Non-isothermal kinetics of the organocatalytic ring-opening polymerization of ε-caprolactone with metal-free α‑hydroxy acids: Eco-friendly and facile synthesis process

Metal-free and green α‑hydroxy acids (AHA) such as l-malic (MA), DL-mandelic acid (MDL), and citric acid (CA) were successfully and effectively utilized as an effective initiator for the solvent-free ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The performance of AHAs in the polymerization of ε-CL was completely and powerfully investigated via the non-isothermal differential scanning calorimetry (DSC). The proceed of ROP of ε-CL with AHAs could be real-time monitored by the obtained polymerization exotherms at different heating rates. The polymerization exotherms obtained from the ROP of ε-CL with CA occurred at a lower temperature range than MA, and MDA, respectively. From the kinetics study, the average activation energy (E_a) values for the ROP of ε-CL with CA (38.0 ± 1.8 kJ mol⁻¹) were lower than MA (45.2 ± 3.6 kJ mol⁻¹) and MDA (48.8 ± 6.2 kJ mol⁻¹). Using the first-order model fitting, the values of pre-exponential factor (lnA₀) for the ROP of ε-CL with CA, MA, and MDA were 7.0 ± 0.3, 8.5 ± 0.2, and 8.9 ± 0.3, respectively. The effectiveness of AHAs in the synthesis of PCL was clarified by conducting a larger-scale (4.0000 g) polymerization using conventional heating and microwave (MW) irradiation methods. From conventional heating, the green AHAs produced PCL with the number average molecular weight (M_n) and dispersity (Đ) values in the range of 5.18 × 10³ - 1.43 × 10⁴ g mol⁻¹ and 1.14–2.02, respectively. The irradiation by MW could enhance the synthesis of PCL by reducing the synthesis time. By using the MW power of 450 W and irradiation time of 30 min, the PLC was obtained from the ROP of ε-CL with MA and MDA initiators, and the M_n of the obtained PCL from MW heating was 4.28 × 10³ - 8.45 × 10³ g mol⁻¹. The mechanism for the ROP of ε-CL with all AHAs was proposed through the activated monomer mechanism.