Abstract: A hydroxycarboxylic acid copolyester of R-stereospecific configuration contains a majority of first repeating units capable of forming a highly crystalline high-melting homopolyester and a minority of second repeating units capable when randomly copolycondensed with said first repeating units of lowering the melting point of said homopolyester and is characterised by one or more of: higher crystalline melting point, shorter crystallising time, higher notched impact strength at age 1 month, as compared with corresponding random polyester, and said majority repeating units in the polyester chain being present in blocks longer than correspond to their over-all molar proportionality. The copolyester may be made by fermentation, controlling the distribution of repeating units by reference to the pattern of feeding substrates corresponding to the respective repeating units.

Abstract: A process for producing poly-3-hydroxyalkanoate (PHA) by culturing Alcaligenes on a low water-solubility aliphatic carboxylic acid and, or hydrolysable derivative of low solubility in pure water, by fermenting the organism in a growth step on a nutrient medium containing inter alia phosphorus in a quantity corresponding to the intended quantity of bacterial cells to be grown until cell growth stops or slows substantially, then in a PHA accumulation stage fermenting the grown cells by feeding said low-solubility carbon source while monitoring pH and adjusting it by addition of ammonia and/or alkaline alkali metal compound until a design quantity of PHA has been produced, and recovering PHA from the product. Further phosphorus may be fed to the accumulation stage at a rate sufficient to permit some bacterial growth but insufficient to permit growth to the exclusion of PHA accumulation.