Thermal polymerization of levoglucosan

The uncatalyzed polymerization of neat levoglucosan (1,6-anhydro-β-D -glucopyranose) was studied at 190–210°C under argon, and the course of the reaction was followed by analyzing samples at various reaction times; the residual levoglucosan was determined by GLC, and the optical rotation of the mixture was also measured. The plots of percentage reaction versus time were sigmoid, indicating the occurrence of an autocatalytic reaction. A. kinetic equation of the form, dA/dt = k₂A₁]² + k₁A_t] (A₀] ? A_t]), where A₀] and A_t] denote the concentrations of levoglucosan at zero time and time t, was found to fit the results. A reaction scheme is proposed, consisting of a slower dimerization (k₂) followed by a faster reaction between levoglucosan and the dimers and other oligomers formed (k₁). This scheme results from the mechanistic assumption that axial hydroxyl groups in the unchanged levoglucosan are less reactive than equatorial hydroxyl groups present only in the oligomers, while the highest reactivity is ascribed to the primary hydroxyl at C₆, again only present in the oligomers. The relation between the mechanisms of the reactions occurring and the structure of the product are discussed.