# Better understanding of the maleinization of oils using model fatty compounds: Insights from NMR, SEC, and HRMS analyses

Shamseldin A Mohamed, Laurène Andric, Virginie Silvestre, Dorothée Laurenti, Christophe Geantet, Justine Cantot, Aurélie Lespes, Guillaume Chollet, Jasmine Hertzog, Clémence Queffelec, et al.

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## ► To cite this version:

Shamseldin A Mohamed, Laurène Andric, Virginie Silvestre, Dorothée Laurenti, Christophe Geantet, et al. Better understanding of the maleinization of oils using model fatty compounds: Insights from NMR, SEC, and HRMS analyses. Industrial Crops and Products, 2026, 239, pp.122469. 10.1016/j.indcrop.2025.122469. hal-05412920

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# Better understanding of the maleinization of oils using model fatty compounds: Insights from NMR, SEC, and HRMS analyses

Shamseldin A. Mohamed<sup>a</sup>, Laurène Andric<sup>a</sup>, Virginie Silvestre<sup>a</sup>, Dorothée Laurenti<sup>b</sup>, Christophe Geantet<sup>b</sup>, Justine Cantot<sup>c,d</sup>, Aurélie Lespes<sup>d</sup>, Guillaume Chollet<sup>d</sup>, Jasmine Hertzog<sup>e,f</sup>, Clémence Queffelec<sup>a,f</sup>, Emmanuel Chailleux<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Maleinization reaction
Maleic anhydride (MA)
Lipids
Triglyceride
FT-ICR MS
NMR
Ene reaction
Radical reaction

## ABSTRACT

The chemical modification of vegetable oils through maleinization reaction offers a promising route to sustainable bio-based materials for diverse industrial applications. However, the underlying mechanisms of the maleinization reaction such as the ene, Diels-Alder, and radical pathways, remain insufficiently understood, limiting process optimization and broader adoption. In this study, we employed a model compound approach using methyl oleate (C18:1), methyl linoleate (C18:2), and their corresponding triglycerides to systematically investigate the maleinization reaction with maleic anhydride (MA) under controlled conditions. Advanced analytical techniques, including nuclear magnetic resonance (NMR) spectroscopy, size-exclusion chromatography (SEC), and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were performed to elucidate reaction mechanisms, quantify product distributions (when possible), and assess structural modifications. Our results demonstrated that the ene reaction is the predominant pathway for C18:1 substrates while radical mechanisms contribute to minor products. The Diels-Alder cycloaddition is negligible under the studied conditions. The extent of maleinization increases with MA equivalents, as confirmed by quantitative ¹³C NMR and FT-ICR MS analyses while SEC reveals some polymerization.

## 1. Introduction

The chemical modification of vegetable oils has gained significant attention in recent years as a promising approach to develop sustainable and renewable materials for various industrial applications. Among these modification processes, the maleinization reaction stands out as an effective method for enhancing the properties and expanding the potential uses of vegetable oils (Mohamed et al., 2025). This reaction involves the addition of maleic anhydride to the unsaturated bonds of fatty acid chains, resulting in the formation of maleinized vegetable oils with unique characteristics and improved functionality (Mohamed et al., 2025; Gaglieri et al., 2021; Carbonell-Verdu et al., 2017a; Huang et al., 2013; Biermann et al., 2012; Zhou et al., 2023a). The maleinization of vegetable oils is of particular interest due to its ability to create bio-based materials with enhanced thermal stability, adhesion

properties, and compatibility with other polymers (Carbonell-Verdu et al., 2017b). These modified oils find applications in diverse fields, including lubricants, coatings, adhesives, and polymer composites (Gaglieri et al., 2021; Amos et al., 2021; Alam et al., 2025). Furthermore, the use of renewable vegetable oils as starting materials aligns with the principles of green chemistry and contributes to the development of more environmentally friendly products (Gaglieri et al., 2021). Recent studies have focused on optimizing the maleinization process to improve conversion rates, yields, and selectivity while reducing reaction times and energy consumption. For example, the use of heterogeneous catalysts was explored as well as alternative energy sources such as microwave irradiation to enhance the efficiency of the reaction (Gaglieri et al., 2021; Zhou et al., 2023b). Additionally, the development of water-soluble derivatives from maleinized vegetable oils has opened up new possibilities for applications in aqueous processes (Zhou et al.,

<footnote><sup>a</sup> Nantes Université, CEISAM, UMR 6230, Nantes 44000, France</footnote>

<footnote><sup>b</sup> IRCELYON, UMR 5256, CNRS-Université Claude Bernard Lyon 1, Villeurbanne 69626, France</footnote>

<footnote><sup>c</sup> Université Gustave Eiffel, MAST-MIT, Bouguenais 44344, France</footnote>

<footnote><sup>d</sup> ITERG (Institut des corps Gras & Produits apparentés), Canéjan, 33610, France</footnote>

<footnote><sup>e</sup> Université de Lorraine, LCP-A2MC, Metz 57000, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: jasmine.hertzog@univ-lorraine.fr (J. Hertzog), clemence.queffelec@univ-nantes.fr (C. Queffelec).</footnote>
2023b).

