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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:  ΔH°depol = −43 kJ·mol-1, ΔS°depol = 66 J·K-1 mol-1, and ΔG°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 × 103), 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:  ΔH°pol = 12 kJ·mol-1, ΔS°pol = −32 J·K-1·mol-1, and ΔG°pol = 25 kJ·mol-1. These results contradict the polymerization experiment. The factors which may contribute to a decrease of ΔG°pol are discussed.

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Summary Reactions such as branching and crosslinking leading to nonlinear polymers play an important role in many polymerization productions, but they increase the sensitivity of the processes and increase the requirements for their design and control. It is therefore desirable to have an accurate and simple means to model these processes. However, the simplest method for the description of the polymers: the method of moments was said to fail in the nonlinear case because of the so called closure problem. It will be shown here how to extend the method of moments to nonlinear processes in the pregel region − except scission − and thus provide a simple means for the simulation and optimization of the main parameters of a polymer distribution. Furthermore, the assumption of one radical per polymer molecule is no longer needed. The formulas allow average polymer properties to be easily included in flow sheet simulations or computational fluid dynamics codes and allow checking other more elaborate codes for the description of polymer distributions. Examples will be given for free radical polymerizations. An approximation in case of scission will be discussed together with an application to reversible polycondensation.