Homolytic scission, thermal degradation

It has been noted that initial work on the thermal degradation of PET suggested that homolytic (i.e., free radical-based) scission did not have a role, but this was not confirmed and, in any case, certain conditions may exist where such reactions might come into play. At very high temperatures (or under certain conditions in processing machinery where formation of mechanoradicals may occur), homolytic scission of the polyester chain may occur [7-10]. The weakest link in the PET chain would appear to be the sequence carbonyl-oxygen-methylene, which may be expected to homolytically cleave in two possible ways ... 

In general, a homolytic reaction is proposed, i.e., chain scission followed by hydrogen abstraction. Only aromatic hydrogens are available, so this leads to formation of large percentages of char by the end of the thermal degradation process. 

Notwithstanding this complexity, some semiempirical rules could be identified by analogy with small organic model molecules, and by considering that thermal degradation is a homolytic bond scission process initiated by thermally activated molecular vibrations ... 

Conversely, the thermal degradation reactions of polyamides such as PA-6,10, for example, produce mostly linear or cyclic oligomeric fragments and monomeric units. Primary polyamide chain scission (C(O)-NH or NH-CH2 bonds), hydrolysis, homolytic scission, intramolecular C-H transfer and ds-elimination are all suggested from the product distribution as possible mechanisms 502570. For PA-12, it has been found that lactams (cyclic monomer and dimer) are the major primary products of the thermal degradation however, olefinic nitriles and a-olefins were also found. In the case of PA-6,12 the formation of cyclic oligomers has been found to occur as a result of thermal degradation, but also some dinitriles have been identified 502570. ... 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

The degradation of a chain can be induced chemically, thermally, mechanically, ultrasonically, or photolytically. Chemical reactions are, for example, the hydrolyses of polyesters, polyamides, or cellulose, or the ozonolysis and subsequent ozonide scission of polydienes. Thermal scissions occur homolytically or heterolytically according to the temperature and the initial polymer. 

As outlined in a simplified mechanisms in Fig. 4.1, degradation proceeds through a radical chain mechanism (2,3). Initiation typically occurs through exposure to heat generated during production. Trace metal impurities such as copper or iron accelerates radical formation. Reactive hydroperoxides are formed after reaction of the carbon-centered radical with oxygen. Thermally induced homolytic cleavage of hydroperoxides leads to additional reactive radical formation and subsequent polymer chain scission. 

There are many similar attributes of thermal-, photo- and radiafion-degradafion. They progress on the similar mechanisms based on the homolytic or heterolytic scissions and the formation of degradation initiators for loop process of oxidative degradation. The discrepancies between these three ageing ways consist of the concentration of primary free radicals and their distribution in the material. The depth of degradation is only some microns, but the diffusion of chain promoters, hydroperoxides, towards the inner layers of materials leads to a parabolic distribution of oxidation products around the symmetry axe. 

Radical reaction pathways govern the crosslinking and degradative reactions of UHMWPE. For these reactions to occur, macroradicals must be induced in the polymer, for example by thermal decomposition of hydroperoxides or by high-energy radiation, which leads to homolytic bond scissions with the production of alkyl macroradicals. In previous chapters, we have often referred to crosslinking... 

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