Macromolecules
Article
group), yet the relative placements of the functional groups
vary (i.e., ortho vs para position). Synthetic conditions were
developed to prepare thiol−ene networks with high yield and
conversion. The thermal and mechanical behaviors of the
thiol−ene networks were investigated. This work develops
fundamental relationships between the molecular structure of
the phenolic acids and the physical properties of the resulting
networks. Gaining such knowledge is an important first step
toward the widespread implementation of biobased phenolic
acids in thiol−ene polymer applications.
to the flask (the molar ratio of K CO to phenolic acid was 2.20
2
3
to 1.00). After 3 min of stirring, allyl bromide (19.3 g, 159
mmol) was added dropwise with a syringe (the molar ratio of
allyl bromide to the phenolic acid was 2.20 to 1.00). The
solution was stirred at room temperature for 48 h. Distilled
water (340 mL) was added and the product was isolated by
extraction with ethyl acetate (3x), washing with saturated brine,
drying over MgSO , and concentration in vacuo, followed by
4
drying in a vacuum oven at 50 °C, until the NMR peaks
associated with DMF (7.96 ppm, 2.94 ppm, 2.78 ppm) were
not observed. The yield was 84% for allyl (2-allyloxy)benzoate
and 88% for allyl (4-allyloxy)benzoate.
EXPERIMENTAL DETAILS
■
Allyl (2-Allyloxy)benzoate (Referred to as “Allylated SA” in
Materials. All chemicals were purchased from Sigma-
Aldrich unless otherwise noted below. The phenolic acids
used in this study were also purchased from Sigma-Aldrich,
though they were chosen for their prevalence in various plant
sources. All reagents were used as received without further
purification unless noted. Two phenolic acids were employed in
this study: salicylic acid (SA, ≥ 99%, FG/Halal/Kosher) and 4-
hydroxybenzoic acid (4HBA, 99%, ReagentPlus). The chemical
structures of both phenolic acids are shown in Figure 1.
1
This Manuscript). H NMR (400 MHz, DMSO-d ): δ 7.65
6
(
dd, J = 7.7, 1.8 Hz, 1H), 7.50 (ddd, J = 8.5, 7.4, 1.8 Hz, 1H),
7
1
5
.12 (dd, J = 8.5, 0.6 Hz, 1H), 7.00 (ddd, J = 7.7, 7.4, 0.6 Hz,
H), 6.04−5.93 (m, 2H), 5.44 (ddt, J = 17.3, 1.9, 1.9 Hz, 1H),
.37 (ddt, J = 17.2, 1.7, 1.7 Hz, 1H), 5.24−5.20 (m, 2H), 4.72
(
ddd, J = 5.3, 1.7, 1.3 Hz, 2H), 4.61 (ddd, J = 4.7, 1.9, 1.3 Hz,
2
1
6
1
1
6
H) ppm. 13C NMR (100 MHz; DMSO-d ): δ 165.4, 157.1,
6
33.6, 133.2, 132.7, 130.8, 120.3, 120.2, 117.7, 117.0, 113.8,
8.6, 64.9 ppm. FTIR (ATR): 3083, 3017, 2987, 2936, 2877,
728, 1648, 1601, 1582, 1489, 1450, 1424, 1411, 1377, 1360,
301, 1245, 1165, 1133, 1102, 1073, 996, 930, 854, 755, 706,
−
1
70, 654 cm .
Allyl (4-Allyloxy)benzoate (Referred to as “Allylated 4HBA”
1
in This Manuscript). H NMR (400 MHz, DMSO-d ): δ 7.93
6
(
2
d, J = 9.0 Hz, 2H), 7.07 (d, J = 9.0 Hz, 2H), 6.10−5.98 (m,
H), 5.44−5.35 (m, 2H), 5.30−5.24 (m, 2H), 4.76 (ddd, J =
Figure 1. Chemical structures of phenolic acids used in this study: (a)
salicylic acid (SA) and (b) 4-hydroxybenzoic acid (4HBA).
5
.3, 1.5, 1.5 Hz, 2H), 4.66 (ddd, J = 5.2, 1.5, 1.5 Hz, 2H) ppm.
