the polymer bonds and the structure of the polymer chain. Pathways may be described as random scission, unzipping and side group elimination. Polyolefins such as polyethylene and polypropylene follow the random scission pathway and break into pieces of the original molecule to form... 

Thermal degradation of polymers usually proceeds via a number of possible mechanisms, which can generally be grouped into three classes (a) random scission, (b) depolymerisation, and (c) side group elimination. 

Side-group elimination (Figure 8A), which involves loss of pendant groups on the polymer backbone to form a polyene. Further reaction involves scission, aromatization, and char formation. Typical is the case of PVC, which eliminates HCl followed by backbiting to form benzene, toluene, naphthalene, and other aromatics. 

Another important example of a polymer which gives coloration due to side-group elimination is polyacrylonitrile (5), which colors slowly when heated and very rapidly on exposure to alkalis. This is usually attributed to the formation of conjugated, cyclic structures by reactions of the form... 

During pyrolysis, polymeric materials may degrade via a number of mechanisms that are generally grouped into three classes random scission, depolymerisation, and side group elimination. 

It has also been reported [a.l59] 863996 that some degradation occurs by side-group elimination, leading to the formation of unsaturated products. It has also been claimed that side-group elimination is a more dominant process than chain scission initiation. 

Many polymers break down in successive stages as the temperature is raised. For example, a side group elimination reaction will normally be followed at higher temperatures by fragmentation of the residual, modified polymer chain. The observed behaviour may then depend on whether the temperature is raised gradually or rapidly. 

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