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

Nathaporn Cheechana; Wachara Benchaphanthawee; Natthapol Akkravijitkul; Puracheth Rithchumpon; Thiti Junpirom; Wanich Limwanich; Winita Punyodom; Nawee Kungwan; Chanisorn Ngaojampa; Praput Thavornyutikarn; Puttinan Meepowpan

doi:10.3390/polym13244290

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

Polymers (Basel). 2021 Dec 8;13(24):4290. doi: 10.3390/polym13244290.

Authors

Nathaporn Cheechana^{1

2}, Wachara Benchaphanthawee^{1

2}, Natthapol Akkravijitkul^{1

2}, Puracheth Rithchumpon^{1

2}, Thiti Junpirom¹, Wanich Limwanich³, Winita Punyodom^{1

4

5}, Nawee Kungwan^{1

4

5}, Chanisorn Ngaojampa¹, Praput Thavornyutikarn¹, 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, 128 Huay Kaew Road, Chiang Mai 50300, Thailand.
⁴ Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, 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.

Abstract

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 (M_w) of PCL 20,130 g mol^-1 (Đ~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.

Keywords: bis(n-(n′-butylimidazolium)alkane dicationic ionic liquids; density functional theory; metal-free catalyst; ring-opening polymerization; ε-caprolactone.