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The chemical composition distributions (CCDs) of biosynthesized poly(3-hydroxybutyrate-co-3-hydroxyvalerate), P(3HB-co-3HV), samples with different average compositions were determined by temperature rising elution fractionation (TREF). P(3HB-co-3HV) samples were supplied by Sigma–Aldrich Co. Inc. According to catalog data, the 3HV contents were about 8, 14, and 24 mol%, respectively. The TREF condition was as follows: the sample concentration was 0.3% dissolved in acetonitrile at 75 °C; the column temperature was slowly (2 °C/h) decreased from 75 °C to 0 °C to crystallize the sample. The fractions were eluted with acetonitrile by stepwise increasing the temperature by 5 °C. The chemical compositions of the original samples and the fractions were determined by 1H NMR. Cumulative CCD curves were constructed from the fractionation data for the respective samples. It became clear that these samples had bimodal CCDs with two discrete peaks, that is, one part in the wide composition range around 10–15 mol% of 3HV contents and another part at different compositions.

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