Renewable polyamides and polyurethanes derived from limonene

Article abstract of DOI:10.1039/c2gc36557j

The addition of cysteamine hydrochloride to (R)-(+)- and (S)-(-)-limonene is described as a versatile and effective way to obtain new amine functionalized renewable monomers for polyamide and polyurethane synthesis. Thus, through different combinations, limonene, fatty acid, as well as Nylon 6,6 copolymers were prepared and their structure-thermal property relationships were studied. GPC and DSC characterization revealed that these monomers can be used to synthesize polyamides with good and adjustable thermal properties. Moreover, the synthesized diamines have successfully been transformed into dicarbamates via a phosgene-free route and their behaviour in polycondensation was studied. A number of linear renewable polyurethanes, from amorphous to semi-crystalline, were thus obtained via an isocyanate-free route, demonstrating new potential uses for these renewable resources. Also these polyurethanes were characterized in detail and their thermal properties were studied.

Full text of DOI:10.1039/c2gc36557j

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Renewable polyamides and polyurethanes derived
from limonene
Cite this: DOI: 10.1039/c2gc36557j
Maulidan Firdaus and Michael A. R. Meier*
The addition of cysteamine hydrochloride to (R)-(+)- and (S)-(−)-limonene is described as a versatile and
effective way to obtain new amine functionalized renewable monomers for polyamide and polyurethane
synthesis. Thus, through different combinations, limonene, fatty acid, as well as Nylon 6,6 copolymers
were prepared and their structure–thermal property relationships were studied. GPC and DSC characteri-
zation revealed that these monomers can be used to synthesize polyamides with good and adjustable
thermal properties. Moreover, the synthesized diamines have successfully been transformed into dicarba-
mates via a phosgene-free route and their behaviour in polycondensation was studied. A number of
linear renewable polyurethanes, from amorphous to semi-crystalline, were thus obtained via an isocya-
nate-free route, demonstrating new potential uses for these renewable resources. Also these poly-
urethanes were characterized in detail and their thermal properties were studied.
Received 2nd October 2012,
Accepted 23rd November 2012
DOI: 10.1039/c2gc36557j
www.rsc.org/greenchem
pure make limonene a very useful feedstock for the synthesis
of new important chemicals for use as renewable solvents
Introduction
Polyamides and polyurethanes are among the most important (replacing aromatic, mineral oils, etc.), fragrances, flavours,
polymeric materials, and since they display versatile proper- pharmaceuticals, and chiral intermediates, which can be
ties, they can be used for many applications, such as tubings, further modified into diverse compounds via isomerization,
footwear, industrial machinery, coatings and paints, elastic addition, epoxidation, or hydration–dehydration reactions.6,7
fibers, rigid insulations, soft flexible foam, medical devices,
Limonene has also been successfully incorporated into
and many others.1,2 Both polyamides and polyurethanes can novel polymers as a bio-based monomer or monomer precur-
be produced from petroleum resources or renewable feed- sor. For example, Sharma and Srivastava reported on the
stocks. However, due to the depletion of petroleum resources radical copolymerization of limonene with styrene,8 methyl
and their escalating prices as well as environmental impact, methacrylate,9 acrylonitrile,10 N-vinyl pyrrolidone,11 and other
more research on alternative, preferably renewable, resources co-monomers.12 Alternating polycarbonate copolymers were
as well as the design of synthetic pathways that are more sus- synthesized from (R)-limonene oxide and carbon dioxide using
tainable and environmentally benign is urgently needed.
β-diiminate zinc complexes.13 Aikins and Williams reported
Terpenes, which are readily available in large scale, have a the radiation-induced cationic polymerization of α-pinene
great potential to substitute currently used petrochemicals. oxide, β-pinene oxide, and limonene oxide resulting in low
A very common terpene, limonene, is an inexpensive natural molecular weight polyethers with high monomer conver-
compound and is considered as an economically feasible sions.14 We recently described a simple approach to obtain a
renewable feedstock. Limonene can for instance be obtained wide range of alcohol and/or ester functionalized renewable
as a by-product of the citrus fruit industry.3 As a result of monomers from (R)-(+)- and (S)-(−)-limonene, as well as
citrus fruit processing and other minor contributors, more (−)-β-pinene via thiol–ene additions.15 Polycondensation of the
than 15 million tons of citrus waste accumulate annually.4 The resulting monomers gave polyesters with molecular weights up
world production of limonene is estimated to be over 70 000 tons to 25 kDa. However, only very few examples have been reported
per year.5 In addition to being abundant, the chemical struc- on employing limonene derivatives as a monomer for the syn-
ture, having two naturally occurring double bonds and a chiral thesis of polyamides or polyurethanes. Polyamides based on
center, and its ability to be readily obtained stereochemically 1,8-diamino-p-menthane, a readily available limonene deriva-
tive, have been synthesized via interfacial condensation with
various diacid chlorides, which led to low molecular weight
polymers.16 Polyurethane–urea microcapsules utilizing limo-
Karlsruhe Institute of Technology, Institute of Organic Chemistry, Fritz-Haber-Weg 6,
nene oil were synthesized by interfacial polymerization for
textile applications.17 However, in this report, limonene was
Building 30.42, 76131 Karlsruhe, Germany. E-mail: m.a.r.meier@kit.edu;
http://www.meier-michael.com
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used as an active agent and not as a monomer. More recently, thio]acetate (3S),15 2-[2′-((1′′R)-3′′-((2′′′-hydroxyethyl)thio)-4′′-
Mülhaupt and co-workers developed a versatile route to linear methylcyclohexyl)propylthio]ethanol (9R),15 2-[2′-((1′′S)-3′′-
as well as cross-linked terpene-based non-isocyanate poly ((2′′′-hydroxyethyl)thio)-4′′-methylcyclohexylpropyl)thio]ethanol
(hydroxyurethanes) and prepolymers derived from a cyclic (9S),15 dimethyl icosanedioate (4),15 (E)-dimethyl icos-10-ene-
limonene dicarbonate monomer.18
dioate(5),33 methyl 11-(2-aminoethylthio)undecanoate (6),24
Considering the olefin functional groups of limonene, and (E)-icos-10-ene-1,20-diol (11) were prepared as previously
introducing amine functional groups with a simple and reported. (R)-(+)-Limonene (Sigma, 97%), (S)-(−)-limonene
efficient methodology would be highly desirable. In this case, (Aldrich, 96%), cysteamine hydrochloride (Fluka, >97%),
thiol–ene additions are a suitable tool since these reactions 2,2-dimethoxy-2-phenylacetophenone (DMPA, Aldrich, 99%),
often display click reaction features and are a very useful K2CO3 (Aldrich, 99%), Na2SO4 anhydrous (Acros Organics,
approach for various transformations.19,20 The high efficiency 99%), dimethyl carbonate (Aldrich, 99%), 1,5,7-triazabicyclo-
of thiol–ene additions allows the introduction of different [4.4.0]dec-5-ene (TBD, Aldrich, 98%), silica gel 60
functional groups, which can be employed to obtain poly- (0.035–0.070 mm, Aldrich), dimethyl adipate (7, Aldrich,
esters,15,21 polyurethanes,22,23 polyamides,24 and other poly- ≥99%), hexamethylenediamine (8, Aldrich, 98%,), 1,6-hexane-
mers.25,26 In a previous report, we successfully introduced an diol (10, Acros Organics, 97%), and trifluoroacetic anhydride
amine functionality to fatty acids via a thiol–ene addition.24 (TFAA, Aldrich) were used as received. All solvents (technical
Since the introduction of an amine functional group to limo- grade) were used without further purification.
