Depolymerization of poly[(R)-3-hydroxybutyrate] to cyclic oligomers and polymerization of the cyclic trimer: An example of thermodynamic recycling

Biotechnologically produced poly[(R)-3-hydroxybutyrate] (PHB) was subjected to catalytic depolymerization in the melt and in solution. In the melt depolymerization yields crotonic acid beside linear oligomers according to an ester pyrolysis reaction independent of the catalyst used. In solution cyclic oligomers were obtained via back-biting reactions. Efficient catalysts for this reaction are dibutyltin dimethoxide and p-toluenesulfonic acid. The thermodynamic parameters of the depolymerization of PHB to yield (R,R,R)-4,8,12-trimethyl-1,5,9-trioxacyclododeca-2,6,10-trione (TBL) were determined from calorimetric measurements by Lebedev et al, as a function of temperature at standard pressure (p = 101.325 kPa) and are at T = 400 K: Delta H degrees(depol) = -43 kJ . mol(-1), Delta S degrees(depol) = 66 J . K-1 mol(-1), and Delta G degrees(depol) = -70 kJ . mol(-1). The experimental results are in full agreement with these values. Polymerization of TBL in the melt with dibutyltin dimethoxide as initiator results in poly[(R)-3-hydroxybutyrate] (PHB') of low molecular weight (M(n) = 5 x 10(3)), the spectroscopic characteristics are identical with those of the biotechnologically produced material. The thermodynamic parameters of the polymerization were determined from calorimetric measurements by Lebedev et al. as a function of temperature; they are at standard pressure (p = 101.325 kPa) and a polymerization temperature of T = 400 K: Delta H degrees(pol) = 12 kJ . mol(-1), Delta S degrees(pol) = -32 J . K-1. mol(-1), and Delta G degrees(pol) = 25 kJ . mol(-1). These results contradict the polymerization experiment. The factors which may contribute to a decrease of Delta G degrees(pol) are discussed.