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

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.

Ring‐Opening Polymerization of ϵ‐Caprolactone by Benzoic Acid in the Presence of Benzyl Alcohol as Initiator

“…It gave a PCL with the M n = 7700 g/mol and PDI = 1.25, as determined by 1 H NMR spectroscopy and GPC. Compared to benzoic acid-catalyzed ɛ-CL polymerization procedures with higher initiator amount, [31] higher molecular weight was obtained under lower reaction temperatures in this work.…”

“…Based on above experiments, a plausible mechanism of the ring-opening polymerization of ɛ-CL catalyzed by benzoic acid is proposed as shown in Scheme 2. [31] According to previous reported works, the organic acid-catalyzed ROP of ɛ-CL is an activated monomer mechanism leading to well-defined polyesters. The carbonyl group of a monomer is activated via hydrogen bonding to the carbonyl hydroxyl proton of benzoic acid.…”

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

“…1,6‐ Bis (1 H ‐imidazol‐1‐yl)hexane (1,6‐ bis [Bim], 6b ) was synthesized using 60% sodium hydride dispersion in mineral oil (3.85 g, 88.14 mmol) in THF (10 mL) was slowly applied to a flask containing the solution of imidazole ( 4 ) (5.00 g, 73.45 mmol) in THF (20 mL) at 0 °C for 1 h. 1,6‐Dibromohexane ( 5b ) (5.7 mL, 36.73 mmol) was added at room temperature, and the mixture was further refluxed for 72 hours, according to published literature [ 42–44 ] as condition B . After the reaction was complete, the resulting solution was filtered to remove by product as NaBr and washed with THF.…”

“…(1,, 6a) was prepared using imidazole (4), NaOH, and 1,4-dibromobutane (5a) as starting materials with minor modifications as condition A, according to published literature. [40][41][42] To a 250 mL round bottom flask equipped with a Dean-Stark apparatus, a condenser, and a magnetic stir bar place on an oil bath equipped with a magnetic stirrer, Imidazole (4) (5.00 g, 73.45 mmol) and NaOH (2.93 g, 73.26 mmol) were added with 40 mL of DMSO and stirred at 150 °C for 2 h. Then, the solution of 1,4-dibromobutane (5a) (36.73 mmol) in DMSO (10 mL) was slowly added into the previous reaction mixture at 80 °C via a dropping funnel. The reaction mixture was vigorously stirred for 24 h at 100 °C.…”

“…Then, the reaction was filtered to remove a by-product as solid NaBr and washed with toluene (10 mL). The filtrate was evaporated to dryness and subsequently recrystallized from deionized water to obtain 1,4-bis(1H-imidazol-1-yl)butane (1,4- (1,, 6b): Condition B 1,6-Bis(1H-imidazol-1-yl)hexane (1,6-bis[Bim], 6b) was synthesized using 60% sodium hydride dispersion in mineral oil (3.85 g, 88.14 mmol) in THF (10 mL) was slowly applied to a flask containing the solution of imidazole (4) (5.00 g, 73.45 mmol) in THF (20 mL) at 0 °C for 1 h. 1,6-Dibromohexane (5b) (5.7 mL, 36.73 mmol) was added at room temperature, and the mixture was further refluxed for 72 hours, according to published literature [42][43][44] as condition B. After the reaction was complete, the resulting solution was filtered to remove by product as NaBr and washed with THF.…”