Biodegradable addition polymers

W. J. Bailey s(52,53) work with ketene acetals deserves mention as potentially a route to biodegradable addition polymers. Its novelty resides in the instability of the vinyl radical and rearrangement to introduce a polyester linkage into a radically produced polymer. As we shall see in the next section, polyesters are biodegradable hence, their Introduction into a polymer with a C-C backbone produces weak links which fracture the polymer into oligomers which we have seen are biodegradable. This chemistry is exemplified schematically, below. 

This indicates the possibility of making addition polymers biodegradable by the introduction of ester linkages in to the backbone. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-1,3-dioxepane (1, Scheme I), made possible the introduction of ester groups into the backbone of addition polymers, this appeared to be an attractive method for the synthesis of biodegradable addition polymers. 

Since the monomer I would copolymerize with a wide variety of comonomers with the introduction of an ester group into the main chain, this appeared to make possible the preparation of biodegradable addition polymers. Copolymerization of ethylene and the ketene acetal I at 120°C produced a series of copolymers containing ester groups in the backbone of the copolymer, again with quantitative ring opening. 

Similar polyacetals were prepared by BASF scientists from CO-aldehydic aUphatic carboxyUc acids (189,190) and by the addition of poly(hydroxycarboxyhc acid)s such as tartaric acid to divinyl ethers (191) as biodegradable detergent polymers. 

The skeleton of the non-degradable polymers containing a high percentage of biodegradable additives survive the degradation and pollute the environment. 

Addition polymers with C-C backbones do not biodegrade to any significant extent. 

Addition polymers with hetero-atoms in the backbone biodegrade. 

Muzafarov, Golly and Moller 646 have prepared similar poly(alkoxysilanes) that are easily hydrolyzed under acidic conditions and are thus biodegradable. Additional branched organosilicon polymers of interest include those of Ishikawa et al. fi4c ... 

During the same period, commercialization of thermoplastic starch polymer blends was pursued by Novamont, a division of the Ferruzzi Group of Italy.162-172 Their products, marketed under the trade name Mater-Bi, are typically comprised of at least 60% starch or natural additive and hydrophilic, biodegradable synthetic polymers.64,165 It is stated that these blends form interpenetrated or semi-interpenetrated structures at the molecular level. Properties of typical commercial formulations have properties similar to those in the range of low- and high-density PE. Blends of Mater-Bi products with biodegradable polyesters have been claimed for use as water impervious films.173... 

This chapter deals with polymers synthesized from oilseed sources. However, to provide the reader with an appreciation of the area of renewable, biodegradable polymers and the place within this area that polymers from oil seeds occupy in terms of functionality, price, and acceptability, some other polymers from major renewable sources are also discussed. The most well-known and widely used renewable biodegradable polymers are those from polysaccharides. The principal polysaccharides of interest to polymer chemists are starches and cellulose, both of which are polymers of glucose. In addition to these, fibers, polylactic acid (PLA), and triacylglycerols of oils are of particular interest for the development of biodegradable industrial polymers. 

In addition to chemical hydrolysis, hydrolysis by enzymes can operate as an alternative degradation process. It has become widely accepted that biodegradable synthetic polymers tend to be designed to mimic those structures prevailing in nature, since enzymes produced by microbial populations may not discriminate between polymers of similar structure.11 Synthetic nonpolypeptidic, chiral polyamides could mimic natural peptides or proteins, resulting in biodegradable products useful in biomedicine. 

The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are the condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose. Condensation polymerization is occasionally used to form simple hydrocarbons. This method, however, is expensive and inefficient, so the addition polymer of ethene, i.e., polyethylene, is generally used. Condensation polymers, unlike addition polymers, may be biodegradable. The peptide or ester bonds between monomers can be hydrolyzed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces. The most commonly known condensation polymers are proteins and fabrics such as nylon, silk, or polyester. 

---