Article pubs.acs.org/Macromolecules
Anionic Polymerizable Surfactants from Biobased ω‑Hydroxy Fatty Acids Jing Hu,† Zhennan Jin,† Tzu-Yin Chen,† Jennifer D. Polley,† Michael F. Cunningham,*,‡ and Richard A. Gross*,§ †
Polytechnic Institute of New York University, Six Metrotech Center, Brooklyn, New York 11201, United States Department of Chemical Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6 § Department of Chemistry and Biology, Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute (RPI), 4005B BioTechnology Bldg., 110 Eighth Street, Troy, New York 12180, United States ‡
S Supporting Information *
ABSTRACT: Biobased ω-hydroxytetradecanoic acid prepared via an efficient yeast-catalyzed ω-hydroxylation reaction was converted by a onestep reaction to the polymerizable surfactants ω-acryltetradecanoic acid (MA-1) and ω-maleate tetradecanoic acid (MA-2). MA-1 is a single polarheaded surfactant, whereas MA-2 is a bolaamphiphile with carboxylic acid polar groups at both chain ends. MA-1 gave a distinct critical micelle concentration (cmc) at 253 mg/L, whereas for MA-2, the surface tension decreased monotonically and a distinct cmc was not observed even up to 1800 mg/L. Experimental determination of the reactivity ratios for MA-1 and MA-2 with styrene showed that for MA-1 copolymers that approximate random structures were formed while MA-2 tends to form copolymers with an alternating nature. Emulsion polymerizations conducted with varying amounts of MA-1 and MA-2 (1−10 wt % with respect to styrene) gave colloidally stable latexes with particle sizes ranging from 52 to 155 nm. In emulsion polymerizations using either MA-1 or MA-2 at more than 5 wt % to monomer, a linear increase in latex particle volume with conversion was observed and the particle number remained constant, establishing that the polymerizations proceeded without significant aggregation or secondary particle nucleation. Potentiometric titration and 1H NMR were used to measure MA-1 and MA-2 conversions during polymerization as well as how the surfactants were distributed between the particle surface, aqueous phase, and particle interior. Observed differences were rationalized based on the comparative structures of MA-1 and MA-2 and their corresponding partitioning behavior.
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INTRODUCTION Polymerizable surfactants are designed to stabilize oil phase particles during emulsion polymerizations while also participating as comonomers. By copolymerization, surfactants become covalently bound to emulsion particles, thereby preventing their leaching from films formed from the latex. Since the first study of polymerizable surface-active agents in 1956,1 a broad range of polymerizable surfactants have been prepared and evaluated. Polymerizable moieties incorporated within surfactants include styrenic,2 maleate,3−11 (meth)acrylic,12,13 allyloxy,14 and crotonate11 groups. The polar head groups of these surfactants may be charged (anionic, cationic) or nonionic and include hydrophilic groups such as sulfonate,2,3 sulfate,11,12 maleate,4,5 carboxylic acid, ammonium,12 methyl sulfate,6 PEO (poly(ethylene oxide)),9,14,15 amido esters,10 and others. By using polymerizable surfactants instead of their nonreactive counterparts, improvements in corresponding latex film properties often result, including stability against low temperatures5,14 and electrolytes,3,9,11 water resistance,7,11,14 and mechanical resistance.7 © 2013 American Chemical Society
Desired characteristics of polymerizable surfactants are as follows:16,17 (i) providing effective surface properties and emulsification activity that stabilizes latex particles during emulsion polymerization; (ii) having proper polymerization reactivity below that of monomers within the particles so polymerizable surfactants are not consumed prematurely so they are buried within particles; and (iii) obtaining a large fraction of the surfactant molecules covalently linked and located at the latex surface upon completion of the polymerization, leaving minimal unreacted surfactant remaining in the aqueous phase. The effectiveness of polymerizable surfactants will also be a function of the polymerization process and the monomer structure(s). Hence, the design of polymerizable surfactants must be carefully considered to ensure they exhibit the desired performance characteristics. Unzué11 compared surfactants with methacrylate, maleate, and crotonate unsatuReceived: June 22, 2013 Revised: November 20, 2013 Published: December 31, 2013 113
