Polyesters from Bio-Aromatics - American Chemical Society

Chapter 24

Downloaded by CENTRAL MICHIGAN UNIV on September 23, 2015 | http://pubs.acs.org Publication Date (Web): June 18, 2015 | doi: 10.1021/bk-2015-1192.ch024

Polyesters from Bio-Aromatics Ha T. H. Nguyen, Elizabeth R. Suda, Emma M. Bradic, Jessica A. Hvozdovich, and Stephen A. Miller* The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, 318 Leigh Hall, University of Florida, Gainesville, Florida 32611-7200, United States *E-mail: [email protected].

Ferulic acid, a naturally occurring hydroxycinnamic acid possessing antioxidant properties, is abundantly available from lignin and lignocellulose. It is the starting material for the synthesis of acetylferulic acid and acetyldihydroferulic acid monomers. These monomers, when copolymerized at various feed ratios, produce copolymers with tunable thermal and physical properties. Some copolymers exhibit thermal properties comparable to commercially available fossil fuel-based packaging plastics—particularly polyethylene terephthalate (PET) and polystyrene (PS). For example, the glass transition temperature can be tuned from 78 °C to 153 °C and increases with increasing ferulic acid content and decreasing dihydroferulic acid content. With promising properties and scalable feedstock availability, copolyesters from substituted hydroxycinnamic acids could prove to be sustainable replacements for a variety of non-renewable and non-degradable commodity plastics.

Introduction Polymer chemists have an increasing motivation to develop bio-based commodity plastics that are not built from fossil fuels and that degrade more readily in the natural environment into benign by-products. An overarching goal is to synthesize polymers from abundant and inexpensive biorenewable monomers, yielding materials with thermal and mechanical properties which mimic or excel those of commodity polymers. © 2015 American Chemical Society In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Two polymers of interest to our research group are polyethylene terephthalate (PET) and polystyrene (PS) (1), which account for about 7% (2) and 9% (3) of the global plastics market, respectively. A primary driver for this interest is that these high-volume commodity plastics have thermal properties—specifically higher glass transition temperatures—in a range that bio-based polylactic acid (PLA, Tg = 55 °C) (4) cannot match. Since the key aromatic components of PET (terephthalic acid) and PS (styrene) derive from non-renewable petroleum, utilization of aromatic monomers from biorenewable feedstocks is a logical approach. Lignin, which makes up about 30% of a typical tree, is the second most abundant organic polymer on earth and thus, appears to be the optimal source of renewable aromatics (5). It is the major by-product of the paper pulping industry, but nearly all of it is burned onsite for energy production. In addition to lignin, the bran of grains offers attractive opportunities to harvest potentially useful aromatic monomers (6). One such monomer, ferulic acid (4-hydroxy-3-methoxycinnamic acid), can be extracted from lignin or lignocellulose and is probably the most abundant hydroxycinnamic acid in the plant world (7). Besides being naturally occurring and abundantly available from non-food resources, ferulic acid also possesses antioxidant properties which protect against oxidative stress in cells, as well as the progression of age-related diseases (8). Accordingly, ferulic acid, derived from rice bran and rice bran oil, is sold in tablet form as a dietary supplement (9). Our first successful ferulic acid-based PET mimic was poly(dihydroferulic acid) (PHFA), which exhibited a glass transition temperature (Tg = 73 ˚C) quite similar to that of PET (67 °C) and a melting temperature (Tm = 234 ˚C) about 30 °C lower than that of PET (10). In pursuit of further developing ferulic acid as a building block for sustainable polymers, herein we reveal a copolymerization strategy utilizing ferulic acid (FA) and dihydroferulic acid (HFA) at various feed ratios; this allows refinement of the glass transition temperature, including a match of the Tg value for polystyrene (95-100 ˚C) en route to values exceeding 150 ˚C (11, 12). This control of the thermal properties—over a large temperature range—invites these copolymers to be polystyrene replacements, in addition to high temperature packaging applications such as pasteurized milk containment (70 °C) and other hot-fill bottling and canning processes. Moreover, the hydrolytic degradation products of the copolymers are ferulic acid and dihydroferulic acid, both of which are present in tea, coffee, whole grains, and other antioxidant rich foods. Our previous synthesis of poly(dihydroferulic acid) (PHFA) began with vanillin and extended the carbon framework to that of ferulic acid via a Perkin reaction with acetic anhydride (10). Interestingly, Rhodia-Solvay performs the reverse conversion of ferulic acid to vanillin in their manufacture of bio-based, natural vanillin, trademarked as Rhovanil® (13–15). Thus, it seemed more direct and proper to use ferulic acid (FA) as the starting material. Accordingly, the monomers acetylferulic acid (AFA) and acetyldihydroferulic acid (AHFA) were synthesized as depicted in Figure 1.

