Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid

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Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid: A new Chemical Route for Plant Oil-based Epoxies Cong Li, Yonghui Li, Xiaoxia Cai, Hongwang Wang, Stefan H. Bossmann, Jonggeun Sung, and Xiuzhi Susan Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01656 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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ACS Sustainable Chemistry & Engineering

Main Article

Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid: A new Chemical Route for Plant Oil-based Epoxies Cong Li,[a] Yonghui Li,[a] Xiaoxia Cai,[a] Hongwang Wang,[c] Stefan H Bossmann,[c] Jonggeun Sung,[a] and Xiuzhi Susan Sun*[a][b]

[a] Dr. C. Li, Dr. Y. Li, Dr. X. Cai, Dr. J. Sung, Prof. X. S. Sun Bio-Materials and Technology Lab, Department of Grain Science and Industry, Kansas State University,1980 Kimball Ave, BIVAP Innovation Center, Manhattan, Kansas 66506 (USA) * Correspondence to: Xiuzhi Susan Sun, E-mail: [email protected], Phone: 785-5324077, fax: 785-532-7193 [b] Prof. X. S. Sun Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS66506 (USA) [c] Dr. W. Wang, Prof. S. H. Bossmann Department of Chemistry, Kansas State University, Manhattan, KS66506 (USA)

Submitted to ACS Sustainable Chemistry & Engineering

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Main Article ABSTRACT Plant oil is one of the world’s most abundant renewable resources, however, its derived epoxies are low in thermal resistance and mechanical strength. In this work, a new chemical route referred to “competitive nucleophilic attack (CNA)” was discovered to achieve plant oil based epoxy with high thermal resistance and mechanical strength as well as many other unique properties comparable to diglycidyl ether of bisphenol A (DGEBA), one of the most popular petroleum based epoxies. The CNA route was realized by using 10-undecenoic acid (UA), a plant derived monomer as building block reacting with alicyclic oxirane chemicals, such as 4-ethenyl-7-oxabicyclo[4.1.0]heptanes (ECP) to achieve epoxy monomers with ether-bridged cycloaliphatic ring structure. A newly formed hydroxyl (NFH) involves in the nucleophilic attack upon oxonium to compete with UA anion during the UA-ECP reaction. The resultant epoxy is UV curable in a few seconds, possessing high tensile strength (~48 MPa), high glass transition temperature (~142 °C), high transparency (~90 %) as well as low viscosity (~1.9 Pa·s). These properties are superior to the plant oil based epoxies published and comparable to or better than commercial DGEBA. Structure analysis revealed that the ether-bridged cycloaliphatic ring structure via CNA route played a key role in maximizing the network performance. With CNA feature, chain structure can be further regulated via introducing methyl group to hinder the NFH nucleophilic attack, achieving a conversion of epoxy resin from rigid to semi-ductile. This finding suggests that CNA strategy could be a new direction to design bio-based epoxies using all possible chemicals with acid-alkene structures from various renewable resources rather than plant oils only. KEYWORDS: Bio-based Epoxy, Plant Oil, Nucleophilic Attack, Renewable, Bisphenol A 2 ACS Paragon Plus Environment

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Main Article INTRODUCTION Industry has been constantly searching for sustainable alternatives to petroleum based epoxies. The global market value for epoxy resin was about US $18.6 billion in 2013, and is projected to reach US $25.8 billion by 2018 for a range of applications including epoxy composites and the adhesive market.1 Diglycidyl ether of bisphenol A (DGEBA), a petroleum chemical, is the most widely used epoxy resin, accounting for almost 90% of the epoxy resin market. Bisphenol A (BPA), the main component of DGEBA is derived from petrochemicals and appears to be an estrogen receptor agonist, which has been strictly limited in many countries.2, 3 To date, bio-based epoxy resins have been investigated using a variety of building blocks derived from plant oil, woody biomass, natural polyphenols, terpene, starch and sugar.2-24 However, compared to DGEBA that has low viscosity (e.g., 5 Pa·s) and high tensile strength (e.g., 40 MPa) as well as broad glass transition range (70 °C~ 170 °C, depending on the epoxy value, curing agents or modifiers), none of the bio-based epoxies possesses all these superior properties at a given formula. For example, the epoxies derived from natural polyphenols, terpene, and woody biomass normally have high glass transition temperatures similar as DGEBA, but are viscous and difficult to be processed at ambient temperature; some starch and sugar derived epoxies have relatively lower viscosities, but are not satisfied in thermal resistance. Plant oil is one of the world’s most abundant renewable resource, however, its derived epoxies show low Tgs due to the long aliphatic chains and the low reactivity of epoxy groups.17,18 Though efforts have been made and further increases in Tgs have been achieved, the resultant effects are limited. For instance, Gerbase et al. polymerized epoxidized soybean oil (ESO) resins with