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Scheme 1. Synthetic Routes To Prepare (a) ω-Acryltetradecanoic Acid (MA1) and (b) ω-Maleate Tetradecanoic Acid (MA2)
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rated groups in the semicontinuous emulsion polymerizations of styrene, butyl acrylate, and acrylic acid. The crotonatefunctionalized surfactant gave relatively stable latexes but was quite unreactive. Increasing the reactivity of the surfactant by using a maleate moiety resulted in stable latex particles similar to those obtained using SDS (sodium dodecyl sulfate), the reference surfactant. Highly reactive methacrylic surfactants gave unstable latex particles with large amounts of coagulum. This behavior was attributed to reaction by the surfactant molecules too early in polymerizations such that a large fraction of these molecules became buried within the particles. Different polymerization processes to incorporate polymerizable surfactants at latex surfaces have been reported such as miniemulsion,12 seeded semicontinuous,7,8,10,15,18 and mixed surfactant system polymerizations.5 Our research team has developed a mild and efficient biotechnological route to convert fatty acids, such as tetradecanoic acid (C14), to ω-hydroxytetradecanoic acid. This was accomplished by developing an engineered Candida tropicalis strain whose background activity for conversion of ωhydroxy fatty acids to their corresponding ω-carboxy diacids was dramatically reduced. To reach this goal, 16 genes were removed from the C. tropicalis genome. After reinsertion of a suitable gene encoding a cytochrome P450 enzyme, the new C. tropicalis strain produced over 160 g/L ω-hydroxytetradecanoic acid from the C14-methyl ester with less than 5% formation of the corresponding diacid.19 Through this route, other ωhydroxy fatty acids differing in chain length and degree of unsaturation degree are becoming available. These molecules represent platform chemicals that can be used to build polymers,20,21 polyol polyester precursors, lactones, and more. In this paper we report studies on biobased polymerizable surfactants derived from the conversion of C14-methyl ester to ω-hydroxytetradecanoic acid catalyzed by engineered C. tropicalis.19 One-step chemical conversion of this heterobifunctional precursor gave MA-1 and MA-2 with polymerizable acrylic and maleate moieties, respectively (Scheme 1). MA-2 is a unique maleate surfactant consisting of the nonpolar (−(CH2)13−) lipid moiety flanked at the respective chain ends by maleate and carboxylate functionalities. Surface tensions of surfactant solutions were measured. Bulk copolymerizations were conducted to determine the reactivity ratios of MA1 and MA2 with styrene. Emulsion polymerizations of styrene with different concentrations of MA1 and MA2 were performed with monitoring of monomer conversion and particle size. Batch polymerizations using MA1 and MA2 as sole surfactants gave colloidal stable latexes with high monomer conversion and particle sizes ≤160 nm. To determine the relative fractions of MA1 and MA2 that reside within particles, at particle surfaces, and in the aqueous phase of latexes, an approach was used that combined information from 1H NMR and pH titration.
EXPERIMENTAL SECTION
Materials. ω-Hydroxytetradecanoic acid (>99% pure) was produced by a whole-cell biotransformation using Candida tropicalis and purified in our lab as previously reported.19 Triethylamine (99%), acryloyl chloride (97%), maleic anhydride (99%), and hydroquinone (99%) were purchased from Sigma-Aldrich (St. Louis, MO) and were used as supplied. Styrene (99%) was purchased from Sigma-Aldrich and was purified by distillation. Potassium persulfate (99%) was purchased from Sigma-Aldrich and was purified by methanol recrystallization. All solvents were of HPLC grade and were used as received without any further purification. Synthesis of ω-Acryltetradecanoic Acid (MA1). ω-Hydroxytetradecanoic acid (5 g, 20.5 mmol) was dissolved in 100 mL of tetrahydrofuran (THF). Triethylamine (81.9 mmol, 4 equiv) was added to the solution, and acryloyl chloride (61.5 mmol, 3 equiv) was added dropwise at 5 °C. After acryloyl chloride addition, the reaction temperature was elevated slowly to room temperature, and the reaction mixture was stirred overnight. The reaction was monitored by using thin-layer chromatography. After evaporating the THF, purified product was obtained by recrystallization from n-hexane. 