Peroxidisable Polymers

Four main types of polymer are currently accepted as being environmentally degradable. They are the photolytic polymers, peroxidisable polymers, photo-biodegradable polymers and hydro-biodegradable polymers. Commercial products may be composite materials in which hydrolysable and peroxidisable polymers are combined (e.g. starch-polyethylene composites containing prooxidants). The application, advantages and limitations of each group will be briefly discussed. 

This order reflects the increasing ease of abstraction of the weakest P-H bonds (namely the methylenic hydrogens) in reaction (4) [14]. Consequently blends of saturated polymers with rubbers or co-polymers of saturated and unsaturated polymers peroxidise more rapidly than the saturated polymers themselves and this kind of modification has sometimes been used to increase the rate of bioassimilation of polymers through environmental peroxidation. 

Some organic water-soluble polymers, of which poly(vinyl alcohol), PVA, and poly(ethers) [ e.g. poly(ethylene glycol), PEG] are the most important, are rapidly peroxidised both abiotically and by microorganisms at ambient temperatures. 

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. 

Although E/CO initially photodegraded to fragments more rapidly than S-G and E-St, photo-degradation of the transition metal ion catalysed systems continued to a much lower molar mass. After fragmentation, the peroxidised polymers were incubated in the absence of any other source of carbon with three microorganisms isolated from soil in the vicinity of discarded polyethylene. Two were bacteria (Nocardia asteroides and Rhodococcus rhodochrous) and one was a fungus Cladosporium cladosporioides). It recently has been shown by Delort and co-workers that biofilm formation is very rapid on the surface of peroxidised polyethylene (Fig. 1). 

It will be seen in Section 4 that the rate of the abiotic and hence the biodegradation process can be readily controlled by antioxidants whereas no comparable control process has yet been developed for hydro-biodegradable polymers. A second conclusion was that starch plays no part in the biodegradation of a polyethylene matrix until the latter has been extensively peroxidised in the presence of transition metal ions. Similar conclusions have been reached by Wool [56] who showed that in the absence of PE degradation in starch-PE blends, biodegradation is controlled by the rate of migration... 

Chiellini et al. [58] extracted thermally peroxidised polyethylene with acetone and measured the rate of mineralization of the solvent free extracts in forest soil. This is compared with cellulose and a number of low molar mass control hydrocarbons in Fig. 2. Surprisingly, the peroxidation products were converted to carbon dioxide and water more rapidly than cellulose. The extracted polyethylene degraded at a similar rate to the pure hydrocarbons and it is evident from this work that the rate controlling process in the overall sequence of degradation reactions is the initial peroxidation of the polymer. It has been demonstrated [19] that the exposure of peroxidised PE to an abiotic water-leaching environment did not remove the peroxidation products from the polymer, whereas bioassimilation began immediately (see Fig. 2)... 

Increasing peroxidisability and biodegradability Scheme 2 Relative stability of hydrocarbon polymers in the environment[8]... 

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