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Main Article succinic anhydride, maleic anhydride, hexahydrophthalic anhydride and phthalic anhydride at an epoxide-anhydride equivalent ratio of 1:1. The resulting resins had Tgs in the range from 5 to 65 °C.19 Webster et al. developed a sucrose linseedate epoxy and cured it with 4-methyl hexahydrophthalic anhydride with an improved Tg of 104 °C.20 Zhang et al. synthesized a glycidyl ester type epoxy using the Diels–Alder reaction of tung oil fatty acid and fumaric acid to further improve Tg to 131°C.21 In this work, we discovered a new route for plant oil-based epoxy preparation. When 10-undecenoic acid (UA), a renewable derivate from castor oil was simply blended with 4-ethenyl-7-oxabicyclo[4.1.0]heptanes (ECP) at 80°C for 48h, a catalyst-solvent-free reaction occurred and, a new epoxy precursor (CUA) was formed. The resultant new epoxy was UV curable and had a glass transition temperature of 142 °C. More encouragingly, compared to DGEBA, this new bio-based epoxy demonstrated lower viscosity, higher UV-curing reactivity, better optical appearance, and more robust mechanical properties. We initially expected that UA anion would dominate the nucleophilic attack upon ECP oxonium (for nucleophilicity, see Figure S1), however, a hydroxyl group was formed during the reaction. As another important nucleophile, this newly formed hydroxyl (NFH) significantly involved in the nucleophilic attack competition, leading to an ether-bridged cycloaliphatic ring structure (Scheme 1a). Competitive nucleophilic attack (CNA) occurred in UA-ECP reaction plays a key role in achieving this plant oil-based epoxy with the unique properties mentioned above. With the feature of CNA, we selected limonene 1, 2-epoxide (MECP) instead of ECP, a citrus oil-derived material with methyl groups grafted onto the cyloaliphatic ring, to react with UA, aiming at further regulating the chain structure of epoxy. Compared to the


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Main Article ECP, the methyl group on the MECP hindered the nucleophilic attack of NFH upon oxonium, therefore, a linear monomer (MCUA) structure was formed (Scheme 1b). As expected, the resultant epoxy (EMCUA) presented relatively lower mechanical strength and higher ductility than the CUA derived epoxy (ECUA). On the other hand, its tensile strength of 32 MPa is still comparable to DGEBA because of the presence of NFH, by which cross-linked structure was achieved through the reaction of NFH and epoxy ring. We characterized the chemical structures of these two epoxy precursors (CUA and MCUA). Chemical synthesis pathways of CUA and MCUA were investigated to understand the influence of steric effects on the nucleophilic attacks and consequently final structures of the bio-based epoxy resins. We proposed that the competitive nucleophilic attack (CNA) pathway could be regulated by the cycloaliphatic ring structures and hence the epoxy properties. With this strategy, new bio-based epoxy could be expected by using any chemicals containing acid-alkene structure derived from all possible renewable resources. Here, for the first time, we report the possibility of how tender plant oil-derived material can be constructed into an epoxy with unique properties comparable to commercial DGEBA via CNA strategy. Experimental Section Materials. 10-undecenoic acid (UA, 98%), 4-ethenyl-7-oxabicyclo[4.1.0]heptanes (ECP, 98%), and (+)-limonene 1,2-epoxide (MECP, 98%) were purchased from SigmaAldrich (USA). Diglycidyl ether of bisphenol A (DGEBA) with epoxy equivalent weight of 175 g mol-1was obtained from Dow Chemical Company. M-chloroperoxybenzoic acid (m-CPBA, 70-75%) was purchased from Acros Organics. The epoxidized soybean oil (ESO, Vikoflex® 7170) was provided by ArkemaInc., and cationic initiator (PC-2506,[4-(2-



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Main Article hydroxyl-1-tetradecyloxy)-phenyl], phenyliodoniumhexafluorantimonate) was kindly provided by Polyset Inc. Synthesis of UA-ECP Epoxy Precursor CUA. UA (100.0 g) and ECP (67.4 g) were charged into a 250 mL round-bottomed flask equippedwith a reflux condenser, and the mixture was stirred with a magnetic bar at 80 °C for 48 h and transferred to a 2000 mL separating funnel. 400 mL hexane and 800 mL sodium carbonate solution (10 wt %) were then added to separate the residual UA from the product. Hexane solvent was then removed and collected through high vacuum rotary evaporation at 60 °C, and a colorless CUA liquid with a yield rate of 82 % (relative to pure ECP) was obtained.NMR spectra were performed as: 1H NMR (CDCl3, δ ppm) 5.72 (s, 5H), 4.92 (s, 10H), 4.80 (m, 2H), 4.67 (m, 2H), 3.77 (m, 1H), 3.65 (m, 1H), 2.74 (m, 2H), 2.37 (m, 2H), 2.24 (s, 4H), 1.97 (s, 4H), 1.83 (m, 2H), 1.55 (s, 4H), 1.28 (s, 37H);

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C NMR (600 MHz, CDCl3): δ =

173.5 (2C), 142.0 (3C), 138.8 (2C), 173.5 (2C), 114.1 (2C), 113.9 (3C), 74.0-72.9 (2C), 68.7-67.4 (2C), 52.8.0-51.3 ppm (2C). Synthesis of UA-MECP Epoxy Precursor MCUA. The synthesis and purification procedures of MCUA were the same as that of CUA, and a colorless liquid with a yield rate of 70% (relative to pure MECP) was obtained. NMR spectra were performed as: 1H NMR (CDCl3, δppm) 5.76 (s, 2H), 4.92 (s, 4H), 4.78 (m, 1H), 4.67 (s, 4H), 4.04 (s, 2H), 2.25 (m, 4H), 1.99 (m, 4H), 1.86 (m, 2H), 1.67 (s, 4H), 1.57 (m, 4H), 1.49 (s, 2H), 1.25 (s, 30H), 1.13 (s, 2H). 13C NMR (600 MHz, CDCl3): δ= 172.7 (1C), 148.8 (1C), 139.0 (1C), 114.1 (1C), 108.8 (1C), 82.4-75.2 (1C), 70.3-69.7 ppm (1C). Synthesis of UA-ECP Epoxy ECUA. CUA (50.0 g), m-CPBA (163 g) and 300 mL dichloromethane were charged into a 1000 mL round-bottomed flask with a reflux


