Isocyanate-free route to poly(carbohydrate-urethane) thermosets and 100% bio-based coatings derived from glycerol feedstock

Article abstract of DOI:10.1021/acs.macromol.6b01485

Glycerol serves as the exclusive bio feedstock for the preparation of high purity sorbitol tricarbonate (STC) as new intermediate for poly(carbohydrate-urethane) thermosets and 100% bio-based non-isocyanate polyhydroxyurethane (NIPU) coatings. In this process, glycerol-based acrolein is dimerized, carbonated, and oxidized, thus producing the highly reactive diepoxy functional ethylene carbonate (DOC), which by facile chemical CO₂ fixation yields high purity STC. Opposite to most state-of-the-art multifunctional five-membered cyclic carbonates and regardless of the feedstock used for its manufacture, STC enables amine curing at ambient temperature even in the absence of catalysts. According to FT-IR and NMR spectroscopic analyses of the amine/carbonate reaction kinetics, the internal cyclic carbonate group is 3 times more reactive with respect to the two terminal carbonate groups. This is attributed to the electron-withdrawing effect of terminal cyclic carbonates. Curing STC with a blend of bio-based flexible and rigid diamines such as dimer fatty acid-based diamine (Priamine 1074) and isophorone diamine affords poly(carbohydrate-urethane) thermosets and NIPU coatings exhibiting substantially improved thermal and mechanical properties.

Full text of DOI:10.1021/acs.macromol.6b01485

Article
pubs.acs.org/Macromolecules
Isocyanate-Free Route to Poly(carbohydrate−urethane) Thermosets
and 100% Bio-Based Coatings Derived from Glycerol Feedstock
Stanislaus Schmidt,†,‡ Benjamin S. Ritter,† D. Kratzert,§ Bernd Bruchmann,‡,∥ and Rolf Mulhaupt*,†,‡
̈
†Institute for Macromolecular Chemistry, Stefan-Meier Strasse 31, D-79104 Freiburg, Germany
‡Freiburg Materials Research Center (FMF), Stefan-Meier Strasse 21, D-79104 Freiburg, Germany
§Institute for Inorganic and Analytical Chemistry, Albertstrasse 21, D-79104 Freiburg, Germany
∥JONAS - Joint Research Network on Advanced Materials and Systems, BASF SE, Carl-Bosch-Strasse 38, D-67056 Ludwighafen,
Germany
S
* Supporting Information
ABSTRACT: Glycerol serves as the exclusive bio feedstock for
the preparation of high purity sorbitol tricarbonate (STC) as new
intermediate for poly(carbohydrate−urethane) thermosets and
100% bio-based non-isocyanate polyhydroxyurethane (NIPU)
coatings. In this process, glycerol-based acrolein is dimerized,
carbonated, and oxidized, thus producing the highly reactive
diepoxy functional ethylene carbonate (DOC), which by facile
chemical CO2 fixation yields high purity STC. Opposite to most
state-of-the-art multifunctional five-membered cyclic carbonates
and regardless of the feedstock used for its manufacture, STC
enables amine curing at ambient temperature even in the absence
of catalysts. According to FT-IR and NMR spectroscopic analyses
of the amine/carbonate reaction kinetics, the internal cyclic
carbonate group is 3 times more reactive with respect to the two terminal carbonate groups. This is attributed to the electron-
withdrawing effect of terminal cyclic carbonates. Curing STC with a blend of bio-based flexible and rigid diamines such as dimer
fatty acid-based diamine (Priamine 1074) and isophorone diamine affords poly(carbohydrate−urethane) thermosets and NIPU
coatings exhibiting substantially improved thermal and mechanical properties.
