Polymer molecular degradation products

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

Applications Rather intractable samples, such as organic polymers, are well suited to FD, which avoids the need for volatilisation of the sample. Since molecular ions are normally the only prominent ions formed in the FD mode of analysis, FD-MS can be a very powerful tool for the characterisation of polymer chemical mixtures. Application areas in which FD-MS has played a role in the characterisation of polymer chemicals in industry include chemical identification (molecular weight and structure determination) direct detection of components in mixtures off-line identification of LC effluents characterisation of polymer blooms and extracts and identification of polymer chemical degradation products. For many of these applications, the samples to be analysed are very complex... 

The mechanisms of photochemical degradation of a polymer are rather difficult to study as the observed degradation is typically a result of the effect of light on the contaminants in the polymer rather than the pure polymer. Studies typically look for the degradation products, either low molecular weight volatiles or radicals, or work to identify the wavelength of light absorbed by the system via spectroscopic methods. 

Pyrolysis GC/MS is limited in application to those studies in which the compounds formed are capable of being analysed by GC, that is it is only reasonably suitable for low molecular weight products. Many synthetic polymers degrade (pyrolyse) by processes that may simply be described as either random scission (e.g., polyolefins), unzipping to produce mostly monomers (e.g., PMMA)... 

A popular theory with azo materials is that their degradation products are always aromatic amines, like azo dyes. Ueda and co-workers observed that the azo bonds in segmented polyurethenes were reduced to hydrazo intermediates after incubation with human feces, since no decrease in the molecular weight was observed [73]. It was then theorized that drug release from pellets coated with these azo polymers was due to both a conformational change and a breakdown of the film structure. Other studies also concluded that the polymers were reduced to hydrazo intermediates or were completely degraded to aromatic amines depending upon their hydrophilic/ hydrophobic nature. 

Polymerization of N-vinylimidazole deviates from the polymerization of conventional vinyl polymers such as styrene or methyl methacrylate. The process leads usually to low molecular weight products, which is due to degradative addition. This process can be illustrated by the reaction ... 

Amide urethane, and ester groups in the polymer chain, such as those present in nylons and polyesters may be hydrolyzed by acids to produce lower-molecular-weight products. Polyacetals are also degraded by acid hydrolysis, but ethers, such as polyphenylene oxide (PPO), are resistant to attack by acids. 

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