Investigation of Methyl Methacrylate Grafting on Model Single Fatty

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Investigation of Methyl Methacrylate Grafting on Model Single Fatty Acid Alkyds Qianhe Wang, Brittany Pellegrene, and Mark D. Soucek* Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States

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S Supporting Information *

ABSTRACT: Four model alkyds were prepared by the fatty acid process using a single fatty acid (stearic, oleic, linoleic, or linolenic acid) with phthalic anhydride and glycerol. These model alkyds were reacted with methyl methacrylate (MMA) in the presence of benzoyl peroxide or azobis(isobutyronitrile). 1H NMR, 2D gradient heteronuclear multiple quantum coherence NMR, matrix assisted laser desorption ionization mass spectrometry, solvent extraction, and gas chromatography were used to evaluate each model alkyd system. The conversion of MMA was quantified, and the grafting mechanism of MMA onto each model alkyd was elucidated. In general, both polymerization rates and MMA conversions were inhibited in the presence of alkyd resins. For oleic alkyd model systems, the grafting site was primarily located at the double bond on the fatty acid chain. Both the homopolymerization and copolymerization of the MMA are significantly retarded by chain transfer of the hydrogen from the double allylic site in the linoleic and linolenic model systems. For the linoleic alkyd and linolenic alkyd model systems, the grafting reaction predominately occurred at the activated double allylic methylene group on the fatty acid chain via hydrogen abstraction by the primary radical and MMA radical, followed by grafting in a termination reaction with a propagating MMA radical.

1. INTRODUCTION Alkyds are essentially fatty acid modified polyesters. They were first synthesized by General Electric in 1914 for electrical fluids. In 1926, alkyds were first used as protective films.1 Alkyds are one of the most versatile and low-cost coating materials and are used extensively in architectural and industrial coatings.2,3 Alkyds are commonly derived from seed oils such as soybean, linseed, and sunflower. These seed oils contain a mixture of fatty acids, usually including stearic, oleic, linoleic, and linolenic acids. It should be noted that these acids all have 18 carbons but with different degrees of unsaturation, which will contribute to different drying characteristics and grafting mechanisms.1 As more modern polymers were developed, there was a push to formulate these new polymeric materials with alkyds. Poly(methyl methacrylate)s (pMMA) were developed in 1928 and have suitable mechanical properties for blending with alkyds. However, pMMA is not miscible with alkyds. The resultant phase separation leads to haziness and lower gloss.4,5 Compatibilization can be achieved by having a concentration of hybrid polymers that have both alkyd and acrylic components, as shown in Figure 1. Over the past decade, several studies have been focused on the interpretation of the grafting mechanism of acrylics onto alkyd resins under a variety of conditions. For a typical mixture of fatty acid in an alkyd, there are four potential graft sites for MMA, as shown in Figure 2. First, the activated methylene unit © XXXX American Chemical Society

Figure 1. Alkyd resin−MMA hybrid.

allylic to one double bond can be hydrogen abstracted and form graft copolymer. Second, the activated methylene unit allylic to two double bonds can be more easily abstracted by free radicals. Another option is direct addition to the double bond on the fatty acid. The fourth site is the glycerol on the polyester backbone which can graft via abstracting the hydrogen connected to the secondary carbon.6 Dziczkowski et al.6,7 reported the synthesis of acrylated alkyd via free radical polymerization of soybean oil alkyd with MMA Received: May 15, 2018 Revised: July 30, 2018 Accepted: August 1, 2018

A

DOI: 10.1021/acs.iecr.8b01941 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

study. It was found that no grafting across the alkyd double bonds occurred, regardless of methacrylic or acrylic monomer usage. There was evidence only of grafting via hydrogen abstraction at the doubly allylic site. The difference in grafting mechanisms was attributed to the change in reaction medium and the high monomer surface area in the miniemulsion system.6,7,21 In this study, four different model alkyds were synthesized via the fatty acid process with stearic, oleic, linoleic, or linolenic acid with phthalic anhydride and glycerol. Each model alkyd consisted of only one fatty acid, and therefore, they were useful in elucidating the contribution of each type of fatty acid: saturated, unsaturated with one double bond, unsaturated with doubly allylic methylene unit, or unsaturated with two doubly allylic methylene units, respectively, to the grafting mechanism. Two initiators, benzoyl peroxide (BPO) and azoisobutyronitrile (AIBN), were used to evaluate the effect of initiator. Alkyd−MMA hybrids were characterized with 2D gradient heteronuclear multiple quantum coherence (gHMQC) 1H NMR, matrix assisted laser desorption ionization (MALDI) mass spectrometry, solvent extraction, and gas chromatography.