polyhydroxyurethanes (NIPU).16 The formation of NIPU from
glycidyl ethers is illustrated in Scheme 1. Unlike conventional
polyurethane production, NIPU synthesis does not require toxic
INTRODUCTION
■
Today most poly(carbohydrate−urethane) and poly(carbohydrate−
ester−urethane) polymers are produced by curing saccharide-based
polyols with isocyanates.1−4 For industrial use it is highly attrac-
intermediates such as phosgene and highly moisture sensitive
isocyanates. However, in sharp contrast to isocyanates, most
tive to exploit bioresources such as glycerol, which is an abundant
side product of biodiesel production.5−10 The two largest
cyclic carbonates are considerably less reactive and require amine
cure at elevated temperatures and catalyst addition.17−19
Fleischer et al. reported on bio-based NIPUs derived from
glycidyl ethers of pentaerythritol (PEC) and trimethylolpropane
(TMC).8 An overview on cyclic carbonate and NIPU syntheses
including the use of glycerol and saccharide feedstocks was given
suppliers of glycerol in the world are Germany and Malaysia.11
Currently, glycerol is employed as bio-based feedstock for
manufacturing of polyglycerol, acrylic acid, acrolein, epichlor-
ohydrin, and glycidol.12 Acrolein, available by dehydration of
glycerol and other routes, is produced worldwide in 350 kt/a
scale.13 Besides acrylic acid production, one of the most
important industrial application for acrolein is its con-
version into DL-methionine, an amino acid which is essential
for animal growth and employed as animal food additive in meat
production.14 Applications of glycerol in polymer synthesis span
from the production of polyglycerol to epichlorohydrin, e.g., in
Solvay’s Epicerol process, which is industrially exploited as
intermediate for producing epoxy resins.15 Hence, bio-based
epichlorohydrin is employed to convert biopolyols such as glyc-
erol and saccharides into bio-based epoxy resins, which on carbo-
natization by chemical fixation of carbon dioxide affords multi-
functional cyclic carbonates as intermediates for non-isocyanate
by Blattmann and Mulhaupt.20 Sorbitol tricarbonate is known
̈
since the 1960s, but it has not yet been exploited as intermediate
in NIPU synthesis.21 Most likely, this is attributed to difficulties
encountered in the direct carbonatization of saccharides and
its tedious purification.22 Such synthetic problems are also
well-known for the esterification of saccharides such as sorbi-
tol, yielding complex reaction mixtures.22−27 Moreover, unlike
saccharide−glycidyl ether-based cyclic carbonates, which are
Received: July 12, 2016
Revised: September 20, 2016
© XXXX American Chemical Society
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Scheme 1. Synthesis of Conventional Cyclic Carbonates Derived from Glycidyl Ethers by Chemical Fixation of CO2 and Its
Conversion with Amines to NIPU
Scheme 2. Preparation of Sorbitol Tricarbonate (STC) Derived from Glycerol Feedstock
In general, the kinetic studies were made by dissolving the carbonates
in DMSO (2 mol/L). The reaction was started by addition of the
equimolar amount of the amine species. The zero point was set by
mixing the carbonate and the equimolar amount of the alcohol related to
the used amine (e.g., ethylene glycol for ethanolamine). The exami-
nation of the values were made by relation of the cyclic carbonate
(1800 cm−1) to the CH2 (2925 cm−1) signals.
NMR Spectroscopy. NMR spectroscopy measurements were made
with the Avance II spectrometer from Bruker. The spectra were
recorded with a frequency of 300 MHz by using different deuterated
solvents like acetone-d6, CDCl3, and DMSO-d6.
In general, the kinetic studies in the NMR tube were done with the
conversion of STC with octylamine. Therefore, STC (0.1 g, 3.8 mmol)
was dissolving in DMSO-d6. Shortly before the measurement was
started, octylamine (0.05 g, 3.8 mmol) was added into the NMR tube.
For the duration of 9 h, spectroscopic analyses performed every 30 min
for 1H NMR and every 15 min for 13C NMR spectra are included in the
Supporting Information.
Evaluation of the conversion rate of external and internal cyclic
carbonate groups was determined by the conversion of STC and
dodecylamine. Therefore, STC (0.10 g, 3.8 mmol) was dissolved in
DMSO-d6 (1 mL), and dodecylamine (0.71 g, 3.8 mmol) was also
dissolved in DMSO-d6 (1 mL). Both solutions were added into one flask
and stirred for 24 h at room temperature. Hence, the product was
analyzed by one and two-dimensional NMR experiment. All spectra are
included in the Supporting Information.
Thermogravimetric Analysis. Analysis was made with a thermoscale
STA 409 (Netzsch). The degradation and the mass loss were detected in
a temperature range between 50 and 650 °C under an air atmosphere
with a heating rate of 10 K/min.
Differential Scanning Calorimetry. The thermal properties were
obtained by using a DSC-1 PerkinElmer and also by Netzsch DSC 204
F1 Phoenix (heating rate 10 or 20 K/min).
