Autoxidation chain scission

Since alkoxy radicals are known precursors to chain scission in autoxidation (24), the "hot" alkoxy radicals formed as shown should undergo facile chain scission or fragmentation. The chain scission is illustrated for the alkoxy radical derived from either the ethylene or propylene monomer unit in EPM ... 

Thermal Degradation. Thermal degradation follows different paths depending upon whether moisture and oxygen are present. Above 140 °C, however, moisture does not influence degradation. Continuous exposure to heat below the pyrolysis temperature can produce chain scission and autoxidation within the cellulose chains (23,24). To produce thermally degraded samples for evaluation, cotton fabrics were exposed in a forced convection oven at 168° 4°C. Fabric was removed at intervals ranging from 24r-211 hr for examination. 

The decomposition may occur either uni- or bimolecularly to form alkoxy and peroxy radicals. These oxy radicals abstract a labile hydrogen from the hydrocarbon to produce either an alcohol or a hydroperoxide. The alkyl radical thus formed readily adds oxygen to reform a peroxy radical, and the process continues. When the autoxidation occurs In the absence of an antioxidant, the termination of the kinetic chain occurs chiefly by the combination of two peroxy radicals. This termination Is a source of alkoxy radicals which can undergo chain scission and give rise to volatile products and carbonyl groups. 

Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Exposure of polyolefins to UV radiation in the presence of air leads to autoxidative reactions with the incorporation of oxygenated groups into the polymer. The increase in UV absorbance (measured at 310 mn) of irradiated polypropylene is reported to be too low to measure for wavelengths of irradiation in the range of 260-600 nm. Even at 260 nm, only a minimal increase was observed [121], For polypropylene containing flame retardants, either DBDE (decabromodiphenyl oxide) or TBA (tetrabromobisphenol A) at a 2% level, the maximum efficiency of chain scission was observed at X. = 280 and 260 nm, respectively. 

The primary photolytic reactions postulated are not themselves self-propagating, but provide the variety of radical species involved in subsequent oxidation and chain scission reactions characteristic of PET. The extent of oxidation reaction will depend in part on the balance of scission reactions in the initial process. Once established, the autoxidation cycle has a very long kinetic chain length, and will result in a rapid build-up of hydroperoxide in the polyester. 

Several more complex species may also result from addition of H- to the polymer, especially on the aromatic rings. The presence of oxygen will lead to an autoxidation cycle to be established. The relative importance of chain scission versus crosslinking in PET will be very dependent on conditions, e.g., humidity, oxygen levels, crystallinity, dosage, dosage rate and temperature. 

Photo-initiated oxidation accompanied by scission With PE and PP, light-initiated oxidation is the predominant reaction, and extensive main-chain scission, sometimes accompanied by cross-linking, takes place with rapid loss of mechanical integrity. The autoxidation cycle associated with photo-initiated scission is discussed in detail in Eigure 6.3. 

That epoxide groups are also present in oxidized natural rubber has been demonstrated by Golub et al (1975) by the use of H NMR and NMR spectroscopy. The presence of occasional epoxide groups is not believed to have a great effect on the properties of natural rubber but it may be noted in passing that, in theory, it provides sites for cross-linking by such materials as amines and acid anhydrides which are well known as epoxide resin hardeners. At the present time the above reaction sequence is of more interest in that it provides a route for converting peroxy radicals into alkoxy radicals and is believed by some workers to play an important role in chain scission due to autoxidation. 

The bulk of the discussion so far has centred round main-chain scission as opposed to cross-link scission. It is of interest to establish the extent to which the latter also occurs during autoxidation. 

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