nene via a thiol–ene addition was not yet reported, we wanted
General methods and instrumentation
to employ the same methodology to obtain limonene derived
diamines, which are suitable AA-type monomers for polyamide Thin layer chromatography (TLC) was performed on silica gel
synthesis.
TLC-cards (layer thickness 0.20 mm, Fluka). Permanganate
With respect to polyurethane synthesis, a green route reagent was used as a developing solution. 1H NMR and 13C
without using diisocyanates is preferable since diisocyanates NMR spectra were recorded in CDCl3 on Bruker AVANCE DPX
are usually very toxic.27 Several non-isocyanate routes for pre- spectrometers operating at 300, 400, 500, or 600 MHz. Chemi-
paring polyurethanes have been reported including the use of cal shifts (δ) are reported in parts per million relative to the
di-tert-butyltricarbonate,28 carbonylbiscaprolactam,29 cyclocar- internal standard tetramethylsilane (TMS, δ = 0.00 ppm). The
bonates,18,30 and carbamates.31,32 Along with the idea to syn- relaxation time (d1) was set to 5 seconds for the analysis of the
thesize
a diamine from limonene, the introduction of polymers. Where necessary, NMR analysis of the polymers
carbamate functional groups to this terpene is also of great were conducted after TFAA treatment; for this, necessary
interest. To the best of our knowledge, the introduction of amounts of the polymer and deuterated solvent were placed in
dicarbamate functions to limonene and the subsequent an NMR tube and TFAA was added dropwise with continuous
preparation of non-isocyanate derived polyurethanes have not shaking until a homogeneous solution was obtained. FAB
yet been reported.
(Fast-Atom-Bombardment)-mass spectra were measured with a
Thus, in this work, we synthesized new renewable diamine MAT95 of the company Finnigan. High resolution mass spectra
monomers from (R)-(+)- and (S)-(−)-limonene via a thiol–ene (HRMS) with electron impact ionization (EI) were recorded on
addition. In order to obtain renewable polyamides, we copoly- a GC-TOF. Analytical GC characterization was carried out with
merized the synthesized diamines with different renewable a Bruker 430 GC instrument equipped with a capillary column
diesters via polycondensation. Copolymers of Nylon 6,6 were FactorFourTM VF-5 ms (30 m × 0.25 mm × 0.25 μm), using
also prepared to show the possible use of these new monomers flame ionization detection. The oven temperature program
as co-monomers for common Nylons. Moreover, the trans- was: initial temperature 95 °C, hold for 1 min, ramp at 15 °C ×
formation of the resulting diamines into dicarbamates was min−1 to 220 °C, hold for 4 min, ramp at 15 °C × min−1 to
accomplished via a phosgene-free route and the obtained 300 °C, hold for 2 min. The injector transfer line temperature
AA-type monomers were employed in polycondensation with was set to 220 °C. Measurements were performed in split–split
different diols in order to prepare a number of linear renew- mode using hydrogen as the carrier gas (flow rate 30 mL
able polyurethanes via this non-isocyanate and phosgene-free min−1). GC-MS (EI) chromatograms were recorded using a
route. All polymers were characterized by DSC and GPC to Varian 431 GC instrument with a capillary column Factor-
compare the molecular weights and thermal properties of the FourTM VF-5 ms (30 m × 0.25 mm × 0.25 μm) and a Varian 210
different polyamides and polyurethanes.
ion trap mass detector. Scans were performed from 40 to 650
m/z at a rate of 1.0 scan × s−1. The oven temperature program
was: initial temperature 95 °C, hold for 1 min, ramp at 15 °C ×
min−1 to 200 °C, hold for 2 min, ramp at 15 °C × min−1 to
325 °C, hold for 5 min. The injector transfer line temperature
was set to 250 °C. Measurements were performed in split–split
Experimental part
Materials
Methyl 2-[2′-((1′′R)-3′′-((2′′′-methoxycarbonylethylthio)-4′′-methyl- mode (split ratio 50 : 1) using helium as the carrier gas (flow
cyclohexyl)propylthio]acetate (3R),15 methyl 2-[2′-((1′′S)-3′′- rate 1.0 mL × min−1). Two GPC systems, running in THF or
((2′′′-methoxycarbonylethylthio)-4′′-methylcyclohexyl)propyl)- HFIP, were used for polyurethane and polyamide analyses,
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respectively. THF system: a gel permeation chromatography Characterization of monomers
(GPC) system LC-20 A from Shimadzu equipped with a SIL-20A
auto sampler, PLgel 5 mm MIXED-D column (Polymer Labora-
tories, 300 mm × 7.5 mm, 100 Å, 1000 Å, 10 000 Å) and a
RID-10A refractive index detector in THF (flow rate 1 mL ×
min−1) at 50 °C. All determinations of molar mass were per-
formed relative to PMMA standards (Polymer Standards
Service, Mp 1100–981 000 Da). HFIP system: a Tosoh EcoSEC
HLC-8320 GPC system with hexafluoroisopropanol (HFIP,
Chempur, 99.9%) containing 0.1 wt% potassium trifluoroace-
tate (Sigma Aldrich, 98%) as the solvent. The solvent flow was
0.40 mL × min−1 at 30.0 °C. The analysis was performed on a
3-column system: PSS PFG Micro precolumn (3.0 × 0.46 cm,
10.000 Å), PSS PFG Micro (25.0 × 0.46 cm, 1000 Å) and PSS PFG
Mirco (25.0 × 0.46 cm, 100 Å). The system was calibrated with
linear poly(methyl methacrylate) standards (Polymer Standard
Service, MP 102–981 000 Da). Differential scanning calorimetry
(DSC) experiments were carried out with a DSC821e (Mettler
Toledo) calorimeter, under a nitrogen atmosphere, at a
heating rate of 10 °C × min−1 up to a temperature of 250 °C,
and using a sample mass in the range 5–7 mg. The glass tran-
sition temperature, Tg, is reported as the midpoint of the heat
a, b, c, or d
When possible, “
” is used to differentiate between
1
different diastereomers. In the H NMR spectra, “ A ” and “
”
B
differentiates between two protons connected to the same
carbon. The 13C NMR assignment is unambiguous with
respect to each nucleus, but laborious when it comes to diff-
erentiating between a particular carbon atom belonging to
different diastereomers. The two major diastereomers are
a
b
c
marked with “ ” and “ ”, and the minor ones with “ ” and
d
“
”. The assignment of 1H and 13C NMR spectra has been
done using the following number codes for each nucleus in
the diamine and dicarbamate products:
2-[2′-((1′′R)-3′′-((2′′′-Aminoethyl)thio)-4′′-methylcyclohexyl)-
capacity change. The melting temperature, Tm, is recorded as propylthio]ethanamine (1R). Slightly yellow viscous liquid
the minimum (endothermic transitions are represented down- (84%).
