Kinetics and thermodynamics studies of the ring-opening polymerization of ε-caprolactone initiated by titanium(IV) alkoxides by isothermal differential scanning calorimetry

Abstract

The structure–reactivity relationship of titanium(IV) alkoxides (Ti(OR)4; R = n-propoxide, n-butoxide, tert-butoxide and 2-ethylhexoxide) in the ring-opening polymerization of ε-caprolactone (ε-CL) has been successfully investigated by differential scanning calorimetry (DSC) technique. Based on isothermal method, the kinetic results demonstrated that the polymerization rate decreased with increasing chain length and bulkiness of alkoxy group of Ti(OR)4. The determined activation energy (Ea) from autocatalytic model (f(α) = αm(1 − α)n) for titanium(IV) n-propoxide, n-butoxide, tert-butoxide and 2-ethylhexoxide were found to be 77.7, 97.2, 105.2 and 97.9 kJ/mol. From thermodynamics analysis of transition state (TS) formulation, the obtained activation enthalpy (∆H≠) values revealed that the titanium(IV) n-propoxide required the lowest energy to form the TS with ε-CL. From the obtained activation entropy (∆S≠) values, it was found that the stability of TS of ε-CL with titanium(IV) n-propoxide was higher than n-butoxide, 2-ethylhexoxide and tert-butoxide. From the overall results, it is clearly indicated that the steric hindrance of Ti(OR)4 initiators plays an important role on the kinetics and thermodynamics of polymerization process. The reactivity of Ti(OR)4 initiators was determined in the order of: titanium(IV) n-propoxide > n-butoxide ≈ 2-ethylhexoxide > tert-butoxide. The polymerization mechanism of all Ti(OR)4 initiators with ε-CL was proposed through the coordination-insertion mechanism. Graphical abstract: [Figure not available: see fulltext.]