It has been known for a long time that, at sufficiently high temperature, solid sodium chloroacetate undergoes a polymerization reaction to produce polyglycolide and sodium chloride. However, an understanding of this reaction and its mechanism was not previously possible, as sodium chloroacetate is microcrystalline and structural information could not be determined from conventional single-crystal X-ray diffraction methods. In this paper, we report the structure of sodium chloroacetate; this advance has been possible through the application of new methodology, based on Monte Carlo sampling techniques, that we have developed recently for crystal structure determination from powder diffraction data. The reported structure of sodium chloroacetate provides a direct rationalization, based on topochemical principles, of the polymerization reaction to produce polyglycolide.