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

Abstract

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.

Room‐Temperature Lewis Acid Organocatalysts for Bulk Ring‐Opening Polymerization of Bis‐Lithium N‐Heterocyclic Carbenes Complexes Activated on ε‐Caprolactone: Synthetic, Experimental, and Density Functional Theory of Mechanistic Studies

“…Furthermore, the analysis of certain substances revealed a high degree of conformity with existing research that has been published in the relevant literature. [ 48 ]…”

“…In a previous study conducted by Akkravijitkul and co‐workers, [ 48 ] the use of 1,4‐ bis ‐lithium NHCs as catalysts in the bulk ring‐opening polymerization (ROP) of ε ‐caprolactone ( ε ‐CL) with BnOLi as an initiator at room temperature was found to quickly synthesize a linear homopolymer of PCL with high molecular weight and acceptable dispersity. Based on this finding, we were interested in modifying the catalyst structures to investigate the effect of linker groups in the catalyst systems for bulk ROP of ε ‐CL.…”