The discovery and chemical identification, in the 1920s, of the aliphatic polyester: poly(3-hydroxybutyrate), PHB, as a granular component in bacterial cells proceeded without any of the controversies which marked the recognition of macromolecules by Staudinger. Some thirty years after its discovery, PHB was recognized as the prototypical biodegradable thermoplastic to solve the waste disposal challenge. The development effort led by Imperial Chemical Industries Ltd., encouraged interdisciplinary research from genetic engineering and biotechnology to the study of enzymes involved in biosynthesis and biodegradation. From the simple PHB homopolyester discovered by Maurice Lemoigne in the mid-twenties, a family of over 100 different aliphatic polyesters of the same general structure has been discovered. Depending on bacterial species and substrates, these high molecular weight stereoregular polyesters have emerged as a new family of natural polymers ranking with nucleic acids, polyamides, polyisoprenoids, polyphenols, polyphosphates, and polysaccharides. In this historical review, the chemical, biochemical and microbial highlights are linked to personalities and locations involved with the events covering a discovery timespan of 75 years.

Pseudomonas okovorans was grown in homogeneous media containing n-alkanoic acids, from formate to decanoate, as the sole carbon sources. Formation of intracellular poly(jI-hydroxyalkanoates) was observed only for hexanoate and the higher n-alkanoic acids. The maximum isolated polymer yields were approximately 30% of the cellular dry weight with growth on either octanoate or nonanoate. In most cases, the major repeating unit in the polymer had the same chain length as the n-alkanoic acid used for growth, but units with two carbon atoms less or more than the acid used as a carbon source were also generally present in the polyesters formed. Indeed, copolymers containing as many as six different types of ,-hydroxyalkanoate units were formed. The weight average molecular weights of the poly(Pi-hydroxyalkanoate) copolymers produced by P. oleovorans ranged from 90,000 to 370,000. In spite of the higher cell yields obtained with octanoate and nonanoate, the use of hexanoate and heptanoate yielded higher-molecular-weight polymers. These copolyesters represent an entirely new class of biodegradable thermoplastics.

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Lamellar single crystals of both natural poly[(R)-β-hydroxybutyrate], PHB, and synthetic poly[(R,S)-β-hydroxybutyrate] of various tacticities were degraded using enzymes isolated from the fungus Aspergillus fumigatus and the bacterium Pseudomonas lemoignei. Degradation was monitored by both turbidimetric and titrimetric assays. Despite their highly ordered state, single crystals of bacterial PHB were observed to degrade completely; no decrease in molecular weight was observed in the partly degraded polymer. By contrast, the single crystals of synthetic PHB showed only partial degradation, with some decrease in molecular weight; faster and more substantial degradation was observed for the most isotactic material, whereas single crystals from a nearly atactic sample were essentially inert. These results differ from those found for PHB films, implying that tacticity response is linear when crystallinity effects have been normalized. The observed results are consistent with preferential degradation from the crystal edges rather than the chain folds of the lamellar surface and support the hypothesis of a combined endo−exo degradation mechanism for these two depolymerases.