Figure 2. Potential graft sites for MMA in four model alkyds.

and BA in butanol. The soybean oil alkyd was synthesized using a monoglyceride process. Because the predominant fatty acid was linoleic acid, the double allylic methylene site was the principle grafting site, as shown in Scheme 1. Direct addition of acrylic monomers onto the fatty acid double bonds was not observed. Gan et al.8,9 reported copolymerization of oleic alkyd resin with MMA in toluene in the presence of benzoyl peroxide. From 1H NMR results, the decrease of the oleic double bond concentration was observed, and the grafting of both monomers was found to be across the double bonds. However, the 1H NMR spectrum was too complicated with overlapping resonances to fully elucidate the atom connectivity between alkyd and acrylic resins. The grafting mechanism of alkyd−acrylic hybrids in miniemulsion systems has been investigated by Schork et al.10−14,21 The grafting site, monomer conversion, and cross-linking mechanism were investigated. When acrylate monomers were used, grafting was shown to occur directly across the double bonds in the alkyd’s pendant fatty acid chains. In the case of methacrylates, it was found that grafting took place by hydrogen abstraction of the doubly allylic hydrogens due to the steric hindrance around the radical center. The steric hindrance prevented the monomer from adding directly across the resinous double bond, as occurs in the case of acrylates.10−14 Dziczkowski et al.6,7 investigated a solventborne alkyd−acrylic hybrid system with a detailed 2D NMR

2. EXPERIMENTAL SECTION 2.1. Materials. Stearic acid (98.5%), oleic acid (90.0%), linoleic acid (99.0%), linolenic acid (99.0%), glycerol (99.9%), phthalic anhydride (PA, 99.0%), p-xylene (98.5%), 1-butanol (99.4%), diethyl ether (99.0%), benzoyl peroxide (BPO, 99.9%), azoisobutyronitrile (AIBN, 98%), potassium hydroxide (99.0%), benzoic acid (99.5%), acetonitrile (99.9%), acetone (99.9%), inhibitor removal resin, and methyl methacrylate (MMA, 99.0%) were all purchased from Sigma-Aldrich and used as received except MMA, which was purified by running through a column of inhibitor removal resin, and BPO and AIBN, which were recrystallized from chloroform and methanol, respectively, prior to use. Chloroform-d (CDCl3, 99.8%, 0.05% v/v TMS) was supplied by Cambridge Isotope Laboratories, Inc. 2.2. Instrumentation. Varian NMRS 500 MHz spectrometer was used to record the 1H and gHMQC NMR spectra. CDCl3 containing 0.1 wt % TMS was used as solvent. MALDItime-of-flight (TOF) mass spectrometry was performed on a

Scheme 1. Grafting Mechanism for MMA via Hydrogen Abstraction at Methylene Unita

a

Reprinted from ref 7, Copyright 2012, with permission from Elsevier. B


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ducted for all eight model alkyd systems with the absence of MMA, as shown in Table 1.

Bruker Ultrflex-III tandem time-of-flight (TOF/TOF) mass spectrometer (Bruker Daltonics, Billerica, MA, United States) equipped with a Nd:YAG laser emitting at 355 nm. A Varian CP-3800 gas chromatography with flame ionization detector (FID) detector was used to analyze and record quantitative data for the reaction. 2.3. Synthesis of Model Alkyd. Stearic acid (41.18 g, 0.145 mol), oleic acid (41.0 g, 0.145 mol), linoleic acid (40.6 g, 0.145 mol), or linolenic acid (40.31 g, 0.145 mol), PA (25.0 g, 0.169 mol), glycerol (15.3 g, 0.166 mol), and xylene (4 g, 5 wt % of total weight of other reactants) were formulated according to eqs 1−4.1 ÄÅ ÅÅ ÅÅ ÅÇ

[x(1 − L)2L]E1 ÉÑ − 1ÑÑÑÑE1 + E2 + ÑÖ ÄÅ É ÅÅ1 − 2R ÑÑÑ ÅÅ Ñ x Ñ Å ÑÖ A1(fatty acid) = Ç 1+R

R=

{( ) 1 L

Table 1. Polymerization Recipe for Acrylated Alkyda acrylated alkyd component alkyd MMA n-butanol

Bx (glycerol) =

R 1+R

6.0 g 3.0 g, 0.03 mol 4.4 g, 0.06 mol

BPO

( )Ex} x 2

(1)

AIBN

( )

A 2 (phthalic anhydride) =

AIBNinitiated

(2R /x) 2+R

0.02 g, 1.2 × 10−4 mol

BPO-initiated

control AIBN

control BPO

6.0 g 3.0 g, 0.03 mol 4.4 g, 0.06 mol 0.03 g, 1.2 × 10−4 mol

6.0 g

6.0 g

4.4 g, 0.06 mol

4.4 g, 0.06 mol 0.03 g, 1.2 × 10−4 mol

0.02 g, 1.2 × 10−4 mol

a

Each column represents four different model alkyds made with various fatty acids.

