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- bis NHC) 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][PF 6 ]), benzyl alcohol (BnOH), and n -butyl lithium ( n BuLi) ([1,4- bis [Bim][PF 6 ]]/[BnOH]/[ n BuLi]) 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][PF 6 ] 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][PF 6 ]]/[BnOH]/[ n BuLi] 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- bis NHC 1,1,3 , 1,4- bis NHC 1,2,3 , and 1,4- bis NHC 1,2,4 systems.