However, some fundamental chemical reaction mechanisms are not fully understood. According to the literature, three primary mechanisms have been proposed for the maleinization reaction: the ene, the Diels-Alder, and radical reactions (Wu and Musa, 2016). Additionally, some polymerization processes may occur during the process. However, there is ongoing debate among researchers regarding the precise nature and relative importance of these mechanisms (Biermann et al., 2007a; Ste-fanoiu et al., 2008a; Holmberg et al., 1982; Bickford et al., 1948a, 1948b, 1942; Nagaruka and Yoshitomi, 1972; Quesada et al., 2003). The specific reaction pathway appears to be heavily influenced by the number and the conjugation of unsaturated bonds present on the substrate, and by the reaction temperature. This dependence on unsaturation and conjugation suggests that the reaction mechanism can vary significantly depending on the type of vegetable oil or fatty acid used as a starting material (Lanero et al., 2022). For instance, oils rich in polyunsaturated fatty acids may favor different reaction pathways compared to those containing primarily monounsaturated fatty acids. This complexity in reaction mechanisms presents both challenges and opportunities for researchers seeking to optimize the maleinization process. Understanding the interplay between these different reaction pathways is crucial for controlling the properties of the resulting materials and for developing more efficient and sustainable production methods. Previous attempts to characterize this reaction using waste cooking oil and conventional analytical tools such as NMR and infrared spectroscopy have proven to be challenging (Debello et al., 2024).

The present study focuses on gaining a deeper understanding of the maleinization reaction with model unsaturated fatty compounds that can be found in vegetable oils. The complex nature of vegetable oils, which are blends of various fatty acids, makes the interpretation of characterization results particularly difficult. In light of these challenges, we propose a novel strategy to understand the maleinization reaction in vegetable oils. Our approach consists of two key elements:

- Utilizing model compounds with known compositions and known double bond positions.

- Employing complementary advanced molecular identification technique: Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), size-exclusion chromatography (SEC), and NMR spectroscopy.

- Focusing on only one reaction temperature

Regarding the model oils, we have focused on methyl oleate (C18:1) and linoleate (C18:2) fatty acid methyl esters, and on the corresponding triglyceride forms. The chosen methyl ester fatty acid models represent the dominant unsaturation types found in widely used vegetable oils. This choice therefore reproduces the key reactivity classes present in real feedstocks. Carbon-chain length was not systematically varied because it has only a minor effect on the intrinsic reaction pathway. Free fatty acids were not included as separate model compounds, since the maleinization mechanism primarily involves the unsaturated chain and is not significantly influenced by the carboxylic functionality. In addition, free acids may undergo side reactions under the reaction conditions complicating mechanistic interpretation.

This innovative approach aims to provide more precise insights into the maleinization process of vegetable oils.

## 2. Materials and methods

### 2.1. Chemicals

Methyl oleate (C18:1) and linoleate (C18:2), triglycerides of oleic acid and linoleic acid were purchased from Sigma Aldrich (≥99% purity) and used without further purification. Maleic anhydride (MA) (≥99% purity) was also obtained from Sigma Aldrich and used as received. EM HOSO refers to the sunflower oil that has been

transesterified into methyl ester was provided by ITERG (see protocol below). The oil is composed of 99% methyl esters, and its fatty acid composition is the same as before transesterification (89% of oleic acid and 11% other fatty acids), all in the form of methyl esters.

### 2.1.1. Transesterification of HOSO

HOSO (50 g) and 20 mL of MeOH were mixed in a glass reactor. Then, 250 mg of MeONa (0.5 wt%/oil) were added and the mixture was heated at reflux for 2 h. Then, the mixture was poured in a separatory funnel and the lower layer corresponding to MeOH and glycerol was removed. The upper layer was then washed twice with water then recovered, and evaporated under vacuum. The EM HOSO was then characterized by gas chromatography (GC) and iodine value. Iodine value of EM HOSO is ca. 83.7 gI₂/100 g.

### 2.2. Methods

#### 2.2.1. Maleinization reaction in a sealed tube

The oil bath was preheated to 220 °C and insulated with cotton and aluminum foil to minimize heat loss. The fatty compound was first added in the sealed tube (~15 mL, L × O.D. 10.2 cm × 25.4 mm, Ace pressure tube from Sigma Aldrich) using a plastic pipette, taking care to avoid the inner walls, followed by the addition of maleic anhydride (see Table 1). The tube was then purged with argon for 5 min to remove atmospheric oxygen. After purging, the sealed tube was placed in the preheated oil bath and stirred continuously for 4 h allowing the reaction to proceed under an inert atmosphere. At the end of the reaction, the mixture was poured into a pre-weighed vial that was put under vacuum (P < 1 mbar) for 30 min to remove unreacted maleic anhydride. Maleic anhydride evaporates at about 50–70 °C, under a vacuum of < 1 mbar, whereas its boiling point, at atmospheric pressure, is roughly 200 °C (Mohamed et al., 2025).

In the following, MO, ML, MOtag, and MLtag correspond respectively to methyl oleate (C18:1), methyl linoleate (C18:2), triolein (C18:1 triglyceride), and trilinolein (C18:2 triglyceride).