(2)

1

H NMR samples were prepared by dissolving 10 mg of each sample with 0.7 mL of d-chloroform, and a pulse width of 90° was used to acquire 32 scans with a relaxation delay of 25 s between each scan to allow complete relaxation of the monomeric and polymeric hydrogens. Conversion was calculated from the proton integration of vinyl (δ 5.6 and 6.1 ppm) and methoxy hydrogens of unreacted acrylic monomer (δ 3.7 ppm) and acrylic polymer (δ 3.6 ppm). 2D gHMQC samples were prepared by dissolving 50 mg of each sample with 0.7 mL of d-chloroform, and an acquisition time of 0.128 s and a relaxation delay of 1 s with 45° pulse width were used to acquire 512 increments with a scan per increment of 8. All NMR spectra were referenced relative to the resonance signal of TMS (δ 0.0 ppm). 2.5. Separation of Grafted Alkyd and Acrylic Homopolymer. The degree of grafting of each model alkyd system was determined by solvent extraction in Soxhlet extractor using diethyl ether as solvent. Vacuum dried sample (1 g) was wrapped with filter paper and inserted into an extraction thimble. During the extraction, alkyd and alkyd− MMA copolymers are extracted by diethyl ether, while the MMA homopolymer is not and remains in the extraction thimble. After 24 h of extraction with refluxing diethyl ether, the residual sample in the thimble was dried in a vacuum oven and weighed. Then, the residual pMMA was extracted again in diethyl ether to confirm that pMMA does not dissolve in diethyl ether in a measurable amount. The degree of grafting is calculated based on the residual weight and the total MMA weight in each system as following eqs 5 and 6:

(3)

(4)

where R is alkyd constant for formulating alkyd composition, L is oil length of desired alkyd, x is the functionality of polyol, E1 is the equivalent molecular weight of fatty acid, E2 is the equivalent molecular weight of phthalic anhydride, Ex is the equivalent molecular weight of polyol, A1 is the molar equivalent of fatty acid, A2 is the molar equivalent of phthalic anhydride, and Bx is the molar equivalents of glycerol. Each alkyd has an oil length of 58%. Reactants were charged into a 250 mL four-neck roundbottom flask equipped with a mechanical stirrer, reflux condenser, thermometer, Dean−Stark trap, and inert gas inlet. The setup was slowly heated to 220 °C. The reaction progress was monitored by acid number (AN) according to ASTM D1639-90 until AN was under 10 mg (KOH)/g (sample) was achieved.15 A slightly yellow viscous liquid was obtained after solvent removal. 1H NMR samples were prepared by dissolving 10 mg of each sample with 0.7 mL dchloroform, and a pulse width of 90° was used to acquire 32 scans with a relaxation delay of 1s. 1H NMR (500 MHz) δ (ppm) 0.85−1.05 (m, 3H, −CH3), 1.05−1.53 (m, 20H, −CH2−), 1.53−1.78 (m, 2H, −OCOCH2CH2−), 1.78−2.22 (m, 4H, cis −CH2CHCHCH2−), 2.22−2.41 (m, 2H, −OCOCH 2 CH 2 −), 2.60−2.85 (m, 4H, cis −CH CHCH2CHCH−), 4.06−4.94 (m, 6H, −OCH2CHCH2O−), 5.23−5.45 (m, 2H, cis −HCCH−), 7.27 (s, CHCl3), 7.38−7.98 (m, 4H, −OOCPhCOO−); 2.4. Synthesis of Model Alkyd−MMA Copolymer. Model alkyd resin (6.0 g) and n-butanol (2.20 g, 0.03 mol) were charged into a 250 mL four-neck round-bottom flask equipped with a reflux condenser, mechanical stirrer, and pump/syringe system. The reaction mixture was slowly heated to 220 °C. A mixture of n-butanol (2.20 g, 0.03 mol), MMA (3.0 g, 0.03 mol), and BPO (0.03 g, 1.2 × 10−4 mol) or AIBN (0.02 g, 1.2 × 10−4 mol) was introduced into the flask continuously over 1 h by pump/syringe. The reaction conditions were maintained for 24 h and then cooled to ambient temperature. Eight control experiments were con-

MMA% =

(total MMA)(conversion of MMA) (total MMA)(conversion of MMA) + alkyd + initiator

(5)

× 100% grafting degree =

(total polymer weight)(MMA%) − residual weight (total polymer weight)(MMA%) × 100%

(6)

After solvent extraction, the residue in the filter paper was characterized by 1H NMR. Extracted polymer dissolved in diethyl ether was retrieved by solvent removal and characterized by 1H NMR: 1H NMR (500 MHz) δ (ppm) 0.78−1.53 (m, 3H, −CH3), 1.73−1.98 (m, 2H, −CH2−), 3.47−3.57 (m, 3H, −OCH3), 7.27 (s, CHCl3) C


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Industrial & Engineering Chemistry Research Scheme 2. Synthesis of Model Alkyd Resin with One Fatty Acid

2.6. Hydrogen Abstraction Measurement. Quantitative analysis of benzoic acid and isobutyronitrile were carried out via gas chromatography. The injector temperature was set at 250 °C, FID detector temperature was 300 °C, the carrier gas is helium, and flow rate is 0.9 mL/min. Column oven temperature was 50 °C initially for 2 min and was increased to 280 °C with a rate of 15 °C/min and then held for 2 min. The sample injection volume was 1 μL. Monomer concentration was calculated from the standard calibration curve. The standard benzoic acid solutions in acetonitrile were made with known concentration of 25, 50, 100, and 150 ppm (μg/ mL).

