Chain Multiplication of Fatty Acids to Precise Telechelic Polyethylene

Article abstract of DOI:10.1002/anie.201702796

Starting from common monounsaturated fatty acids, a strategy is revealed that provides ultra-long aliphatic α,ω-difunctional building blocks by a sequence of two scalable catalytic steps that virtually double the chain length of the starting materials. The central double bond of the α,ω-dicarboxylic fatty acid self-metathesis products is shifted selectively to the statistically much-disfavored α,β-position in a catalytic dynamic isomerizing crystallization approach. “Chain doubling” by a subsequent catalytic olefin metathesis step, which overcomes the low reactivity of this substrates by using waste internal olefins as recyclable co-reagents, yields ultra-long-chain α,ω-difunctional building blocks of a precise chain length, as demonstrated up to a C₄₈ chain. The unique nature of these structures is reflected by unrivaled melting points (T_m=120 °C) of aliphatic polyesters generated from these telechelic monomers, and by their self-assembly to polyethylene-like single crystals.

Full text of DOI:10.1002/anie.201702796

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Accepted Article
Title: Chain Multiplication of Fatty Acids to Precise Telechelic
Polyethylene
Authors: Timo Witt, Manuel Häußler, Stefanie Kulpa, and Stefan
Mecking
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201702796
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10.1002/anie.201702796
Angewandte Chemie International Edition
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Chain Multiplication of Fatty Acids to Precise Telechelic
Polyethylene
Timo Witt†, Manuel Häußler†, Stefanie Kulpa, Stefan Mecking*
Abstract: Fatty acids are unique feedstocks for the generation of their length corresponds to the typical thickness of phospholipid
functionalized aliphatic mid- and long-chain compounds. The bilayers, which offers possibilities for synthetic structures
maximum chain length achievable is practically limited by the length mimicking these ubiquitious structures of biological systems.[15]
of the fatty acid. We reveal a novel strategy starting from common Further, such chain lengths are also found as the typical thickness
monounsaturated fatty acids that provides ultra long aliphatic of crystalline lamellae in polymers, which determines their
α,ω-difunctional building blocks by a sequence of two scalable materials properties. This suggests that there are specific lengths
catalytic steps that virtually double the starting materials’ chain length. of a methylene sequence, which show an extraordinary strong
The central double bond of the well-known α,ω-dicarboxylic self- driving force for crystallization. Artificial ultra long-chain
metathesis products of fatty acid is shifted selectively to the compounds with these lengths thus might be powerful building
statistically much disfavored α,β-position in a catalytic dynamic blocks for various crystalline materials.
isomerizing crystallization approach. ‘Chain Doubling’ via
a
However, the very limited perspectives for accessing such
subsequent catalytic olefin metathesis step that overcomes this compounds efficiently has discouraged broader studies of their
substrates’ low reactivity by using waste internal olefins as recyclable properties and potential. Reported strategies for the synthesis of
co-reagents yields ultra long-chain α,ω-difunctional building blocks of aliphatic α,ω-difunctional building blocks with a chain length >C30
a precise chain length, as demonstrated up to a C48 chain. The unique commonly comprise tedious multi-step reaction sequences. E.g.
nature of these structures is reflected by unrivaled melting points (Tm multiple cycles of Wurtz coupling of α-halo ω-alcohols can be
= 120 °C) of aliphatic polyesters generated from these telechelic used for the preparation of a C44 diol.[16] Catalytic methods have
monomers, and by their self-assembly to polyethylene-like single also been investigated, as reported by Augustin and Schäfer in a
crystals.
Glaser coupling starting from oleic acid. [17] Yet again, laborious
steps for the preparation of a terminal acetylene are required to
access the desired (bis-triunsaturated) C36 α,ω-diester.
Additionally, the reagents and by-products involved in all these
procedures are prohibitive to performing them on larger scales.
As an example of entirely different polymerization approaches,
catalyzed chain growth of ethylene can yield difunctional chains.