13
C NMR (100 MHz, DMSO-d ): δ 165.0, 162.2, 133.1, 132.9,
6
1
31.3, 121.9, 118.0, 117.7, 114.7, 68.5, 64.8 ppm. FTIR (ATR):
Other chemicals used were N,N-dimethylformamide (DMF,
BDH, ≥ 99.8%, ACS reagent), potassium carbonate (K CO , ≥
3086, 3050, 2989, 2941, 2873, 1714, 1649, 1605, 1581, 1509,
1455, 1421, 1361, 1313, 1268, 1251, 1169, 1102, 1011, 995,
929, 847, 769, 697, 668, 633 cm .
2
3
−1
9
9.0%, ACS reagent), allyl bromide (97%), ethyl acetate (BDH,
≥
99.5%, ACS grade), magnesium sulfate (MgSO , BDH, ≥
Polymer Synthesis. The UV curing of thiol−ene networks
4
16
9
9.0%, anhydrous reagent grade), 2,2-dimethoxy-2-phenyl-
followed procedures similar to those reported in ref. The
allylated phenolic acid (1.00 g, 4.59 mmol) was mixed with
PETMP (1.18 g, 2.29 mmol, stoichiometry based on equal
molar functional groups) and 1 wt % of the photoinitiator
DMPA (0.0218 g, 0.0851 mmol) at room temperature in a 20
mL vial (using magnetic stirring), which was covered by
aluminum foil. The mixture was placed in the following sample
holders appropriate for each characterization experiment: (a)
between two glass slides with a 0.4 mm glass spacer for TGA,
DSC, DMA and ATR-FTIR, (b) between two NaCl windows
(32 mm diameter, 3 mm thick) with a 0.05 mm Teflon spacer
for transmission-mode FTIR, and (c) in a Teflon dogbone-
shaped mold following ASTM D638 (bar type 5, thickness 0.4
mm) for tensile testing. The sample was exposed under
continuous 365 nm UV light (4 W, Spectroline ENF-240C) for
15 min and transferred to a convection oven at 150 °C for a
specified period of time, summarized in Table 1.
acetophenone (DMPA, 99%), and pentaerythritol tetrakis(3-
mercaptopropionate) (PETMP, > 95%).
Nuclear Magnetic Resonance (NMR). The following
NMR experiments were performed on a JEOL ECA-400
instrument using deuterated dimethyl sulfoxide (Cambridge
1
Isotope Laboratories, Inc., 99.9% D) as the solvent: H NMR
13
(
400 MHz), C NMR (100 MHz), DEPT 135, COSY, HSQC,
and HMBC. Chemical shifts were referenced to the solvent
Fourier Transform Infrared Spectroscopy (FTIR). FTIR
spectra were recorded on a Thermo Scientific Nicolet 4700
spectrometer in transmission mode as well as using an
attenuated total reflection (ATR) stage (containing a
Germanium crystal). The OMNIC Series software was used
to follow selected peaks at 1.928 cm− resolution using 32
scans. FTIR spectra were collected on allylated monomers and
thiol−ene networks (prior to exposure of the sample with UV,
after 15 and 30 min of UV exposure, and after 15 min of UV
exposure followed by annealing at 150 °C for 10 min).
Monomer Synthesis. The allylation of phenolic acids was
1
Network Synthesized from Allylated SA. FTIR (ATR):
2953, 2929, 1732, 1600, 1582, 1491, 1467, 1452, 1420, 1388,
1352, 1302, 1244, 1166, 1135, 1085, 1047, 1028, 931, 757, 704,
−1
663 cm .
Network Synthesized from Allylated 4HBA. FTIR (ATR):
22
conducted following literature procedures. Phenolic acid
10.0 g, 72.4 mmol) was dissolved into 340 mL DMF in a 1000
2954, 2921, 1736, 1710, 1605, 1580, 1510, 1468, 1421, 1385,
1353, 1316, 1273, 1251, 1168, 1140, 1105, 1028, 928, 849, 770,
(
−
1
mL glass round-bottom flask equipped with a rubber septum
697, 644 cm .
and a magnetic stirring bar. The temperature was maintained at
Dynamic Mechanical Analysis (DMA). The dynamic
mechanical behavior of cured thiol−ene films (following the
0
°C using an ice bath. K CO (22.0 g, 159 mmol) was added
2 3
B
Macromolecules XXXX, XXX, XXX−XXX