3. RESULTS The objective of this study was to elucidate the grafting site and mechanistic pathway for grafting MMA onto alkyds. Four model alkyds were prepared using only one of the fatty acids typically found in seed oil, as shown in Scheme 2. The alkyds were prepared via the fatty acid method with glycerol, phthalic anhydride, and stearic, oleic, linoleic, or linolenic acid. In this way, the grafting behavior of each fatty was deconvoluted from the mixture. 3.1. MALDI Mass Spectra of Model Alkyds. Because alkyds are synthesized by a polycondensation mechanism with a number of side reactions, alkyds typically possess a very wide PDI. This makes alkyds very difficult to characterize by GPC. As a consequence, they are usually characterized by acid and hydroxyl number.16,17 MALDI-TOF mass spectrometry is a soft ionization method which produces a single charged ion without fragmentation even for high molecular weight samples. However, it is still difficult to achieve a good resolution with alkyds using MALDI-TOF-MS. In this study, model single fatty acid alkyds were synthesized to simplify the complex structures of alkyds. Therefore, mass to charge ratio (m/z) of each model alkyd can be more easily distinguished. The molecular weight of each alkyd molecule can be calculated by adding the molecular weight of fatty acid, phthalic anhydride, glycerol, and subtracting out the water evolved. By comparison of calculated molecular weight and m/ z values in Figure 3, alkyd molecular structures can be deduced. Tables 2 and 3 indicate that there are multiple forms of alkyd molecules in the oleic model alkyd derived from fatty acid process. Expanded tables and other spectra can be seen in the Supporting Information. The most abundant linear oleic alkyd molecule has three fatty acids, one phthalic anhydride, and two glycerols. The most abundant cyclic oleic alkyd molecule contains two fatty acids, three phthalic anhydrides, and three glycerols. The mass of the repeating unit in an oleic

Figure 3. MALDI-TOF mass spectrum of oleic model alkyd resin.

acid alkyd is 486.3 Da. All the arrows in Figure 3 represent the mass difference of one repeating unit in oleic model alkyd. Ring formation of alkyds has been reported in literature. Cyclization of alkyd is especially prominent for ortho-phthalic anhydride-based alkyds.18 However, there is no direct evidence that shows the exact molecular weight of linear and cyclic alkyd molecules. To the best of our knowledge, this is the first time such detailed structural information about alkyds is available through MALDI-TOF-MS techniques. 3.2. 1H NMR. To monitor the polymerization of MMA and the amount of pMMA grafted to the alkyd, conversion of MMA monomers was measured for all systems. Conversion in all systems was calculated from the integration of 1H NMR spectroscopy as shown in Table 4. The accuracy of conversion calculation from NMR was found to be within ±0.3%. Conversion versus time plots are shown in Figures 4 and 5. From Table 4 and Figures 4 and 5, it was observed that the polymerization rate and final conversion with the initiation using AIBN are slightly higher than those with BPO. This is expected because AIBN has a shorter half-life. Also, BPO is capable of hydrogen abstraction, resulting in retardative chain transfer, which can contribute to a lower final conversion. Compared to stearic and oleic systems, the polymerization of linoleic and linolenic systems was significantly retarded due to the presence of double allylic methylene sites. To determine the contribution of double bond and double allylic sites to grafting, the loss of double bond and double allylic hydrogens was measured from the integration of 1H NMR. Reacted double bond (RDB) is the fraction of double bonds in model alkyds that were reacted during copolymerizaD


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Industrial & Engineering Chemistry Research Table 2. Mass Composition of Alkyd Resinsa stearic alkyd

oleic alkyd

linoleic alkyd

fatty acid

phthalic anhydride

glycerol

water evolved

linear or cyclic

1135.892 1221.698 1357.946

1130.017 1217.796 1352.015

1123.807 1213.656 1345.886

3 2 3

1(2) 3(6) 2(4)

2(6) 3(9) 3(9)

5 8 7

linear cyclic linear

a

In parentheses: functionality of the molecule.

Table 3. Chemical Structure of Oleic Alkyd for Each m/za

a

R is the fatty acid.

Table 4. Conversion of MMA combinations MMA + AIBN MMA + BPO homo MMA + AIBN homo MMA + BPO

stearic (%)

oleic (%)

linoleic (%)

linolenic (%)

73.4 67.2

73.5 58.9

52.4 50.6

50.1 49.4

90.1 89.7

Figure 5. Conversion−time plot of stearic/oleic/linoleic/linolenic alkyd copolymerized with MMA initiated by BPO.