wards) of the endothermic melting peak.
1H NMR (600 MHz, CDCl3) δ 2.90–2.85 (H3′′a, H3′′b),
2.84–2.75 (m, –S–CH2–CH2–NH2), 2.62–2.46 (m, –S–CH2–CH2–
NH2), 2.29 (dt, J = 21.8, 13.0 Hz, HA1′a, HA1′b, H1′c, H1′d, HB1′a,
HB1′b), 2.18–2.07 (m, H3′′c, H3′′d), 1.93 (dd, J = 24.2, 8.5 Hz,
HA2′′c, HA2′′d), 1.87–1.78 (m, HA2′′a, HA2′′b), 1.78–1.69 (m, H1′′a,
Synthesis of monomers
General procedure for addition of cysteamine hydrochloride H1′′b), 1.69–1.61 (m, H4′′a, H4′′b), 1.61–1.44 (m, –NH2, HA6′′a,
to (R)-(+)-limonene or (S)-(−)-limonene. A mixture of (R)- HA6′′b, H2′a, H2′b, HA6′′c, HA6′′d), 1.44–1.30 (m, HB2′′a, HB2′′b,
(+)-limonene or (S)-(−)-limonene (1.00 g, 7.30 mmol), cystea- HA5′′a, HA5′′b, HA5′′c, HA5′′d, H2′c, H2′d), 1.30–1.17 (m, HB5′′a,
mine hydrochloride (3.0–6.0 equiv.), and DMPA (0.01–0.2 HB5′′b, HB5′′c, HB5′′d, H4′′c, H4′′d), 1.17–1.07 (m, HB6′′a, HB6′′c,
equiv.) was dissolved in a minimal amount of ethanol. The HB2′′c, HB2′′d), 1.03 (d, J = 5.6 Hz, H7′′c, H7′′d), 0.96 (d, J =
mixture was exposed to a hand-held UV-lamp (2 × 4 W, λ = 6.5 Hz, H7′′a, H7′′b), 0.95–0.90 (m, HB6′′b, HB6′′d, H3′c, H3′d),
365 nm). Samples were taken periodically to test for conversion 0.90 (d, J = 6.4 Hz, H3′a, H3′b).
of the double bonds using 1H NMR. Ethanol was then
13C NMR (101 MHz, CDCl3, δ in ppm): δ 50.84 (C3′′c, C3′′d),
removed and the mixture was dissolved in dichloromethane 50.58 (C3′′a), 50.42 (C3′′b), 42.14 (Cminor diast), 41.98 (Cminor diast),
and washed with a K2CO3 solution until the pH of the solution 41.90 (Cminor diast), 41.67 (C2′c), 41.52 (C2′d), 41.45 (–S–CH2–
became 10 (to neutralize the hydrochloride salt and obtain CH2–NH2), 41.14 (–S–CH2–CH2–NH2), 41.12 (–S–CH2–
free amines) and washed two times with water. The organic CH2–NH2), 40.77 (–S–CH2–CH2–NH2), 40.75 (–S–CH2–CH2–
layer was dried over anhydrous Na2SO4 and the solvent was NH2), 39.17 (–S–CH2–CH2–NH2), 37.90 (Cminor diast), 37.84
removed under reduced pressure. The products 1R and 1S (Cminor diast), 37.81 (Cminor diast), 37.68 (C4′′c), 37.57 (C2′a, C2′d),
were purified by column chromatography on silica gel with 37.49 (C4′′d), 37.06 (C2′′a), 36.97 (C1′a), 36.82 (C1′c), 36.81 (C1′d),
hexane–ethyl acetate (1 : 1) followed by methanol as eluents.
36.76 (C1′b), 36.73 (–S–CH2–CH2–NH2), 36.63 (Cminor diast),
Methoxycarbonylation of 1R or 1S with dimethyl carbonate. 36.53 (–S–CH2–CH2–NH2), 36.50 (–S–CH2–CH2–NH2), 36.48
Reactions were performed in a carousel reaction stationTM (C4′′a, C4′′b), 35.33 (C2′′c), 35.24 (C1′′a), 35.21 (C1′′b), 34.96
RR98072 (Radleys Discovery Technologies, UK). In a carousel (C2′′b), 34.55 (Cminor diast), 34.12 (C2′′d), 32.42 (Cminor diast),
tube was added 1 or 2 (1.27 g, 4.37 mmol), dimethyl carbonate 32.32 (Cminor diast), 31.60 (Cminor diast), 31.48 (Cminor diast), 31.43
(3.94 g, 43.7 mmol), and TBD (60.8 mg, 0.43 mmol). The (Cminor diast), 31.40 (Cminor diast), 31.16 (Cminor diast), 29.77 (C6′′a,
mixture was stirred magnetically at 80 °C. The reactions were C6′′c), 29.59 (5′′a, C5′′c), 29.44 (C5′′b, C5′′d), 29.27 (Cminor diast),
monitored by TLC until completion using hexane–ethyl acetate 27.64 (C6′′b), 27.02 (C6′′d), 20.55 (C7′′c), 20.52 (C7′′d), 20.06
(6 : 4) as an eluent. Excess dimethyl carbonate was then (C7′′a, C7′′b), 15.67 (C3′a), 15.64 (C3′c) 15.50 (C3′b), 15.43 (C3′d).
removed by vacuum evaporation. The crude reaction mixture
was purified by column chromatography with hexane–ethyl C14H30N2S2 [M + H+] calc. 291.1923, found 291.1926.
FAB-MS of C14H30N2S2 (M + H+ = 291.2). HRMS (FAB) of
acetate (6 : 4) as an eluent to obtain pure products 2R and 2S.
Purity according to GC analysis: > 99.5%.
