A green approach toward oleic- and undecylenic acid-derived polyurethanes

Article abstract of DOI:10.1002/pola.24671

Naturally occurring oleic and undecylenic acids were used as raw materials for the synthesis of novel polyurethanes (PUs). The application of environmentally friendly thiol-ene additions to 10-undecenoate and oleate derivatives was studied with the goal of obtaining renewable diols. The resulting monomers were then polymerized with 4,4′-methylenebis (phenylisocyanate), in N,N-dimethylformamide solution using tin (II) 2-ethylhexanoate as catalyst, to produce the corresponding thermoplastic PUs (TPUs). Also, ultrasound irradiation has been tested to improve the synthesis of PU. Under these conditions, TPUs were obtained in high yields (80-99%) with weight-average molecular weights in the 36-83 kDa range. The chemical structures of PUs were assessed by FTIR and NMR spectroscopy. The thermal and mechanical properties of the synthesized TPUs have been studied and they showed a clear dependence on the structure of the parent diol. MTT test was carried out to asses the potential cytotoxicity of the prepared PUs, indicating no cytotoxic response.

Full text of DOI:10.1002/pola.24671

A Green Approach Toward Oleic- and Undecylenic
Acid-Derived Polyurethanes
´
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R. J. GONZALEZ-PAZ, C. LLUCH, G. LLIGADAS, J. C. RONDA, M. GALIA, V. CADIZ
Departament de Qu´ımica Anal´ıtica i Qu´ımica Orga`nica, Universitat Rovira i Virgili, Campus Sescelades,
Marcel.l´ı Domingo s/n., 43007 Tarragona, Spain
Received 14 February 2011; accepted 14 March 2011
DOI: 10.1002/pola.24671
Published online 8 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Naturally occurring oleic and undecylenic acids
were used as raw materials for the synthesis of novel polyur-
ethanes (PUs). The application of environmentally friendly
thiol-ene additions to 10-undecenoate and oleate derivatives
was studied with the goal of obtaining renewable diols. The
resulting monomers were then polymerized with 4,40-methyle-
nebis (phenylisocyanate), in N,N-dimethylformamide solution
using tin (II) 2-ethylhexanoate as catalyst, to produce the corre-
sponding thermoplastic PUs (TPUs). Also, ultrasound irradia-
tion has been tested to improve the synthesis of PU. Under
these conditions, TPUs were obtained in high yields (80–99%)
with weight-average molecular weights in the 36–83 kDa range.
The chemical structures of PUs were assessed by FTIR and NMR
spectroscopy. The thermal and mechanical properties of the syn-
thesized TPUs have been studied and they showed a clear de-
pendence on the structure of the parent diol. MTT test was carried
out to asses the potential cytotoxicity of the prepared PUs, indicat-
C
V
ing no cytotoxic response. 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 2407–2416, 2011
KEYWORDS: oleic acid; polyurethanes; renewable resources;
thermoplastics; thiol-ene; undecylenic acid
INTRODUCTION To date, a wide range of industrial materials
such as solvents, fuels, synthetic fibers, and chemical prod-
ucts are being manufactured from petroleum resources.
However, rapid depletion of fossil and petroleum resources
is encouraging current and future chemists to orient their
research toward designing safer chemicals, products, and
processes from renewable feedstock with an increased
awareness of environmental and industrial impact.1
viewed by industry as a viable alternative to hydrocarbon-
based feedstocks. In sustainable materials, PUs are currently
prepared starting from renewable polyols, while the second
partner, isocyanate, is mainly made from petroleum resour-
ces.9,10 There is limited literature available concerning the
synthesis of isocyanate compounds based on plant oils. Ku¨se-
foglu and Çayli11 reported the functionalization of soybean
oil with isocyanate moieties, and demonstrated that these
plant-based isocyanates are suitable for PU preparation. On
the other hand, recently Narine and co-workers12 have
developed methodologies for the synthesis of isocyanates
and polyols from vegetable oils and corresponding biobased
PUs entirely from lipid feedstock. Moreover, concerning noni-
socyanate methods for the preparation of PUs derived from
plat oils, two methodologies have been described: the reac-
tion of cyclic carbonates with amines13 and the more recent
self-condensation approach of AB-type monomers.14
Nature offers an abundance of opportunities for designing
novel monomers and shaping structural and functional poly-
mers in its wide variety of raw materials.2 Presently, their
relative use for synthesis of monomers and polymers com-
pared to petrochemicals is small. Natural oils, such as vege-
table oils provide interesting feedstock—triglyceride fatty
acids—that beyond their use in food allow additional chem-
istry that yields either opportunities for replacing petro-
chemicals or may be directly used to synthesize bioinspired
materials.3–6 Fatty acids have been used in various classes of
biodegradable polymers but have been largely confined to
