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Thermal degradation processes of microbial copolyesters were studied in the temperature range 100-200 °C by monitoring the time-dependent changes in molecular weights of melt samples. Two types of copolyesters, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV); 3HV = 0-71 mol %) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB); 4HB = 0-82 mol %), were used in this study. All copolyester samples used were thermally unstable at temperatures above 170 °C, and their molecular weights decreased rapidly with time. The time-dependent changes in molecular weights during the thermal degradation followed the kinetic model of random chain scission at ester groups. The rates of random chain scission were independent of the compositions of the copolyesters but dependent strongly on temperature. These copolyester samples were thermally stable at temperatures below 160 °C. It has been suggested that the microbial copolyesters with melting temperatures below 160 °C are applicable to conventional plastics processing methods.

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IntroductionThe bacterial copolyester of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) has attracted industrial attention as a possible candidate for large-scale biotechnological production, since the copolyester is an environmentally degradable thermoplastic'). The copolyester has been produced from propionic acid in Alcaligenes eutrophus, and the range of compositions has been shown to vary from 0 to 47 mol-Vo 3HVRecently, we have found that copolyesters with a wide range of compositions up to 90 mol-qo of 3HV units are produced in A . eutrophus by using pentanoic acid as carbon source4). This communication reports the biosynthesis of an interesting terpolyester of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and Shydroxyvalerate (5HV) in A . eutrophus from 5-chloropentanoic and pentanoic acids. Experimental partA. eutrophus (NCIB 11599) was first grown at 30°C in a nutrient-rich medium (100 cm3) containing 1,0 g of yeast extract, 1,0 g of polypeptone, 0,5 g of meat extract, and 0,5 g of (NH&S04. The cells were harvested after 24 h and washed with water. Under these culture conditions an accumulation of polyesters in the cells was not observed. To promote polyester synthesis, about 0,4 g quantities of the washed cells were transferred into a nitrogen-free mediums) containing 5-chloropentanoic acid (ClCH2CH2CH2CH2COOH) and pentanoic acid (CH,CH2CH2CH2COOH). The cells were cultivated in this medium (100 cm3, pH = 7 3 ) for 48 h at 30 "C, harvested by centrifugation, washed with acetone, and finally dried under vacuum at room temperature. Polyesters were extracted from the dried cells with hot chloroform in a Soxhlet apparatus and purified by reprecipitation with hexane.The ' H NMR spectra of polyester samples were recorded on a JEOL GX-500 spectrometer operating at 500 MHz, using a CDCl, solution of polyester (5 mg/cm3). The spectra were obtained at 27OC, 45 pulse ( 3 3 ps), 6,O s pulse repetition, 7000 Hz spectral width, 32K data points, and 128 accumulations. The I3C NMR spectra of polyesters were recorded on a JEOL FX-100 spectrometer operating at 25 MHz, using a CDCl, solution of polyester (50 mg/cm3). The spectra were obtained at 27 "C, 45 O pulse (8,5 ps), 5,O s pulse repetition, 5000 Hz spectral width, 8 K data points, and 10000 accumulations. \ 0173-2803/87/$01 .OO8 h) Tab. 1. Biosynthesis of terpolyesters of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and 5-hydroxyvalerate (5HV) in Alculigenes eutrophus at 3OoC Sample Wt. of carbon sourcea) in g Wt. of dry Polyester Polyester compositionc) in mol-yo [tll d, cells in g contentb)

Unusual copolyesters of 3-hydroxybutyrate (3HB) and 4-hydroxybutyrate (4HB) with a range of compositions varying from 0 to 49 mol % 4HB were produced by Alcaligenes eutrophus in a nitrogen-free culture solution of 4-hydroxybutyric or 4-chlorobutyric acid. The copolyesters were characterized by solution NMR, solid-state 13C NMR, and differential scanning calorimetry. The compositions of the copolyesters were determined by 500-MHz NMR and two-dimensional homonuclear ( ) correlated spectroscopy. The copolymers were shown to have a random sequence distribution of 3HB and 4HB units by analysis of the 125-MHz 13C NMR spectra. The solid-state 13C NMR spectra and thermal analyses showed that the crystallinity of copolyester decreases with an increase in the fraction of 4HB units.

Poly(/3-hydroxybutyrate) (PHB) and a copolyester of /3-hydroxybutyrate and /3-hydroxyvalerate are isolated from Alcaligenes eutrophus H16 grown in nitrogen-free culture media containing CHgCOONa and CHgCH2COONa as carbon sources. The chain dynamics of PHB (Mw = 420000) in chloroform are studied by 13C NMR measurement at 25 MHz. The carbon-13 spin-lattice relaxation times (TJ and nuclear Overhauser enhancements (NOE) indicate that the average correlation times (tc) for segmental motion of the PHB molecule are in the range (6-8) x 10_u s at 27 °C, as expected for flexible linear polymers. The conformational structure generated by rotation about CH2-CH bonds of PHB is determined in chloroform and dimethylformamide over a temperature range of 20-100 °C by analysis of vicinal coupling in the 500-MHz NMR_spectra. In both solvents the gauche (G) and trans (T) conformers are found to exist predominantly, the G conformer being suppressed. The sequence distributions of the monomeric units in the copolyester of /3-hydroxybutyrate and /3-hydroxyvalerate are determined by analysis of 125-MHz 13C NMR spectra. The diad and triad sequence distributions are accounted for in terms of first-order Markov statistics.

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