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

Natthapol Akkravijitkul; Nathaporn Cheechana; Puracheth Rithchumpon; Thiti Junpirom; Wanich Limwanich; Kanarat Nalampang; Praput Thavornyutikarn; Winita Punyodom; Puttinan Meepowpan

doi:10.1021/acs.joc.2c01062

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

J Org Chem. 2022 Sep 16;87(18):12052-12064. doi: 10.1021/acs.joc.2c01062. Epub 2022 Sep 7.

Authors

Natthapol Akkravijitkul^{1

2}, Nathaporn Cheechana^{1

2}, Puracheth Rithchumpon^{1

2}, Thiti Junpirom¹, Wanich Limwanich³, Kanarat Nalampang¹, Praput Thavornyutikarn¹, Winita Punyodom^{1

4

5}, Puttinan Meepowpan^{1

4

5}

Affiliations

¹ Department of Chemistry, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand.
² Graduate School, Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand.
³ Faculty of Sciences and Agricultural Technology, Rajamangala University of Technology Lanna, Chiang Mai 50300, Thailand.
⁴ Center of Excellence for Innovation in Chemistry (PERCH-CIC), Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand.
⁵ Center of Excellence in Materials Science and Technology, Chiang Mai University, 239 Huay Kaew Road, Chiang Mai 50200, Thailand.

PMID: 36073019
DOI: 10.1021/acs.joc.2c01062

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-bisNHC_1,1,3, 1,4-bisNHC_1,2,3, and 1,4-bisNHC_1,2,4 systems.