Scalable and Room-Temperature Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Active Lithium Tetramethylene-Tethered Bis[N-(N'-butylimidazol-2-ylidene)] N-Heterocyclic Carbene as a Lewis Acid Organocatalyst.

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 (Mn) 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.

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 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 (D~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.