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Main Article condenser and a magnetic stirrer. The reactants were mixed at 0 °C for 3 h and then kept at 25 °C for 48 h. 600 ml ethyl ether was added into the mixture after removing dichloromethane by vacuum rotary evaporator. Then 300 mL sodium sulfite solution (10 wt %), 500 mL saturated sodium bicarbonate solution and 500mL saturated sodium chloride solution were subsequently used to wash the product. Ethyl ether and residual water in organic layer were then removed and collected by vacuum rotary evaporator, and finally a clear and colorless ECUA liquid was obtained with a yield rate of 84% with respect to the CUA. NMR spectra were performed as: 1H NMR (CDCl3, δ ppm) 4.81 (m, 2H), 4.68 (m, 2H), 3.81 (m, 2H), 2.81 (m, 3H), 2.72 (m, 2H), 2.65 (s, 5H), 2.46 (m, 3H), 2.37 (m, 2H), 2.22 (m, 4H), 1.77 (m, 2H), 1.48 (m, 4H), 1.21 (s, 45H);

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C NMR (600

MHz, CDCl3): δ = 172.7 (2C), 72.6-71.1 (2C), 67.5-66.1 (2C), 55.7-55.2 (5C), 52.2 (2C), 46.9 (2C), 46.2-45.8 ppm (3C). Synthesis of UA-MECP Epoxy EMCUA. The synthesis and purification procedures of EMCUA were the same as that of ECUA, and a clear and colorless liquid with a yield rate of 91% (relative to the MCUA) was obtained. NMR spectra were performed as: 1H NMR (CDCl3, δppm) 4.71 (m, 1H), 3.96 (m, 1H), 2.80 (m, 2H), 2.64 (m, 2H), 2.53 (m, 2H), 2.44 (m, 2H), 2.36 (m, 2H), 2.17 (m, 4H), 1.58 (m, 2H), 1.51 (m, 4H), 1.42 (s, 34H), 1.15 (s, 2H), 1.04 (s, 2H);13C NMR (600 MHz, CDCl3): δ = 172.7 (1C), 82.0-74.5 (2C), 69.6 (1C), 59.1 (1C), 53.2 (1C), 52.2 (1C), 47.0 ppm (1C). UV Polymerization. Cationic photo-initiator (PC-2506, 3wt%) was added into DGEBA, ESO, ECUA and EMCUA, respectively. The mixture was mixed homogenously with the aid of a Vortex mixer and sonicator, then coated onto a 15.24 cm ×25.4cm glass plate with a film casting knife (model 4302, BYK-Gardner USA) set at



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Main Article 160 µm wet thickness. The resin containing photo-initiator was cured with a Fusion 300S UV system (300 W/inch power, D bulb, UVA radiation dose 1.7-1.8J/cm2) equipped with a LC6B benchtop conveyor at conveyor speed of 213 cm/min. The corresponding cured resins were named as uv-DGEBA, uv-ESO, uv-ECUA and uv-EMCUA, respectively. Characterizations. Fourier transform infrared (FTIR) measurements were done with a Perkin-Elmer Frontier FT-IR/NIR spectrometer. Spectra acquisitions were based on 32 scans with data spacing of 2.0 cm-1. 2D-FTIR spectra were performed and calculated by 2Dshige (Shigeaki Morita, Kwansei-Gakuin University, 2004e2005). 1H NMR were performed using a Bruker 300 MHz spectrometer at room temperature.1H-1H COSY spectra were obtained with 128 increments and four scans for each increment.13C NMR spectra were recorded using a Bruker 600 MHz spectrometer at room temperature. Deuterated chloroform (CDCl3) was used as solvent for NMR tests. The contents of epoxy precursors CUA and MCUA were determined

by gas

chromatography (GC) using a Shimadzu GC-2010 plus GC system (Shimadzu, Columbia, MD) equipped with a flame ionization detector (FID).Helium was used as the carrier gas at a flow rate of 1.5 ml/min. The injector and column temperatures were ramped from 80 °C to 300 °C at 7 °C/min with the detector temperature held at 380 °C. Rheological behaviors were measured using a Bohlin CVOR150 rheometer (Malvern Instruments, Southborough, MA) with a parallel plate (PP20, 20-mm plate diameter and 500-µm gap). Frequency sweep was conducted at 25 °C with a strain of 0.5 % and an angular frequency range of 0.1-30 rad/s. The photocalorimetric measurements of resins were performed with a TA Q200 DSC coupled to a photocalorimeter accessory (PCA, OmniCure S2000, TA instruments) equipped with a high-pressure 200-W mercury lamp. The UV