polyanhydrides, polyesters, and poly(ester-anhydrides). In
these polymers, fatty acid monomers obtained from natural
sources were incorporated in the polymer backbone to
obtain the desired properties.7
Classically, the reaction between a thiol and a double bond
has received significant attention as candidate for many
applications including coatings, adhesives, dental materials,
and imprinting lithography. Resurgence over the past deca-
des has occurred in response to the benefits thiol-ene
coupling presents for polymer synthesis: tolerance to many
different reaction conditions/solvents, clearly defined reac-
tion pathways/products, and facile synthetic strategies from
a range of easily obtained starting materials.15 Thus, thiol-
ene chemistry has recently emerged as a powerful tool for
Vegetable oils are becoming extremely important as renew-
able resources for the preparation of polyols required for the
polyurethane (PU) industry.8 Polyols from natural oils, such
as soybean, castor, and palm oils are increasingly being
Correspondence to: V. Ca´diz (E-mail: virginia.cadiz@urv.cat)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2407–2416 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
synthetic chemistry and polymer functionalization that has
the potential to fall within the realm of click chemistry.16–18
The thiol-ene coupling makes use of the high nucleophilicity
of the sulfhydryl moiety and proceeds under physiological
conditions. The formed thioether linkage is very stable under
physiological conditions and resists a strong basic or acidic
environment and is also stable toward reducing agents; how-
ever, it is susceptible toward oxidizing agents. The robust na-
ture of thiol-ene chemistry allows for the preparation of
well-defined materials with few structural limitations and
synthetic requirements.19 While both heat and light have
been used to generate radicals that initiate the thiol-ene rad-
ical chain process, the use of light has enormous advantages
for small molecule synthesis, surface and polymer modifica-
tion, and polymerization reactions. A vast array of work has
been performed in an effort to understand and implement
radical-mediated thiol-ene reactions, primarily focusing on
the photoinitiated reactions. This large body of literature is
detailed in very recent review articles.20–23
formamide (DMF) was dried with CaH2 for 24 h and freshly
distilled before use. Phosphate-buffered solution (PBS) of pH
7.4 from Sigma (Steinheim, Germany) was used as received.
Thermanox (TMX) control disks were supplied by Labclinics
S.L. (Barcelona, Spain), and aqueous solutions of Triton X-100
were supplied by Aldrich (Steinheim, Germany). Tissue cul-
ture media, additives, and 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide (MTT) were purchased from
Sigma (Steinheim, Germany). The fetal bovine serum was
obtained from Gibco (Paisley, UK), and L-glutamine, penicillin,
and streptomycin were from Sigma (Steinheim, Germany).
Synthesis of Allyl Oleate (OLA)
To a 250-mL round-bottom flask, 26 g (0.092 mol) of
oleic acid, an excess of allyl alcohol 25 mL (0.37 mol), and p-
toluensulphonic acid as a catalyst were added, and the mix-
ture was refluxed and magnetically stirred for 8 h. Once the
reaction was completed, the mixture was washed with ethyl
ether and 10% sodium bicarbonate solution, dried over
anhydrous magnesium sulphate and filtered. The solvent was
evaporated off under reduced pressure. The product was
purified by column chromatography using hexane:ethyl
acetate, 8:2, as eluent, to afford pure allyl oleate (OLA) as
viscous oil, in a 80% yield.
In the last decade, many authors have reported the use of
vegetable oils as feedstock for UV-curable systems,24–26 and
although UV-curable chemistries based on thiol-ene function-
ality offer many advantages,27,28 only recently thiol-ene UV-
curable coatings using vegetable oils is reported.29
1H NMR [CDCl3, tetramethylsilane (TMS), d (ppm)]: 5.88 (m,
ACH¼¼C), 5.35 (m, AHC¼¼CHA, 2H), 5.25 (m, C¼¼CH2), 4.58
In particular, thiol-ene click chemistry of fatty acid deriva-
tives,30,31 obtained from plant oils, is a promising route that
can be used for the synthesis of novel chemical intermedi-
ates from renewable resources. Our research applies the
thiol-ene click chemistry of unsaturated fatty acid ester
derivatives with hydroxyl-functionalized thiols for its ability
to add hydroxyl functionality in lieu of double bounds. This
methodology provides a green approach toward novel plant-
derived diols.32
(d,
AOCH2),
2.37
(t,
ACH2ACOA),
2.01
(m,
ACH2ACH¼¼CHA, 4H), 1.62 (t, ACH2ACH2ACOOA), 1.38–
1.22 (m, ACH2A, 20 H), 0.93 (CH3A). 13C NMR [CDCl3, TMS,
d (ppm)]: 173.80 (s), 132.15 (d), 130.00 (d), 129.80 (d),
118.02 (t), 64.73 (t), 34.10 (t), 32.15 (t), 32.1 (t), 29.81 (t),
29.62 (t), 29.55 (t), 29.25 (t), 29.21 (t), 29.12 (t), 29.00 (t),
27.21 (t), 27.15 (t), 24.5 (t), 22.32 (t), 14.1 (q).
Synthesis of 3-(2-Hydroxyethylthio)propyl
In this work, we report UV light-mediated synthesis of four
diols from oleic acid and 10-undecenoic acid derivatives.