Stress−Strain Characterization. Mechanical characterization of
casted dog bones was studied by using a Zwick Z005 (Ulm, Germany,
ISO527A1, 5 mm/min). The measurement occurred at room temper-
ature. Young’s modulus and elongation at break were determined by
taking the statistical average of at least five test samples.
cis/trans-1,2-Divinylethylene Glycol (DVG, 2). Acrolein (30 mL,
90%, 0.40 mol) was added to a mixture of THF (900 mL) and saturated
aqueous ammonium chloride (540 mL). Zinc (52.9 g, 0.808 mol) was
added, and the mixture was allowed to stir for 16 h at room temperature.
viscous liquids and easy to handle in NIPU synthesis, the pure
sorbitol tricarbonate is a crystalline solid, melts at high tem-
perature of 230 °C close to its decomposition, has low solubility
in nonpolar media, and is highly immiscible with most NIPU
components.21,28,29
Here, we present synthetic strategies for preparing high purity
sorbitol tricarbonate and for tailoring novel poly(carbohydrate−
urethane) thermosets derived from glycerol feedstock. As
illustrated in Scheme 2, acrolein (1), which is industrially
available by the dehydration of glycerol, was dimerized by means
of the zinc-catalyzed McMurry coupling reaction to produce
divinyl ethylene glycol (DVG, 2). Then DVG was converted into
divinyl ethylene carbonate (DVC, 3) by transesterfication with
diethyl carbonate following procedures reported by Trost
et al.30,31 Finally, the two terminal carbonate groups were
introduced via epoxy-mediated CO2 conversion with the highly
reactive diepoxy carbonate (DOC, 4,5-diepoxy-2-yl-1,3-dioxo-
lan-2-on, 4), readily available by DVC oxidation. Alternatively,
STC is also available by conversion of sorbitol with diphenyl
carbonate (see Supporting Information). Sorbitol tricarbonate
(STC, 5) and its corresponding liquid prepolymers were cured
with bio-based diamines to produce poly(carbohydrate−
urethane) polymers tailored for enabling cure at ambient
temperatures and for meeting the demands of coating appli-
cations.
EXPERIMENTAL SECTION
■
Materials. Acrolein 90%, formic acid 95%, mCPBA 70−77%,
1-dodecylamine 99%, and dichloromethane 99% were obtained from
Sigma-Aldrich. DMSO 99% and peracetic acid 38−40% were obtained
from Merck, MMPP 80% from SAF, THF from Carl Roth, TBAB 99%
from AlfaAesar, and 2-methyltetrahydrofuran 99% from AppliChem.
1-Octylamine 99% was obtained from Fluka. Carbon dioxide (N45) was
obtained from Air Liquid. All deuterated solvents were purchased by
Deutero GmbH. Priamine1074 was obtained from Croda GmbH.
Characterization. ATR-FTIR Spectroscopy. Fourier transform
infrared analysis was made with Vector 22 from Bruker. The attenuated
total reflection measurements were recorded by using 30 scans between
4000 and 800 cm−1.
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Afterward, the mixture was filtered, the phases were separated, and
the aqueous phase was extracted with CH2Cl2 (3 × 250 mL). The
combined organic phases were dried over magnesium sulfate, and the
solvent was removed in a vacuum. The product, a colorless oil (15.5 g,
70%), was purified by vacuum distillation (80 °C and 3 mbar). 1H NMR
(300 MHz, CDCl3): δ = 2.5 (br s, 2H, −OH), 3.9−3.95 (dd, 4.77 Hz,
1 Hz, 1H, 3-CH), 4.10−4.17 (dd, 4.77, 1 Hz, 1H, 4-CH), 5.15−5.35
(m, 4H, 1/6-CH2), 5.75−5.88 (m, 2H, 2/5-CH2) ppm.
recrystallized in acetone. The product was isolated as a colorless,
crystalline powder (70%).
General Procedure of NIPU Preparation. Sorbitol tricarbonate
was mixed with the respective amine at room temperature for 1 min.
This mixture was homogenized with a rolling mill at room temperature
2·(90 μm/30 μm), 2·(15 μm/5 μm). The process destroys the crys-
talline structure of STC and accelerates the forming of STC/diamine
prepolymers. The homogeneous resign was degassed at 60 °C for 5 min
and casted into a mold. Cure was carried out at 80 °C for 16 h. The
conversion of the reaction progress was determined by ATR-FTIR
spectroscopy using the appearance of cyclic carbonate groups at
1800 cm−1 and the urethane bond at 1700 cm−1.
cis/trans-1,2-Divinylethylene Carbonate (DVC, 3). cis/trans-1,
2-Divinylethylene glycol (DVG, 2) (15.00 g, 131.4 mmol) was dissolved
in diethyl carbonate (19.4 g, 19.9 mL, 164 mmol, 1.25 equiv), and
potassium carbonate (0.054 g, 0.395 mmol) was additionally added.
