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Biodegradable and biocompatible polyhydroxyalkanoates (PHAs) are promising alternatives to conventional plastics. Based on the chain length of their monomers they are classified as  short chain length  ( scl- ) or  medium chain length  ( mcl- ) PHA polymers. The type of monomers, the composition and the molecular weight (MW) define the polymer properties. To accelerate the use of PHA as a bulk material, the downstream associated costs need to be minimized. This study focuses on the evaluation of non-halogenated solvents, especially acetone as a  scl -PHA non-solvent, for the recovery of poly(3-hydroxybutyrate- co -3-hydroxyhexanoate) – P(HB- co -HHx) – with an  mcl -HHx content >15 mol% and a MW average ( M  w ) < 2 × 10 5  Da. Solvents and precipitants were chosen regarding zeotrope formation, boiling point differences, and toxicity. Non-halogenated solvent-precipitant pairs were evaluated regarding the MW characteristics (MWCs) of the extracted polymer. Acetone and 2-propanol as a low toxic and zeotropic solvent-precipitant pair was evaluated at different extraction temperatures and multiple extraction times. The extraction process was further evaluated by using impure acetone for the extraction and implementing a multi-stage extraction process. Additionally, P(HB- co -HHx) extracted with three different solvents was characterized by  1 H and  13 C-APT NMR. The screening of precipitants resulted in a negative influence on the MWCs by ethanol precipitation for extractions with acetone and ethyl acetate, respectively. It was observed, that extractions with acetone at 70°C extracted a higher fraction of PHA from the cells compared to extractions at RT, but the  M  w  was decreased by 9% in average. Acetone with a 2-propanol fraction of up to 30% was still able to extract the polymer 95% as efficient as pure acetone. Additionally, when acetone and ethyl acetate were used in a multi-stage extraction process, a two-stage process was sufficient to extract 98–99% of the polymer from the cells.  1 H and  13 C-APT NMR analysis confirmed the monomer fraction and structure of the extracted polymers and revealed a random copolymer structure. The presented strategy can be further developed to an ecological and economically feasible PHA downstream process and thus contributes to the commercialization of low-cost PHAs.

Recovery of the PHA Copolymer P(HB-co-HHx) With Non-halogenated Solvents: Influences on Molecular Weight and HHx-Content