Ring-Size Distribution in the Depolymerization of Poly(butylene Terephthalate)

Poly(butylene terephthalate) (PBT) was depolymerized in refluxing o -dichlorobenzene using stannoxane catalysts to produce good yields of cyclic oligomers. We have studied how the ring size distribution varies as a function of the starting concentration of monomer units. Over a concentration range of 0.05 to 0.2 M, there is little change in the ring size distribution. This result suggests that the depolymerizations were performed above the critical monomer concentration. Application of a recent model by Mandolini (an adaptation of the Jacobson−Stockmayer model) for thermodynamically controlled cyclizations contradicts the experimental results by suggesting that the experiments were run below the critical monomer concentration. It appears that Mandolini's model may not be appropriate, in its present form, for depolymerizations. We further studied the experimental data using RIS modeling in which the probability of zero end-to-end distance for linear oligomers was used to calculate macrocyclization equilibrium constants. Reasonable agreement was observed between calculated and experimental data with greater deviation for smaller ring sizes, as might be expected. Incorporation of the width of the distribution and skewness into the calculation depressed the macrocyclization values.