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Main Article wavelength was adjusted to 320–500 nm using a cut-off filter, and a light intensity of 100 mW/cm2 was used. Approximately 10 mg of each resin was accurately weighed in an open aluminum pan, and an empty aluminum pan was used as reference. The sample was equilibrated at 25 °C for 0.5 min without UV, then UV-irradiated for 20 min under a nitrogen atmosphere. Light transmittances were measured by UV spectrometer (HewlettPackard 8453) with 0.16 mm thickness for specimens. Dynamic mechanical analysis (DMA) was performed with a DMA (Q800 New Castle, DE) using a tension mode. A 5µm amplitude was applied to guarantee the measurement was within the linear viscoelastic region. Samples were heated from -50 °C to 270 °C using a heating rate of 3 °C/min and frequency of 1Hz. Tensile strength and elongation at break were measured according to ASTM D882-12 using a tensile tester (TT-1100, ChemInstruments, Fairfield, OH) with a specimen dimension of 40 ×8 × 0.16 mm and a grip separation rate of 2.54 cm min-1. Five specimens were tested for each sample to obtain an average. RESULTS AND DISCUSSION Bio-based Epoxy Precursors UA-ECP Precursor CUA. The epoxy ring of cycloaliphatic oxide is prone to be protonated to form the oxonium ion intermediate and can be opened via nucleophiles’ attack because of the high strain on the three-membered ring. 10-undecenoic acid (UA), a renewable material derived from castor oil was used as the nucleophile in this research and 4-ethenyl-7-oxabicyclo[4.1.0]heptanes (ECP) was initially selected as the cycloaliphatic oxide. Reaction was confirmed by the decreased FTIR signals centered at 809 cm-1 (stretching C-O-C of epoxy group) and 2678 cm-1 (stretching O-H of carboxylic acid dimer) respectively,25 combined with the increase of the band centered at 3454 cm-1 9 ACS Paragon Plus Environment


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Main Article (stretching O-H of hydroxyl group) (Figures 1a and b). This result confirmed the nucleophilic attack of UA upon the epoxide ring of ECP and the formation of a new hydroxyl group on the cycloaliphatic ring defined as “newly formed hydroxyl” (NFH). Since the stoichiometric coefficient of UA to ECP is equal to 1.0, the disappearance of epoxy peaks should be theoretically equal to the depletion of carboxylic acid peaks if the reaction proceeded in accordance with the molar ratio of 1:1. However, the epoxy peak (centered at 809 cm-1) almost disappeared after a 48-hour reaction, while the carboxylic acid dimer peak (centered at 2678 cm-1) was still observable. This unexpected result implied that a complex reaction existed in the UA-ECP system, which was characterized by a two-dimensional 2D-FTIR analysis (see Figure S2). 1

H-NMR spectra of CUA were presented in Figure1c. The proton signals from the

internal and terminal sides of double bonds are located at 5.85-5.60 ppm and 5.07-4.83 ppm, respectively. The signals of protons connected to the C=O group are located at 2.30-2.19 ppm. By integrating the corresponding peak areas, the molar ratio of double bond to C=O was calculated as 1: 0.44. If CUA was derived from a reaction of UA and ECP according to a 1:1 reactant stoichiometry, the corresponding molar ratio of double bond to C=O group should be 1: 0.5. Therefore, the molar ratio of 1: 0.44 indicated that CUA should be a multi-component system containing both linear and branched monomers, as the typical chemical structures shown in Figure 1c. Since the nucleophilic attack of UA upon ECP was based on a 1:1 stoichiometry, the formation of branched structure implied that the NFH group involved in the nucleophilic attack upon oxonium ion to compete with UA anion. Such nucleophilic attack phenomenon is defined as “competitive nucleophilic attack” (CNA) chemistry route. Based on the molar ratio of


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Main Article double bond to C=O group of 1: 0.44, the molar ratio of branched structure to linear structure was estimated as about 1: 3.

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C NMR of CUA also confirmed the similar

information as that obtained from 1H-NMR (see Figure S3).

Figure 1. (a) Time evolution of FTIR spectra for the stoichiometric (1:1) UA-ECP system at 80 °C. (b) Kinetics of variations of the epoxy ring, COOH and OH functions (time dependences of the stretching C-O-C band intensity for epoxy ring, the stretching O-H band for carboxylic acid dimer and the stretching O-H band for hydroxyl group). (c) 1

H NMR spectra of CUA (typical chemical structures are shown). Specific components in CUA were further revealed by GC-MS spectrometry and

detailed analysis was discussed in Figure S4. . The existence of isomers was detected and



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Main Article illustrated by the transition state model proposed by John C. Leffingwell, etc.26 (Scheme S1) UA-MECP epoxy precursor (MCUA). Limonene 1, 2-epoxide (MECP) is a derivative from citrus oils, and the methyl group grafted cycloaliphatic ring is expected to hinder the nucleophilic attack of NFH upon oxonium ion, from the steric point of view, in comparison with the hydrogen atom in ECP (Figure S6). From the nucleophilicity point of view, compared to the negatively charged undecenoic acid anion, the neutral hydroxyl group is less capable to achieve the nucleophilic attack upon the methyl-grafted oxonium ring (see Figure S1). Therefore, the CNA pathway in UA-MECP should be hindered compared with that in UA-ECP system. The time evolution of FTIR spectra (Figures. 2a and b) showed that the epoxy groups centered at 840 cm-1 were still notable after 48 hours reaction, indicating that MECP had not been fully consumed, while in the UA-ECP system, the epoxy peak almost disappeared. 1H NMR spectra (Figure 2c) displayed the detail chemical structure of MCUA (product derived from the reaction of UA and MECP), in which the peaks at 5.10-4.85 ppm belong to the protons H1, H1', H2 and H2' from the double bonds of UA segment, and the peaks at 4.77-4.48 ppm belong to the protons H29, H29', H30 and H30' from the double bonds of MECP segment. By integrating the corresponding peak areas, the molar ratio of double bond from UA segment to double bond from MECP segment was calculated as 1: 0.996, very close to 1. A similar result was also obtained from the corresponding

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C NMR (see Figure S7). Since the

nucleophilic attack of UA upon MECP was based on a 1:1 stoichiometry, the molar ratio of 1: 0.996 obtained in MCUA proton spectra indicated that the NFH group did not achieve a nucleophilic substitution upon oxonium ion and the nucleophilic reaction was


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Main Article solely processed between UA anion and MECP based on a 1:1 stoichiometry. This phenomenon was further confirmed in details in the supporting information (Figure S8).

