Polymerization of functional cyclic monomers and investigation of initiator efficiency by computational chemistry

In this study, the ring opening polymerization (ROP) of biodegradable and biocompatible functional lactones and also initiator efficiency was investigated experimentally and computationally. The aim is to illuminate the polymerization process with computational chemistry, to predict the most effective initiator and to confirm it experimentally. Poly(α-angelica lactone), PAL, was synthesized by ring opening polymerization in the presence of SnAcet2 and ZnAcet2, but a low molecular weight product (1000-1500 g/mol) was obtained. However, initiation and propagation stages of the ROP mechanism were studied by semi empirical and DFT methods. The reaction barriers arising in the bond breaking steps was found to be 30.2 kcal/mol and 23.4 kcal/mol by means of the ωB97X-D method for SnAcet2 and ZnAcet2, respectively. The thermodynamics of the process was discussed by calculating the enthalpy of polymerization of the reaction. In addition, the ionic polymerization reaction barrier of α-angelica lactone was also found to be 24.6 kcal/mol with the ωB97X-D method. Accordingly, it was theoretically revealed for the first time that the high molecular weight polymer could not be synthesized. Poly(δ-valerolactone), PVL, was synthesized by ring opening polymerization using SnAcet2, PbAcet2.3H2O, CdAcet2.2H2O, NiAcet2.4H2O and CuAcet2.H2O initiators and the highest molecular weight (45438 g/mol, 91% yield) is obtained with the initiator CdAcet2.2H2O. Toxicity tests were performed for PVLs synthesized with acetate complexes containing Sn, Cd and Pb. The initiator activity was investigated by NBO and LUMO analyses using several DFT methods. In the presence of SnAcet2, the reaction mechanism was studied with the wB97X-D method and the reaction barrier was calculated as 28.9 kcal/mol. According to the computational data, it was determined that the most effective initiator was CdAcet2.2H2O. The δ-valerolactone and glycidol copolymer were first synthesized in the presence of SnAcet2. In addition, the -OH group in the glycidol structure was protected with tosyl-chloride and the poly (δ-valerolactone-co-glycidol) copolymer was also synthesized for the first time. However, due to the steric effect of the protecting group, low molecular weight copolymer was obtained (~1500 g/mol). Glycidol was found to be 40.3 kcal/mol by polymerization reaction with SnAcet2, whereas Gibbs free energy exchange was found to be -10 kcal/mol. According to the theoretical data, this reaction is thermodynamically spontaneous.