Limitations of these approaches are imprecise chain lengths and
imperfect end group fidelity. Additionally, in most cases specific
functionalized magnesium reagents and additional post-
polymerization steps are required which are not well suited for
large scales.[18-20]
Naturally occurring fatty acids are, due to their particular
molecular structure, a feedstock of choice for the generation of
molecules comprising long methylene sequences with an
attached functional group. The generation of sebacic acid from
castor oil is a well established example. This mid-chain
dicarboxylic acid is used as a monomer building block for high
performance nylons[1] or biodegradable polyesters.[2] Recently
developed catalytic routes allow for the preparation of other long-
chain α,ω-difunctional compounds from readily available fatty
acids, such that the product comprises the full length of a fatty
acid chain. E.g. dicarboxylic acids HOOC-(CH2)n-COOH with n =
18 or 19 can be generated from the C18 fatty acids oleic or stearic
acid.[3,4]
We now report an approach that allows for the ‘doubling’ of the
chain length of readily available fatty acid starting materials,
employing catalyzed steps exclusively which also do not consume
further reagents and do not generate excessive amounts of
Beyond these chain lengths, ultra long-chain compounds with
methylene sequences that significantly exceed the length of a
typical fatty acid chain appear relevant from different perspectives.
For instance, such structural motifs occur as ceramide[5,6] – a main
component of the stratum corneum of the epidermis layer of the
human skin, and they are part of the cellular walls of several algae
undesired
side-products.
With
this
technique,
α,ω-difunctional building blocks with a C48 chain can be accessed
that open opportunities as exemplified by self-assembled
structures and crystalline materials.
Self-metathesis of unsaturated fatty acid derivatives, as a well-
established and industrially used process,[21] allows for the
synthesis of internally unsaturated dicarboxylic compounds.
These reactions, generally leading to equilibrium mixtures of
products and starting material, can be driven in a favorable
direction by choosing appropriate reaction conditions.[22,23] By
application of Hoveyda-Grubbs 2nd generation complex (Cat1) as
a metathesis catalyst, the reaction equilibrium of oleic acid (1a)
can be driven to the diacid (and stoichiometric amounts of
octadecene) by its precipitation from the reaction mixture.
Our concept for ‘Chain Multiplication’ relies on a selective shift of
the double bond toward a chain end, starting from the above self-
metathesis product. From here, an olefin metathesis step will
result in a virtual doubling of the chain length, with expulsion of
only a small C4 fragment (Scheme 1). The internal double bond of
the chain-doubled products offers itself for e.g. hydrogenation to
generate ultra long-chain α,ω-difunctional saturated compounds.
species (algaenans)[7-11] and of plants (cutin and suberin)[12-14]
.
The highly hydrophobic nature of the α,ω-difunctional building
blocks here is exploited to generate an insoluble polymer,
protecting the organisms from undesired influences from their
environment as well as from water loss. From a different angle,
[*]
T. Witt, M. Häußler, S. Kulpa, Prof. Dr. S. Mecking
Chair of Chemical Materials Science, Department of Chemistry
University of Konstanz
Universitätsstraße 10, 78457 Konstanz, Germany
E-mail: stefan.mecking@uni-konstanz.de
[†]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201702796
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Scheme 1. ‘Chain Doubling’ strategy for the preparation of ultra long-chain α,ω-diester 4a.
To promote isomerization of the starting material’s internal double
bond, we employed [(dtbpx)Pd(OTf)2] (Cat2), bearing the
electron-rich, bulky diphosphine 1,2-bis{(di-tert-butylphosphino)-
methyl}benzene (dtbpx) ligand, as a highly active catalyst.[24-26]
Upon exposure of diester 2a to a methanolic solution of 0.5 mol%
of Cat2, an equilibrium of the isomers of the unsaturated diester
differing in the position of the double bond along the chain is
attained rapidly. The slightly higher stability of the target α,β-
unsaturated ester 3a reflects in a slight preference for 3a in an
otherwise statistical isomerization equilibrium. That is, at 30 °C
the α,β-unsaturated isomer 3a is present in 11.7% with a (E/Z)
ratio of 55 (cf. SI Figure S1).
proceed with satisfactory yields and 4a was only formed to a
minor extent.[28] Apparently, the electron deficient character of the
olefinic bond in 3a reduces its reactivity to such an extent that no
significant self-metathesis occurs. [29] To overcome this problem,
intermittent alkenolysis with electron-richer and more reactive
short-chain olefins was performed (Scheme 2). Readily available,
low-cost 2-butene or a mixture of internal hexene isomers,
respectively, were employed (for further information on
alkenolysis of 3a cf. Supporting Information).
