In this work, we successfully synthesized high thermal sTable 1,n-bis(N-(N′-butylimidazolium)alkane bishexafluorophosphates (1,n-bis[Bim][PF6], 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/nBuOH/1,4-bis[Bim][PF6] 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/nBuOH/1,4-bis[Bim][PF6] 400/1.0/0.5 mol% at 120 °C for 72 h led to the polymerization conversions higher than 95%, with the molecular weight (Mw) 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.

The Lewis acid organocatalytic system of lithium tetramethylene-tethered bis[N-(N′-butylimidazol-2-ylidene)] N-heterocyclic carbene (1,4-bisNHC) including lithium benzyloxide and benzyl alcohol has been successfully utilized in the ring-opening polymerization (ROP) of ε-caprolactone (CL) for the first time. The catalytic performance of this organic catalyst in the synthesis of high-molecular-weight polymers was investigated via bulk polymerization using different combinations of tetramethylene-tethered bis[N-(N′-butylimidazolium)] hexafluorophosphate (1,4-bis[Bim]­[PF6]), benzyl alcohol (BnOH), and n-butyl lithium (nBuLi) ([1,4-bis[Bim]­[PF6]]/[BnOH]/[nBuLi]) with the molar ratios of 0:2:2, 1:1:3, 1:2:3, and 1:2:4. The results showed that the molar ratio of 1:2:3 efficiently and rapidly initiated the bulk ROP of CL at room temperature with a high molar ratio of CL to 1,4-bis[Bim]­[PF6] of 3000/1 and produced the highest number of average-molecular-weight (M n) poly­(ε-caprolactone) (103,057 g mol–1) with the dispersity (D̵) and %conversion of 1.73 and 98% in a short period of time (152 s). From comparative studies, the relative polymerization rates of the bulk ROP of CL with different [1,4-bis[Bim]­[PF6]]/[BnOH]/[nBuLi] molar ratios was determined in the following order: 1:2:4 > 1:1:3 > 1:2:3 > 0:2:2. For mechanistic investigation, the bulk ROP mechanism of CL with our organic catalyst was proposed through the intramolecular bis-lithium–carbene interaction pathway for 1,4-bisNHC1,1,3, 1,4-bisNHC1,2,3, and 1,4-bisNHC1,2,4 systems.

This work investigates the high catalytic activity of bi‐functional lithium N‐heterocyclic carbene (bisLiNHCs) complexes in bulk ring‐opening polymerization (ROP) of ε‐caprolactone (ε‐CL) monomer at room temperature. The study employs a bulk ROP technique using tetramethylene‐tethered bis[N‐(N′‐alkyl/arylimidazolium)] bishexafluorophosphate (bis[Bim][PF6]), benzyl alcohol (BnOH), and n‐butyl lithium (nBuLi) in molar ratios of 1:2:4. The results show that 1,4‐bisLiNHC (3a) has the highest catalytic capability, producing the highest molecular weight of PCL with acceptable dispersity in only 93 s (59 361 g mol−1, Đ = 1.8). However, longer and aromatic linker chains (3b‐3g) are found to be less effective due to the increased steric effect for complex formation with the monomer, leading to longer reaction times and lower molecular weight PCL with wider dispersity. Additionally, the study conducts a mechanistic analysis using the DFT method with B3LYP (6‐31G(d,p)) as the basis set. The most plausible mechanism proposed involves continuous and coordinated insertion with low energy barriers.