2.5 wt % of MA-1 hexane solution was prepared at 40 °C and stirred for 15 min. The solution was filtered before storing at −20 °C for overnight. White solid powder precipitation of purified MA-1 product was obtained with melting point 48.9 °C in 83% yield. 1H NMR (300 MHz, CDCl3, δ, ppm): 6.38 (m, 1H, CH2CH−CO−), 6.13 (m, 1H, CH2CH−CO−), 5.84 (m, 1H, CH2CH−CO−), 4.15 (t, 2H, −O−CH2−CH2−), 2.35 (m, 2H, −CH2−COOH), 1.8−1.2 (m, 22H, −O−CH2−(CH2)11−CH2−COOH). Underlining denotes the hydrogen that corresponds to the given chemical shift. 13C NMR (300 MHz, CDCl3, δ, ppm): 180.0 (−CO−OH), 166.4 (CH2CH−CO−O−), 130.5 (CH2CH−), 128.6 (CH2CH−), 64.8 (−CO−O−CH2−), 34.1 (−CH2−COOH), 29.6−24.7 (−O−CH2−(CH2)11−CH2− COOH). Underlining denotes the carbon that corresponds to the given chemical shift. Synthesis of ω-Maleate Tetradecanoic Acid (MA2). ωHydroxytetradecanoic acid (5 g, 20.5 mmol) was dissolved in 100 mL of toluene at 60 °C. Maleic anhydride (2.2 g, 22.6 mmol, 1.1 equiv) was then added to this solution. After stirring for 10 h, the reaction mixture was cooled to room temperature. White solid product with melting point 93.1 °C was precipitated from cool toluene at 10 °C. No further purification was required. The yield was 88%. 1H NMR (300 MHz, CDCl3, δ, ppm): 6.32 (d, 2H, −CO−CHCH−CO−), 4.19 (t, 2H, −CO−O−CH2−), 2.29 (t, 2H, −CH2−COOH), 1.8−1.2 (m, 22H, −O−CH2−(CH2)11−CH2−COOH). 13C NMR (300 MHz, CDCl3, δ, ppm): 180.3 (−CO−OH), 167.3 (OH−CO−CH), 166.4 (CH−CO−O−), 134.1 (OH−CO−CH), 130.3 (CH−CO− O−), 66.9 (−CO−O−CH2−), 34.0−24.6 (−O−CH2−(CH2)12− COOH). Emulsion Polymerization. The recipes for emulsion polymerizations of styrene are given in Table 1. The pH of each batch was adjusted to 8.9−9.1 before reaction. Each batch (40 g) of emulsion in a reaction flask was purged with argon before the reaction. To start the reaction, the emulsion was placed in an oil bath at 80 °C and was constantly stirred for 15 h at 180 rpm with a mechanical stirrer. A syringe was used to withdraw about 2 mL of reaction solution through a rubber septum at various times. A small amount of hydroquinone (1 wt % solution) was mixed with each drawn sample to terminate the 114
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precipitating with methanol twice. 1H NMR analyses were conducted after samples were freeze-dried. Using the Fineman−Ross method, the consumption curves of each monomer were plotted, and the molar fraction F1 of monomer 1 (styrene) in the copolymer at the beginning of the reaction was calculated from the slopes of the curves at the origin. Styrene Polymerization Conversion Measurement. About 1.5 mL of each latex solution was weighed precisely for solid content determination. Then, styrene conversion was calculated from the equation
Table 1. Recipes for Emulsion Polymerization compositions (wt, g)
system styrene
4.0
surfactant (MA1 or MA2)
0.04−0.4
aqueous phase (buffered solution, 100 nM, NaHCO3) KPS
36 0.104
notes ∼10 wt % to total 1−10 wt % to styrene − 1 mol % to styrene
polystyrene × 100% styrene in feed total solids − solids other than polystyrene = × 100% styrene in feed
conversion = polymerization reaction. After polymerizations, pH values of latex solutions were recorded with a pH meter. Nuclear Magnetic Resonance (NMR). Proton (1H) and carbon (13C) NMR spectra were recorded on a Bruker DPX300 NMR spectrometer at 300 MHz in deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO) as solvents. The chemical shifts (ppm) for 1H and 13C NMR were referenced relative to tetramethylsilane (TMS, 0.00 ppm) as the internal reference. Mass Spectrometry. Samples were dissolved in 100% methanol at a concentration of 0.1 μg/mL and directly injected into an LTQOrbitrap XL mass spectrometer (Thermo Fisher Scientific, San Jose, CA) coupled with electrospray ion source in negative mode with a flow rate at 5.0 μL/min. External calibration of mass spectra routinely produced a mass accuracy of better than 3 ppm. All mass spectra were acquired at a resolution of 60 000 within the 100−1000 Da mass range. Surface