The mass of MA was calculated according as follow: m MA (g) = n<sub>fatty acid</sub> * neq × M<sub>MA</sub>

#### 2.2.2. Maleinization reaction in a glass reactor

The glass reactor consists of a 500 mL heated reactor that opens into two parts with five necks, a thermocouple measuring the temperature inside the mixture, and a shaft with a stirring blade driven by a motor for agitation. EM HOSO was charged into the reactor (20 g) fitted with an argon balloon, the thermocouple, the condenser, and a bubbler. MA

<caption>**Table 1**</caption>

Characteristics of the studied samples with the masses of fatty compound and corresponding masses of MA used in the sealed tube, also given in equivalent. Samples were heated at 220 °C, for different reaction times.

<table><thead><tr><th>Name</th><th>Starting material mass (g)</th><th>Maleic anhydride mass (g)</th><th>Equivalent</th></tr></thead><tbody><tr><td>MO-220-1eq-4h</td><td>MO = 1.00</td><td>0.33</td><td>1</td></tr><tr><td>MOtag-220-1eq-4h</td><td>MOtag = 1.00</td><td>0.33</td><td>1</td></tr><tr><td>ML-220-1eq-4h</td><td>ML = 0.80</td><td>0.53</td><td>1</td></tr><tr><td>MLtag-220-1eq</td><td>MLtag = 0.80</td><td>0.53</td><td>1</td></tr><tr><td>MO-220-0.5eq-4h</td><td>MO = 1.00</td><td>0.16</td><td>0.5</td></tr><tr><td>MO-220-2eq-4h</td><td>MO = 1.00</td><td>0.66</td><td>2</td></tr><tr><td>ML-220-0.5eq-4h</td><td>ML = 0.80</td><td>0.26</td><td>0.5</td></tr><tr><td>MOtag-220-0.5eq</td><td>MOtag = 1.00</td><td>0.16</td><td>0.5</td></tr><tr><td>MLtag-220-0.5eq</td><td>MLtag = 0.80</td><td>0.26</td><td>0.5</td></tr></tbody></table>
(6.89 g, 1 eq according to iodine value) was added when the temperature reached 220 °C to avoid MA sublimation. After 4 h of stirring, the heating is stopped, the mixture is then transferred into a flask and is distilled under vacuum (P < 1 mbar) at mild temperatures 50–70 °C range for 30 min.

## 2.3. NMR

Different spectrometers were used (the 300 MHz as a routine instrument, the 400 MHz for shorter acquisition of ¹³C, and the 700 MHz for quantification).

### 2.3.1. 300 MHz NMR

¹H and ¹³C NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer fitted with a 5 mm i.d. BBO probe carefully tuned to the recording frequency of 300.13 MHz (for ¹H) and 75.468 MHz (for ¹³C). The temperature of the probe was set at room temperature (around 293–294 K). The spectra are referenced to the solvent in which they were run (7.26 ppm for ¹H CDCl₃ and 77.16 ppm for ¹³C CDCl₃).

### 2.3.2. 400 MHz NMR

¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer fitted with a 5 mm i.d. BBFO+ probe carefully tuned to the recording frequency of 400.16 MHz (for ¹H) and 100.62 MHz (for ¹³C).

### 2.3.3. 700 MHz NMR

¹H and ¹³C NMR spectra were recorded on a Bruker Avance III HD 700 MHz spectrometer fitted with a 5-mm QCI (H,F,C,N) cryogenic probe carefully tuned to the recording frequency of 700.28 MHz (for ¹H) and 176.086 MHz (for ¹³C). The temperature of the probe was set at 303 K.

The 700 MHz was used for the quantification. The peak areas were recorded in quantitative ¹³C-{¹H} ¹D NMR spectra. For all quantification evaluations by ¹³C-{¹H} NMR, proton signals are only decoupled during the signal acquisition (inverse-gate pulse scheme) to minimize the nuclear Overhauser effect (NOE) induced by the ¹H composite-pulse decoupling sequence (¹H-CPD).

Areas of ¹³C peaks corresponding to different carbon sites were determined by the curve-fitting process implemented within Perch (Perch NMR Software, University of Kuopio, Finland, p.AC030385E). Each resonance was smoothed by adjusting its chemical shifts, its amplitude, its width at half maximum (FWHM), and using Lorentzian-shaped lines.

Longitudinal relaxation times ($T_1$) were determined for each sample. The longest $T_1$ value measured was 12 s. For quantitative NMR acquisition, a relaxation delay ($d_1$) of 60 s and an acquisition time (aq) of 1 s were applied corresponding to at least $5 \times T_1(\max)$. With this delay, quantitative signal integration are ensured. Proton decoupling was applied without the introduction of NOE effects.