The reaction mixture was heated to 115 °C as long as ethanol could be
distilled off. As no ethanol development could be detected anymore, the
pressure was reduced to 1 mbar and the product (16.0 g, 0.114 mol,
RESULTS AND DISCUSSION
■
From Glycerol to Sorbitol Tricarbonate (STC). Following
Trost’s synthetic procedure (see Scheme 2), acrolein (1) was
dimerized to produce cis/trans-1,2-divinyl ethylene glycol (DVG, 2),
which was converted into the corresponding 1,2-divinyl carbo-
nate (DVC, 3) by transesterification with diethyl carbonate.30,31
Owing to the electron-withdrawing effect of the cyclic carbo-
nate group, the reactivity of the double bonds was rather low with
respect to oxidation reactions. Hence, magnesium monoperox-
yphthalate (MMPP) and Na2WO4/H2O2 failed to produce
DOC. Albeit both peracetic acid and the more reactive performic
acid afforded DVC double-bond conversion at 45 °C, no DOC
1
95%) was gained at 105 °C as colorless oil. H NMR (300 MHz,
CDCl3): ν = 4.63−4.65 (dd, 2.95 Hz, 6.61 Hz, 1H, 1-CH), 5.07−5.13
(dd, 2.95 Hz, 6.61 Hz, 1H, 1′-CH), 5.33−5.49 (m, 4H, 3/3′-CH2),
5.64−5.90 (m, 2H, 2/2′-CH) ppm.
Dimethyldioxirane (DMDO). In a two-neck bottle flask water
(175 mL), NaHCO3 (29.0 g, 345 mmol), and acetone (96 mL, 75.8 g,
1.31 mol) were stirred and cooled with an ice bath to 5 °C. To this
mixture oxone (60 g, 195 mmol) was added in different portions every
3 min. After the last addition the mixture was stirred for an additional
3 min, the ice path was removed, and the pressure was reduced to
100 mbar. The boiling gas was condensed at −78 °C. The resulting
yellow liquid was dried over K2CO3/molecular sieve (4 Å) and stored at
−78 °C.
1
was detected. According to H NMR data, the main products
were α,β-hydroxy esters, as is evident from the multiplet signal at
4 ppm. Obviously, the harsh reaction conditions and long
reaction times caused ring-opening of the epoxy groups to
produce esters. Furthermore, this ester formation was confirmed
by means of GC/MS analyses. For the first time vinyl group
conversion >90%, and successful epoxidation was achieved when
using dimethyldioxirane (DMDO) which represents a green
non-nucleophilic oxidizing agent, prepared by oxidation of
acetone with potassium peroxymonosulfate (Oxone).32,33 DVC
was added to the solution of DMDO and acetone and stirred at
room temperature. Even at long reaction time of 48 h, the
1H NMR spectroscopic analysis did not reveal any side-products
in DOC formation. In lab synthesis, DMDO was successfully
substituted by m-chlorobenzoic peracid (mCPBA) which is
well-known as highly reactive epoxidation reagent in organic
synthesis.34 Typically, DVC was dissolved in CH2Cl2 and cooled
to 0 °C. After adding mCPBA, the reaction mixture was allowed
to warm up to 60 °C. The reaction progress was observed by
1H NMR spectroscopy (see Figure 1). The signals at 5.33−
5.49 ppm (m, 4H, 2/2′-CH2), 5.64−5.90 ppm (m, 2H, 2/2′-CH)
correspond to the vinyl groups. With increasing reaction time the
intensity of the double bond signals decreased, whereas the
signals at 2.9−3.1 ppm (m, 4H, a,a′-CH2), 3.3−3.6 ppm (m, 2H,
b,b′-C−H), corresponding to the epoxy groups, increased.
Owing to the absence of stereoselectivity, the signals at 4.7−
4.8 ppm (m, 1H, c,c′-C−H) corresponding to the hydrogen
atoms allocated next to the epoxide and cyclic carbonate groups
were broadened. There was no indication for side-reactions.
While a reaction time of 1 day at 60 °C gave 50% double-bond
conversion, full conversion was observed after 3 days. DOC was
formed in essentially quantitative yields as verified by 1H NMR
spectroscopy.
The concentration of DMDO in the mixture was identified by
oxidation of triphenylphosphine (50.2 mg, 0.19 mmol) with 1 mL of
DMDO−acetone solution and calculated afterward with the 1H NMR
spectroscopy. A concentration of 0.08 mol/L 0.09% of DMDO could be
determined.
General Procedure for All Epoxidation Reactions. DVC was
dissolved and cooled to 0 °C. To this mixture the oxidant was slowly
added in three portions. The crude mixture was first warmed at ambient
temperature and then heated to the necessary reaction temperature. The
progress was observed by 1H NMR spectroscopy.
