Abiotic peroxidation

In the case of PVA, which is one of the few carbon-chain polymers that does not need to be abiotically peroxidised before biological attack, polyketones are the initial products and these are catabolysed to carboxylic acids and bioassimilated by bacteria e.g. Pseudomonas). Kawai has shown (personal communication) that the PEGs behave rather similarly and are bioassimilated by Sphingomonas. The active enzymes are believed to be PEG dehydrogenase coupled with cytochrome c, the oxidase enzyme of the baeterial respiratory system. Since the polyethers are also very peroxidisable abiotically, abiotic peroxidation may also play a part in the overall process. 

Bioassimilation of polymers is primarily controlled by the rate of abiotic peroxidation or hydrolysis. Information is available in the published literature which could form the basis of biodegradation tests but has so far not been incorporated in standard test methods. However, in the long-term it is important that science-based test methods should be developed to permit users of degradable materials in agriculture, shipping and packaging to take advantage of these new materials. 

Abiotic peroxidation of cis-PI occurs at ambient temperatures so long as oxygen is present in the system. However, oxo-biodegradation also proceeds... 

It has been pointed out that some actinomyctes can utilise CO2 as a source of carbon . It is therefore necessary to equate microbial growth and associated formation of protein to loss of weight of the substrate. Table 2, taken from the work of Heisey shows that there is indeed a broad correlation between mass loss and protein formation and Delort and co-workers have shown that loss of carboxylic acids formed during abiotic peroxidation of PE correlates with the formation of protein and polysaccharides, almost certainly associated with the cross-linked bacterial cell wall structure. 

Nocardia and P. aeruginosa were shown to break the cw-PI chain by an oxidative mechanism since aldehyde groups were found to accumulate during microbial degradation. This is always the first product formed during the abiotic peroxidation of cw-PI and the evidence suggests that the bacteria initiate a radical-chain peroxidation. This will be discussed further in the context of polyolefin biodegradation. 

Chlorinated polymers are much more resistant to abiotic peroxidation than pure hydrocarbon polymers and nitrile rubbers, although susceptible... 

A good deal is now known about the kinetics of abiotic peroxidation and stabilisation of carbon-chain polymers and it is possible in principle to extrapolate to the time for ultimate oxidation from laboratory experiments. As already indicated, the key determinant of the time to bioassimilation is the antioxidant and if this is chosen to optimize the service life, bioassimilation can also be achieved as in the case of wood, straw, twigs, etc. It seems that straw is a particularly appropriate model for the biodegradation of the polyolefins since, like the polyolefins, it fully bioassimilated in biologically active soil over a period of about ten years. The most important conclusion from recent work is that nature does not depend on just one degradation mechanism. Abiotically initiated peroxidation is just as important, at least initially as biooxidation. 

The hydroxyl radical is one of the most reactive free radicals known in chemistry. It is 10 more reactive than hydroperoxyf and it extracts a hydrogen atom at every encoimter with a hydrocarbon. It is thus also one of the most potent initiators of peroxidation known. It is not surprising then that the subsequent polymer degradation reactions are dominated by abiotic peroxidation chemistry. 

As with the poly (dienes), the polyolefins peroxidise to biodegradable products through the intermediate unstable hydroperoxides. Scheme 3 illustrates this process for poly(ethene), (PE). Poly(propene) (PP) peroxidises much more rapidly than PE due to the tertiary carbon atom (Scheme 2) and the higher proportion of vicinal hydroperoxides formed [6,9]. In-chain hydroperoxide formation continues steadily in all the polyolefins provided oxygen is available and the rate of biodegradation correlates with the rate of abiotic peroxidation. 

Since abiotic peroxidation and bio-oxidation occur together during the... 

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