Oleic and 10-undecenoic acids are the major products of
high oleic sunflower oil saponification and castor oil pyroly-
sis,33 respectively. We will focus our discussion on the syn-
thesis of monomer diols which have been used in the syn-
thesis of PUs as well as characterization and properties of
the resulting PUs.
11-(2-hydroxyethylthio) Undecanoate (UDA-diol):
Thiol-Ene Coupling of UDA with 2-Mercaptoethanol
In a 25-mL flask, 5.0 g (22 mmol) of UDA reacted with 4.3 g
(55 mmol) of mercaptoethanol. The radical initiator, DMPA,
was added in the proportion of 0.3% mol init./mol C¼¼C. The
amount of acetonitrile necessary to dissolve the photoinitiator
was added. The reaction was carried out at room
temperature, without deoxygenation, by irradiation with two
9 W UV lamps (k ¼ 365 nm). After few minutes, a white solid
was precipitated. The completion of the reaction was con-
EXPERIMENTAL
1
firmed by H NMR by the complete disappearance of the dou-
Materials
ble bond signals that appear in the region of 5–6 ppm. The
mixture was crystallized from ether, filtered, washed with
cold ether and hexane, and dried under vacuum (yield 98%).
Methyl oleate (OLM) and methyl 10-undecenoate (UDM) were
synthesized from oleic acid (90% Mallinckrodt, St. Louis, MO)
and 10-undecenoic acid (98%, Aldrich, Steinheim, Germany)
following standard methods. Allyl 10-undecenoate (UDA) was
synthesized following a previously reported procedure.32 The
following chemicals were purchased from Aldrich (Steinheim,
Germany) and used as received: lithium aluminum hydride,
LiAlH4 (95%), acetonitrile, allyl alcohol (>98%), 2,2-dime-
thoxy-2-phenylacetophenone (DMPA; 99%), 2-mercaptoetha-
nol (99%), tin (II) 2-ethylhexanoate, and 4,40-methylenebi-
s(phenylisocyanate) (MDI). Tetrahydrofuran (THF) was
distilled from sodium immediately before use; N,N-dimethyl-
1H NMR [CDCl3, TMS, d, ppm)]: 4.15 (t, AOCH2), 3.73 (t,
ACH2AOH), 3.70 (t, ACH2AOH), 2.73 (t, HOCH2ACH2ASA),
2.72 (t, HOCH2ACH2ASA), 2.59 (t, AOA(CH2)2ACH2ASA),
2.51 (ACH ACH2AS), 2.29 (CH2ACOOA), 1.93 (m, ACH2A),
2
1.59 (m, ACH2A, 4H), 1.38–1.26 (m, ACH2A, 12 H). 13C
NMR [CDCl3, TMS, d (ppm)]: 173.96 (s), 62.79 (t), 60.40 (t),
60.28 (t), 35.43 (t), 35.40 (t), 34.40 (t), 31.73 (t), 29.84 (t),
29.51 (t), 29.45 (t), 29.32 (t), 29.26 (t), 29.22 (t), 28.94 (t),
28.91 (t), 28.23 (t), 25.06 (t).
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ARTICLE
Synthesis of 3-(2-Hydroxyethylthio)propyl 9- and
10-(2-hydroxyethylthio) Octadecanoate as a Mixture
of Isomers (OLA-diol): Thiol-Ene Coupling of OLA
with 2-Mercaptoethanol
OLA-diol was synthesized following the procedure described
for UDA-diol but higher ratio of radical initiator was used
(1.7% mol init./mol C¼¼C). The product was purified by col-
umn chromatography using hexane:ethyl acetate, 1:1, as elu-
ent, to afford OLA-diol as viscous oil, in a 80% yield.
was added as the viscosity increased. After 30 min, excess
LiAlH4 was decomposed by the addition of 20 mL of ethyl
acetate dropwise, then a saturated 10% H2SO4 aqueous solu-
tion (60 mL) was added, the phases were separated, and the
aqueous layer was extracted with ethyl acetate. The com-
bined organic phase was washed with a saturated aqueous
NaCl solution, dried over anhydrous magnesium sulfate, fil-
tered, and the solvent was removed under reduced pressure.
The solid product was recrystallized from heptane to obtain
a white solid (yield 80%).
1H NMR [CDCl3, TMS, d (ppm)]: 4.10 (t, ACOOACH2), 3.66
(q, HOCH2A), 3.63 (q, HOCH2A), 2.67 (t, HOCH2ACH2ASA),
2.65 (t, HOCH2ACH2ASA), 2.53 (t, AOA(CH2)2ACH2ASA),
2.50 (m, ACHASA), 2.23 (t, ACH2ACOOA), 1.85 (m,
AOACH2ACH2ACH2AS) 1.56–1.25 (m, ACH2A, 28H), 0.81
(t, CH3). 13C NMR [CDCl3, TMS, d (ppm)]: 174 (s), 62.55 (t),
60.50 (t), 60.25 (t), 45.80 (d), 35.04 (t), 34.99 (t), 34.98 (t),
34.10 (t), 33.75 (t), 31.72 (t), 29.68 (t), 29.64 (t), 29.61 (t),
29.52 (t), 29.42 (t), 29.31 (t), 29.22 (t), 29.15 (t), 28.80 (t),
28.01 (t), 26.60 (t), 25.00 (t), 22.65 (t), 14.11 (q).
