The role of tri-n-butyltin(IV) n-butoxide/aluminum(III) tri-s-butoxide mixed initiators in the non-isothermal ring-opening polymerization of ε-caprolactone: from small-scale to larger-scale polymerization

The solvent-free ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with the tri-n-butyltin(IV) n-butoxide/aluminum(III) tri-s-butoxide mixed initiators [nBu₃SnOnBu/Al(OsBu)₃] was successfully studied by the non-isothermal differential scanning calorimetry for the first time. The nBu₃SnOnBu/Al(OsBu)₃ mixed initiators were completely dissolved in ε-CL and could be utilized to produce high molecular weight poly(ε-caprolactone) (PCL). The presence of the Al(OsBu)₃ in the mixed initiators did not interfere with the initiation temperature and could increase the propagation rate of ε-CL. The activation energies (E_a) obtained from the peak methods of Kissinger and Ozawa for the ROP of ε-CL initiated by the nBu₃SnOnBu/Al(OsBu)₃ (1.0:2.0 mol%) (74.2–78.6 kJ/mol) were lower than nBu₃SnOnBu/Al(OsBu)₃ (1.0:1.0 mol%) (75.8–80.3 kJ/mol) and the single nBu₃SnOnBu initiator (78.3–82.8 kJ/mol), respectively. From the proton-nuclear magnetic resonance spectroscopy (¹H-NMR), the polymerization mechanism was proposed through the classic coordination–insertion mechanism that comprised: (i) the coordination of ε-CL and nBu₃SnOnBu with Al(OsBu)₃ and (ii) the nucleophilic attacked of n-butoxy group (–OnBu) from nBu₃SnOnBu to the carbonyl carbon of ε-CL. From the small-scale synthesis (4 g) of PCL, our nBu₃SnOnBu/Al(OsBu)₃ mixed initiators could control the polymerization of ε-CL by adjusting the concentration of Al(OsBu)₃. The number and weight average molecular weights (M_n and M_w) of PCL increased with decreasing concentration of mixed initiator. The nBu₃SnOnBu/Al(OsBu)₃ mixed initiators produced PCL with M_n, M_w, dispersity (Đ), and %yield in the range of 1.3 × 10⁴–4.0 × 10⁴ g/mol, 2.9 × 10⁴–6.5 × 10⁴ g/mol, 1.65–2.26 and 50–95%, respectively. The larger-scale polymerization (250 g) of ε-CL with the nBu₃SnOnBu/Al(OsBu)₃ mixed initiators was preliminary conducted at 150 °C for 48 h. PCL with higher M_n (7.3 × 10⁴ g/mol) and M_w (1.2 × 10⁵ g/mol) was synthesized. The nBu₃SnOnBu/Al(OsBu)₃ mixed initiators acted as an effective candidate for the production of the high molecular weight PCL via solvent-free polymerization.