402 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 1. Synthesis of acetylferulic acid (AFA) and acetyldihydroferulic acid (AHFA) from ferulic acid (FA).

In a previous literature preparation, the acetylation of ferulic acid was accomplished with excess acetic anhydride (2.8 or 3.6 equivalents) (16) and a large excess of pyridine (15.8 or 18.6 equivalents) (17) over a long reaction time of 3 to 24 hours. The same product was successfully produced, in 90% yield, by using stoichiometric ratios (1:1:1) of ferulic acid:acetic anhydride:pyridine for a significantly shorter reaction time, about 5 minutes. The hydrogenation step to afford acetyldihydroferulic acid was optimized to employ only 0.5 mol% of Pd/C catalyst, yielding product in 94% yield.

Figure 2. Transesterification copolymerization of acetylferulic acid and acetyldihydroferulic acid to yield copoly(ferulic acid/dihydroferulic acid). 403 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

404


Table 1. Polymerization and Characterization of Copoly(Ferulic Acid/Dihydroferulic Acid) Tg b (°C)

Tm b (°C)

T50% c (°C)

Mw d (Da)

Mn d (Da)

PDI

Yield (%)

0

78

243

408

9,400

3,400

2.8

85.6

10

9

80

223

408

22,600

6,300

3.6

85.3

80

20

18

80

212

412

18,600

5,200

3.6

80.4

4

70

30

32

79

n.o.

395

8,900

3,300

2.7

75.2

5

60

40

37

87

n.o.

414

9,200

3,000

3.0

77.0

6

50

50

46

98

n.o.

417

15,600

3,600

4.3

79.9

7

40

60

59

108

n.o.

420

13,900

3,500

3.9

81.4

8

30

70

71

118

n.o.

418

20,100

3,100

6.5

83.3

9

20

80

78

125

n.o

430

13,200

2,600

5.1

85.4

10

10

90

86

140

n.o.

442

20,000

3,500

5.8

86.6

11

0

100

100

153

n.o.

459

13,000

2,800

3.9

73.5

Entry

AHFA (mol%)

AFA (mol%)

1

100

0

2

90

3

Inc. of AFA (mol%)

a

a Incorporation of acetylferulic acid in the copolymer determined by 1H NMR. b T and T g m determined using Differential Scanning Calorimetry (DSC); n.o. = not observed. c Temperature at 50% weight loss of polymer determined using thermal gravimetric analysis (TGA) under nitrogen. d Mw and Mn determined by Gel Permeation Chromatography (GPC) in hexafluoroisopropanol (HFIP) versus poly(methyl methacrylate) (PMMA) standards.

In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


A patent from 1980 reported that polyferulic acid (PFA) had a high glass transition temperature of 150 ˚C, but was rather intractable with no suitable solvents described (18). We reasoned that incorporation of more rigid ferulic acid units into a poly(dihydroferulic acid) (PHFA) polymer would augment the glass transition temperature, as well as improve the tractability and solubility—to the extent that a more random structure would result. Consequently, a series of copolymers, generalized in Figure 2, was targeted via transesterification copolymerization.

Figure 3. Glass transition temperatures observed and plotted for polydihydroferulic acid (0% AFA feed), polyferulic acid (100% AFA feed) and copolymers thereof, copoly(ferulic acid/dihydroferulic acid). Tg values for several common commodity plastics are given for comparison: polylactic acid (PLA), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), and polycarbonate (PC).

Copolymerizations of acetylferulic acid and acetyldihydroferulic acid in feed ratios every decade (every 10%) from the homopolymer of acetyldihydroferulic acid to the homopolymer of acetylferulic acid were catalyzed by 1.2 mol% of anhydrous zinc acetate (Figure 2). These copolymerizations were performed under one atmosphere of nitrogen for 5 hours and vacuum was subsequently applied for 6 hours, which liberated acetic acid during a temperature ramp from 150 to 250 ˚C using an oil bath. As revealed in Table 1, the copolymers were obtained in good yields (73 to 87%) and with acceptable molecular weights. Mw 405 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


ranged from 9,200 to 22,600 with half of the samples above Mw = 15,000. The polydispersity values (PDI) were somewhat large for well-behaved step-growth polymerizations and ranged from 2.7 to 6.5. During these melt polymerizations, the reaction viscosity seemed especially high for those with larger acetylferulic acid feed fractions—possibly explaining the broader PDI values for entries 8, 9, and 10 (Table 1). The simple magnetic stir bar employed was likely inadequate and future studies with mechanical stirring and more efficient mixing are anticipated to increase molecular weights and corral the broad PDI values.