Figure 2. (a) Time evolution of FTIR spectra for the stoichiometric (1:1) UA-MECP system at 80 °C. (b) Kinetics of variations of the epoxy ring, COOH and OH functions (time dependences of the stretching C-O-C band intensity for epoxy ring, the stretching O-H band for carboxylic acid dimer and the stretching O-H band for hydroxyl group). (c) 1

H NMR spectra of MCUA (typical chemical structures are shown). (d) 1H-1H COSY

spectra of MCUA GC-MS spectrometry suggests three types of isomers exist in MCUA precursor (Figure 3). Three significant peaks were recognized in the retention times of 25~30 min in the 13 ACS Paragon Plus Environment


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Main Article GC chromatogram (other minor peaks’ confirmation see Figure S9). For each peak, MECP (m/z=152) and UA (such as m/z= 55, 83 and 96) featured mass signals were identified in the corresponding MS spectrum, indicating a chemical structure containing MECP and UA components. Besides, the corresponding MS spectra of these three peaks were very similar, suggesting the possibility of identical components in them; however, these components did not appear at the same retention time because of the isomerization, which was caused by the different substituted positions of UA anion upon epoxide ring, as explained by the transition state model for the UA-ECP precursor CUA (Scheme S1). Because of the existence of methyl substituent on the epoxy ring, in addition to the two possible substitution approaches based on the transition state, the formation of partial positive charge on the tertiary carbon atom results in partial trans-epoxides undergo a nucleophilic attack at the tertiary carbon site instead of the secondary carbon site.27 1

H-1H COSY spectra of MCUA (Figure 2d) provide a clear evidence that UA anion

attacked not only the higher hindered tertiary carbon on the epoxide ring, resulting in isomers α, γ (case 1), and but also the less hindered secondary carbon on the epoxide ring, resulting in isomer β (case 2), as steric molecular structures shown in Figure 3. For case 1, as the structure shown in Figure 2c, the proton 27 (H-C(OH)) can couple with the vicinity proton 28 (on hydroxyl group) and the protons 25, 26 (on cyclohexane ring), respectively. Correspondingly, two correlated peaks should be observed in the 1H-1H COSY spectra, which were evidenced by the two correlated peaks ((f1=1.85 ppm, f2=4.82 ppm) and (f1=1.71 ppm, f2=4.82 ppm)) on the 1H-1H COSY spectra (detailed chemical shift assignments see Figure 2c). For case 2, because of the connection of ester group, the proton 27’ (H-C(OOC)) only coupled with the vicinity protons 25’, 26’(on cyclohexane


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Main Article ring). Correspondingly, only one correlated peak should appear in the 1H-1H COSY spectra, which were confirmed by the correlated peak (f1=1.71 ppm, f2=4.06 ppm) on the 1

H-1H COSY spectra (detailed chemical shift assignments see Figure 2c). Based on the

integrated peak area ratio of protons 27 to 27’ in 1H NMR spectra, the molar ratio of case 1 to case 2 was estimated as 7:3. This means case 1 accounts for the majority of MCUA.

Figure 3. GC-MS chromatogram of MCUA. Steric molecular structures of three possible isomers in MCUA are shown (case 1: α and γ, substitution occurred on the tertiary carbon. case 2: β, substitution occurred on the secondary carbon).



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Main Article According to those analytical analyses, the features of CNA pathway in CUA and MCUA synthesis process are proposed in Scheme 1. First, the epoxy ring of ECP or MECP was protonated (SN1 route) by the proton dissociated from UA. Then the UA anion as a nucleophile attacks the electrophilic carbon on the epoxy ring to realize a ring opening (SN2 route).28 In Scheme 1a, the newly formed hydroxyl (NFH) group acts as a nucleophile achieving the nucleophilic attack upon the oxonium ion in competition with the nucleophilic attack caused by UA anion. Therefore, branched monomers were formed co-existing with linear monomers in the UA-ECP epoxy system. Distinct from ECP, the MECP has a bulky methyl group grafted on the cycloaliphatic ring that would hinder the nucleophilic attack by the NFH group upon oxonium ion (Scheme 1b). Therefore, only linear structured monomer was obtained in the UA-MECP epoxy system. In summary, the CNA pathway via NFH and UA anion upon the oxonium ion plays a key role in manipulating the epoxy precursor structure and hence the properties of epoxy resins. For the UA-ECP system, linear structure and branched structure coexist. For the UA-MECP system, only linear structure exists. The NFH group competition with UA anion in the nucleophilic attack upon oxonium, can be promoted or hindered to achieve either linear and branched co-existing epoxy monomers (i.e., UA-ECP epoxy) or linear epoxy monomer (i.e., UA-MECP epoxy).