Table 5. RDB and RDA Values of Oleic, Linoleic, and Linolenic Systemsa control AIBN control BPO oleic−MMA−AIBN oleic−MMA−BPO linoleic−MMA−AIBN linoleic−MMA−BPO linolenic−MMA−AIBN linolenic−MMA−BPO

Figure 4. Conversion−time plot of stearic/oleic/linoleic/linolenic alkyd copolymerized with MMA initiated by AIBN.

tion with MMA. Reacted double allylic (RDA) is the fraction of double allylic sites in linoleic and linolenic model alkyd that were reacted during copolymerization with MMA. It is important to note that the loss of these double allylic hydrogens is not necessarily related to grafting, as this could also happen as alkyds cross-link with one another. However, it was found in control experiments that the RDB and RBA values were negligible, so this can be disregarded. All the RDB and RDA values are shown in Table 5. In all eight control experiments, RDB and RDA values are negligible in the absence of MMA. In oleic−MMA−AIBN and oleic− MMA−BPO systems, there are decreases of double bond

a

RDB (%)

RDA (%)

0 0 7.6 6.0

0 0

41.3 40.6 31.5 28.1

RDB: reacted double bonds; RDA: reacted double allylic.

integration of 7.6 and 6.0%. In linoleic and linolenic systems, the decreases of double bond integration were negligible. However, there are decreases of double allylic hydrogen integration, which indicates grafting by hydrogen abstraction is occurring in linoleic and linolenic systems. E


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Industrial & Engineering Chemistry Research 3.3. 2D gHMQC NMR. To resolve the overlapping resonances in proton NMR, 2D gHMQC NMR was used. By using 2D gHMQC spectra, the carbon to hydrogen atom bonding can be identified and used to deduce the grafting mechanism. 2D gHMQC NMR can detect cross correlations between proton nuclei and carbon nuclei separated by one bond. This method shows one resonance for each pair of correlated nuclei. gHSQC detects a spectrum similar to that of gHMQC with different protocols; the only difference is that gHMQC is superior for medium sized molecules, and gHSQC is better for large molecules.19 Because all the model alkyds spectra are similar, the example of gHMQC NMR spectrum of linoleic model alkyd was shown in Figure 6 (see supplementary data for other spectra). The

Figure 7. 2D gHMQC NMR spectrum of MMA grafted oleic model alkyd.

Figure 6. 2D gHMQC NMR spectrum of linoleic model alkyd.

resonances at δ 0.5−3.50 (1H)/10.0−45.0 (13C) ppm correspond to the aliphatic hydrogens on fatty acid side chains and their adjacent carbons (outlined in black in Figure 6). The resonances at δ 4.0−5.5 (1H)/60.0−70.0 (13C) ppm correspond to the hydrogens on the polyol backbone and their adjacent carbons (outlined in red in Figure 6). The figure is expanded for this region due to important changes here when grafting occurs. The resonances at δ 5.5−8.50 (1H)/70.0− 145.0 (13C) ppm correspond to the vinyl hydrogens on fatty acid side chains and aromatic hydrogens on phthalic anhydride segment and their adjacent carbons (outlined in blue in Figure 6). For all model alkyd systems, 2D gHMQC spectra were expanded to the region where important connectivity is detected which is the proton/carbon coupling detected in the proton region of δ 3.0−6.0 ppm and the carbon region from δ 45−90 ppm as shown in Figures 7−9. In stearic and oleic alkyd−MMA spectra, there is a new resonance at around δ 3.6(1H)/52.0(13C) ppm compared to model alkyd spectra. This new resonance shows correlation between the methyl protons of MMA and its adjacent carbon, as shown in Figure 7. This resonance indicates that MMA was either homopolymerized or copolymerized in stearic and oleic alkyd systems. In the model linoleic alkyd−MMA and linolenic alkyd− MMA 2D gHMQC spectra, there were new resonances observed at δ 3.6(1H)/62.0(13C) ppm. This new resonance

Figure 8. 2D gHMQC NMR spectrum of MMA grafted linoleic model alkyd.

has been previously reported as the correlation between the double allylic proton and its adjacent carbon in MMA grafted linoleic or linolenic model alkyd, as shown in Figures 8 and 9. This correlation indicates that MMA was grafted to linoleic or linolenic model alkyds by abstracting hydrogen from double allylic position followed by termination via recombination with a propagating MMA radical. Analysis of these 2D gHMQC NMR has shown that the grafting occurs at the doubly allylic site or directly to the double bond. The potential grafting sites shown in Figure 2 also mentioned that an allylic hydrogen or the backbone hydrogen in the glycerol could also potentially participate in the grafting reactions. However, no evidence of this was seen in the analysis of these NMR spectra. 3.4. Quantitative Evaluation of Graft and Homo Polymerization. To separate MMA homopolymer from grafted copolymer, solvent extraction was used, and the degree of grafting was calculated. The degree of grafting is a measure of the efficiency of grafting monomers onto model alkyds. F


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Figure 9. 2D gHMQC NMR spectrum of MMA grafted linolenic model alkyd.