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2-[2′-((1′′S)-3′′-((2′′′-Aminoethyl)thio)-4′′-methylcyclohexyl)- (Cminor diast), 38.05 (C4′′c), 37.97 (C4′′d), 37.87 (C2′a), 37.77 (C2′b),
propylthio]ethanamine (1S). Slightly yellow viscous liquid 37.50 (C2′′a), 37.44 (C1′a), 37.30 (C1′c), 37.20 (C1′b), 37.11 (Cminor diast
(81%).
37.03 (C1′d), 36.93 (C4′′a), 36.87 (C4′′b), 35.72 (C2′′c), 35.60
)
1H NMR (600 MHz, CDCl3, δ in ppm): δ 2.91–2.87 (H3′′a, (C2′′d), 35.44 (C1′′a), 35.37 (C1′′b), 34.96 (C2′′b), 32.97 (–S–CH2–
H3′′b), 2.86–2.75 (m, –S–CH2–CH2–NH2), 2.61–2.50 (m, –S–CH2– CH2–NH), 32.91 (–S–CH2–CH2–NH), 32.80 (–S–CH2–CH2–NH),
CH2–NH2), 2.66–2.39 (m, HA1′a, HA1′b), 2.33 (dd, J = 8.3, 3.4 Hz, 30.39 (C6′′c), 30.28 (C6′′a), 30.00 (C5′′a), 29.95 (C5′′c), 29.83
H1′c, H1′d), 2.31 (dd, J = 8.7, 2.0 Hz, HB1′a), 2.27 (dd, J = 13.1, (C5′′b), 29.75 (C5′′d), 28.07 (C6′′b), 27.48 (C6′′d), 20.88 (C7′′c,
8.8 Hz, HB1′b), 2.19–2.08 (m, H3′′c, H3′′d), 2.00–1.89 (m, HA2′′c, C7′′d), 20.43 (C7′′a, C7′′b), 16.04 (C3′c), 15.97 (C3′a, C3′b),
HA2′′d), 1.88–1.80 (m, HA2′′a, HA2′′b), 1.76 (ddd, J = 12.2, 10.3, 15.85 (C3′d).
3.6 Hz, H1′′a, H1′′b), 1.71–1.62 (m, H4′′a, H4′′b), 1.62–1.47 (m,
FAB-MS of C18H35N2S2O4 (M + H+ = 407.2). HRMS (FAB) of
HA6′′a, HA6′′b, H2′a, H2′b, HA6′′c, HA6′′d), 1.47–1.31 (m, –NH2, C18H35N2S2O4 [M + H+] calc. 407.2033, found 407.2036.
HB2′′a, HB2′′b, HA5′′a, HA5′′b, HA5′′c, HA5′′d, H2′c, H2′d), 1.31–1.17
N1,N2′′′-Dimethoxy-carbonyl-2-[2′-((1′′S)-3′′-((2′′′-aminoethylthio)-
(m, HB5′′a, HB5′′b, HB5′′c, HB5′′d, H4′′c, H4′′d), 1.17–1.01 (m, 4′′-methylcyclohexyl)propyl)thio]ethanamine (2S). Colourless
HB6′′a, HB6′′c, HB2′′c, HB2′′d), 1.04 (d, J = 6.3 Hz, H7′′c, H7′′d), viscous liquid (79%).
0.97 (d, J = 6.7 Hz, H7′′a, H7′′b), 0.95–0.88 (m, HB6′′b, HB6′′d,
H3′c, H3′d), 0.92 (d, J = 6.8 Hz, H3′a, H3′b).
1H NMR (400 MHz, CDCl3, δ in ppm): 5.39–4.93 (m, NH),
3.65 (s, OCH3), 3.42–3.23 (m, –S–CH2–CH2–NH), 2.97–2.90
13C NMR (101 MHz, CDCl3, δ in ppm) δ 50.63 (C3′′c, C3′′d), (H3′′a, H3′′b), 2.70–2.58 (m, –S–CH2–CH2–NH), 2.56 (dd, J = 7.8,
50.36 (C3′′a), 50.20 (C3′′b), 41.78 (Cminor diast), 41.70 (Cminor diast), 4.7 Hz, H1′a), 2.35 (td, J = 12.6, 8.3 Hz, H1′b), 2.41–2.28 (m,
41.47 (C2′c), 41.33 (C2′d), 41.28 (–S–CH2–CH2–NH2), 40.96 H1′c, H1′d), 2.18 (ddd, J = 21.6, 10.6, 3.2 Hz, H3′′c, H3′′d),
(–S–CH2–CH2–NH2), 40.94 (–S–CH2–CH2–NH2), 40.60 (–S–CH2– 2.02–1.94 (m, HA2′′c, HA2′′d), 1.93–1.75 (m, HA2′′a, HA2′′b, H1′′a,
CH2–NH2), 40.57 (–S–CH2–CH2–NH2), 38.98 (–S–CH2–CH2– H1′′b), 1.75–1.63 (m, H4′′a, H4′′b), 1.64–1.50 (m, HA6′′a, HA6′′b,
NH2), 37.70 (Cminor diast), 37.64 (Cminor diast), 37.61 (Cminor diast), H2′a, H2′b, HA6′′c, HA6′′d), 1.50–1.35 (m, HB2′′a, HB2′′b, HA5′′a,
37.48 (C4′′c), 37.36 (C2′a, C2′b), 37.28 (C4′′d), 36.86 (C2′′a), 36.76 HA5′′b, HA5′′c, HA5′′d, H2′c, H2′d), 1.34–1.17 (m, HB5′′a, HB5′′b,
(C1′a), 36.62 (C1′c, C1′d), 36.56 (C1′b), 36.52 (–S–CH2–CH2–NH2), HB5′′c, HB5′′d, H4′′c, H4′′d), 1.17–1.01 (m, HB6′′a, HB6′′c, HB2′′c,
36.42 (Cminor diast), 36.29 (–S–CH2–CH2–NH2, C4′′a, C4′′b), 35.14 HB2′′d), 1.05 (d, J = 6.5 Hz, H7′′c, H7′′d), 0.99 (d, J = 6.7 Hz, H7′′a,
(C2′′c, C2′′d), 35.03 (C1′′a), 35.00 (C1′′b), 34.75 (C2′′b), 34.34 H7′′b), 0.93 (d, J = 3.3 Hz, H3′a), 0.96–0.92 (m, HB6′′b, HB6′′d,
(Cminor diast), 33.91 (C2′′d), 32.22 (Cminor diast), 32.12 (Cminor diast), H3′c, H3′d), 0.92 (d, J = 3.3 Hz, H3′b).