1H NMR [CDCl3, TMS, d (ppm)]: 3.71 (t, HOCH2ACH2ASA),
3.64 (t, HOCH2A), 2.73 (t, HOCH2ACH2ASA), 2.51 (t,
ASACH2A),
1.58
(m,
ACH2ACH2OH),
1.5
(m,
ACH2ACH2ASA), 1.37–1.25 (m, ACH2A, 14H). 13C NMR
[CDCl3, TMS, d (ppm)]: 63.30 (t), 60.30 (t), 35.55 (t), 33.00
(t), 31.80 (t), 29.93 (t), 29.74 (t), 29.67 (t), 29.65 (t), 29.59
(t) 29.38(t), 29.02 (t), 25.93 (t).
Synthesis of 9- and 10-(2-Hydroxyethylthio)
octadecan-1-ol (OLM-diol)
Synthesis of Methyl 11-(2-Hydroxyethylthio)undecanoate
(UDM-OH): Thiol-Ene Coupling of UDM
with 2-Mercaptoethanol
UDM-OH was synthesized following the procedure described
for UDA-diol but in this case no radical initiator was used.
The product was obtained as a white solid with 95% yield.
OLM-diol was synthesized following the procedure described
for UDM-diol. The product was dissolved in ethyl ether (5
ꢂ
mL) and crystallized (ꢁ20 C) with 75 % yield.
1H NMR [CDCl3, TMS, d (ppm)]: 3.69 (t, HOCH2ACH2ASA),
3.66 (t, HOCH2A), 2.72 (t, HOCH2ACH2ASA), 2.58 (m,
ACHASA), 1.58–1.25 (m, ACH2A, 30H), 0.88 (t, ACH3). 13C
NMR [CDCl3,TMS, d (ppm)]: 63.00 (t), 60.67 (t), 45.80 (d),
34.99 (t), 34.90 (t), 33.80 (t), 32.72 (t), 31.86 (t), 29.68 (t),
29.57 (t), 29.48 (t), 29.44 (t), 29.35 (t), 29.28 (t), 26.77 (t),
26.71 (t), 26.69 (t), 25.73 (t), 22.65 (t), 14.11 (q).
1H NMR [CDCl3, TMS, d (ppm)]: 3.73 (t, HOACH2ACH2ASA),
3.66 (s, AOCH3), 2.73 (t, HOCH2ACH2ASA), 2.51 (t,
ASACH2), 2.30 (t, ACH2ACOO), 1.63–1.27 (m, ACH2A, 16H).
13C NMR [CDCl3, TMS, d (ppm)]: 174.51 (s), 60.44 (t), 51.59
(q), 35.36 (t), 34.22 (t), 31.80 (t), 29.86 (t), 29.52 (t), 29.45
(t), 29.32 (t), 29.28 (t), 29.23 (t), 28.93 (t), 25.05 (t).
General Procedure for Polyurethanes Synthesis
A dry 50 mL round-bottom flask was charged with 12 mL
of DMF, 6 mmol of diol (UDM-diol, OLM-diol, UDA-diol, or
OLA-diol), 6 mmol of MDI, and 2%, w/w (with respect to
MDI) of tin (II) 2-ethylhexanoate. The flask was immersed
into a 50 ꢂC preheated silicone oil bath with magnetic stir-
ring. The reaction was continued for 24 h, and the PUs
were isolated as white solids by precipitation into diethyl
ether. Purification of PUs was carried out by dissolving the
polymer in the minimum volume of chloroform or THF and
reprecipitation into diethyl ether. The pure polymers were
Synthesis of Methyl 9- and
10-(2-Hydroxyethylthio)octadecanoate
as a Mixture of Isomers (OLM-OH): Thiol-Ene
Coupling of OLM with 2-Mercaptoethanol
OLM-OH was synthesized following the procedure described
for UDA-diol but higher ratio of radical initiator was used
(1.7% mol init./mol C¼¼C). The product was obtained as vis-
cous oil with 80% yield.
1H NMR [CDCl3, TMS, d (ppm)]: 3.69 (t, HOCH2ACH2ASA),
3.66 (s, AOCH3), 2.71 (t, HOCH2ACH2ASA), 2.57 (m,
ACHASA), 2.29 (t, ACH2ACOO), 1.63–1.25 (m, ACH2A, 28H),
0.87 (t, CH3). 13C NMR [CDCl3, TMS, d (ppm)]: 174.53 (s), 60.89
(t), 51.64 (q), 45.95 (d), 35.14 (t), 34.27 (t), 33.95 (t), 32.03 (t),
29.83 (t), 29.75 (t), 29.68 (t), 29.62 (t), 29.54 (t), 29.48 (t),
29.42 (t), 29.35 (t), 29.22 (t), 26.95 (t), 25.05 (t), 22.83 (t),
14.28 (q).
dried under vacuum and stored in
a desiccator until
needed. Films were solution cast from DMF and dried at
50 ꢂC for 1 day and then in a vacuum oven until constant
weight.