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Main Article

Scheme 1. Competitive nucleophilic attack (CNA) chemical pathways for the synthesis of (a) CUA and (b) MCUA. 17 ACS Paragon Plus Environment


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Main Article Bio-based Epoxy Monomers – ECUA and EMCUA. The UA-ECP and UA-MECP epoxy precursors CUA and MCUA were epoxidized, respectively to obtain ECUA and MECUA epoxy monomers using m-chloroperoxybenzoic acid (m-CPBA) as shown in Scheme 2. Detailed 1H NMR chemical shift assignments were listed in Figure 4. Double bonds with chemical shift higher than 4.5 ppm were vanished and epoxide-featured signals appeared within 2.9~2.3 ppm, indicating the epoxidation of CUA and MCUA occurred. 13C NMR spectra also confirmed the epoxidation of CUA and MCUA (see Figure S10).

Scheme 2. Epoxidation of CUA and MCUA into (a) ECUA and (b) EMCUA.

Figure 4. 1H NMR spectra of ECUA and EMCUA (typical chemical structures are shown). 18 ACS Paragon Plus Environment

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Main Article The viscosities of DGEBA, ECUA and EMCUA epoxy monomers as a function of shear rate were determined in comparison to that of DGEBA (Figure 5). Compared with DGEBA, the complex viscosities of ECUA and EMCUA were considerably lower than that of DGEBA, which is preferred for easy downstream processing, and consequently, easier to apply onto the substrates for UV-curing. The viscosity of EMCUA was lower than ECUA when shear rate was above 20 rad/s, which was probably attributed to the higher amount of linear structures in the EMCUA monomer compared to ECUA.

Figure 5. Complex viscosities (Ƞ†) of epoxies determined at 25 °C as a function of shear rate. UV polymerization of ECUA and EMCUA epoxy monomers Epoxy films were prepared via the curing of monomers containing photo-initiator with Fusion 300S UV system, and the conversion of epoxy group after curing was evaluated by FTIR spectroscopy with Attenuated Total Reflectance (ATR) in Figure S11. The curing characteristics of epoxy monomers under ultraviolet light were evaluated using PCA-DSC (Figure 6). Apparent exothermic peaks were observed for all the samples, indicating effective epoxy curing occurred under UV radiation. The exothermic peak times of ECUA and EMCUA were shorter than that of DGEBA (Figure 6 and Table 1), suggesting EMCUA and ECUA possessed higher reactivity than DGEBA during UV 19 ACS Paragon Plus Environment


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Main Article curing. This fast curing feature can be attributed to the built-in NFH groups in the ECUA and EMCUA systems. The steric requirement for a hydroxyl group (i.e., NFH in this case) attacking upon an epoxy oxonium ion is relatively less compared to that between two epoxy rings, therefore, a faster curing behavior of ECUA and EMCUA is expected, 29 which was supported by additional experiments described in the supporting information section (Figure S12). A similar phenomenon was also found in Sangermano’s work.30 ESO possessed the slowest curing speed and lowest exothermic enthalpy. The bulky fatty acid ester chains of ESO hindered the propagation of epoxides during curing.

Figure 6. Exothermic behaviors of epoxy monomers containing of 3 wt% photo-initiator (PC-2506) radiated through UV light as a function of time. Table 1 Exotherm parameters of epoxy monomers in the UV-curing process. Sample

DGEBA

ESO

ECUA

EMCUA

Exothermic peak (min)

0.98

1.21

0.88

0.82

Onset of leveling off (min)

6.49

10.79

7.86

6.12

Exothermic enthalpy (J/g)

312.7

263.1

336.1

332.9


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Main Article Furthermore, the built-in NFH groups not only accelerated the curing speed, but also suppressed the “orange skin” effect on the cured epoxy film surface (Figure 7a), while “orange skin” marks were found on the surface of uv-DGEBA film (highlighted by red circle). The “orange skin” mechanism is complicated. One possible reason could be the internal stresses generated during UV polymerization.31 For the DGEBA system, the chain propagation continued through the nucleophilic attack of the epoxy ring upon the growing oxonium ion chain end; however, for ECUA and EMCUA, the participation of NFH groups in the nucleophilic attack led to a termination of the growing oxonium ion chain and transfer into a new chain (see mechanism of Scheme S2), which is likely a cause for the notable improvement in film appearance. Compared to the yellowing color of uv-DGEBA, the newly developed epoxy resins exhibited a clear and colorless appearance, with a higher light transmission throughout the visible wavelength range (Figure 7b).

Figure 7. (a) Appearances of epoxy resins after UV-curing and (b) transparency comparison. 21 ACS Paragon Plus Environment


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Main Article The uv-ECUA showed a remarkable glass transition temperature (Tg) of 142 °C (Figures 8a and b) which was higher than those previous published Tg values of plant oilbased epoxy resins.19-21 Moreover, uv-ECUA had a higher crosslink density (νe) and moduli E’ in the rubbery state than that of uv-DGEBA (Figure 8a), implying a good thermal resistance. Here, νe was calculated based on the theory of rubber elasticity according to the equation below:

