The bacterial plastic poly(3-hydroxybutyrate) (PHB) has many practical applications but its commercial potential is presently limited, partly due to the challenge of achieving appropriate mechanical performance characteristics. One method of tailoring the physical properties of the polymer is to control its molecular weight, which can be achieved by the addition of chain transfer agents such as poly(ethylene glycol) (PEG) to the culture medium during production. For the first time, PEG has been compared directly with alternative chain transfer agents, all of which contain at least one hydroxyl group. Methanol, ethanol and isopropanol all reduced the molecular weight of PHB produced by transgenic Escherichia coli by the same amount as PEG (similar to 60%). PEG-treated cultures grew to lower density and produced less PHB than control or alcohol-treated cultures. Glycerol and sodium acetate were also tested, but only reduced the molecular weight by 15-24%. Naturally occurring short chain alcohols such as methanol and ethanol are a superior choice of chain transfer agent than the currently preferred PEG, as they achieve the same effect with reduced toxicity and are substantially less expensive. (C) 2013 Society of Chemical Industry