Tension of Surfactant Solutions. Surface tensions of aqueous surfactant solutions were determined by the plate method using a Krüss digital tensiometer K10 at room temperature. Surface tensions of several solutions at different concentrations were measured. The critical micelle concentrations (cmc) of surfactants were obtained from log plots of surface tension versus surfactant concentration. Copolymerization Reactivity Ratio Measurement. Copolymerization reactions were performed in bulk at 70 °C in the presence of azobis(isobutyronitrile) (AIBN) initiator. Under a N2 atmosphere, each solution was prepared with a given molar fraction f1 of monomer 1 (Table 2), and equal volumes were placed in five sealed vials. These vials were placed in an oil bath at 70 °C and stirred magnetically. After 30 min, reactions were stopped by pouring the reaction mixture into a 30 times volume excess of methanol. Precipitated polymer products were dried and weighed. Polymer conversions were maintained below 10%. The precipitates were further purified by dissolving in CHCl3 and
Particle Size and Morphology of Latex Particles. The particle sizes and distributions of the latexes were measured three times at 90° by a Coulter N4 plus particle size analyzer. The calculations of particle sizes and standard derivations were made by the size distribution processor (SDP) analysis from the instrument software. The morphologies of latex particles were studied using a Fei Tecnai Spirit transmission electron microscope (TEM) (College of Staten Island, City University of New York). Particle Number Calculation. The particle number per liter of water (Np, L−1) was calculated according to the following equation, which can be further simplified as
Np =
where C is the fractional conversion (if conversion is 50%, C = 0.50); (m/w) the monomer-to-water ratio, g/L; D the average particle diameter, nm; and ρ the polymer density (polystyrene ρ = 1.05 g/ cm3). Latex Purification. In order to remove low molecular weight species (e.g., residual monomer and salts) in the aqueous phase from the latex emulsions, they were dialyzed using tubing with a 10 000 molecular weight cutoff. Emulsions were poured into dialysis molecular porous membrane tubes and then placed in deionized water for 2 weeks, with water changes four times a day. Potentiometric Titration of Latex Solution. A VWR sympHony pH meter was used for titration. A specified amount of dry latex particles was dispersed in water, and the mixture was titrated to pH > 10. The mixture was stirred for at least 30 min to complete protonation of acid groups. The mixture was then titrated with HCl solution (0.010 M). Potentiometric titrations plots of pH as a function of HCl added volume were drawn, as was the first derivative of the curve ∂pH/∂VHCl. The amount of weak acid groups accessible to the aqueous solution, or at the latex surface, was determined by calculating the difference in added HCl between equivalence point 1 and equivalence point 2 in the potentiometric first-derivative curve.22
Table 2. Molar Fractions of Styrene in the Feed f1 and in the Copolymer F1 for Copolymerization Reactions between Surfactants and Styrenea surfactant
f1
F1
x = f1 f1[(1 − F1)/ (F1(1 − f1)2)]
y = f1[(2F1 − 1)/ F1(1 − f1)]
MA-1
0.987 0.975 0.963 0.952 0.887 0.987 0.975 0.963 0.951 0.887
0.991 0.982 0.973 0.962 0.926 0.959 0.907 0.901 0.867 0.780
55.6 27.9 18.8 15.1 4.9 262.2 158.1 75.5 59.1 17.4
77.8 38.5 25.5 18.8 7.2 75.2 35.2 23.3 16.6 5.6
MA-2
6C(m/w) C(m/w) = 1.8 × 1021 × πρD3 D3
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RESULTS AND DISCUSSION
Synthesis of Polymerizable Surfactants. Two biobased polymerizable surfactants derived from ω-hydroxytetradecanoic acid were synthesized according to Scheme 1. Preparation of ωacryltetradecanoic acid (MA-1) was accomplished by a one-step reaction. Acryloyl chloride reacted directly with the terminal hydroxy group in the presence of the base reagent triethylamine. Recrystallization was required to purify the product. The 1 H NMR and 13C NMR spectra of MA-1 are in agreement with the product structure. Furthermore, the mass spectral MA1 peak was [M1-H]−exp = 297.2062 Da. This is in excellent agreement with MA1’s elemental composition (C17O4H29), [M1-H]−thero = 297.2060 Da. Yields over multiple synthetic preparations ranged from 80% to 83%.
