Scalable and Room-Temperature Ring-Opening Polymerization of e-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 epsilon-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(epsilon-caprolactone) (103,057 g mol(-1)) with the dispersity (D?) 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 [1,4bis[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 bislithium-carbene interaction pathway for 1,4-bisNHC(1,1,3), 1,4-bisNHC(1,2,3), and 1,4-bisNHC(1,2,4) systems.