4,5-Diepoxy-2-yl-1,3-dioxolan-2-one (DOC, 4). cis/trans-1,2-Di-
vinylethylene carbonate (12 g, 0.09 mol) was dissolved in CH2Cl2
(120 mL), and the mixture was cooled to 0 °C. mCPBA (70−77%,
2.2 equiv, 0.42 mol, 45 g) was added to the mixture in different portions.
The mixture was allowed to stir for 5 min at 0 °C. Afterward, the reaction
was warmed slowly to 60 °C. The epoxidation process was finished after
3 days reflux at 60 °C.
The precipitated 3-chlorobenzoic acid (mCBA) was removed by
filtration, and the mother lye was mixed with isohexane (30 mL).
Dichloromethane was removed under vacuum, and the remaining
mixture of isohexane was stirred for 15 min at 40 °C. The residue was
washed over 30 times with fresh isohexane. The product, colorless oil
(13.4 g, 0.77 mol, 92%) was gained after removal of residual isohexane in
vacuum. 1H NMR (300 MHz, CDCl3): ν = 2.9−3.1 (m, 4H, 1,1′C-H2),
3.3−3.6 (m, 2H, 2,2′ C-H), 4.7−4.8 (m, 1H, 3,3′ C-H) ppm.
Sorbitol Tricarbonate (STC, 5). General Procedure. In an
autoclave, the bisepoxide 4,5-divinyl-2-yl-1,3-dioxolan-2-one (70%)
was dissolved and treated with a catalytical amount of 0.5−1 wt %
TBAB. The autoclave was fluted with CO2 to 40 bar, and the mixture was
allowed to stir at 80 °C. Afterward, the mixture was cooled down to
room temperature The precipitated powder was washed with water and
cold acetone. The dried product could be isolated as a slightly beige
1
powder (50%). H NMR (300 MHz, acetone-d6): ν = 4.49 (dd, 2H,
C−H2), 4.53 (t, 2H, C−H2), 5.1 (m, 2H, C−H) ppm.13C NMR
(300 MHz, acetone-d6): ν = 65 (C−O), 74 (C−O), 76 (CO), 154 ppm.
Sorbitol Tricarbonate Made by Transesterification of Sorbitol with
Diphenyl Carbonate. D-Sorbitol (50.00 g, 0.274 mol), diphenyl car-
bonate (3.3 equiv, 193.69 g, 0.904 mol), DMSO (70 mL, 1.1 mol/L),
and a catalytic amount of K2CO3 (1 mol %, 150 mg) were heated to
120 °C at 30 mbar. During the conversion DMSO and phenol were
distilled off very slowly. The residue brown solid product was
The yellowish and highly viscous DOC was purified by column
chromatography using a mixture of isohexane and ethyl acetate
(60:40) and characterized by means of GC/MS and 1H NMR to
further confirm the successful DOC formation. Since DOC was
exceptionally reactive and highly sensitive to nucleophiles, DOC
did not tolerate the addition of bases used in conventional
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As shown in Scheme 2, DOC was converted into STC by
catalyzed chemical fixation of CO2 in the presence of 0.5−1 wt %
tetrabutylammonium bromide (TBAB) as catalyst and 40 bar of
CO2 pressure. In green solvents like 2-methyltetrahydrofuran,
THF, and DMSO, STC was obtained within 3 days at 60 and 80 °C
in quantitative yields. On increasing the temperature above 100 °C,
no STC was formed. Using other solvents and K2CO3 as catalyst
at temperatures above 80 °C failed to produce STC owing to side
reactions. The gained products did not contain any epoxide
groups which led to the assumption that the side reaction
was carried out by the opening of the epoxide rings. Further
characterization on byproduct formation was not investigated.
In THF or 2-methyltetrahydrofuran pure STC precipitated from
the reaction mixture and was readily recovered by filtration.
1
The high purity of STC was confirmed by means of H NMR
spectroscopy (see Figure 2). STC contains three cyclic carbonate
1
rings. By means of H NMR spectroscopy it was possible to
distinguish between the internal and the two terminal cyclic
carbonate groups. The hydrogen atoms allocated the terminal
cyclic carbonates are marked as H1, H2, H3, H6, H7, H8 and
correspond to the two signals, at 4.45 ppm (dd, CH2) and 4.67 ppm
(t, CH2). This coupling is observed, since the rings have different
configurations. The hydrogen atoms of the internal cyclic carbonate
are marked with H4, H5 and correspond to a multiplet signal at
5.20 ppm. The downfield shift is attributed to the electron-
withdrawing effect of the adjacent terminal cyclic carbonate rings.
