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

doi:10.3390/polym13244290

Nathaporn Cheechana, Thiti Junpirom + Show 9 more

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https://doi.org/10.3390/polym13244290

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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 (Mw) 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.

Highlights

Biodegradable polymeric materials with optimized physico-chemical and degradation properties have become more important in terms of new biomedical technologies in recent decades
The structure 6a–6d were characterized by 1H-NMR in MeOH-d4, which showed an important signal of two N-methylene protons (NCH2) at 4.19–4.31 (t, J = 7.5 Hz, 4H), and the two methine protons (N–CH=N) of imidazoles moiety at 8.80–9.22 (s, 2H) ppm
The synthesized DILs were completely characterized by FITR, 1H-NMR, 13C-NMR

Summary

Introduction

Biodegradable polymeric materials with optimized physico-chemical and degradation properties have become more important in terms of new biomedical technologies in recent decades. Woodruff and Hutmacher reported the wide range of biomedical applications involving PCL, including drug delivery, medical devices, and tissue engineering of bone, cartilage, blood vessels, skin, and nerve [3–5]. The synthesis of biodegradable polymers, such as polylactones and polylactide, can be synthesized by several methods, e.g., polycondensation of hydroxycarboxylic acids or ring-opening polymerization (ROP) of cyclic ester monomers. The initiator or catalyst for ROP is essential to initiate or activate the polymerization of cyclic esters to obtain high molecular weight polyesters with low polydispersity. These general initiators or catalysts for ROP are alkoxides based on metals, such as Al [10–21], Sn [11,22–27], Zr [11,12,22,23,28], Zn [11,23,29–32], Sb [11,22], and rare earth or lanthanide metals [11,33–36]

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