Organocatalytic Ring-Opening Polymerization of ε -Caprolactone Using bis ( N -( N '-butylimidazolium)alkane Dicationic Ionic Liquids as the Metal-Free Catalysts: Polymer Synthesis, Kinetics and DFT Mechanistic Study.

In this work, we successfully synthesized high thermal stable 1,n- bis ( N -( N '-butylimidazolium)alkane bis hexafluorophosphates (1,n- bis [Bim][PF 6 ], n = 4, 6, 8, and 10) catalysts in 55-70% yields from imidazole which were applied as non-toxic DILs catalysts with 1-butanol as initiator for the bulk ROP of ε -caprolactone (CL) in the varied ratio of CL/ n BuOH/1,4- bis [Bim][PF 6 ] from 200/1.0/0.25-4.0 to 700/1.0/0.25-4.0 by mol%. The result found that the optimal ratio of CL/ n BuOH/1,4- bis [Bim][PF 6 ] 400/1.0/0.5 mol% at 120 °C for 72 h led to the polymerization conversions higher than 95%, with the molecular weight ( M w ) of PCL 20,130 g mol -1 ( Đ ~1.80). The polymerization rate of CL increased with the decreasing linker chain length of ionic liquids. Moreover, the mechanistic study was investigated by DFT using B3LYP (6-31G(d,p)) as basis set. The most plausible mechanism included the stepwise and coordination insertion in which the alkoxide insertion step is the rate-determining step.