## 2.4. SEC

Size exclusion chromatograms (SEC) were obtained on a Thermo-Fisher Scientific system equipped with a quaternary pump CN (VC-P21-A), a RefractoMax 521 refractive index detector, a split autosampler CT (VC-A12-A) set to 25°C, and an Ultimate 3000 column oven set to 40°C. A PLgel 5 µm guard and four PLgel 5 µm 500 Å columns from Agilent were used in THF at a flow rate of 1 mL/min. For all figures below, the plots have been normalized based on the starting material (SM) peak, except for Fig. 7, where the SM has been completely consumed. In this case, normalization was performed using the product peaks instead.

## 2.5. FT-ICR MS

Samples were first dissolved to a concentration of 10 mg/mL in

acetone to be then diluted in methanol for a final concentration of 0.01 mg/mL (methanol and acetone, HPLC grade, Carlo Erba Reagents).

Achieved solutions were characterized by ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS 7 T, SolariX 2xR, Bruker Daltonics, Bremen, Germany) in positive-detection electrospray ionization (ESI - Bruker Daltonics). This detection mode was chosen as it is more adapted to ester detection, here in the form of sodium adducts [M+Na]⁺ (Zhang et al., 2011; Ashraf-Khorassani et al., 2015). FTMS-Control V2.3.0 (Bruker Daltonics) was used to control both ion source and instrument parameters. Prior acquisition, mass spectrometer was externally calibrated and ICR detection cell was shimmied and gated with a 0.1 mg/mL sodium trifluoroacetate solution. Two acquisition methods were used, the first one (a) being adapted to low-mass compounds such as methyl esters and the second one (b) being more adapted to triglyceride compounds. Twenty scans were accumulated on a *m/z* 107.5 – 1500 (a) or 107.5 – 3500 (b) range, with a 4 megaword time-domain. Mass resolutions of 430 000 and 150,000 were achieved at *m/z* 300 and 900, respectively. Samples were infused into the ESI source with a 3 µL/min flow rate and the capillary voltage was set at 3.9 kV. ESI source was tuned with drying gas temperature and flow rate at 200 °C and 4 L/min, respectively, and the nebulizer pressure at 0.5 bar.

FT-ICR MS/MS experiments were performed on some ions of interest in order to have structural information. The ion was isolated in the quadrupole to be fragmented by collision-induced dissociation (CID), which consists in accelerating the ion in the collision cell filled with argon. Collision energy used for fragmentation was adapted to each ion to obtain *MS*² spectrum comprising both the products and precursor ions, with the intensity of the last one being below those of the product ions.

DataAnalysis 5.0 software (Bruker, Daltonics) was used to internally calibrate the FT-ICR mass spectra with a list of known compounds – fatty acids, ester fatty acids, and triglycerides - with a standard deviation of mass error ≤ 100 ppb. Molecular formulae were assigned by Smart-Formula Editor (Bruker Daltonics) with a ± 1 ppm mass accuracy window.

# 3. Results and discussion

## 3.1. Maleinization reaction

As previously mentioned, the maleinization reaction can proceed via several mechanistic pathways depending on the substrate and reaction conditions. One prominent pathway is the ene reaction, where MA reacts with the allylic hydrogen of unsaturated fatty acid chains, resulting in the formation of a new C-C bond and migration of the double bond; this is particularly relevant for lipids like methyl oleate and occurs predominantly at elevated temperatures, usually at 220 °C (Farmer, 1947; Teeter et al., 1948). Another mechanism is the [4 + 2] Diels-Alder cycloaddition, which takes place with conjugated dienes present in some unsaturated fatty acids or their derivatives (e.g., oleostearic acid), leading to cyclic adducts of succinic anhydride (Biermann et al., 2007b). In addition, a free radical mechanism has been proposed, particularly in the presence of radical initiators such as organic peroxides, where MA reacts with lipid radicals formed via hydrogen abstraction, although this pathway is less selective and may lead to polymerization or side reactions (Amos et al., 2021; Eren et al., 2003; Tran et al., 2005). These pathways are not mutually exclusive and may coexist under certain conditions, contributing to the structural diversity of maleinized products.

Thus, the maleinization of model unsaturated fatty acids from oleic acid (C18:1) and linoleic acid (C18:2) and their corresponding triglycerides, was investigated varying maleic anhydride equivalents. Reactions were performed at 220 °C, as classically reported in the literature, (Farmer, 1947; Teeter et al., 1948) for 4 h with 0.1–2 equivalents of maleic anhydride per mol of fatty compound. With C18:1
fatty acid, the Diels-Alder mechanisms cannot occur since there is only one double bond. For the C18:2 fatty acid, as the double bond are not conjugated, Diels-Alder mechanisms is not expected to be the primary mechanism.

## 3.2. C18:1

Methyl oleate was initially chosen as a model compound to elucidate the reaction mechanism and to quantify the extent of maleinization using NMR spectroscopy. To investigate the influence of maleic anhydride (MA) amount, the reaction was performed in a sealed tube with varying MA equivalents, ranging from 0.1 to 2. As the amount of MA increased, the reaction product gradually changed in color, from pale yellow (0.1 eq.) to dark brown (2 eq.), indicating increased modification. A control experiment (blank reaction) was also conducted, in which methyl oleate was heated alone at 220 °C without the addition of MA, in order to assess the thermal stability of the compound. ¹H NMR analysis of the thermally treated sample showed no significant changes in the chemical structure. However, FT-ICR MS analysis revealed the presence of minor autoxidation products, which were not detectable by NMR.