Degree of grafting is shown in Table 6 and was calculated by eqs 5 and 6. Figure 10. Initiator byproducts (a) benzoyl peroxide (b) azobis(isobutyronitrile).20

Table 6. Degree of Grafting of Each System sample

stearic (%)

oleic (%)

linoleic (%)

linolenic (%)

MMA + AIBN MMA + BPO

5.0 14.5

10.1 15.7

60.6 72.4

58.4 74.7

Table 7. Benzoic Acid Product Weight Percentage in Four Samples

It was observed that the degree of grafting is generally proportional to the number of double bonds in the model alkyd systems. Stearic and oleic alkyd hybrid systems have very low degree of grafting due to the lack of active grafting sites. Linoleic and linolenic alkyd hybrid systems show a dramatic increase in the degree of grafting because the double allylic methylene sites provide additional grafting pathways. Compared to model linoleic alkyd systems, the extra double allylic site in linolenic alkyd does not seem to contribute to a significantly higher degree of grafting. It was also observed that the BPO initiated systems have a degree of grafting higher than that of the AIBN for the same model alkyd systems. 3.5. Evaluation of Hydrogen Abstraction by Initiator. The dissociation product of BPO and AIBN will be primarily benzoyloxy- and isobutyronitrile-free radicals at 80 °C, as shown in Figure 10.20 The dissociation of initiator will form primary radical. Primary radicals can either initiate polymerization or undergo hydrogen abstraction, which can then chain transfer to initiate another polymer chain. When direct hydrogen abstraction occurs, benzoic acid and isobutyronitrile are formed. To quantitatively measure the hydrogen abstraction by the primary radical, gas chromatography was used. Table 7 shows quantitative measurement of benzoyl peroxide initiator conversion to benzoic acid. The percentages are the amount of benzoic acid present in the alkyd/acrylic hybrid following reaction relative to the benzoyl peroxide added to the reaction initially. The equations are detailed in the Supporting Information. In the BPO initiated systems, hydrogen abstraction by primary radical was dependent on the structures of fatty acids. In the AIBN initiated systems, primary radical hydrogen abstraction products were not detected, and thus, the AIBN system was independent of type of fatty acid used. It was

sample name

stearic/MMA

oleic/MMA

benzoic acid (%)

1.4

3.6

linoleic/MMA linolenic/MMA 44.0

41.9

observed that the amount of hydrogen abstraction product of BPO in stearic and oleic model alkyd was minimal, and the amount of hydrogen abstraction product of BPO in linoleic and linolenic model alkyd were at least 10 times larger than those in stearic and oleic alkyd, which indicated a high degree of hydrogen abstraction by BPO for the linoleic and linolenic model alkyd model systems. Compared to linoleic model alkyd system, the extra double allylic site in linolenic model alkyd systems did not lead to more direct hydrogen abstraction by the primary radical.

4. DISCUSSION In this study, monomer conversion, degree of grafting, and hydrogen abstraction by primary radical are studied to deduce mechanisms of grafting MMA onto model alkyd resins. Model alkyds were synthesized by fatty acid process and characterized by MALDI-TOF mass spectrometry, deconvoluting the complex structures of alkyds. This is also the first time such a comprehensive study with modern instrumentation has been reported for investigating the mechanism of grafting MMA onto alkyds. 4.1. Stearic Model Alkyd Systems. Stearic model alkyd systems showed retardation of MMA polymerization compared to MMA polymerized without the presence of stearic model alkyd. The MMA final conversions of stearic alkyd−MMA− AIBN and stearic alkyd−MMA−BPO systems are 73.4 and 67.2%. The degree of grafting for stearic alkyd−MMA−AIBN and stearic alkyd−MMA−BPO are 5.0% and 14.5%, respectively. The low degree of grafting is expected as stearic G


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Figure 11. Hypothetical mechanism of grafting MMA onto the glycol unit of stearic model alkyd.6

Figure 12. Expected mechanism of grafting MMA onto double bonds in oleic model alkyd.6

hydrogen abstraction by primary radical, it is supposed that hydrogen abstraction by propagating MMA radical is responsible for generating graft sites. It is surmised that in the stearic alkyd−MMA−BPO system, grafting sites are mainly generated via primary radical hydrogen abstraction and propagating MMA radical hydrogen abstraction. In the stearic

acid is a saturated fatty acid; MMA monomers polymerize into MMA homopolymer more readily than grafting onto a stearic alkyd. The amount of hydrogen abstraction by BPO is 1.42%, and that by AIBN is negligible. As stearic alkyd−MMA−AIBN system still has 5.0% of grafting without the contribution of H


