Thermodynamic control of the polymerizability of five-, six-, and seven-membered lactones.
The thermodynamics of polymerization of epsilon-caprolactone and 1,4-dioxan-2-one has been investigated theoretically and compared with that recently reported for delta-valerolactone and gamma-butyrolactone. Specifically, the ability of these monomers to polymerize has been related to the strain of the rings, the Gibbs free energy of simple models for ring-opening reactions of the cyclic lactones, and the conformational preferences of linear model compounds of the corresponding homopolyesters. The results are fully consistent with the lack of polymerizability of gamma-butyrolactone, while the ring openings of epsilon-caprolactone and delta-valerolactone have been found to be exergonic processes. Polymerizability of 1,4-dioxan-2-one has been found to be favored, even though less than that of epsilon-caprolactone and delta-valerolactone. Two factors explain these features: (i) the strain of the ester group in the lactones increases with the exergonic character of the ring-opening process, and (ii) the stability of coiled conformations in model compounds follows this order: poly-4-hydroxybutyrate > poly(1,4-dioxan-2-one) > poly-6-hydroxycaproate approximately poly-5-hydroxyvalerate. Finally, the influence of the environment on the polymerizability of the three cyclic lactones is discussed in detail.