The ¹H NMR spectrum (Figure S1) with 1 equivalent of MA is similar to that reported by Stefanoiu et al. (2008b). To determine the reaction mechanism, the ¹³C NMR spectrum provides more informative and readable data. In our case, four new peaks were observed at 126.1, 126.3, 136.6, and 136.9 ppm (Fig. 1). According to Stefanoiu et al. (2008b). and Lackinger et al. (2011a) those new signals correspond to the four sp² carbons resulting from the “ene” reaction (carbons 8 and 9 or carbons 10 and 11 in the structures III and IV in Figure S2) and the ones at 127.3, 127.5, 136.2 and 136.5 ppm to the radical reaction (carbons 9 and 10 in the structures I and II in Figure S2). Compared to “ene” products, radical products are formed in significantly lower quantities (Figure S2).

The influence of the amount of MA has been studied and different

equivalents were used to conduct the reaction, and the % of maleinized compound obtained was calculated. The quantification was done on ¹³C spectra, and the peak areas were measured in quantitative ¹³C-{¹H} ¹D NMR spectra. Then, areas of ¹³C peaks corresponding to different carbon sites were determined by the curve-fitting process implemented within Perch (see material and methods section and Figures S3 and S4). For 0.1 equivalent MA (entry 1) there was no noticeable grafting of maleic anhydride on the fatty acid, and only after increasing the equivalent of MA to 0.5 (entry 2) maleinization products are formed at about 11 %. The trend continued, with entry 3 and 4, where increasing the maleic anhydride equivalent to 1 and 2, respectively, led to higher incorporation of MA at around 40 % and 69 % of maleinized products.

Finally, we conducted the reaction with 1 equivalent of MA at 200 °C (entry 5) as a measure to avoid MA deposition from gaseous phase directly to solid on the reactor walls. Unsurprisingly, the kinetic of the reaction was slower than at 220 °C and the incorporation of MA was similar to the reaction operated with 0.5 eq at 220 °C. This is not what was observed by Amos et al. (2021). where the incorporation of MA was higher at 200 °C than at 220 °C.

For each run, some unreacted maleic anhydride was evidenced by the peak at 7.04 ppm. In this regard, we had to distill off the excess of MA under reduced pressure (Lackinger et al., 2011a, 2011b, 2012). In our experiments, the peak at 6.5 ppm corresponding to maleic acid was not detected, while MA in presence of water - originating from atmosphere - can easily form maleic acid (C₄H₄O₄) (Fig. 2) (Rosenfeld and Murphy, 1967; Wojcieszak et al., 2015).

Finally, the maleinization reaction was as well performed in an “open” glass reactor with 1 eq. of MA at 220 °C for 4 h on the EM HOSO, as described in the material and method section. Even though MA condensed and deposited on the cooling system and on the head space of the reactor, the NMR quantification of the maleinized compound was around 33 %, close to the conversion calculated in the sealed reactor with the same ¹H NMR profile (Figure S5). These results indicate that the choice between a sealed tube and an open glass reactor does not affect

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Zoom on the 120–140 ppm zone of the ¹³C spectrum obtained after the maleinization of MO with 1 eq. of MA in a sealed tube at 220 °C for 4 h (measured at 16.45 Tesla).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Hydration reaction of maleic anhydride into maleic acid (a) and succinic anhydride moiety into succinic acid moiety (b).</figcaption></figure>

the maleinization reaction of model fatty compounds.

In addition to the signals from the maleinized and starting com-
pounds, the ¹H NMR spectrum showed the presence of broad signals
suggesting the formation of some oligomeric species as it has already
been observed in the literature (Amos et al., 2021). In the glass reactor,
this phenomenon is less pronounced suggesting much fewer oligomeric
species formed during the maleinization reaction (Figure S6).

SEC and FT-ICR MS were as well performed on the compound
focusing this time only on 0.5 and 1 equivalent of MA. The maleinized
products were observed in all cases.

In SEC, the profile obtained for maleinized methyl oleate revealed
that (i) the reaction was not complete (peak at 33.17 min) as observed
by NMR, (ii) oligomerization occurs (29 min < peak < 31 min), and (iii)
the maleinized product was formed (peak at 31.69 min) and was more
intense for 1 eq. of MA (Fig. 3).

In FT-ICR MS, methyl oleate (C₁₉H₃₆O₂) is observed at *m/z* 319.260764 in the form of [M+Na]⁺ (Figure S7), as well as its corresponding dimer C₃₈H₇₂O₄. After heating this compound at 220 °C, the signal at *m/z* 319.260764 is still observed but a lower intensity, while signals at *m/z* 335.25573 and 351.250675 were more intensely observed and were respectively assigned as C₁₉H₃₆O₃Na⁺ and C₁₉H₃₆O₄Na⁺, which seem to be oxidation products of methyl oleate promoted by the heating (Figure S7b). After addition of 0.5 and 1 eq. of MA, new signals were observed notably at *m/z* 435.271702, which was assigned as C₂₃H₄₀O₆Na⁺ (Fig. 4). This molecular formula corresponds to the reaction product between methyl oleate and MA that underwent hydration forming the acidic form (Fig. 2). Regarding dimer formation, as the achieved molecular formula corresponds to twice the monomer element, it indicates that one double bound might be involved in the dimerization between the two monomers.