Instrumentation
The FTIR spectra were recorded on a JASCO 680 FTIR spec-
trophotometer with a resolution of 2 cmꢁ1 in the absorbance
mode. An attenuated total reflection (ATR) accessory with
thermal control and a diamond crystal (Golden Gate heated
single-reflection diamond ATR, Specac. Teknokroma) was
used to determine FTIR spectra. 1H (400 MHz), 13C (100.5
MHz) NMR spectra were obtained using a Varian Gemini 400
spectrometer with Fourier transform, CDCl3 as solvent, and
TMS as internal standard.
Synthesis of 11-(2-Hydroxyethylthio)undecan-1-ol
(UDM-diol)
A 250-mL, two-necked, round-bottom flask equipped with a
Teflon-coated magnetic bar and a pressure-equalized drop-
ping funnel was charged with LiAlH4 (0.42 g, 0.011 mol) and
anhydrous THF (15 mL) under argon. UDM-OH (3 g, 0.011
mol) dissolved in 15 mL of anhydrous THF was added
slowly with stirring for 1 h. Anhydrous THF (3 ꢀ 10 mL)
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OLEIC- AND UNDECYLENIC ACID-DERIVED PUs, GONZALEZ-PAZ ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Calorimetric studies were carried out on a Mettler DSC821e
and DSC822e thermal analyzer using N2 as a purge gas (20
mL/min) at scanning rate of 10 ꢂC/min. Thermal stability
studies were carried out on a Mettler TGA/SDTA851e/LF/
1100 with N2 as a purge gas. The heating rate in the TGA
dynamic mode was 10 ꢂC/min. Isothermal measurements
were carried out at 240, 250, and 260 ꢂC. Degradation
kinetics of polymers was studied using TGA with experimen-
tal data being processed using the Flynn method, which
includes three isothermal and one dynamic TGA curves.34
This method provides the activation energy dependence with
temperature.
MTT Assay for Polymers
TMX, Triton, and disks of copolymers were set up in 5 mL of
DMEM, fetal calf serum (FCS)-free medium. They were
placed on a roller mixer at 37 ꢂC, and the medium was
removed at different time periods (1, 2, and 7 days) and
replaced with another 5 mL of fresh medium. All the extracts
were obtained under sterile conditions. Human fibroblasts
were seeded at a density of 11 ꢀ 104 cells/mL in complete
medium in a sterile 96-well culture plate and incubated to
confluency. Then, the medium was replaced with the corre-
ꢂ
sponding eluted extract and incubated at 37 C in a humidi-
fied air with 5% CO2 for 24 h. A solution of MTT was pre-
pared in warm PBS and filtered before use. MTT, 10 lL, was
added to all wells to give a final concentration of 0.5 mg/
Mechanical properties were measured using a dynamic me-
chanical thermal analysis apparatus (TA DMA 2928) in the
controlled force-tension film mode. The tensile assays were
performed on rectangular films (5 ꢀ 3 ꢀ 0.2 mm3) meas-
ꢂ
mL, and the plates were incubated at 37 C, 5% CO2 for 4 h.
Excess medium and MTT were removed, and 100 lL of
DMSO was added to all wells to dissolve the MTT taken up
by the cells. This was mixed for 10 min, and the absorbance
was measured with a Biotek ELX808 IU plate reader, using a
test wavelength of 570 nm and a reference wavelength of
630 nm. The cell viability was calculated from equation 1:
ꢂ
uring the strain while applying a ramp of 3 N/min at 35 C.
A preload force of 0.01 N and a soak time of 5 min were
used.
Sonication Techniques
The main sources of ultrasound used were a Branson 2510
horn system, operating at 42 KHz and used in the usual con-
figuration whereby the horn was immersed to a depth of
about 1.5 cm in the reaction mixture. Thermostatting around
ambient temperature was achieved to 61 ꢂC by water bath
surrounding the reaction vessel.
Relative cell viability ¼ 100 ꢀ ðODS ꢁ ODBÞ=ODC (1)
where ODS, ODB, and ODC are the optical densities of forma-
zan production for the sample, blank (DMEM without cells),
and control (Tween solution in free serum-supplemented
DMEM), respectively. Results were normalized with respect
to a negative control (TMX ¼ 100%) and statistically tested
with ANOVA (p < 0.05).