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and BA will have slower polymerization rates, lower final monomer conversion, and allows more grafting, which is in harmony with our results. The degree of grafting of MMA in the presence of linoleic and linolenic model alkyd systems is much higher than that in stearic and oleic model alkyd systems, although MMA has a lower conversion in the presence of linoleic and linolenic alkyd. A possible interpretation is that retardative chain transfer in the presence of double allylic hydrogen will produce more grafting sites, and more MMA will graft onto those graft sites. The initiation of BPO, which is reported as a highly efficient hydrogen abstractor, usually results in a higher degree of grafting than AIBN.20 In linoleic alkyd−MMA−AIBN and linoleic alkyd−MMA− BPO systems, there are on average 2.1 and 2.6 mol of MMA on 1 mol of reacted graft site. In linolenic alkyd−MMA−AIBN and linolenic alkyd−MMA−BPO systems, there are on average 1.3 and 1.9 mol of MMA on 1 mol of reacted graft site. Linoleic alkyd systems have more MMA grafted to each grafting site than linolenic alkyd systems as linolenic model alkyd have more grafting site than linoleic model alkyd. Generally, the number of grafted MMA on each grafting site will decrease as the degree of unsaturation increases. BPO initiated systems have more MMA grafted per grafting site compared to AIBN initiated systems. Also, there is a significant increase in the amount of hydrogen abstraction by BPO in both linoleic and linolenic model alkyd systems. This large amount of primary radical hydrogen abstraction is consistent with the low MMA conversions and high degree of grafting in the presence of linoleic and linolenic model alkyds due to retardative chain transfer to double allylic sites. It can also be noted from Table 6 that the conversion of BPO to benzoic acid by hydrogen abstraction for linoleic and linolenic alkyds are similar, although linolenic alkyd has one more double allylic site than linoleic alkyd. It is surmised that the grafting onto the double allylic site will stabilize the adjacent double allylic site on the linolenic acid side chain. Thus, the additional double allylic site in linolenic acid will not facilitate more hydrogen abstraction. Hydrogen abstraction from the glycol is possible to some extent; however, the generated free radical tends to chain transfer to a lower energy state, which is the double allylic hydrogen. AIBN is reported as incapable of abstracting hydrogen from allylic sites.17 In this study, isobutyronitrile is not detected in all model alkyd hybrid systems initiated by AIBN, which supports that 2-cyanoprop-2-yl radical generated from thermal dissociation of AIBN is not able to abstract any hydrogens from alkyds by primary radical attack. In this case, MMA propagating radicals are possible hydrogen abstractors instead. Unlike oleic model alkyd systems, linoleic and linolenic model alkyd systems show that the loss of double bond integrations on fatty acid chain during polymerization are negligible. Instead, there is significant decreases of double allylic hydrogen integration in all linoleic and linolenic model systems (see Table 5). Also, a new resonance signal was observed at δ 3.6(1H)/62.0(13C) ppm in 2D gHMQC spectrum which, accordingly, is the corresponding resonance signal of the predicted mechanism for hydrogen abstraction from the double allylic site. After the hydrogen is abstracted, MMA propagating radical may terminate with the generated radical site by recombination. It should be noted that in the 1H NMR spectrum the resonance signal of double allylic graft site

alkyd−MMA−AIBN system, grafting sites are generated only via propagating MMA radical hydrogen abstraction as AIBN radicals have less tendency to abstract hydrogen, and isobutyronitrile was not detected by gas chromatography, which indicates primary hydrogen abstraction is not possible for AIBN initiated stearic alkyd−MMA systems. The new radical generated from hydrogen abstraction could combine with another propagating MMA radical to form stearic alkyd− MMA graft copolymer, as shown in Figure 11. 4.2. Oleic Model Alkyd Systems. The final conversions and degree of grafting of oleic alkyd−MMA−AIBN and oleic alkyd−MMA−BPO systems are similar to those in the stearic alkyd system. Not surprisingly, the amount of hydrogen abstraction by BPO is 3.55%, and that by AIBN is negligible, as the relative high reactivity of the double bonds will lead to direct addition onto double bonds on oleic acid side chains instead of abstracting hydrogen from the polyol backbone and allylic site. In oleic alkyd−MMA−AIBN and oleic alkyd− MMA−BPO systems, there are decreases of double bond integration of 7.6 and 6.0% that indicate direct addition across vinyl groups on the pendant fatty acid. It was surmised that copolymerization is initiated via direct addition onto double bonds, and MMA may propagate either through the generated radical or by termination with propagating MMA radicals, as shown in Figure 11. From MMA conversion, degree of grafting, and consumed graft site, the number of MMA per graft site can be estimated, as shown in eq 7. In oleic alkyd−MMA−AIBN and oleic alkyd−MMA−BPO systems, there are on average 2.7 and 3.0 mol of MMA on 1 mol of reacted graft site. no. MMA per graft site total mol MMA × MMA conversion% × grafting degree% = total mol graft site × consumed graft site%

(7)

