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 (nBuSnCl(3)) initiator was successfully utilized in the ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) under solvent-free conditions. Its reactivity was investigated by the non-isothermal differential scanning calorimetry (DSC). Effect of nBuSnCl(3) concentration on the ROP of epsilon-CL was investigated by the Kissinger and model fitting methods. The obtained activation energy (E-a) values for the ROP of epsilon-CL with 1.0 mol% of nBuSnCl(3) were higher than 2.0, and 4.0 mol%, respectively. Furthermore, the degree of aggregation (m) of nBuSnCl(3) in the ROP of epsilon-CL was determined by the DSC technique. From the obtained m values at 120 and 140 degrees C, nBuSnCl(3) presented in a non-aggregated state in the non-isothermal ROP of epsilon-CL. The performance of nBuSnCl(3) was also compared with the di-n-butyltin(IV) dichloride (nBu(2)SnCl(2)) and tri-n-butytlin(IV) chloride (nBu(3)SnCl). The number of chloride and n-butyl groups adjacent to the Sn active center significantly affected the solvent-free ROP of epsilon-CL. From the non-isothermal DSC measurement, the polymerization exotherms for the ROP of epsilon-CL with nBuSnCl(3) occurred and were completed at a lower temperature range than nBu(2)SnCl(2), and nBu(3)SnCl, respectively. The E-a value for the ROP of epsilon-CL initiated by nBu(3)SnCl was higher than nBu(2)SnCl(2,) and nBuSnCl(3), respectively. The values of pre-exponential factor (lnA) for the ROP of epsilon-CL initiated by nBu(3)SnCl, nBu(2)SnCl(2,) and nBuSnCl(3) were determined from the first order reaction model fitting (f(alpha) = 1-alpha) and compensation effect. The kinetics data demonstrated that the reactivity of nBuSnCl(3) was higher than nBu(2)SnCl(2,) and nBu(3)SnCl due to the lowest steric hindrance and the highest Lewis acidity of nBuSnCl(3). Larger-scale (250 g) polymerization of epsilon-CL was preliminarily investigated by using the highly reactive nBuSnCl(3) initiator. nBuSnCl(3) could effectively produce PCL with weight average molecular weight (M-w) of 7.1 x 10(4) g/mol and 82% yield at 150 degrees C for 24 h. The probable mechanism of the ROP of epsilon-CL with nBu(3)SnCl was proposed via the coordination-insertion mechanism.