Abstract: High molecular weight copolymers containing 3-hydroxybutyrate residue, i.e. units of the formula--O.CH(CH.sub.3).CH.sub.2.CO--and up to 50 mole % of residues of other hydroxy acids, viz units of the formula--O.CR.sup.1 R.sup.2.(CR.sup.3 R.sup.4).sub.n.CO--where n is 0 or an integer and, if n=1 and R.sup.2, R.sup.3, and R.sup.4 =H, R.sup.1 is not methyl.The copolymers are made microbiologically: for part of the cultivation the micro-organism is under conditions such that polymer is accumulated, e.g. by limitation of a nutrient, e.g. nitrogen source, required for growth but not polyester accumulation. For at least part of this period of polymer accumulation the substrate is an acid or a derivative thereof that gives the comonomer units. Proprionic acid, which gives polymers where n=1, R.sup.2 =R.sup.3 =R.sup.4 =H and R.sup.1 =C.sub.2 H.sub.5, is the preferred acid.

Abstract: Poly(.beta.-hydroxybutyric acid) is separated from bacterial cells by drying a finely divided stream or spray of an aqueous suspension of the cells with a gas heated to above 100.degree. C.