Unveiling the Mechanisms of Hydrolytic Ring-Opening Polymerization of Caprolactam and Amino-Assisted Ring Opening of Cyclic Dimers: A DFT Study

At the M06-2X/6-31+G(d,p) computational level, the reactions hydrolytic ring-opening polymerization of caprolactam to form cyclodimers and amino-assisted ring opening of the cyclic caprolactam dimers were studied in detail. For the above-mentioned reactions, free energy barriers were calculated and the possible reaction pathways were discussed. Our calculated results showed that the hydrolytic ring-opening polymerization reaction includes four stages. The first three stages contain two steps: nucleophilic addition and proton transfer. After finishing stage 3, a cyclic caprolactam dimer is formed by the rate-determining step─backbiting of the amino end groups. For the amino-assisted ring-opening reaction of the cyclic caprolactam dimer, the rate-determining step is stage 3 due to its highest free energy barrier. This stage can take place via a concerted or stepwise pathway. Under the experimental conditions, hydrolytic ring-opening polymerization and amino-assisted ring opening of the cyclic caprolactam dimer can be easily carried out. This study provides a theoretical basis for the effective removal of caprolactam oligomers.