Figure 4. Proton Nuclear Magnetic Resonance (1H NMR) spectra of copoly(dihydroferulic acid/ ferulic acid) with acetylferulic acid (AFA) feed fractions of 0%, 50%, 80%, and 100%.

Figure 3 plots the glass transition temperature values observed for copoly(ferulic acid/dihydroferulic acid) as the ferulic acid feed fraction increases from 0% to 100% with 10% increments. As predicted by the Fox equation (19), the glass transition temperatures followed a steady increase as the ferulic acid content increased, reaching a maximum with 100% ferulic acid feed fraction. Inspection of the Figure 3 plot reveals a minimal effect for the feed fraction 406 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


up to 30% of acetylferulic acid. The linear Tg relationship begins at 30% and extends to 100%. The thermal data collected confirmed the suspicion that increased unsaturation—and concomitant increased conformational rigidity—led to decreased chain flexibility and higher glass transition temperatures. It is clear from Figure 3 that this copolymer system can be tuned over a wide range of glass transition temperatures (range = 153 °C – 78 °C = 75 °C) that overlaps well with those of several common commodity plastics, including polyvinyl chloride (82 °C), polystyrene (100 °C), polymethyl methacrylate (105 °C), and polycarbonate (147 °C). Note from Table 1 that increased ferulic acid content decreases the melting temperature (Tm) from 243 °C (0% FA) to 212 °C (20% FA) but copolymers with FA contents above 20% did not display melting endotherms for various DSC trials. The homopolymers and copolymers were analyzed by 1H NMR spectroscopy (in a combination of 1,1,2,2,-tetrachloroethane-d2 and trifluoroacetic acid) in order to quantify the relative abundance of the two monomers incorporated during polymerization. Figure 4 depicts exemplary 1H NMR spectra for the polymers and copolymers with acetylferulic acid (AFA) feed fractions of 0%, 50%, 80%, and 100%. Of particular use are the peaks attributed to the methoxy hydrogens of both ferulic and dihydroferulic repeat units (3.8 and 3.9 ppm) and the methylene hydrogens of dihydroferulic repeat units (3.0 and 3.1 ppm). Relative integration provided the incorporation values reported in Table 1. Figure 5 plots the incorporation fraction of ferulic repeat units as a function of the acetylferulic acid (AFA) feed fraction. The linearity of this plot (see the dashed line) confirms that the incorporation fraction closely matches the feed fraction for these copolymerizations and thereby suggests a random copolymerization of the two comonomers.

Figure 5. The incorporated ferulic content of the copolymers plotted as a function of the acetylferulic acid (AFA) feed fraction. The near linearity (shown by the dashed line) suggests a random copolymerization of acetylferulic acid (AFA) and acetyldihydroferulic acid (AHFA). 407 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Conclusion The naturally abundant phytochemical ferulic acid has been converted into acetylferulic acid and acetyldihydroferulic acid. The transesterification copolymerization of these monomers yields a novel series of biorenewable thermoplastics with tunable glass transition temperatures. Polydihydroferulic acid exhibited a Tg of 78 °C, polyferulic acid exhibited a Tg of 153 °C, and the copolymers exhibited intermediate Tg values. This broad glass transition temperature range suggests that these polymers might be sustainable mimics for important commodity polymers. For example, the copoly(ferulic acid/dihydroferulic acid) prepared with a 50:50 ratio of monomers has a Tg of 98 °C, an excellent match for that of polystyrene (PS). The prepared copolymers were of moderate molecular weight and somewhat broad polydispersity—likely because of the high melt viscosity encountered during polymerization. Future work will employ a polymerization autoclave with mechanical stirring and improved mixing during polymerization. Additionally, mechanical testing and hydrolytic degradation studies will be performed. Ferulic acid is a noteworthy biorenewable feedstock because it is abundantly present in the non-edible components of the world’s two largest crops, sugarcane and corn. Moreover, polyesters made from ferulic acid and dihydroferulic acid should readily undergo hydrolytic degradation yielding not just benign, but beneficial by-products which possess antioxidant properties important to human health.

Acknowledgments This research was supported by the National Science Foundation (CHE1305794), U.S. Bioplastics, Inc. (usbioplastics.com), and the Florida High Tech Corridor Council.

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409 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Polyesters from Bio-Aromatics - American Chemical Society

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