Hydrolytic Degradation Assays
Films of aliphatic PUs with a thickness of approximately
200 lm were prepared by casting at room temperature from
a solution in THF. The films were cut into 10 mm diameter,
20–30 mg weight disks, which were dried under vacuum at
50 ꢂC to constant weight. For incubation, samples were
immersed in vials containing 10 mL of the sodium phos-
phate buffer (pH 7.4), and experiments were carried out
at temperature of 60 ꢂC. After incubation for the scheduled
period of time, samples were removed, rinsed thoroughly
with water, and dried to constant weight. Sample weighing
and gel permeation chromatography (GPC) measurements
were used to follow the evolution of the hydrodegradation.
RESULTS AND DISCUSSION
Synthesis of Allyl 10-Undecenoate and Allyl Oleate
The synthesis of allyl ester of 10-undecenoic acid (UDA) was
carried out by refluxing UDA, which is the major product of
castor oil pyrolysis,33 with an excess of allyl alcohol and using
2% p-toluenesulfonic acid as catalyst, for 6–8 h (Scheme 1).
The synthesis of OLA was carried out in a similar way as
compared to than UDA but using oleic acid as fatty acid. The
1H NMR spectrum of the product confirmed the expected
structure, showing together with the five allyl protons a mul-
tiplet at 5.35 ppm corresponding to the two olefinic protons
of the central double bond.
Biocompatibility of Polymers
The negative control used was tissue culture plastic, TMX,
and the positive control (toxic agent) was a 0.5% aqueous
solution of Triton X-100. Disks of 10 mm diameter and 1
mm thickness of the polymers and the controls were used
for direct and indirect biocompatibility experiments. The
polymers were tested for cytotoxicity assay. All specimens
were sterilized with ethylene oxide. The cells used in the pri-
mary cell culture were human fibroblasts and were cultured
Reactivity of Undecenoates and Oleates
Toward Thiol Addition
The thiol-ene coupling mechanism has been extensively stud-
ied and is known to follow a radical mechanism, in which
the addition of a thiyl radical to a double bond is followed
by chain transfer to thiol.35 The thiol-ene addition product is
formed, with anti-Markovnikov orientation, with the concom-
itant generation of a new thiyl radical. Possible termination
reactions involve typical radical–radical coupling processes.
ꢂ
at 37 C. The culture medium was Dubelcco’s modified eagle
medium (DMEM), rich in glucose, modified with 4-(2-hydroxy-
ethyl)-1-piperazineethanesulfonic acid (HEPES) (Sigma,
Steinheim, Germany) and supplemented with 10% fetal
bovine serum, 200 mM L-glutamine, 100 units/mL penicillin,
and 100 lg/mL streptomycin. The culture medium was
changed at selected time intervals with care to cause little
disturbance to culture conditions.
RS: þ CH2 ¼¼ CHR0 ! R ꢁꢁSCH2 ꢁ C:HR0
(2)
(3)
R ꢁꢁSCH2 ꢁꢁC:HR0 þ HSR ! RSCH2 ꢁꢁCH2R þ RS:
Reactivity in the radical thiol-ene reaction can vary consider-
ably depending on the chemical structure of the ene and
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ARTICLE
SCHEME 1 Synthetic procedure for the preparation of diols.
thiol components. UDA is a monomer with different allylic
and vinylic double bond end groups, and OLA is a diolefinic
monomer with allylic and internal double bonds. It is logical
that the chemical structure of an alkene can significantly
affect its reactivity in thiol-ene reactions because of differen-
ces in the steric strain and ene susceptibility to thiyl attack
and subsequent hydrogen abstraction. It is well known that
terminal alkenes react very rapidly and irreversibly with thi-
ols, achieving complete conversions in few minutes.36,37 The
relative reactivity of two UDA end groups toward addition of
photoinitiated radical with 2-mercaptoethanol and in the
presence of DMPA as photoinitiator was studied, finding
that allylic and vinylic chain ends exhibit different reactiv-
ities toward thiol addition (about 1.8 less the former against
the latter).32 Presumably, this is due to the presence of
an electron-withdrawing group that destabilize the radical
intermediate.
Generally, terminal enes are significantly more reactive
toward hydrothiolation compared to internal enes. Thus,
Hoyle et al.15 reported that 1-hexene is 8ꢀ more reactive
´
OLEIC- AND UNDECYLENIC ACID-DERIVED PUs, GONZALEZ-PAZ ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
nal (a) at around 5.33 ppm. After 5 min (37% conversion), a
new signal (b) appears at 5.36 ppm, which is attributed to
the chemical shift characteristic of C¼¼C with trans configura-
tion. The appearance of this signal confirms that under these
conditions, the addition of a thiyl radical to OLM is a reversi-
ble process that generates a more thermodynamically stable
trans double bond. After 30 min, at the point of 50% conver-
sion the signal of trans is higher than the cis and at the
point of 72% conversion (60 min) only practically the signal
of trans ene is remaining. This signal completely disappears
after 120 min. This insertion–isomerization–elimination reac-
tion sequence is also responsible for the reduced reactivity
observed for OLM.
Following the above considerations, both allyl and methyl
undecenoates were reacted with 2-mercaptoethanol in DMPA
to yield UDA-diol and UDM-diol, respectively. Also both OLA
and OLM yielded OLA-diol and OLM-diol following the proce-
dure described for UDA-diol and UDM-diol but with higher
ratio of radical initiator (Scheme 1).
