Heteroatom Chain Polymers

This class of polymers includes polyesters, which have been widely studied from the initiation of research on biodegradable polymers, polyamides, polyethers, and other condensation polymers. Their chemical linkages are quite frequently found in nature, and these polymers are considered more likely to biodegrade than the carbon chain hydrocarbon-based polymers discussed in the previous section.

Polyesters and Polyamides Bailey and co-workers developed a very clever free-radical route to polyesters that they used to introduce weak linkages into the backbones of hydrocarbon polymers and render them susceptible to biodegrad-ability [101-106]. Copolymerization of ketene acetals with vinyl monomers incorporates an ester linkage into the polymer backbone by rearrangement of the ketene acetal radical as it incorporates into the polymer chain. The ester bond produced is a potential site for biological abiotic attack to yield low-molecular-weight fragments likely to biodegrade. Bailey demonstrated the chemistry with ethy-lene; see Scheme 9, and it has been extended to copolymers of acrylic acid [107, 108]. The biodegradation of the resulting copolymers has not been demonstrated, just claimed.

Water-soluble polyesters and polyamides containing carboxyl functionality are reported to be biodegradable detergent polymers by BASF and may be obtained by condensation polymerization of monomeric polycarboxylic acids such as citric acid, butane-1,2,3,4-tetracarboxylic acid, tartaric acid, and malic acid with polyols [109], amino compounds, including amino acids [110], and polysaccharides [111]. Earlier work by Abe et al. [112] and Lenz and Vert [113] demonstrated the self-condensation of malic acid to biodegradable carboxylated polyesters regardless of the ester linkage, alpha or beta, formed. Procter & Gamble has patented succinylated polyvinyl alcohol [114] as a detergent polymer.

Scheme 9. Copolymerization with ketene acetals.

Nonionic water-soluble biodegradable polyamides are reported by Bailey [115], Chiellini et al. [116], and Ahmed for disposable fibers and webs [117].

Polyanionic polyamides are available by the condensation of polycar-boxyamino acids such as glutamic acid and aspartic acid. Though both homopolymers are known and claimed as biodegradable, aspartic acid is more amenable to a practical industrial thermal polymerization since it has no tendency to form an internal N-anhydride as is the case with glutamic acid. An alternative synthesis for polyaspartic acid is from ammonia and maleic acid.

Acid-catalyzed condensation of L-aspartic acid yields an authenticated biodegradable polymer [118] by standard biodegradation test methodology. The noncatalyzed polymerization and the ammonia/maleic acid processes give partially (ca 30 wt % residue remains in the Sturm test) biodegradable polymers due to the molecules being highly branched and more resistant to enzymatic attack.

The two structures starting from L-aspartic acid are shown in Scheme 10. Pathway A is the acid-catalyzed thermal condensation, and B is the noncatalyzed thermal condensation. The polysuccinimide intermediates hydrolyze at the points indicated to give mixtures of a, ¡3 poly (d,l- aspartic acid) salts. Regardless of the stereochemistry of the starting aspartic acid, d or l, the final polymeric product is always the dl racemate.

Many synthesis patents and publications for polyaspartic acids from aspartic acid [119-124] and ammonia/maleic [125, 126] processes have issued, and the products find use in many applications including dispersants [127, 128] and detergents [129, 130]. BASF has an aspartic acid copolymer patent with carbohydrates and polyols [131], and Procter & Gamble [132] has a patent for poly(glutamic acid), both for biodegradable detergent co-builders.

CO2H

-co2h A

-co2h

(Linear and biodegradable) B O

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