31.40 (Cminor diast), 31.28 (Cminor diast), 31.22 (Cminor diast),
13C NMR (101 MHz, CDCl3, δ in ppm): δ 157.06 (CO),
30.96 (Cminor diast), 29.59 (C6′′a, C6′′c), 29.40 (C5′′a, C5′′c), 29.25 52.48–52.07 (group of singlets, OCH3), 51.44 (C3′′c), 51.39
(C5′′b, C5′′d), 29.08 (Cminor diast), 27.46 (C6′′b), 26.84 (C6′′d), 20.37 (C3′′d), 51.28 (C3′′a), 51.16 (C3′′b), 42.08 (C2′c), 41.96 (C2′d), 40.72
(C7′′c), 20.34 (C7′′d), 19.88 (C7′′a, C7′′b), 15.60 (C3′a), 15.48 (C3′c), (–S–CH2–CH2–NH), 40.32 (–S–CH2–CH2–NH), 39.51 (–S–CH2–
15.32 (C3′b), 15.24 (C3′d).
CH2–NH), 38.30 (Cminor diast), 38.25 (Cminor diast), 38.21 (Cminor diast),
FAB-MS of C14H30N2S2 (M + H+ = 291.3). HRMS (FAB) of 38.19 (Cminor diast), 38.10 (C4′′c), 38.02 (C4′′d), 37.92 (C2′a),
C14H30N2S2 [M + H+] calc. 291.1923, found 291.1927.
Purity according to GC analysis: > 99.5%.
37.81 (C2′b), 37.55 (C2′′a), 37.50 (C1′a), 37.35 (C1′c), 37.25 (C1′b),
37.17 (Cminor diast), 37.08 (C1′d), 36.98 (C4′′a), 36.92 (C4′′b), 35.77
N1,N2′′′-Dimethoxy-carbonyl-2-[2′-((1′′R)-3′′-((2′′′-aminoethylthio)- (C2′′c), 35.64 (C2′′d), 35.49 (C1′′a), 35.41 (C1′′b), 35.00 (C2′′b),
4′′-methylcyclohexyl)propyl)thio]ethanamine (2R). Colourless 33.05 (–S–CH2–CH2–NH), 32.99 (–S–CH2–CH2–NH), 32.89 (–S–
viscous liquid (82%).
CH2–CH2–NH), 32.86 (–S–CH2–CH2–NH), 30.47 (C6′′c), 30.33
1H NMR (500 MHz, CDCl3, δ in ppm) δ 5.38–5.03 (m, NH), (C6′′a), 30.05 (C5′′a), 29.98 (C5′′c), 29.87 (C5′′b), 29.79 (C5′′d),
3.66 (s, OCH3), 3.38–3.22 (m, –S–CH2–CH2–NH), 2.95–2.89 28.12 (C6′′b), 27.52 (C6′′d), 20.94 (C7′′c), 20.91 (C7′′d), 20.49
(H3′′a, H3′′b), 2.69–2.57 (m, –S–CH2–CH2–NH), 2.55 (dd, J = 8.5, (C7′′a), 20.45 (C7′′b), 16.07 (C3′c), 16.01 (C3′a, C3′b), 15.90 (C3′d).
3.8 Hz, H1′a), 2.33 (ddd, J = 15.3, 12.4, 8.3 Hz, H1′b), 2.38–2.29
FAB-MS of C18H35N2S2O4 (M + H+ = 407.1). HRMS (FAB) of
(m, H1′c, H1′d), 2.22–2.11 (m, H3′′c, H3′′d), 2.01–1.93 (m, HA2′′c, C18H35N2S2O4 [M + H+] calc. 407.2033, found 407.2036.
HA2′′d), 1.92–1.72 (m, HA2′′a, HA2′′b, H1′′a, H1′′b), 1.72–1.63 (m,
H4′′a, H4′′b), 1.62–1.50 (m, HA6′′a, HA6′′b, H2′a, H2′b, HA6′′c,
Synthesis of polymers
HA6′′d), 1.50–1.34 (m, HB2′′a, HB2′′b, HA5′′a, HA5′′b, HA5′′c, HA5′′d,
General procedure for synthesis of polyamides. All polycon-
H2′c, H2′d), 1.33–1.17 (m, HB5′′a, HB5′′b, HB5′′c, HB5′′d, H4′′c, densation reactions were performed in a carousel reaction
H4′′d), 1.17–1.01 (m, HB6′′a, HB6′′c, HB2′′c, HB2′′d), 1.03 (d, J = 6.4 stationTM RR98072 (Radleys Discovery Technologies, UK). The
Hz, H7′′c, H7′′d), 0.97 (d, J = 6.7 Hz, H7′′a, H7′′b), 0.92 (d, J = 3.9 corresponding monomers (see the entries in Table 1) and 0.05
Hz, H3′a), 0.96–0.91 (m, HB6′′b, HB6′′d, H3′c, H3′d), 0.90 (d, J = equiv. (relative to ester groups) of TBD were added to the car-
3.9 Hz, H3′b).
ousel tube and placed in the reactor system. The polymeri-
13C NMR (126 MHz, CDCl3, δ in ppm) δ 157.04 (CO), zation was performed at 140 °C for 24 h by applying
52.36–52.04 (group of singlets, OCH3), 51.38 (C3′′c), 51.34 continuous vacuum (10 5 mbar). When low boiling point
(C3′′d), 51.22 (C3′′a), 51.10 (C3′′b), 42.03 (C2′c), 41.91 (C2′d), 40.96 monomers were used, the polymerization was started at lower
(–S–CH2–CH2–NH), 40.67 (–S–CH2–CH2–NH), 40.28 (–S–CH2– temperatures and the temperature was gradually increased to
CH2–NH), 38.25 (Cminor diast), 38.20 (Cminor diast), 38.15 140 °C. The polymers (P1–4) were purified by precipitation of a
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Table 1 Analytical data of the synthesized polyamides
Polymer
M1 (equiv.)
M2 (equiv.)
M3 (equiv.)
Mn a (Da)
PDIa
Tm b (°C)
Tg b (°C)
Polymer type
P1
P2
P3
P4
P5
P6
P7
P8
1R (1)
1S (1)
1R (1)
1S (1)
1R (1)
1S (1)
1R (1)
1S (1)
1R (1)
1S (1)
1S (3)
1S (1)
1R (1)
1S (1)
3R (1)
3S (1)
3S (1)
3R (1)
5 (1)
5 (1)
4 (1)
4 (1)
7 (2)
7 (2)
7 (4)
7 (4)
7 (1)
7 (1)
6430
7870
7430
6740
9330
10 630
5500
5870
8960
9150
12 020
9850
9480
10 200
2.10
1.97
2.28
1.98
1.96
1.95
1.97
1.86
1.97
1.69
1.46
1.46
2.35
2.03
41.4c
41.5c
41.5c
41.5c
Limonene
57.7; 84.0d
57.6; 83.4d
102.0e
Limonene/linier diester
Limonene/Nylon 6,6
Limonene/fatty acid
101.5e
P9
8 (1)
8 (1)
8 (1)
8 (3)
6 (2)
6 (2)
150–215f
150–215f
120–170g
204–238h
40.9f
42.4f
41.9g
41.7h
5.9c
P10
P11
P12
P13
P14
7.7c
a GPC data of precipitated polymers. b DSC data recorded at 10 °C min−1 c Second heating scan. d 0.5 h annealing at 45 °C. e 1 h annealing at
.