The evolution of the oleic acid to OLA-diol could be followed
1
by H NMR (Fig. 2). The spectrum (A) of the initial oleic acid
FIGURE 1 Expansion of the 400 MHz 1H NMR spectra during
the photoinitiated thiol-ene coupling between 2-mercaptoetha-
nol and methyl oleate.
than trans-2-hexene and 18ꢀ more reactive than trans-3-
hexene, clearly highlighting that steric effects are important
when considering reactivity. However, these differences in
reactivity are not due entirely due to steric effects. As
reported previously,38–41 addition of thyil radical to cis C¼¼C
bonds is reversible and is accompanied with an isomerisa-
tion process, that is, thyil radicals can be used as means of
converting cis C¼¼C bonds to trans C¼¼C bonds with high effi-
1
ciency. A detailed analysis of the H NMR spectra during the
2-mercaptoethanol addition to methyl oleate (OLM) con-
firmed that this process takes place. As shown in Figure
1(a), pure OLM double bond with cis configuration gives sig-
FIGURE 2 1H NMR spectra of (A) OL; (B) OLA; and (C) OLA-
diol.
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ARTICLE
TABLE 1 Polycondesation Results and Some Properties of PUs
SECc
Solubilityd
DMSO CHCl3
PU
Diol
T (ꢂC)
t (h)
Yield (%)
Mn (g/mol)
Mw (g/mol)
Mw/Mn
H2O
THF
PU1a
PU2a
PU3a
UDM
OLM
UDA
UDA
OLA
50
50
50
40
50
24
24
24
7
99
80
99
99
92
83,293
36,340
50,930
71,210
61,860
188,670
84,050
2.1
2.2
1.9
1.6
1.9
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
þ
þ
þ
þ
þ
ꢁ
þ
þ
þ
þ
þ
þ
þ
þ
þ
96,310
b
PU30
PU4a
a
120,820
120,020
24
c
Polycondensation reaction carried out in DMF by conventional
heating.
Number and average molecular weights determined by GPC in THF
against polystyrene (PS) standards.
Solubility at 25 ꢂC: þ soluble, ꢁ insoluble.
b
d
Polycondensation reaction carried out in DMF by ultrasound irradia-
tion (42 kHz).
shows the characteristic signals of the internal C¼¼C at 5.35
ppm. The formation of OLA (spectrum B) is observed for the
peaks arising from the methylene next to terminal C¼¼C at
4.58 ppm and the new peaks in the olefinic zone at 5.27 and
5.88 ppm corresponding to protons of terminal C¼¼C. The
success of the thiol-ene coupling to yield OLA-diol (spectrum
C) was noted from the appearance signals at about 2.4 and
2.6 ppm corresponding to the newly formed carbon–sulfur
bonds. More evidence of successful coupling was obtained
from the complete disappearance of signals corresponding to
alkene protons.
cmꢁ1, respectively, and the NH stretching and bending bands
appear at around 3320 cmꢁ1 and 1532 cmꢁ1 respectively.
1H and 13C NMR spectra were in all cases in full concord-
ance with the expected chemical structures of PUs.
The solubility of the PUs is collected in Table 1 for a set of
representative solvents evidencing that these compounds
have high solubility in all solvents except in water. Only PU1
showed no solubility in CHCl3.
The effect on thermal transitions Tg and Tm arising from the
different precursor fatty acid units in the PU chain was
investigated by DSC. PUs samples were quenched from the
melt to make clear the observation of the glass transition. Tg
values measured in such DSC traces are given in Table 2. Tm
values were measured on the heating DSC traces recorded
PUs samples coming directly from the synthesis and are also
given in Table 2. PU1 and PU3 derivatives from undecylenic
acid showed higher crystallinity than PU2 and PU4 deriva-
tives from oleic acid. This can be related to the presence of
pendant chains in PU2 and PU4, that difficult the packing.
Results obtained in the synthesis of PUs studied in this work
as well as some characterization data are given in Table 1.
The synthesized undecylenic and oleic acid-based diols
(UDM-diol, UDA-diol, OLM-diol, and OLA-diol) were polymer-
ized with MDI in DMF solution at 50 ꢂC for 24 h using tin
(II) 2-ethylhexanoate as catalyst. Under these conditions, the
reaction proceeded in one step, and isolation and purifica-
tion of the PUs were carried out by solution–precipitation
cycles and subsequent drying under vacuum; yields were in
the 80–99% range. Weight-average molecular weights of PUs
were in the 36–83 kDa range. Polydispersities oscillated
between 2.2 and 1.9 with perceivable differences between
those with and without ester groups in the polymer chain. In
the case of PU obtained with ultrasound irradiation, the poly-
dispersity was lower.