STC is a crystalline solid melting at 230 °C, at which thermal
decomposition starts. Moreover, it should be noted that STC
is hardly soluble in most common organic solvents with the
exception of DMSO, acetone, acetonitrile, and also ionic liquids
such as 1-ethyl-3-methylimidazolium acetate and cyclic carbo-
nates such as propylene and ethylene carbonate.
Figure 1. Monitoring the epoxidation of DVC with mCPBA at 60 °C by
means of 1H NMR spectroscopy (CDCl3).
epoxidation processes to remove 3-chlorobenzoic acid (mCBA).
Hence, a base-free method for separating mCBA was developed.
Unlike mCBA, DOC was hardly soluble in isohexane. After
filtering off the precipitated 3-chlorobenzoic acid, isohexane
was added to dichloromethane, which was then distilled off to
produce a DOC slurry in isohexane. Extraction with isohexane
enabled the complete separation of 3-chlorobenzoic acid. In
principle, it is feasible to recycle mCBA and the solvents. The
optimized synthesis yielded DOC in 95% yield and purity of 97%.
According to thermal analyses by DSC and TGA, DOC decom-
posed at 195 °C.
In an alternative synthetic process (see more details in the
Supporting Information), high purity STC was also prepared by
transesterification of sorbitol with diphenyl carbonate in order to
examine the STC stereochemistry. Sorbitol contains asymmetric
carbon atoms which dictate the STC configuration. According to
NMR spectroscopy, STC derived from glycerol and sorbitol are
Figure 2. 1H NMR spectrum (DMSO-d6): sorbitol tricarbonate prepared by chemical fixation of CO2 from DOC at 60 °C and 40 bar in THF.
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Figure 3. FT-IR spectroscopic study of STC (OC: 1800 cm−1) bulk conversion with 1-octylamine to produce urethane (OC: 1700 cm−1) for 1, 5,
20, and 150 min at room temperature.
Figure 4. 13C NMR-ig (DMSO-d6): hydroxyurethanes prepared from sorbitol tricarbonate and 1-dodecylamine at room temperature in DMSO
after 24 h.
identical with respect to their chemical shifts but differ with
respect to their signal coupling. Most likely, the acrolein route is
nonstereospecific, thus affording STC with random placement of
the carbonate groups.
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Reaction of STC with Primary Amines. In general, cyclic
carbonates are considerably less reactive than isocyanates and do
not allow amine cure at ambient temperatures. This is pro-
blematic for many applications.17,35
Moreover, each of the external groups can produce two different
products (8) and (6). The ratio between C12 and both carbons C6
and C8 multiply by the statistic factor gave the reactivity of the
internal cyclic carbonate group.
Goldstein et al. investigated the kinetics of aminolysis of
erythritol dicarbonate as well as of sorbitol, mannitol, and
dulicitol tricarbonates with n-butylamine in DMSO.28 Using the
Monte Carlo method to provide the model reaction including
the neighboring group effect, they confirmed that adjacent cyclic
carbonate groups have an accelerating influence regarding the
conversion with primary amines.28
In order to examine the role of vicinal cyclic carbonates, the
amine reaction of STC was compared with that of ethylene
carbonate (EC) and styrene carbonate (SC) by monitoring the
reaction using FT-IR and 1H NMR spectroscopy. As is apparent
from Figure 3 for the bulk reaction of STC with 1-octylamine, the
IR band at 1800 cm−1 corresponds to the carbonyl group of the
cyclic carbonate rings, whereas the signal at 1700 cm−1
corresponds to the urethane group formed by the ring-opening
of cyclic carbonates with 1-octylamine.
At room temperature, STC readily reacted with 1-octylamine.
Already after 1 min the amine-mediated ring-opening accounted
for the formation of urethane groups. After 150 min, the absence
of carbonyl groups of cyclic carbonates indicated full STC con-
version. To classify the acceleration induced by the neighboring
group, model reactions of the aminolysis of STC, EC, and SC
with ethanolamine in DMSO solution (2 mol/L) at room tem-
perature were made. DMSO was used as solvent in order to
prevent precipitation.
IC12,68 ppm
1 (IC8,64 ppm + IC6,71 ppm
1
vintCO
=
=
·2 = 3.2
3
0.30 + 0.33
)
2
(1)
The calculations of initial rates of each carbonate group were not
possible by using NMR spectroscopy. Inverse-gated 13C NMR
experiments are very slow and cannot display fast conversions as
observed for STC with primary amines.
