Determination of the activation parameters for the ring-opening polymerization of ε-caprolactone initiated by Sn(II) and Zn(II) chlorides using the fast technique of DSC

Abstract

The kinetics of the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by 1.0 mol% of tin(II) chloride (SnCl2) and zinc(II) chloride (ZnCl2) was completely investigated by the differential scanning calorimetry (DSC) technique. The fast method to determine the activation parameters such as activation enthalpy (∆H≠) and activation entropy (∆S≠) was successfully developed basing on the transition state theory and isoconversional method. From Friedman isoconversional method, the values of activation energy (Ea) for the ROP of ε-CL initiated by SnCl2 (36.2 ± 1.5 to 40.8 ± 1.5 kJ/mol) were lower than ZnCl2 (51.9 ± 2.8 to 62.3 ± 2.8 kJ/mol). From Starink isoconversional method, the Ea values for the ROP of ε-CL initiated by SnCl2 (32.3 ± 1.1 to 35.8 ± 1.1 kJ/mol) were also lower than ZnCl2 (59.2 ± 0.8 to 62.1 ± 0.8 kJ/mol). By using the compensation parameters, the reconstructed monomer conversion function (f(α)) was fitted with the Avrami-Erofeev nucleation model (A2, f(α) = 2(1-α)[-ln(1-α)]1/2). The values of frequency factor (A) for the ROP of ε-CL with SnCl2 and ZnCl2 were 4.3 × 103 to 1.6 × 104 min−1 and 2.1 × 105 to 5.3 × 105 min−1, respectively. From the developed rate equation, the values of ∆H≠ and ∆S≠ could be conveniently determined from dynamic heating. The obtained ∆H≠ values for the ROP of ε-CL initiated by SnCl2 and ZnCl2 were similar to the Ea values. The ∆S≠ values for the transition state of the ROP of ε-CL initiated by SnCl2 (-152.0 to -161.7 J/mol.K) were lower than ZnCl2 (-129.6 to -153.1 J/mol.K). The kinetics and the stability of transition state information demonstrated that the reactivity of SnCl2 in the ROP of ε-CL was higher than ZnCl2. The mechanism of the ROP of ε-CL with SnCl2 and ZnCl2 was proposed through a classical coordination-insertion mechanism. From poly(ε-caprolactone) (PCL) synthesis via solvent-free polymerization, SnCl2 acted as the highly effective initiator than ZnCl2. The highest number average molecular weight (Mn) of PCLs obtained from the SnCl2 and ZnCl2 initiator were 1.8 × 105 g/mol and 3.6 × 104 g/mol, respectively. The effectiveness of the slow ZnCl2 initiator in the production of high molecular weight and %yield of PCL could be improved by increasing the synthesis temperature. These SnCl2 and ZnCl2 compounds acted as a powerful initiator for the synthesis of PCL. The obtained results from this work were useful for understanding the transition state formulation in the ROP of other cyclic esters with various initiating systems that still in our attention and would be described in our future work.