νe =

E’ (1) 3RT

Where R is the gas constant, E’ is the plateau storage modulus after glass transition, and T is the corresponding absolute temperature. Compared to the uv-DGEBA and uvESO networks, in addition to the typical I and II crosslink structures (Figure 8c, highlighted by ellipse and rectangular shadows, respectively), uv-ECUA network is also composed of typical III and IV crosslink structures (Figure 8c, highlighted by square and round shadows, respectively). The connection of NFH and epoxy ring in the UV curing process results in the typical III crosslink structure, while the intrinsic ether-bridged cycloaliphatic rings consists of the typical IV crosslink structure that has been formed via the CNA pathway in the initial CUA synthesis stage. Typical III and IV crosslink structures render the uv-ECUA epoxy high Tg and high storage modulus. Because of the lack of typical IV crosslink structure (Figure 8c), uv-EMCUA showed a significant decrease in glass transition temperature in comparison with uv-ECUA. These results demonstrate that CNA and NFH play significant key roles in manipulating plant oil-based epoxies. With the CNA pathway in the presence of cycloaliphatic ring structure, the stoichiometric coefficient of UA and ECP can be regulated to achieve more ether-bridged


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Main Article cycloaliphatic structures, hence, we believe it is possible to synthesize plant oil-based epoxy with superior thermal resistance higher than 142 °C.

Figure 8. Temperature dependence of storage moduli (a) and loss factors (b) for uv-ESO, uv-DGEBA, uv-ECUA and uv-EMCUA. (c) Schematic diagram demonstrating the key features of cured epoxy networks. Typical I structure: ellipse shading; typical II structure: rectangular shading; typical III structure: square shading; typical IV structure: round shading. Mechanical properties of uv-ECUA and uv-EMCUA uv-ECUA demonstrates superior mechanical properties with higher tensile strength and elongation at break compared to uv-DGEBA (Figures 9a and c), which is ascribed to 23 ACS Paragon Plus Environment


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Main Article its unique network structure as discussed above. The tensile strength of uv-EMCUA is lower than that of uv-ECUA because uv-EMCUA is lack of the typical IV intrinsic crosslink structure (Figure 8c, uv-ECUA) due to its linear monomer structure; however, uv-EMCUA still has a comparable tensile strength with the uv-DGEBA (Figure 9a). Moreover, uv-EMCUA exhibits a semi-ductile behavior with an elongation at break of about 20%, which could attribute to the linear monomer structure of its epoxy precursor MCUA. The inset in Figure 9a shows that the uv-EMCUA film was integral without cracking and the nail was firmly stuck after penetrating the film. In summary, compared to commercial DGEBA, uv-ECUA showed superior performance across a range of properties, including proccessability, appearance, renewability, thermal resistance, and mechanical strength, which is a promising renewable alternative to bisphenol A-based epoxies (Figure 9b). The uv-EMCUA, with 100% bio-content, also has unique properties in comparison with uv-DGEBA. Figure 9d shows the position of the plant (i.e., castor) oil derived epoxy ECUA in the whole bio-based epoxy family. Compared to the published bio-based epoxies (detailed data are listed in Table S1), ECUA is not only low in viscosity, but also high in thermal resistance, possessing superior properties with great potential as alternatives to DGEBA. Promising bio-based epoxies with higher Tg and low viscosity can be expected via regulating the molar ratio of cycloaliphatic ring and fatty acid chain though CNA strategy.


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Main Article

Figure 9. (a) Typical strain-stress curves and (c) detailed mechanical results regarding elongation at break (%) and tensile strength (MPa) of epoxy resins. (b) Comprehensive performance comparison between DGEBA and bio-based epoxies synthesized in this work. The inset in (a) is uv-EMCUA film holding a nail in it. (d) Mapping of viscosity (X-axis) and glass transition temperature (Tg) (Y-axis) of bio-based epoxy family derived from plant oils, woody biomass, natural polyphenols, terpene, starch and sugars. Data are collected from the published information detailed in the supporting information of Table S1. 25 ACS Paragon Plus Environment


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Main Article CONCLUSIONS For the first time, competitive nucleophilic attack (CNA) is proposed in this work as a useful route to design plant oil-based epoxies. In the CNA process, 10-undecenoic acid (UA), a castor oil derived material was used as the first nucleophile, and the newly formed hydroxyl (NFH) group via the attack of UA upon oxonium acts as the second nucleophile. The competition of the nucleophilic attacks between UA and NFH upon oxonium determines the final epoxy structure, and consequently, distinct properties of the epoxies. For UA-ECP system, unique ether-bridged cycloaliphatic rings was created to obtain ECUA epoxy with superior properties comparable to the commercial petroleum based epoxies, such as DGEBA. In addition, the low viscosity and high Tg demonstrates the unique position of ECUA in the bio-based epoxy family. For UA-MECP system, a linear ester-bridged cycloaliphatic structure was created, and the resultant EMCUA demonstrates a semi-ductile behavior. Obviously, the CNA route is attractive for enabling to achieve plant oil-based epoxy comparable to commercial DGEBA; more importantly, with CNA strategy, any renewable materials containing acid-alkene groups has the potential as building blocks to achieve a bio-based epoxy with high thermal resistance and strength while maintaining low viscosity. Supporting Information Nucleophilicity of 10-undecenoic acid anion and hydroxyl group, 2D FTIR correlation spectra of UA-ECP system, Structure characterizations via 13C NMR, FTIR and CG-MS, Nucleophilic attack demonstration via 3D schematic diagram and model chemicals, Chain propagation mechanism in UV-curing process, Tables of viscosity and glass transition of various bio-based epoxies.


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Main Article ACKNOWLEDGMENTS This research is contribution no. 15-312-J from the Kansas Agricultural Experimental Station. Financial support was provided by USDA/NIFA Biomass Research and Development Initiative program (Grant No. 2012-10006-20230).