Gan et al.9 previously reported free radical copolymerization of methyl methacrylate with short oil oleic alkyd. From integration of 1H NMR, it was concluded that double bonds of the alkyd were involved in copolymerization. Current results also agrees with previous work by Schork et al.14 in miniemulsion systems. A reasonable explanation for the low addition reaction across the double bond is that interaction between functional groups and the reactivity of double bond may vary for different locations and the partial cyclic structures of the model alkyds may hinder the grafting of MMA (Figure 12). 4.3. Linoleic and Linolenic Model Alkyd Systems. For linoleic alkyd−MMA−AIBN and linoleic alkyd−MMA−BPO systems, the final conversions are 52.4 and 50.6%. For linolenic alkyd−MMA−AIBN and linolenic alkyd−MMA−BPO systems, the final conversions are 50.1 and 49.4%. In the presence of linoleic and linolenic model alkyds, the conversion of MMA significantly decreases, and the rate of polymerization is severely retarded. The explanation is that a primary radical or a MMA oligoradical can easily abstract the double allylic hydrogen, creating a relatively unreactive double allylic radical as the radical center is resonance stabilized by the two adjacent double bonds. This retardative chain transfer will slow the polymerization rate and lower the final conversion. The double allylic radical will eventually terminate with MMA radicals by recombination and lead to grafting.16 The retardation of the polymerization of MMA by linoleic acid has previously been reported by Schork et al.17 Their results showed that with higher linoleic acid content, MMA I


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Figure 13. Expected mechanism of grafting MMA onto the diallylic site in linoleic/linolenic model alkyd.

(δ 3.6(1H)/62.0(13C) ppm) is overlapped with the resonance signal of the hydrogen adjacent to the ester group in the MMA molecule (δ 3.6(1H)/52.0(13C) ppm). With the aid of 2D NMR, those two resonance signals were distinguished. Similar results can be found in the literature. Majumdar et al.22 studied acrylate grafted dehydrated castor oil (DCO) alkyd in which the majority portion of DCO alkyd is linoleic alkyd. It was proposed that the grafting is achieved by hydrogen abstraction via primary radical attack, and the grafting site is located at the double allylic site on the fatty acid side chain. The grafting mechanism for linoleic and linolenic model alkyd systems are presumed to be mainly via hydrogen abstraction from the activated methylene position in double allylic sites, and MMA propagating radicals may terminate with those generated sites by recombination as shown in Figure 13. By comparing different mechanisms in four model alkyd systems, it was shown that as long as there are nonconjugated double bonds available, the hydrogen abstraction from the diallylic group is the most favorable reaction. When diallylic hydrogens are not present, a preferred reaction pathway is addition to a double bond. In a saturated alkyd, the grafting will occur only at the glyceride unit via hydrogen abstraction. This is in accordance with the change of enthalpy of the reaction. In general, reactions tend to favor the product with the lowest enthalpy and thus those products with the strongest bonds. The change of enthalpy can be calculated by adding the sum of the bond dissociation energy (BDE) of bonds that are broken and subtracting the sum of the BDE of bonds that are formed (ΔH = ∑ (BDE of bonds broken) − ∑ (BDE of bonds formed)).23 For a free radical to abstract a hydrogen from a double allylic site, the change of enthalpy is −130 kcal/ mol. For double bond addition, the change of enthalpy is around −63 kcal/mol. Therefore, hydrogen abstraction from double allylic sites are favored over double bond addition.

5. CONCLUSION In this study, four model alkyds and alkyd−MMA hybrids were prepared and characterized. The mechanism of grafting MMA onto alkyds is dependent on the structure of the fatty acid. In the stearic and oleic model systems, the polymerization rate and final conversion of MMA are slightly retarded, and the degree of grafting is low due to the lack of grafting sites in those model alkyd systems. In linoleic and linoelnic model systems, chain transfer to double allylic site decreases the final conversion and polymerization rate of MMA significantly. However, the degree of grafting is increased compared to that in stearic and oleic systems as the double allylic hydrogens are prone to be abstracted, generating graft site for MMA. The mechanistic difference between BPO and AIBN is illustrated by the propensity for BPO to hydrogen abstract while propagating MMA radicals chain transfer with the doubly allylic methylene hydrogens in the AIBN systems. In this paper, it is the first time such detailed structural information of alkyds has been available and used to elucidate the contribution of each type of fatty acid during copolymerization with MMA.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01941.

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Mass spectrometry data, NMR conversion calculations, gas chromatography data, and 2D gHMQC NMR data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. J


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Industrial & Engineering Chemistry Research ORCID

(20) Moad, G.; Solomon, D. H. The Chemistry of Radical Polymerization, 2nd ed.; Elsevier: Oxford, 2006. (21) Guo, J.; Schork, F. J. Hybrid miniemulsion polymerization of acrylate/oil and acrylate/fatty acid systems. Macromol. React. Eng. 2008, 2 (3), 265−276. (22) Majumdar, S.; Kumar, D.; Nirvan, Y. P. S. Acrylate grafted dehydrated castor oil alkyd - A binder for exterior paints. J. Coat. Technol. 1998, 70 (879), 27−33. (23) Wade, L. G. Organic Chemistry, 7th ed.; Prentice Hall, Inc.: Upper Saddle River, 2009.

Mark D. Soucek: 0000-0003-3865-4504 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding was provided by the University of Akron. REFERENCES

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Investigation of Methyl Methacrylate Grafting on Model Single Fatty

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