Previously, it was reported that the rates of reaction in the
preparation of PUs from H12MDI and different aliphatic diols
can be accelerated by the use of high intensity ultrasound.42
The source of the effect seems to be related to local heating
around collapsing cavitation bubbles together with the
enhanced mass transfer caused by the fluid motion but it
was likely that an effect took place to modify the mode of
action of the catalysts in these systems. Hence, we carried
out one experience of polymerization of UDA-diol under ul-
trasonic irradiation. The results shown in Figure 3 and Table
1 allow to confirm that sonochemical reaction proceeded
The chemical structures of PUs were assessed by FTIR and
NMR spectroscopy. Characteristic IR absorption bands of
main chains were observed. Thus, the C¼¼O stretching bands
arising from ester and urethane appear at 1728 and 1701
TABLE 2 Thermal Properties of PUs
DSC
TGA (ꢂC)
Ea (J/mol)
Tg (ꢂC)
Tm1 (ꢂC)
DH1 (J/g)
Tm2 (ꢂC)
DH2 (J/g)
T5%
Tmax
240 ꢂC
250 ꢂC
260 ꢂC
PU1
PU2
PU3
PU4
56
28
20
8
115
104
124
–
17
3
141
–
19
–
274
277
269
290
293/361/463
116
151
60
94
97
284/356/463
294/362/462
301/362/459
158
71
141
59
42
–
–
–
–
–
125
110
127
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OLEIC- AND UNDECYLENIC ACID-DERIVED PUs, GONZALEZ-PAZ ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
faster in the early stages and led to higher molecular weight
PU compared to PU obtained by conventional thermal
polymerization.
The hydrolytic degradability of PUs was evaluated by incuba-
tion assays. Neither weight loss nor molecular weight
decrease was observed after 6 months. These results are in
agreement with the hydrophobic character of all synthesized
PUs.
The thermal stabilities of polymers were investigated with
TGA in nitrogen stream, and the results are collected in Ta-
ble 2. All PUs showed three degradation stages and the de-
rivative of the weight loss versus temperature showed three
ꢂ
peaks centered at about 300, 360, and 460 C. The range of
temperatures of the first stage suggests that degradation
starts at urethane linkages, which takes place through the
dissociation to isocyanate and alcohol, the formation of pri-
mary amines and olefins, or the formation of secondary
amines.43
FIGURE 3 Evolution of Mn and Mw versus time under conven-
tional heating and ultrasound irradiation of PU30.
FIGURE 4 Dependence of degradation times in nitrogen on conversion at three temperatures for the polyurethanes.
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ARTICLE
Degradation kinetic studies for the temperature region of
240–260 ꢂC were carried out. Figure 4 shows the depend-
ence of degradation times in nitrogen on conversion at three
different temperatures. Activation energies calculated from
the slope of isothermal curves are collected in Table 2. As
can be seen, activation energies show dependence on the
chemical structure of PUs, being lower for ester group-con-
taining PUs P3 and P4, which should influence the initial
degradation step (below 10% conversion) making the degra-
dation process more complex.
Mechanical properties of PUs including tensile strength,
modulus, and elongation at break were evaluated from
stress–strain curves (Fig. 5). PU4 and PU2 samples show a
smooth transition in their stress–strain behavior similar to
lightly crosslinked amorphous rubbers. The stress–strain
behavior of PU1 and PU3 samples is different showing a
yield point as a result of the presence of crystalline domains
in these samples which act by increasing the rigidity of the
material.
FIGURE 6 MTT cytotoxicity results for control TMX and poly-
urethanes. Results are the mean 6 standard deviation. Statisti-
cal analysis (n ¼ 12) of each polymer was performed with
respect to TMX at a significance level of *p < 0.05.
Cytotoxicity of PUs was evaluated using the MTT assay for
testing the toxicity of eluates. Figure 6 shows that cell viabil-
ity was not affected by the presence of extracts from any PU
systems within 7 days, reaching values higher than 85% of
the control TMX in all polymers. These initial cytotoxicity
tests indicate that these materials are promising for biomedi-
cal purposes, however, further testing is required to ascer-
tain whether they are biocompatible.
leum based materials. MTT test was carried out to asses the
potential cytotoxicity of the prepared PUs, indicating no cyto-
toxic response which indicates that these materials are
promising for biomedical purposes.
The authors express their thanks to CICYT (Comisio´n Intermi-
nisterial de Ciencia y Tecnologı´a) (MAT2008-01412) for finan-
cial support for this work and to J. Parra for cytotoxicity assays.
CONCLUSIONS
In this study, we describe a novel route to obtain diols
derived from fatty acids. The application of thiol-ene addi-
tions to 10-undecenoate and oleate derivatives was carried
to obtain the required monomers. The resulting monomers
were then polymerized with 4,40-methylenebis (phenylisocya-
nate), in DMF solution using tin (II) 2-ethylhexanoate as cat-
alyst, to produce the corresponding thermoplastic PUs
(TPUs). Ultrasound irradiation has been proved to improve
the PU synthesis. The thus prepared PUs were characterized
and revealed good thermal and mechanical properties, mak-
ing them possible candidates for the substitution of petro-
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