Macromolecules Article due to their high EXPERIMENTAL SECTION nich greatly limit ation.Despite Materials.Vanillin, molecules remains a challenge.Fo (EC) Co. the lignin-to ally utilize and ale from lignin.has al used o be i is challen is(( aethy44-methyienebi4,lpbelene dia osphi d in I5 groups using pe itol reacting with 2(013 mol) 11.838 s also reported way to produce diepo Thes me s h at 2 6g08m56ed to 8 their flan tato℃for8t0 ng g)wi polymers includin for 24 h in a v nd phosph svnthetic route is illustrated in Scheme 1 by th Scheme 1.Synthetic Routes of DPI and DP2 opean Unic years hav most of the ph ifested a dec HaN-R-NH2 2.2eqHO-CHO e in glass t nsition and -link density of th 40"C 1h C2HsOH ction of envir nent of phos ho e2 phosphte d report on in this high-pe ance flam ng 0 the base x- 9c。 cted wit te to n ph R=Q入Op EPI and EP2 wer obta ols with epic res DP2 NMR.DDM was used to sure EP1.EP2.and DGEBA EP by DSC) 8张m4364N-助30asO-H,12se-05 nd lim 4,88 h) ray ph y (SEM) stability,glass tnsitioa temperature,and nalysis (TGA),DSC.and tensile testdue to their high molecular weights, insolubility, and composition variability depending on the species or the time of year,30−32 which greatly limited their utilization. Despite a great deal of efforts, decomposition of lignin into small molecules remains a challenge. Fortunately, the lignin-to￾vanillin process has been commercially utilized, and vanillin, as the only monoaromatic compounds produced on an industrial scale from lignin, has already exhibited tremendous potential to be used as a renewable building block for polymers.33,34 However, vanillin has only one group suitable to be introduce epoxy group; consequently, producing diepoxies is challenge. Oxidation or reduction of vanillin’s formyl group was conducted to achieve two suitable groups to introduce two epoxy groups,25,26 and using pentaerythritol reacting with formyl group and “coupling” two vanillin to achieve a difunctional phenol followed by reacting with epichlorohydrin is also a reported way to produce diepoxies.23 These methods are relatively complex, and toxic compounds were utilized; the fundamental mechanical properties of the vanillin-based epoxies were not reported. The second drawback of epoxy resins is their flammability, which blocks their application in the fire resistance required fields.35 Phosphorus-containing compounds have been re￾garded as effective and environment-friendly flame retardants for polymers including epoxy resins, and phosphorus￾containing flame retardants have attracted intensive interests36 after some halogenated fire retardants were banned by the European Union due to their toxicity.37 Recent years have witnessed rapid development of phosphorus-containing epoxy resins, while most of the phosphorus-containing epoxy systems manifested a decrease in glass transition temperature (Tg) than the phosphorus-free systems due to the limited chemical structure design and relatively low cross-link density of the cured epoxy resin.38 Sustainability, reduction of environmental impacts, and green chemistry are increasingly guiding the development of phosphorus-containing materials from renew￾able resources,39 while there is limited report on biobased phosphorus-containing epoxy resins. Thus, in this paper, two novel high-performance flame retardant bio-based epoxy resins containing phosphorus were synthesized from vanillin. Diamines were used as the coupling agents in this work, and the Schiff base intermediates (produced from the coupling reaction between vanillin and diamines), without being extracted from the systems, further reacted with diethyl phosphite to obtain phosphorus-containing difunctional phenols; and then the flame retardant epoxy resins EP1 and EP2 were obtained by reacting these difunctianal phenols with epichlorohydrin. The chemical structures of the monomers were characterized by FTIR, 1 H NMR, and 13C NMR. DDM was used to cure EP1, EP2, and DGEBA (the control) to obtain three thermosetting resins EP1-DDM, EP2- DDM, and DGEBA-DDM. The curing behaviors of the systems were examined by differential scanning calorimetry (DSC). Flame retardancy and flame retarding mechanism of the epoxy resins were investigated by vertical burning and limit oxygen index test and char analysis using inductively coupled plasma optical emission spectrometer (ICP-OES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The thermal stability, glass transition temperature, and mechanical properties were studied by thermogravimetric analysis (TGA), DSC, and tensile test, respectively. ■ EXPERIMENTAL SECTION Materials. Vanillin, 4,4-diaminodiphenylmethane (DDM), tetrabutylammonium bromide, p-phenylenediamine (PDA), and diethyl phosphite were purchased from Aladdin-reagent Co., China. Zinc chloride, epichlorohydrin (ECH), ethanol, petroleum ether, dichloromethane, and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin (DGEBA, trade name DER331, epoxy value 0.53) was supplied by DOW Chemical Company. Synthesis of Tetraethyl(4,4′-methylenebis(4,1-phenylene)- bis(azanediyl))bis((4-hydroxy-3-methoxyphenyl)methylene)- diphosphonate (DP1). DP1 was synthesized by a quintessential condensation reaction between vanillin and 4,4-diamino￾diphenylmethane, followed by an addition reaction with diethyl phosphite. 20 g (0.13 mol) of vanillin was dissolved in 150 mL of ethanol in a 500 mL three-neck flask equipped with a magnetic stirrer and a reflux condenser. 11.83 g (0.06 mol) of 4,4-diamino￾diphenylmethane dissolved in 50 mL of ethanol was added dropwise into the flask over a period of 0.5 h at 25 °C, and they were reacted at 40 °C for 1 h. Then diethyl phosphite (24.86 g, 0.18 mol) and zinc chloride (0.5 g) as the catalyst for the addition reaction were added into the system, and the mixture was heated from 40 to 80 °C under a nitrogen atmosphere and kept at 80 °C for 8 h. Finally, the mixture was poured into 1000 mL of petroleum ether and stirred for 0.5 h after being cooled to room temperature. A white solid was collected by filtration; then the white crystalline product DP1 (29.7 g) with the yield of around 93.3% was obtained after being washed with ethanol twice followed by drying at 70 °C for 24 h in a vacuum oven. The synthetic route is illustrated in Scheme 1. FTIR (KBr), cm−1 : 3364 (N−H); 3225 (O−H); 1278 (PO); 1026 (P−O). 1 H NMR (DMSO-d6, ppm): δ = 1.02 (t, 6H), 1.14 (t, 6H), 3.51 (d, 2H), 3.65 (m, 6H), 3.83 (m, 2H), 3.99 (m, 4H), 4.76 (dd, 2H), 5.96 (dd, 2H), 6.64 (dd, 6H), 6.75 (d, 4H), 6.84 (d, 2H), 7.07 (s, 4H), 8.85 (s, 2H). 13C NMR (DMSO-d6, ppm): δ = 16.68 (dd, 4C), 39.94 (s, 1C), 53.64 (s, 1C), 55.17 (s, 1C), 56.12 (s, 4C), 62.65 (dd, 2C), 113.09 (d, 2C), 114.07 (s, 4C), 115.40 (s, 2C), 121.38 (d, 2C), 128.00 (s, 2C), 129.18 (s, 4C), 130.78 (s, 2C), 145.76 (d, 2C), 146.34 (d, 2C), 147.69 (d, 2C). Scheme 1. Synthetic Routes of DP1 and DP2 Macromolecules Article DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 1893