Signal corresponding to the anhydride form of the reaction product
was also observed, but with lower intensity, as well as the esterification
product between the acidic form and the methanol solvent. It was
noticed that with a higher equivalent of MA, signals relative to malei-
nized methyl oleate products were slightly more intense than what
observed with 0.5 eq. (Table S1), suggesting that 0.5 eq. are enough to

graft maleic anhydride on the unsaturated bond of the fatty acid (11.3
yield Table 2).

3.3. C18:2

Additionally, methyl linoleate (ML) was maleinized in the same
conditions, i.e. in a sealed tube at 220 °C for 4 h. The reaction was
conducted with 0.5 and 1 eq. of MA. The ¹H NMR was more compli-
cated, making quantification impossible in this case (Figure S8). More-
over, the product evolves over time, as evidenced by changes in the ¹H
NMR spectrum (Figure S8), thus no ¹³C were recorded for this product.
However, new peaks were observed corresponding to the maleinized
compound. The products obtained were more viscous and more colored
(dark brown) than the products obtained from MO maleinization.

In SEC, it was observed that the ML was not pure and two peaks at
32.33 and 33.18 min were observed. It suggests that probably the ML
(peak at 33.18 min) is very reactive and as soon as the bottle is opened,
oxidation seems to occur (peak et 32.33 min). It is known that products
with higher iodine value are more reactive (Bickford et al., 1942). It is
established since the '90 s that linoleic acids can go through oxidation
process at room temperature due to its active methylene that can lose H
radicals (Kumarathasan et al., 1992). Otherwise, the same trend as for
MO was observed: the reaction was not complete but more advanced
than for MO and more maleinized compound was formed with 1 eq. of
MA (peak at ca. 31.6 min) compared to 0.5 eq. Finally, oligomeric
species are present (26 min < peak < 31 min) and more pronounced
than for MO (Fig. 5).

In FT-ICR MS, similar trends as with methyl oleate were observed. Oxidation products were observed after simply heating ML at 220 °C for 4 h in the sealed tube, with C₁₉H₃₆O₄, showing the influence of the heat on the oxidation process (Figure S9). Dimer of oxidized ML (C₁₉H₃₄O₄) was observed at *m/z* 675.479365 (C₃₈H₆₈O₈Na⁺) and isolated in the quadrupole of the FT-ICR MS instrument. This sole step was enough to observe the corresponding monomer at *m/z* 349.234780 (C₁₉H₃₄O₄Na⁺) indicating that the bound between the two monomers is easy to break and can be formed in the ESI source. Dimer formation was also observed, by James et al., on similar lipid compounds, in ESI (+), even below the critical micellar concentration (James et al., 2006). This was notably explained by lipid concentration upon ESI desolvation process and gas-phase intermolecular cluster reaction. Addition of MA with methyl ester yielded twice maleinized fatty ester and interestingly, both forms - anhydride and acidic - were observed (Fig. 6). With 1 eq of MA, the intensities of the maleinized compounds are greater than the intensity of methyl linoleate, while with 0.5 eq MA, it is the opposite (Fig. 6 and Table S1). This suggests that a higher MA equivalent leads to improved grafting yield, which is consistent with the SEC results.

3.4. TAG C18:1 and TAG C18:2

Finally, the maleinization reaction was conducted on the triglyceride
forms of C18:1 and C18:2. Regarding the maleinization of MOtag (cor-
responding to the C18:1 triglyceride), NMR was performed and ¹³C
spectrum is given in Figure S10. The NMR spectrum was too complicated
for quantification. Regarding the maleinization of MLtag (corresponding
to the C18:2 triglyceride), the ¹³C spectrum was not recorded because
this product evolved over time. Thus, the reaction products were mainly
characterized by SEC and FT-ICR MS.

<figure><img src="image_6.png" /><figcaption>Fig. 3. SEC profile of MO, maleinized MO with 0.5 and 1 eq. of MA.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. ESI (+) FT-ICR mass spectra of methyl oleate heated at 220 °C for 4 h with 0.5 (a) and 1 (b) MA equivalent.</figcaption></figure>