90 °C. f 0.5 h annealing at 150 °C. g 3 h annealing at 115 °C. h 1 h annealing at 200 °C.
concentrated THF solution of the reaction products into cold
methanol. The polymers (P5–14) were purified by precipitation
of a concentrated HFIP solution of the reaction products into
cold methanol. TFAA treatment was applied prior to the NMR
analysis.
General procedure for synthesis of polyurethanes. The poly-
merizations were performed in a carousel reaction stationTM
RR98072 (Radleys Discovery Technologies, UK) at 120 °C apply-
ing continuous vacuum (10 5 mbar). TBD (0.05 equiv. relative
to carbamate groups) was used as a catalyst. Monomer 2R or
2S (150 mg, 0.37 mmol) and the corresponding diols (1.0
equiv.) were polymerized with TBD (5.00 mg, 0.04 mmol) for
16 h. The polymers (P15–20) were purified by precipitation of a
concentrated THF solution of the reaction products into cold
hexane–ethanol (9 : 1).
Scheme 1 Synthesis of monomers from (R)-(+) and (S)-(−)-limonene.
reactions were performed under UV irradiation.24 Therefore,
we performed the addition of cysteamine hydrochloride to
(R)-(+)-limonene or (S)-(−)-limonene under UV irradiation, using
a small amount of ethanol as a solvent and DMPA as a photo-
initiator (Scheme 1).
Results and discussion
Synthesis of monomers
First, the study focused on optimizing the reaction con-
ditions, in order to obtain complete conversion with small
In our previous report, the reaction of both double bonds of amounts of the thiol compound and DMPA. Initially, 3.0
equiv. of cysteamine (1.5 equiv. per double bond) and 0.01
limonene with hydroxyl or methyl ester functionalized thiols
led to suitable monomers for polycondensation.15 The reaction equiv. of DMPA were added to 1.0 equiv. of (R)-(+)-limonene.
of limonene with functional thiols is attractive since it can Based on 1H NMR analysis, after 4 h reaction, 35% of the
monoaddition product was detected and longer reaction times
provide chiral monomers in a one-step procedure directly from
a renewable feedstock. Taking this into account, we used an (24 h) did not lead to the diaddition product efficiently.
amine functionalized thiol, cysteamine hydrochloride, to Further attempts were carried out with an increased amount of
DMPA and cysteamine to selectively obtain the diaddition
perform a thiol–ene addition reaction employing (R)-(+)-limo-
nene and its (S)-enantiomer as substrates. Instead of cystea- product. When 0.02 equiv. of DMPA and 3.0 equiv. of cystea-
mine in the pure form, the hydrochloride salt form was mine were used, the monoaddition product was still observed
in rather large amounts of 21%, even after 24 hours of reaction
chosen since it is cheaper and gave better yields compared to
the pure form in previous studies.24 It was also reported that time. However, when DMPA was increased up to 0.05 equiv.
the salt form is more stable, which prevents side-reactions.26,34 and 3.0 or 6.0 equiv. of cysteamine was used, complete conver-
sion to the final product was obtained in only 4 hours at
Initially, the thiol–ene addition was performed under solvent-
free conditions at room temperature. Unfortunately, employing ambient temperature. Based on these studies, we concluded
this method did not work here due to a lack of solubility of that around 3.0 equiv. of cysteamine and 0.05 equiv. of DMPA
provided the best results of the thiol–ene addition reaction
both reactants. In a previous report, the addition of cysteamine
hydrochloride to fatty acid methyl esters worked well when the employing the least of the thiol compound and photoinitiator
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Fig. 1 (A) 13C NMR (region 53–50 ppm) and (B) 1H NMR of 1S (top) and 2S (bottom).
to obtain the diamine functional limonene in high yields and carbonate represents a good alternative, because the only by-
selectivity. Purification using column chromatography yielded product of this reaction is an alcohol that can be easily
the pure expected products in 84 and 81% for the (R)- and removed from the reaction mixture.35 For the synthesis of car-
(S)-isomer, respectively.
bamates from dialkyl carbonates and amines, the cyclic guani-
The limonenes used in this study are pure enantiomers. dine 1,5,7-triazabicyclododecene (TBD) proved to be an
The additions of molecules to the endocyclic and exocyclic alternative potent organocatalyst.36 Although several metal cat-
double bonds of limonenes generate three additional stereo- alysts such as Yb(OTf)3 hydrate,35 Zn(OAc)2,37 and Zn(OAc)2/
genic centers. Thus, eight possible diastereomers can be activated carbon38 have been successfully used in the synthesis
formed in these thiol–ene addition reactions. However, in our of carbamates, such metal catalysts may cause problems in
previous report, diaddition of thiols to the double bonds of product separation or are environmentally hazardous and
limonenes generated products consisting mainly of a mixture toxic. Thus, methoxycarbonylation of 1R and 1S with dimethyl
of four diastereomers.15 Thus, due to the high number of pos- carbonate were performed using TBD as a catalyst (Scheme 1).
1
sible diastereomers, complex H and 13C NMR spectra of the The reaction was carried out at 80 °C for 4 h under solvent-free
products were obtained. Two-dimensional NMR experiments conditions and dicarbamates 2R and 2S were obtained in good
(COSY, HSQC, and HMBC) were then necessary for structure yields of 82 and 79%, respectively. After purification using
elucidation. The complete transformation was evidenced by column chromatography, the 1H NMR spectra of 2S (Fig. 1B)
1
the H NMR spectrum of 1S, at which no exocyclic and endo- reveal the formation of the expected carbamate. This can be
cyclic double bond signals were observed and the new peaks at followed by disappearance of the signal at 1.43 ppm corres-
2.86–2.75 (Hb) and 2.61–2.50 (Ha) ppm corresponding to the ponding to the protons of the amine functionality (Hc) and
protons at the α- and β-positions to thioether appeared the two newly emerged signals at 5.39–4.93 and 3.65 ppm cor-
(Fig. 1B). The analysis of the 13C NMR spectra confirmed the responding to NH (He) and methoxy carbamate (Hf), res-
presence of diastereomers. As a representative example, the pectively. Since the reaction did not change the chiralilty of
13C NMR spectra of 1S (Fig. 1A) showed four different signals the compound, based on 13C NMR analysis, a mixture of four
of Cd (two peaks with higher intensities refer to major diastereo- diastereomers of 2S (Cd) was still observed (Fig. 1A).