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Tang Pan, Y.; Wang, D. Ultrastiff Biobased Epoxy Resin with High Tg and Low Permittivity: From Synthesis to Properties. ACS Sustainable Chem. Eng. 2016, 4, 2869−2880. (11) Earls, J. D.; White, J. E.; López, L. C.; Lysenko, Z.; Dettloff, M. L.; Null, M. J. Amine-cured ω-epoxy Fatty Acid Triglycerides: Fundamental Structure–property Relationships. Polymer 2007, 48, 712-719. (12) Montarnal, D.; Tournilhac, F.; Hidalgo, M.; Leibler, L. Epoxy-based Networks Combining Chemical and Supramolecular Hydrogen- bonding Crosslinks. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1133-1141. (13) Maiorana, A.; Spinella, S.; Gross, R. A. Bio-Based Alternative to the Diglycidyl Ether of Bisphenol A with Controlled Materials Properties. Biomacromolecules 2015, 16, 1021-1031. (14) Fache, M.; Auvergne, R; Boutevin, B; Caillol, S. New Vanillin-derived Diepoxy Monomers for the Synthesis of Biobased Thermosets. Eur. Polym. J. 2015, 67, 527-538. (15) Hu, F.; La Scala, J. J.; Sadler, J. M.; Palmese, G. R. Synthesis and Characterization of Thermosetting Furan-Based Epoxy Systems. Macromolecules 2014,


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Main Article

For Table of Contents Use Only

Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid: A new Chemical Route for Plant Oil-based Epoxies

Cong Li, Yonghui Li, Xiaoxia Cai, Hongwang Wang, Stefan H Bossmann, Jonggeun Sung, and Xiuzhi Susan Sun

A competitive nucleophilic attack was found when using 10-undecenoic acid as the building block for the synthesis of bio-based epoxy, which opened a way to overcome the poor performance of plant oil-based epoxy and provided a useful route to design bioresins via competitive nucleophilic attack.



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Figure 1. (a) Time evolution of FTIR spectra for the stoichiometric (1:1) UA-ECP system at 80 °C. (b) Kinetics of variations of the epoxy ring, COOH and OH functions (time dependences of the stretching C-O-C band intensity for epoxy ring, the stretching O-H band for carboxylic acid dimer and the stretching O-H band for hydroxyl group). (c) 1H NMR spectra of CUA (typical chemical structures are shown). 982x770mm (144 x 144 DPI)

ACS Paragon Plus Environment

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Figure2. (a) Time evolution of FTIR spectra for the stoichiometric (1:1) UA-MECP system at 80 °C. (b) Kinetics of variations of the epoxy ring, COOH and OH functions (time dependences of the stretching C-O-C band intensity for epoxy ring, the stretching O-H band for carboxylic acid dimer and the stretching O-H band for hydroxyl group). (c) 1H NMR spectra of MCUA (typical chemical structures are shown). (d) 1H-1H COSY spectra of MCUA 1056x741mm (144 x 144 DPI)



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Figure 3. GC-MS chromatogram of MCUA. Steric molecular structures of three possible isomers in MCUA are shown (case 1: α and γ, substitution occurred on the tertiary carbon. case 2: β, substitution occurred on the secondary carbon). 942x732mm (144 x 144 DPI)


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Figure 4. 1H NMR spectra of ECUA and EMCUA (typical chemical structures are shown). 803x276mm (144 x 144 DPI)



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Figure 5. Complex viscosities (Ƞ†) of epoxies determined at 25 °C as a function of shear rate. 169x115mm (144 x 144 DPI)


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Figure 6. Exothermic behaviors of epoxy monomers containing of 3 wt% photo-initiator (PC-2506) radiated through UV light as a function of time. 178x147mm (144 x 144 DPI)



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Figure 7. (a) Appearances of epoxy resins after UV-curing and (b) transparency comparison. 657x732mm (144 x 144 DPI)


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Figure 8. Temperature dependence of storage moduli (a) and loss factors (b) for uv-ESO, uv-DGEBA, uvECUA and uv-EMCUA. (c) Schematic diagram demonstrating the key features of cured epoxy networks. Typical I structure: ellipse shading; typical II structure: rectangular shading; typical III structure: square shading; typical IV structure: round shading. 992x763mm (144 x 144 DPI)



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Figure 9. (a) Typical strain-stress curves and (c) detailed mechanical results regarding elongation at break (%) and tensile strength (MPa) of epoxy resins. (b) Comprehensive performance comparison between DGEBA and bio-based epoxies synthesized in this work. The inset in (a) is uv-EMCUA film holding a nail in it. (d) Mapping of viscosity (X-axis) and glass transition temperature (Tg) (Y-axis) of bio-based epoxy family derived from plant oils, woody biomass, natural polyphenols, terpene, starch and sugars. Data are collected from the published information detailed in the supporting information of Table S1. 701x769mm (144 x 144 DPI)


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Scheme 1. Competitive nucleophilic attack (CNA) chemical pathways for the synthesis of (a) CUA and (b) MCUA. 460x709mm (144 x 144 DPI)



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Scheme 2. Epoxidation of CUA and MCUA into (a) ECUA and (b) EMCUA. 1028x395mm (144 x 144 DPI)


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Competitive Nucleophilic Attack Chemistry Based on Undecenoic Acid

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