;H OH Polyvinyl alcohol

Scheme 4. Hydrolysis of carboxylated poly(vinyl alcohol) derivatives.

et al. [75]. The current polymers have little biodegradability at preferred performance molecular weights, ca. 5000 Dal for polyacrylic acid and 70,000 Dal for copolyacrylic-maleic acids. This, even though there are data to indicate no harmful environmental effects, has resulted in a massive search for biodegradable replacements. The value of negative results mentioned earlier is strongly reflected here.

Many efforts to radically copolymerize acrylic and maleic acids with a whole range of vinyl monomers to produce biodegradable polymers [76-83], and graft substrates, including polysaccharides [84], have failed to produce completely biodegradable water-soluble polycarboxylates. Therefore, it must be considered that the pioneering biodegradation work of Suzuki et al. [41] with carbon chain backbone functional polymers is probably correct and only low-molecular-weight oligomeric carbon chain polymers are likely to be biodegradable, at least at any reasonable rate. Recent confirmation of Suzuki's work has come from Kawai [85] and research in NSKK and Idemitsu Laboratories in Japan [86, 87].

As indicated earlier, Suzuki et al. [41] ozonized high-molecular-weight polyacrylic acid, polyacrylamide, and polyvinyl pyrrolidone to oligomers with molecular weights less than 1000 Dal and observed a marked increase in their biodegradability. The work of Kawai [85] and Tani [86] (NSKK) was based on carefully synthesized oligomers. The results all indicate that polyacrylic acids are not completely biodegradable above about a degree of polymerization of 6-8 (400-600 Dal). The work reported by Larson and Swift in their methodology [39] development also supports this result.

Other efforts to use radical polymerization to synthesize carbon chain biodegradable carboxylated polymers have been based on combining low-molecular-weight oligomers through degradable linkages and by introducing weak links into the polymer backbone. BASF [88] and NSKK [89] have patented acrylic oligomers chain branched with degradable linkages (X), as in Scheme 5. More recently, Idemitsu has patented variations on these themes [90]. Grillo Werke has patented copolymers of acrylic acid and enol sugars [91]. The degradability of all these polymers has not been categorically established, clearly related to the molecular weight control of the fragmented acrylic oligomers portion.

Scheme 5. Branched acrylic oligomers.

Several miscellaneous carbon chain polymers have been claimed as biodegradable without clear evidence. These include copolymers of methyl methacrylate and vinyl pyridinium salts [92-94], where the pyridinium salt is hypothesized as a "magnet" for bacteria that then cleave the chain into small fragments that ultimately biodegrade completely. Careful analysis indicates absorption on sewage sludge present for the laboratory test; and, since biodegradation was measured by TOC loss, the results were misleading and misinterpreted. The work at Deutsche Gold-und Silber [96] with copolymers of acrolein and acrylic acid post converted by a Cannizzaro reaction to copolymers containing hydroxyl and carboxyl functionality (Scheme 6), indicated high levels of biodegradability by BOD. However, the low-molecular-weight fragments and other organics present certainly added to this speculation and no further work has been reported.

Polymers of a-hydroxyacrylic acid [97] are prepared as indicated the Scheme 7 from the polymerization of alpha chloroacrylic acid and subsequent hydrolysis. The claims of biodegradability for these polymers [98-100] were not substantiated by Mulders and Gilain [97] using radiolabeled carbon polymers. There was no clear differentiation of absorption on sludge versus bacterial cellular absorption in the sewage testing protocol.

Mitsubishi [101] claimed a unique biodegradable polycarboxylate (Scheme 8) containing ethylene, carbon monoxide, and maleic anhydride monomers. The initial degradation step is photoactivation to yield low-molecular-weight fragments as indicated in Table 12.2. Unfortunately, no biodegradation data were reported on these fragments.

Scheme 6. Cannizzaro products from acrylic/acrolein polymers.

Scheme 6. Cannizzaro products from acrylic/acrolein polymers.

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