Adipic acid

Citric acid was purchased from Acros (Thermo Fisher Scientific, Geel, Belgium). Malic acid, ethylene glycol, 1,4-butanediol, and poly(ethylene imide) (PEI, branched, Mn = 600, Mw = 800) were from Aldrich (Milwaukee, WI, USA). Poly(ethylene glycol) (PEG, Mn = 1000) was purchased from Alfa Aesar (Ward Hill, MA, USA). Adipic acid, succinic acid, and glycerol were from Showa (Tokyo, Japan). Ionic liquids were purchased from Aldrich (1-ethyl-3-methylimidazolium dicyanamide, [EMIM]N(CN)₂; 1-ethyl-3-methylimidazolium tetrachloroaluminate, [EMIM]AlCl₄; 1-methyl-3-octylimidazolium chloride, [MOIM]Cl); and trihexyltetradecylphosphonium dicyanamide, [P6,6,6,14]N(CN)₂) and Alfa Aesar (1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF₄; 1-butyl-3-methylimidazolium chloride, [BMIM]Cl; and 1-butyl-3-methylimidazolium bromide, [BMIM]Br). Nine flavonoids (5-methoxyflavone (Met), hesperidin (Hpd), naringin (Nag), catechin (Cat), epicatechin (Epi), hesperetin (Hpt), daidzein (Dai), naringenin (Nar), and quercetin (Que)) were from Aldrich. All metal and phosphate salts were of analytical grade and were obtained from Aldrich and Acros. Purified water (18 MΩ∙cm) from a Milli-Q water purification system (Millipore, Bedford, MA, USA) was used to prepare the standard salt and buffer solutions. All standard solutions were protected from light and kept at 4 °C in a refrigerator.

All reagents and solvents were reagent-grade purchasing from commercial companies and used as received. ε-Caprolactam (>99%) and titanium isopropoxide (98%) were obtained from Acros. N-methyl-ε-caprolactam (96%) and N-acetyl-ε-caprolactam (>99%) were purchased from Tokyo Chemical Industry Co., Ltd. Formic acid (88%) and acetone (98%) were purchased from Fisher Chemical. Adipic acid (99%) was from Showa. Polytetramethylene Ether Glycols were received from Aldrich. Poly(4,4′-methylenebis(phenyl isocyanate),Elasturan^® T 6961 (NCO content 23%) was obtained from BASF.