Benzoic Acid as an Efficient Organocatalyst for the Statistical Ring-Opening Copolymerization of ε-Caprolactone and L-Lactide: A Computational Investigation

Statistical copolymers of L-lactide (L-LA) and epsilon-caprolactone (CL) are of major interest as a result of the desired combination of properties they exhibit for high-added-value applications, including in the biomedical field and in microelectronics. However, the high difference of reactivity between the two monomers makes difficult their statistical insertion in copolymer chains. Here, the ring-opening polymerization and copolymerization (ROP and ROcP, respectively) of L-LA and CL mediated by benzoic acid (BA) are investigated by means of density functional theory (DFT). It is first evidenced that the mechanism involves a hydrogen-bonding dual activation, where the acidic proton of BA activates the carbonyl moiety of the monomer, while the conjugated base of BA activates the alcohol initiator. In accordance with experimental findings, DFT calculations have then revealed a kinetically favored energetic profile for the BA-organocatalyzed ROP of CL compared to L-LA. In addition, energetic profiles of the BA-mediated ROcP of CL and L-LA does not show any preference of the insertion between CL and L-LA, irrespective of the type of growing species. Even though the caproyl unit insertion is kinetically favored by the primary nature of the growing chain end alcohol, this is eventually mitigated by the stabilizing effect of the ester moieties of the lactidyl unit, which is thermodynamically favored. As one effect compensates for the other, the dual activation mechanism involved in this organocatalytic pathway using BA as a weak organic acid is shown to be crucial to achieve truly statistical copolymers based on L-LA and CL.