mers and two peaks with lower intensities merged as one
Synthesis of polyamides
refer to minor diastereomers) in the region from 53 ppm to
50 ppm, indicating that the product mainly consists of a The diamines obtained by double addition of cysteamine
mixture of four diastereomers.
hydrochloride to (R)-(+)-limonene or (S)-(−)-limonene were sub-
Having the diamine compounds derived from (R)-(+)-limo- sequently polymerized in order to obtain renewable poly-
nene and (S)-(−)-limonene, further transformation was per- amides (Scheme 2). 1R and 1S monomers were copolymerized
formed in order to introduce carbamate functional groups. with monomers 3R and 3S (terpene-derived diester) and 4 and
With regard to a sustainable method, the preparation of dicar- 5 (fatty acid-derived diester), which are renewable diester com-
bamates from non-toxic reagents with a low price and an envir- pounds that were already prepared in our previous studies.15
onmentally benign catalyst is highly desirable. Instead of Moreover, the monomers prepared within this report were also
using toxic phosgene or chloroformates, the use of dialkyl copolymerized with 6 and/or 7 and 8 to study how the thermal
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Scheme 2 Synthesis of (R)-(+)- and (S)-(−)-limonene based polyamides and the structures of the other monomers utilized (6, 7, 8).
properties of Nylon 6,6 could be tuned when copolymerized GPC analysis showed that P5 and P6 exhibited molecular
with the renewable monomers bearing cycloaliphatic moieties. weights of about 9300 and 10 600 Da, which are higher than
The cyclic guanidine TBD was chosen as an organocatalyst, for P1–4, clearly indicating that the lower molecular weights
because it was reported to successfully catalyze the synthesis do not result from the lower reactivity of 1R and 1S, but are
of amides from esters and primary amines,39 and based on due to steric hindrance. However, P7 and P8 exhibited much
our experience for the synthesis of polyamides.24,40 All poly- lower molecular weights, i.e. 5500 and 5870 Da, respectively,
merizations were carried out without a solvent and under con- most probably because of the fact that the saturated C-20
tinuous vacuum (∼10 mbar) to efficiently remove methanol.
linear structure of 4 gives rise to very high crystallinity that
We started our investigation by copolymerizing 1R and 1S hinders the further growth of the chains in the polymerization
with 3R and 3S with 0.05 equiv. of TBD per ester group at 120, conditions. It should be noted that P5 and P6 are unsaturated
140, and 160 °C for 24 h. Based on GPC analysis, the best stereospecific polyamides, which can be further modified,
result was obtained at 140 °C. Thus, the following polymeri- e.g. by a grafting-to approach via a thiol–ene addition reaction
zations were performed using these conditions. The polyamides to synthesize tailor-made polymers.41
obtained from the combinations 1(R or S) and 3(R or S) exhi-
bited molecular weights (Mn) between 6500–8000 Da (Table 1).
These polymerizations were also investigated using
1H NMR and confirmed the expected structure of the poly-
The polymerization of 3R and 3S is catalyzed less efficiently mers. As an example for this investigation, the 1H NMR
by TBD, most likely because of the steric hindrance caused by spectra of monomers 1S and 5 and the thereof derived
the cycloaliphatic ring. Thus, rather low molecular weight poly- polymer, P6 (Fig. 2), showed almost quantitative disappearance
amides were obtained in this case. The introduction of long of the signal at 3.66 and at 1.43 ppm corresponding to the
and flexible alkyl chains was thus investigated in order to methyl ester (Ha) and the amine (Hc), respectively, which were
decrease the steric hindrance and to obtain higher molecular transformed to the backbone amide link (Hd). Since most of
weight polymers. Therefore, we copolymerized 1R and 1S with the polymers are not soluble in conventional solvents, the
4 and 5 (Table 1, entries P5–8), which are the linear diester NMR analyses of these polymers were conducted using a
monomers synthesized from the self-metathesis of methyl CDCl3/TFAA solvent mixture. While TFAA was added dropwise
10-undecenoate and its hydrogenation product, respectively. It is under continuous shaking, a homogeneous solution was
worth pointing out that these diesters are castor oil derived, obtained and the polymers were readily dissolved, because of
and thus the polymers obtained thereof are also renewable. the N-acylation of the amide groups, which leads to a break-up
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Fig. 3 DSC traces of some synthesized polyamides (see Table 1 for experimen-
tal conditions).
interactions and the polyamides prepared thereof were amor-
phous polymers that exhibited a Tg value of about 41.5 °C,
independent of the composition. The combinations of 1(R or
S) and 3(R or S) (compare P1–4) did not show any effect on the
thermal behaviour of the polymers. This result reveals that
the chirality of the monomers used does not have an effect on
the thermal properties of the derived polyamides. However,
when terpenes are copolymerized with fatty acids or Nylon 6,6,
significant changes of the thermal properties are observed and
a wide range of thermal characteristics, depending on the
structure, can be obtained. For example, when 1R and 1S
are polymerized with 4 and 5, semicrystalline polyamides were
obtained. Interestingly, the copolymers from 5 exhibited two
Fig. 2 1H NMR spectra of diester 5 (top), diamine 1S (middle) and their
derived polymer P6 (bottom).
of the hydrogen bonding.42 Therefore, the NH group could not
anymore be observed in the 1H NMR spectra.
Based on our previous report, copolymerization of 7 and 8
for Nylon 6,6 formation worked well with TBD-catalyzed
polymerization.24 Therefore, copolymerizations of monomers
1R and 1S with Nylon 6,6 were also performed to see if the
properties of Nylon 6,6 can be modified with a monomer
bearing a cycloaliphatic ring (see entries P9–12). It can be seen
that increasing the ratio of diamine derived from limonene
results in an increase of molecular weight. In order to show
the diversity of the polyamides that can be obtained from fatty
acids and limonene derivatives, we also successfully copoly-
merized 1R and 1S with 6 in the presence of 7 to balance the
functional group stoichiometry. GPC analysis of P13 and P14
revealed molecular weights of about 9500 and 10200 Da and a
polydispersity of 2.35 and 2.03, respectively. Although some-
what low molecular weights were obtained in some cases,
these results show that cysteamine-functionalized monomers
derived from limonene can be used for TBD-catalyzed poly-
amide synthesis. Depending on the used co-monomer higher
molecular weights could be achieved, revealing the significant
impact of steric hindrance on the polymerization behaviour.
Most importantly however, the thermal properties, also of
commercially available polyamides, can be tuned by choosing
appropriate co-monomer ratios, as will be discussed below.