On one hand, in accordance with NMR spectroscopic obser-
vations (see Figure 2), the internal cyclic carbonate has a con-
siderably lower electron density compared to its two neighbors
and is much more reactive with respect to amine reactions.
On the other hand, the internal cyclic carbonate ring is not as
sterically hindered as might be expected. This is in accordance
with the X-ray structure, which shows that sorbitol tricarbonate
crystallizes in the orthorhombic space group P212121. From this
data it is apparent that the internal cyclic carbonate is not shielded
but readily accessible (see Figure 5).
According to the FT-IR spectroscopic analysis, all three cyclic
carbonates reacted with ethanolamine at room temperature.
However, the reactivity was vastly different, as indicated by the
ranking STC ≫ EC > SC. Unlike EC and SC, only STC gave full
carbonate conversion after 210 min. Owing to the steric
hindrance and the inductive effect of the phenyl group, SC was
less reactive. In fact, SC failed to give high conversion even after
prolonged reaction times of 800 min. Interestingly, the STC
reaction was much faster during the first 10 min and markedly
slowed down afterward. In order to examine whether the
two different carbonate groups had different reactivities, the
aminolysis with 1-dodecylamine was monitored by means of
1H NMR and 13C NMR spectroscopy. Typically, STC and
1-dodecylamine were dissolved separately in DMSO-d6. Both
solutions were joined together in a flask and stirred for 24 h at
room temperature to ensure complete conversion. This study
confirmed that both internal and terminal carbonate groups
reacted with 1-dodecylamine at room temperature.
The rate of the conversion for each carbonate group was
calculated from inverse-gated 13C NMR measurements using the
signal intensity of the urethane carbon (carbon 12, see Figure 4)
signal at 68 ppm, resulting from ring-opening of the internal
cyclic carbonate, as compared to the signal intensity of the
urethane carbonyl groups at 64 ppm (carbon 8) and 71 ppm
(carbon 6), corresponding to the reaction products relating
to the ring-opening reaction of the terminal cyclic carbonate
groups.
Figure 5. Molecular structure of STC from X-ray diffraction with
displacement parameters at 50% probability.
Poly(carbohydrate−urethane) Thermosets by STC
Amine Cure. Albeit STC, derived from sorbitol, has been
around since many years and affords poly(carbohydrate−
urethane) on amine reactions, no attempts have been reported
to exploit STC as intermediate for NIPU thermoset formation.
This is not surprising since STC is a crystalline solid which is
immiscible with most NIPU components. Moreover, the high
carbonate functionality accounts for rather short gelation and pot
life times together with the formation of densely cross-linked
NIPU, assumed to exhibit high stiffness but rather low tough-
ness. Accordingly, cure with short chain diamines such as
1,12-dodecadiamine was rather tedious owing to the extremely
narrow processing window, paralleled by short pot life and
miscibility problems. Also long-chain nonpolar fatty acid-based
diamines as Priamine 1074 are highly immiscible with STC.
However, this severe immiscibility problem was solved by forming
of STC/Priamine prepolymers in a three-roll mill. Unlike crys-
talline STC, the resulting prepolymer were liquids, fully miscible
with various amine curing agents. Taking into account that cross-
linking readily progressed at room temperature on mixing pre-
polymer with the amine curing agent, degassing and casting
This study (for more details see Supporting Information)
revealed that the internal carbonate group reacted more than
3.2 times faster with respect to the terminal ones. The conversion
rate was calculated according to eq 1. Hence, the factor of 2 is
necessary to include all different types of the formed reaction
products. Statistically, it must be taken into account that STC
has 2 times more external carbonates than internal ones.
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Unlike brittle poly(carbohydrate−urethanes), formed by
curing STC prepolymers with short chain diamines, the STC
prepolymer formation and cure with flexible Priamine 1074
afforded highly flexible and soft poly(carbohydrate−urethane)
thermosets exhibiting low glass temperature of 29 °C and low
stiffness as reflected by Young’s modulus of only 12 MPa,
paralleled by high elongation at break of 250%. The stiffness of
poly(carbohydrate−urethane) thermosets increased substan-
tially by blending highly flexible diamines together with rigid
diamines such as isophorone diamine (IPDA). As a function of
the IPDA content, the Young’s modulus increased to 630 MPa
(+5000%!) and the glass temperature to 60 °C (+200%),
accompanied by drastically lowered elongation at break.
Poly(carbohydrate−urethanes), prepared by curing STC pre-
polymer with a blend of 40 mol % IPDA and 60 mol % Priamine
1074 were attractive materials regarding coating applications.