<table><caption>**Table 2**
Maleinization experiments conditions and yields.</caption><thead><tr><th>Entry</th><th>Starting material</th><th>T (°C)</th><th>Eq. of MA vs C18:1</th><th>Experimental set-up</th><th>% of maleinized product</th><th>% of "ene" products</th><th>% of radical products</th></tr></thead><tbody><tr><td>1</td><td>C18:1</td><td>220 °C</td><td>0.1</td><td>Sealed tube</td><td>&lt; 1 %</td><td>n.d.</td><td>n.d.</td></tr><tr><td>2</td><td>C18:1</td><td>220 °C</td><td>0.5</td><td>Sealed tube</td><td>11.3 (± 2)</td><td>9.4 (± 1.4)</td><td>1.4 (± 0.2)</td></tr><tr><td>3</td><td>C18:1</td><td>220 °C</td><td>1</td><td>Sealed tube</td><td>40.1 (± 1)</td><td>34.8 (± 0.8)</td><td>5.1 (± 1.5)</td></tr><tr><td>4</td><td>C18:1</td><td>220 °C</td><td>2</td><td>Sealed tube</td><td>68.6 (± 1.1)</td><td>61.5 (± 0.3)</td><td>7.1 (± 0.3)</td></tr><tr><td>5</td><td>C18:1</td><td>200 °C</td><td>1</td><td>Sealed tube</td><td>12.6 (± 1.9)</td><td>11.8 (± 1.4)</td><td>1.3 (± 0.2)</td></tr><tr><td>6</td><td>EM HOSO</td><td>220 °C</td><td>1</td><td>Glass reactor</td><td>33.5 (± 2.9)</td><td>30.9 (± 0.8)</td><td>3.3 (± 0.4)</td></tr></tbody></table>

For MOtag, there is still some starting material remaining but 2 families can be observed at ca. Tr = 28 and 29 min, probably maleinized compounds with different degrees of succinic anhydride grafted (Fig. 7).

In comparison, SEC profiles reveal that significant oligomerization occurs for MLtag, and that MLtag was much more reactive with no starting material present at the end of the reaction compared to MOtag (Fig. 8).

In FT-ICR MS, triolein compound was observed at *m/z* 907.772473 in the form of C₅₇H₁₀₄O₆Na⁺, no dimeric species was observed even after heating at 220 °C for 4 h the starting material. With 0.5 eq. of MA, one and, to a lesser extent, two succinic moieties grafted on MOtag were observed, mainly in the open (acidic) form (+ C₄H₄O₄) at *m/z*

1023.7833343 and 1139.794187, corresponding to C₆₁H₁₀₈O₁₀Na⁺ and C₆₅H₁₁₂O₁₄Na⁺ formula, respectively (Figure S11). The signal at *m/z* 1023.7833343 was isolated to be fragmented. MS² spectrum (Figure S12) shows the loss of 2 oleic acids (C₁₈H₃₄O₂) and a signal at *m/z* 421.256099 assigned as C₂₂H₃₈O₆Na⁺ that can be regarded as an oleic acid that reacted with one MA, in the acidic form. This open form is also confirmed by the 2 neutral losses of 18 Da, that are water molecules obtained from the recondensation of the succinic acid into succinic anhydride. This fragmentation pattern also indicated that succinic acid does not form a bridge between two fatty acid chains but maleinization reaction occurs only on one unsaturated bond of one chain. Nevertheless, these measurements cannot inform on which chain of the
<figure><img src="image_8.png" /><figcaption>Fig. 5. SEC profile of ML, maleinized ML with 0.5 and 1 eq. of MA.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. SEC profile of MOtag, maleinized MOtag with 0.5, and 1 eq. of MA.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 6. ESI (+) FT-ICR mass spectra of methyl linoleate heated at 220 °C for 4 h with 0.5 (a) and 1 (b) MA equivalents.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. SEC profile of MLtag, maleinized MLtag with 0.5, and 1 eq. of MA.</figcaption></figure>

triglyceride specifically.

Mass spectrum of the triolein with 1 eq. of MA indicates that two succinic moieties grafted are present, mainly observed in the acidic form as C<sub>65</sub>H<sub>112</sub>O<sub>14</sub>Na<sup>+</sup>, but no signals relative to 3 succinic moieties grafted were observed (Figure S11). MS<sup>2</sup> analyses performed on the “twice-maleinized” show the losses of two oleic acids and one C<sub>4</sub>H<sub>4</sub>O<sub>4</sub>, and formation of C<sub>22</sub>H<sub>38</sub>O<sub>6</sub>Na<sup>+</sup> (Figs. 9 and 10). These results go in line with what previously observed, i.e., there is no cross-maleinization between two fatty acid chains. Also, the fragmentation mass spectrum demonstrates that there is one succinic acid moiety on two of the three fatty chains. In addition, the successive losses of two water molecules (18.010565 ± 0.5 mDa), from the “twice maleinized” specie at *m/z* 1139.794352 are due to the two succinic acid moieties that return to anhydride form, confirming the presence of two acidic succinic moieties. Two water molecules are also lost after the first oleic acid departure confirming the two MA moieties on the remaining diglyceride before the loss of the C<sub>4</sub>H<sub>4</sub>O<sub>4</sub>.