The thermal properties of the synthesized polyamides were
analyzed by DSC. Table 1 summarizes the thermal properties
of these polymerizations and Fig. 3 shows DSC thermograms
of some of the synthesized polymers. Because of the cyclic
moieties, terpene-derived polyamides showed completely
different properties compared to conventional polyamides.
The cycloaliphatic bulky groups prevented intermolecular
melting endotherms, i.e. Tm−1 = 57.7 and 57.6 °C, and Tm−2
=
84.0 and 83.4 °C for P5 and P6, respectively. Nonetheless,
when 4 instead of 5 was used, an increase of more than
20 degrees in the melting temperature and a single endotherm,
which is about 102 °C for P7 and 101.5 °C for P8, was obtained
after an annealing pre-treatment at 90 °C for both P7 and P8.
Thus, the higher packing capability of the saturated alkyl
chains and the higher flexibility of the unsaturated ones caused
higher melting points on polyamides bearing saturated chain.
In the case of Nylon 6,6 copolymers, different ratios of 1(R or
S) were incorporated into Nylon 6,6 (compare P9–12) in order to
see if their thermal properties can be tuned. The DSC thermo-
grams of all these copolymers displayed clear Tgs in the range
of 40.9–42.4 °C. However, concerning the Tm values of the poly-
mers, multiple endothermic peaks in broad range with lack of a
clear baseline were obtained even after annealing. However,
decreasing the 1S ratio to Nylon 6,6 would increase the melting
transition (compare P11 and P12 in Fig. 3). In P11 it seems that
the crystallization of Nylon 6,6 is completely changed (and hin-
dered) due to the steric bulk, whereas in P12 with a higher
content of Nylon 6,6, a higher melting range is observed; some
part of this polymer behaves as a copolymer with a lower
melting point, whereas most of the polymer still behaves as
Nylon 6,6, with melting at ∼230 °C. These results clearly show
that the copolymerisation of limonene derived monomers can
be used to adjust the properties of commercial polyamides.
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Synthesis of polyurethanes
The prepared polyurethanes (P15–18) consist of bulky
cycloaliphatic units linked with very short segments, which
prevent crystallization and should lead to characteristic
thermal properties. DSC analyses display amorphous
Besides our study of limonene based polyamides, we had a
great interest in renewable polyurethanes synthesized via an
isocyanate-free methodology. Thus, the dicarbamate AA-type
monomers (2R and 2S) derived from limonene were employed
in a transesterification like polymerization with selected diols.
Different renewable diols, which we prepared in our previous
study [i.e. 9R and 9S (limonene-derived), 11 (fatty acid-
derived)],15 and commercial diol 10 were copolymerized with
monomers 2R and 2S (Scheme 3). All polymerizations were
carried out using TBD as a catalyst, without a solvent and under
continuous vacuum (∼10 mbar) for efficient methanol removal.
Noteworthy, along with the use of renewable monomers, the
polyurethanes were obtained via an isocyanate-free route.
Initially, optimization reactions were carried out for the
polymerizations of 2(R or S) with 9(R or S) and the best results
were obtained at 120 °C with 0.05 equiv. (related to carbamate
groups) of TBD, the conditions that were used for all further
polymerization reactions. Polyurethanes P15 and P16 were
obtained after 16 h with molecular weights of 7900 and
6150 Da, respectively. Performing the reaction for longer reac-
tion times (24 h) did not lead to higher molecular weights.
Thus, under the same reaction conditions, polymerizations
were performed using long chain diols 10 and 11. As expected
and similar to the results discussed above for polyamide syn-
thesis, P17–20 were obtained with higher molecular weights in
the range of 8660–12 600 Da (Table 2, Fig. 4). 1H NMR analyses
of the synthesized polymers revealed successful polymeri-
zations. As a representative example (Fig. 5), the 1H NMR
spectra of dicarbamate 2S, diol 11 and the thereof derived
polymer P20, showed almost complete disappearance of the
singlet peak at 3.65 ppm corresponding to the methyl carba-
mate (Hb) transformed to the backbone urethane link (He).
Table 2 Analytic data of the synthesized polyurethanes
Polymer (dicarbamate/diol) Mn a (Da) PDIa Tm b (°C) Tg b (°C)
P15 (2R/9R)
P16 (2S/9S)
P17 (2R/10)
P18 (2S/10)
P19 (2R/11)
P20 (2S/11)
7900
6150
8660
9480
11 620
12 600
1.87
1.79
2.15
2.03
1.81
1.84
18.5c
18.3c
15.9c
14.6c
naf
69.2d
62.9e
naf
a GPC data of precipitated polymers. b DSC data recorded at 10 °C
min−1 c Second heating scan. d 1 h annealing at 50 °C. e 1 h annealing
.
at 55 °C. f Tg not observable by DSC.
Fig. 4 GPC traces of selected synthesized polyurethanes.
Scheme 3 Synthesis of (R)-(+)- and (S)-(−)-limonene based polyurethanes.
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functional groups into (R)-(+)- and (S)-(−)-limonene to produce
new renewable monomers for polyamide synthesis. GPC
results and intensive DSC analysis of the resulting polymers
revealed that renewable polyamides from amorphous to high-
melting semicrystalline polymers with molecular weights up to
12 kDa can be obtained via the selective combination of the
above-mentioned monomers. Moreover, Nylon 6,6 copolymers
were successfully prepared. In addition, the synthesized di-
amines derived from limonenes have been efficiently trans-
formed into dicarbamates via a phosgene-free route, and
subsequently employed in polycondensations with several
renewable diols. A number of linear renewable polyurethanes
were obtained via an isocyanate-free route with mole-
cular weights up to 12.6 kDa. Finally, DSC analysis of these
polyurethanes showed amorphous to semicrystalline struc-
tures. Overall, the great potential of these renewable
resources for polyamide and polyurethane synthesis was
demonstrated.
Fig. 5 1H NMR spectra of dicarbamate 2S (top), diol 11 (middle) and their
derived polymer P20 (bottom).
Acknowledgements
M. F. acknowledges the Indonesian Directorate General of
Higher Education for a PhD scholarship.
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structures for all of them (see Fig. 6) with glass transition
temperatures in the range of 14.6–18.5 °C. However, when the
long chain diol 11 was copolymerized with 2R or 2S, crystalli-
zation occurred. On the second heating scan, the synthesized
polyurethanes (P19 and P20) displayed multiple melting tran-
sitions. Thus, annealing was necessary to verify whether these
multiple melting transitions were due to the occurrence of
metastable crystalline phases or actual crystalline structures.
After an annealing pre-treatment, DSC analysis exhibited a
melting transition at 69.2 and 62.9 °C for P19 and P20, respect-
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polymers by DSC.
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Conclusions
In summary, we have shown that thiol–ene additions offer a
versatile and effective way to introduce primary amine
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