a f1: molar ratio of feeding styrene to total feeding monomers, calculated by feeding ratio. F1: molar ratio of polymerized styrene to total polymer, measuring by 1H NMR. x: the x in the consumption curves, according to the Fineman−Ross method. y: the y in the consumption curves, according to the Fineman−Ross method.
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Similar to MA-1, preparation of ω-maleate tetradecanoic acid (MA-2) from ω-hydroxytetradecanoic acid was accomplished by a one-step reaction. Maleic anhydride readily reacts with the ω-hydroxy group of ω-hydroxytetradecanoic acid without catalyst in toluene at 60 °C. Since the substrates remain soluble upon cooling to room temperature but MA-2 precipitates, the product was easily separated from other substances in the reaction solution. Furthermore, no further purification of this product was required. The 1H NMR and 13C NMR spectra are in agreement with the product structure. Furthermore, the mass spectral MA1 peak was [M2-H]−exp = 341.1958 Da. This is in excellent agreement with theoretical data from the elemental composition of (C18O6H29), [M2H]−thero = 341.1959 Da. Yields over multiple synthetic preparations ranged from 90% to 95%. Compared to the synthesis of MA-1 where the acylating agent used (acryloyl chloride) is highly corrosive and a base catalyst was required, MA-2 was prepared under relatively milder conditions without the need for catalyst or a recrystallization step. Surfactants Surface Tension Measurements. The results of surfactants’ surface tension measurements are displayed in Figure 1. Both curves showed significant decrease
distribution, the reactivity ratios of styrene with each surfactant were determined using the Fineman−Ross method. From the plot of versus y = f1[(2F1 − 1)/(F1(1 − f1))] versus x = f1 f1[(1 − F1)/(F1(1 − f1)2)], the reactivity ratios r1 and r2 were then calculated. The compositions of copolymers from copolymerization ratios with different feed ratios are shown in Table 2. For MA-1 and styrene, the reactivity ratios are r1(Styrene) = 1.4, r2(MA1) = 0.87, and r1(Styrene)r2(MA1) = 1.2 (Figure 2a). The r1 and r2 values were both close to 1, with r1 being slightly larger, indicating the reactivity of each monomer to undergo homopolymerization and cross-polymerization is similar. In contrast, for the copolymerization of MA-2 and styrene, reactivity ratios were r1(Styrene) = 0.3, r2(MA2) = 0.0, and r1(Styrene)r2(MA2) = 0.00 (Figure 2b), indicating both monomers exhibit a tendency to cross-propagate versus homopolymerize and yield a copolymer with an alternating nature. These results are consistent with the structural properties of MA-1 and MA-2 (structures are shown in Scheme 1). In MA-1, the polymerizable group is an acrylic group which is expected to have comparable reactivity to styrene. However, the polymerizable group of MA-2 is a maleic ester, with a highly electronwithdrawing structure that makes homopolymerization difficult.23 Effect of Surfactant Concentration. Emulsion polymerizations were conducted with varying amounts of MA-1 and MA-2 (1−10 wt % with respect to styrene). The pH of each batch was adjusted to 8.9 to 9.1 before reaction. The pH values were measured after the polymerization reaction. As shown in Table 3, all postpolymerization pH values were in the range 8.7−9.2, indicating the pH values remained stable during polymerizations. All emulsion polymerizations run with either MA-1 or MA-2 produced stable latexes with particle sizes ranging from approximately 50 to 150 nm. For MA-1, the particle size ranged from 52 to 155 nm when the surfactant loading was varied from 10 to 1 wt % with respect to styrene. For MA-2, the particle size ranged from 82 to 147 nm when the surfactant loading was varied from 10 to 1 wt % with respect to styrene. Coagulum was minimal (