Trilinolein (C<sub>57</sub>H<sub>98</sub>O<sub>6</sub>), observed here at *m/z* 901.725562 as [M+Na]<sup>+</sup>, yields oxidized products after being heated at 220 °C for 4 h, with compounds having 2–6 additional oxygen atoms, and no dimer was observed (Figure S13). This confirms that compounds with more alterations are more reactive and more prone to oxidation (Bickford et al., 1942; Roman et al., 2013). When 0.5 eq. of MA was added, 3 succinic moieties grafted on the triglyceride were observed in acidic forms

(Fig. 11a). Two succinic moieties grafted were observed in the anhydride configuration, but with a very low intensity at *m/z* 1097.726858 as C<sub>65</sub>H<sub>102</sub>O<sub>12</sub>Na<sup>+</sup>. Addition of 0.75 eq. MA also enabled to observe 3-succinic moieties grafted with succinic acid moieties at *m/z* 1249.760068 as C<sub>69</sub>H<sub>110</sub>O<sub>18</sub>Na<sup>+</sup> (Fig. 11b). Trilinolein with 4 succinic acid moieties was observed at *m/z* 1365.769905 as C<sub>73</sub>H<sub>114</sub>O<sub>22</sub>Na<sup>+</sup>, with 1 eq. of MA (Fig. 11c). Only a two succinic moiety grafted specie was observed in the anhydride form. Products with more than 4 grafted succinic moieties were not observed. This could be explained by steric hindrance reached after a certain given grafting number.

These observations agree with what was obtained with triolein. In fact, maleinization does not occur on all unsaturated bonds, for triolein, 2 bonds out of 3 were shown to react while for trilinolein, 4 bonds out of 6 underwent maleinization. No additional grafting was observed, which can be explained by steric hindrance or need of higher reaction energy. Maleinization yield was shown to increase with increasing MA equivalents (Table S1).

## 4. Conclusion

Four model compounds - methyl oleate and methyl linoleate and their corresponding triglycerides forms - were studied for the maleinization reaction. The completion of the reaction was confirmed by NMR, SEC, and FT-ICR MS. The extent of functionalization was found to increase with the amount of maleic anhydride, which showed the progressive appearance of characteristic succinic anhydride moieties. C18:2-based substrates showed a higher degree of conversion compared to C18:1 analogues under identical conditions, consistent with the higher reactivity of bis-allylic systems. For the C18:1 analogues, it seems that the grafting reached a plateau, suggesting a saturation point where either equilibrium limitations, side reactions, or degradation began to compete. Control (blank) reactions carried out under identical thermal conditions but in the absence of maleic anhydride, showed negligible structural modification, confirming that thermal degradation or autoxidation was minimal under the chosen conditions and that observed changes were due to genuine maleinization.

Concerning C18:2 analogues, as already observed, those compounds are more reactive because of their higher unsaturation degree. However, a plateau is reached and no more than 4 succinic moieties could be grafted and more oligomeric species were formed.

In conclusion, although the controlled formation of ene products is generally preferred for obtaining well-defined and easily functionalizable intermediates, the reaction mechanism is often difficult to control, and both ene and radical pathways may occur simultaneously, opening

<figure><img src="image_12.png" /><figcaption>Fig. 9. Fragmentation FT-ICR mass spectrum obtained by collision induced dissociation at 45 V energy of C<sub>65</sub>H<sub>112</sub>O<sub>14</sub>Na<sup>+</sup> ion at *m/z* 1139.794352, regarded as twice-maleinized compound.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 10. Fragmentation pathways proposed for the twice-maleinized triolein based on the CID-MS² spectrum achieved in ESI (+) FT-ICR MS.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. ESI (+) FT-ICR mass spectra of TAG of linoleic acid heated at 220 °C for 4 h with 0.5 (a), 0.75 (b), and 1 (c) MA equivalents, with the addition succinic anhydride (blue), succinic acid (red), and methyl ester succinic acid (green) moieties.</figcaption></figure>

the path for future research to explore the incorporation of catalysts, initiators or inhibitors and other additives to favor certain mechanism.

Our current efforts are focused on compounds with more unsaturations to evidence the formation of Diels-Alder adducts and to determine the optimal conditions that promote this reaction.

CRediT authorship contribution statement

**Aurélie Lespes:** Formal analysis, Data curation. **Justine Cantot:** Writing – review & editing, Data curation. **Christophe Geantet:** Writing – review & editing. **Dorothée Laurenti:** Writing – review & editing.

**Guillaume Chollet:** Funding acquisition, Formal analysis. **Emmanuel Chailleux:** Writing – review & editing, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Jasmine Hertzog**
(corresponding author): Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Vir- ginie Silvestre: Software, Methodology, Formal analysis, Data curation. Laurène Andric: Methodology, Investigation. Shamseldin A. Mohamed: Methodology, Investigation. Queffelec Clémence (corre- sponding author): Writing – review & editing, Writing – original draft, Validation, Supervision, Funding acquisition, Conceptualization.

Declaration of Competing Interest

The authors declare the following financial interests/personal re-
lationships, which may be considered as potential competing interests:
Clemence Queffelec reports financial support was provided by ANR
(POP project, grant ANR-22-CE43-0010-03). If there are other authors,
they declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work
reported in this paper.

Acknowledgments

This work was funded by ANR (POP project, grant ANR-22-CE43-
0010-03). Financial support from the IR INFRANALYTICS FR2054 for
conducting the research is gratefully acknowledged. This work was
partially supported by the MassLor research infrastructure at the Uni-
versity of Lorraine.

**Appendix A. Supporting information**

Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.indcrop.2025.122469.

Data availability

Data will be made available on request.

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