n-Butyltin(IV) trichloride initiator for the solvent-free ring-opening polymerization of ε-caprolactone: evaluation of the structure-reactivity relationship of tin(IV) derivatives

The n-butyltin(IV) trichloride (nBuSnCl₃) initiator was successfully utilized in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions. Its reactivity was investigated by the non-isothermal differential scanning calorimetry (DSC). Effect of nBuSnCl₃ concentration on the ROP of ε-CL was investigated by the Kissinger and model fitting methods. The obtained activation energy (E_a) values for the ROP of ε-CL with 1.0 mol% of nBuSnCl₃ were higher than 2.0, and 4.0 mol%, respectively. Furthermore, the degree of aggregation (m) of nBuSnCl₃ in the ROP of ε-CL was determined by the DSC technique. From the obtained m values at 120 and 140 °C, nBuSnCl₃ presented in a non-aggregated state in the non-isothermal ROP of ε-CL. The performance of nBuSnCl₃ was also compared with the di-n-butyltin(IV) dichloride (nBu₂SnCl₂) and tri-n-butytlin(IV) chloride (nBu₃SnCl). The number of chloride and n-butyl groups adjacent to the Sn active center significantly affected the solvent-free ROP of ε-CL. From the non-isothermal DSC measurement, the polymerization exotherms for the ROP of ε-CL with nBuSnCl₃ occurred and were completed at a lower temperature range than nBu₂SnCl₂, and nBu₃SnCl, respectively. The E_a value for the ROP of ε-CL initiated by nBu₃SnCl was higher than nBu₂SnCl_2, and nBuSnCl₃, respectively. The values of pre-exponential factor (lnA) for the ROP of ε-CL initiated by nBu₃SnCl, nBu₂SnCl_2, and nBuSnCl₃ were determined from the first order reaction model fitting (f(α) = 1−α) and compensation effect. The kinetics data demonstrated that the reactivity of nBuSnCl₃ was higher than nBu₂SnCl_2, and nBu₃SnCl due to the lowest steric hindrance and the highest Lewis acidity of nBuSnCl₃. Larger-scale (250 g) polymerization of ε-CL was preliminarily investigated by using the highly reactive nBuSnCl₃ initiator. nBuSnCl₃ could effectively produce PCL with weight average molecular weight (M_w) of 7.1 × 10⁴ g/mol and 82% yield at 150 °C for 24 h. The probable mechanism of the ROP of ε-CL with nBu₃SnCl was proposed via the coordination-insertion mechanism.