According to the TGA analysis, all casted materials showed the
same thermal stability with the onset temperatures of T1 = 190 C
and T2 = 420 °C.
The prepolymer blend with the amine curing agent was
coated onto a glass plate using a coating knife and cured at
80 °C for 14 h. Unlike many other NIPU coatings, fully trans-
parent, colorless, and scratch resistant coatings were formed
(see Figure 7). The contact angle of water of 101° indicated that
the incorporation of flexible dimer-fatty-acid-derived segments
rendered the hydrophilic poly(carbohydrate−urethane) hydro-
phobic.
Figure 6. Poly(carbohydrate−urethane) structures obtained by curing
STC prepolymer with dimer-fatty-acid-derived diamine (Priamine
1074).
were performed within 5 min. Figure 6 displays the poly-
(carbohydrate−urethane) network structure containing highly
rigid sorbitol-based segments together with highly flexible
segments corresponding to the incorporation of the dimer-
fatty-acid-based diamine. This balance of rigid and flexible
segments enabled tailoring of poly(carbohydrate−urethane)
mechanical properties ranging from highly stiff to soft as well as
from brittle to flexible and elastomeric.
CONCLUSION
■
Sorbitol tricarbonate (STC), traditionally prepared by phosge-
nation of sorbitol, has been known since many years but requires
tedious purification. Its limited availability in conjunction with
high melting temperature over 200 °C close to its decomposition
temperature, extremely poor solubility, and immiscibility with
the majority of amine curing agents have prevented its appli-
cation in polymer industry. In our research we have successfully
established glycerol- and sorbitol-based synthetic strategies to
produce high purity STC and STC prepolymers as well as the
corresponding poly(carbohydrate−urethane) thermosets and
coatings. Poly(carbohydrate−urethanes), prepared by amine
cure, represent as new class of 100% bio-based nonisocyanate
polyhydroxyurethane materials (NIPU). In addition to our green
phosgene-free carbonatization of sorbitol by means of sorbitol
transesterification with diphenyl carbonate, for the first time
we gain STC from glycerol feedstock. Unlike sorbitol and
other saccharides, glycerol is an abundant industrial byproduct
of biodiesel production and allows to reuse industrial waste
products. In our glycerol-based STC synthesis, acrolein, indus-
trially available by oxidation of glycerol, is dimerized and
carbonated by means of transesterification with diethyl
carbonate. The resulting 1,2-divinylethylene carbonate (DVC)
Table 1 summarizes the mechanical and the thermal properties
of poly(carbohydrate−urethanes) prepared by curing STC with
Table 1. Thermal and Mechanical Properties of STC-Based
Poly(carbohydrate−urethanes) as a Function of the Priamine
1074/Isophorone Diamine (IPDA) Blend Ratio
b
Priamine 1074
[mol %]
IPDA
[mol %]
Tg
Young’s
elongation at
c
c
[°C] modulus [MPa]
break [%]
100
70
0
30
40
60
29
39
50
60
12.0 1.0
220 20
480 40
630 40
250 10
2.5 0.5
5.0 2.0
0.30 0.03
60
40
a
Processing: homogenization and STC prepolymer formation in a
b
c
three-roll mill at rt; curing: 16 h, 80 °C. DSC: 20 K/min. ISO 527A1.
blends of Priamine 1074 and isophorone diamine (IPDA) with
variable IPDA content. Poly(carbohydrate−urethanes) repre-
sent a novel generation of 100% bio-based NIPU materials.
Figure 7. Colorless and optically transparent poly(carbohydrate−urethane) coating derived from STC and Priamine 1074/IPDA (40 mol %/60 mol %).
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is then oxidized to produce the new and highly reactive
4,5-diepoxy-2-yl-1,3-dioxolan-2-one (DOC) in high yields and
with high purity. Finally, two terminal cyclic carbonate groups are
attached to the cyclic carbonate moiety by catalytic carbonatiza-
tion of DOC with CO2. The FT-IR and NMR spectroscopic
investigation of STC model reactions with monoamines such as
1-octylamine and ethanolamine reveals that full amine
conversion is achieved at ambient temperature, thus producing
poly(carbohydrate−urethanes) in essentially quantitative yields.
Opposite to the existing dogma in NIPU chemistry that terminal
carbonate groups are always more reactive with respect to
internal carbonate groups, our NMR model study clearly
confirms that the internal carbonate is 3 times more reactive as
compared to the terminal cyclic carbonate groups. On one hand,
this behavior is attributed to the electron-withdrawing sub-
stituent effect of the terminal cyclic carbonates, as verified by the
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.macromol.6b01485.
Experimental details (PDF)
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