To favor a high conversion of 3a, the short chain olefin co-reagent
was used in excess. In a two-step, one-pot approach first the
cross metathesis was performed with Cat1, followed by an in situ
self-metathesis step of the generated monoesters 5a. The latter
self-metathesis could be induced by simply applying vacuum to
the reaction vessel. By this procedure, more than 95% of the
starting material could be transformed into the desired ultra long-
chain diester 4a. A potential pitfall in this concept is double bond
isomerization at this stage, a ubiquitous reaction in the presence
of ruthenium metathesis catalysts.[30-32] This would lead to
products of variable chain lengths. To this end, isomerization
could be suppressed virtually completely by the choice of
appropriate reaction conditions, particularly reaction times and
the presence of 1,4-benzoquinone. Hereby, selectivities for the
target chain lengths of ≥ 98 % were achieved, as determined by
GC analysis (Figure 1).
In order to achieve a full conversion to 3a, a physical separation
from the continuously catalyzed dynamic equilibrium was
employed. From screening of a range solvents, and temperature
sequences, a crystallization from methanol evolved as condition
of choice. Due to its slightly different polarity and lower solubility,
crystallization of 3a occurs preferentially despite its comparatively
low concentration in the equilibrium of the double bond isomers.
In combination with continuous replenishment by the ongoing
isomerization equilibrium, this provides a net driving force for
conversion of 2a to 3a. In this batch-type procedure, a decreasing
concentration of the double bond isomers in their entirety in
solution with increasing amount of already crystallized material is
compensated by decreasing the temperature, in order to maintain
supersaturation. In this fashion, a virtually complete yield of
crystallized material of 95% could be obtained for diester 3a. The
selectivity for the desired α,β-unsaturated diester was 84%, as
Isolation of the target C32 α,ω-diester was possible by
recrystallization, to yield an overall 86% of 4a in a purity greater
than 99%, as determined by GC analysis (cf. SI Figure S24).
To access even longer chains, we employed erucic acid (1b) as
1
determined by H NMR spectroscopy. That is, by the dynamic
catalytic isomerizing crystallization procedure the thermodynamic
isomer composition can be overcome in favor of the desired α,β-
unsaturated isomer. From the crude products, pure (≥ 99%) 3a
was obtained by recrystallization in 70% overall yield. The
remaining internal isomer material can be recycled to the dynamic
isomerization/crystallization.[27]
With this material in hand, we pursued a ‘Chain Doubling’ to the
desired ultra-long chain structures through olefin metathesis. A
direct self-metathesis of the α,β-unsaturated diesters did not
a starting material. The self-metathesis and subsequent
esterification in methanol of this C22 unsaturated fatty acid yields
an internally unsaturated C26 α,ω-diester. C26 diester 2b can be
used in an double bond isomerization for the preparation of α,β-
unsaturated diester 3b. As the hydrocarbon segment of diester 2b
is significantly longer as compared to the segment of diester 2a
from oleic acid, in the isomerization equilibrium the α,β-
unsaturated isomer is present to a lesser extent of 6.7%
((E/Z) = 57).
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Scheme 2. ‘Chain Doubling’ of α,β-unsaturated diester 3a via cross metathesis with short-chain olefins.
As an additional effect of the longer chain, the decreased solubility
of the compound has to be overcome in a dynamic isomerizing
crystallization. Here, working in ethanol led to a crude yield of 95%
with a selectivity for 3b of 63%. The pure compound, isolated in
57% yield after recrystallization, can be ‘chain doubled’ to C48 α,ω-
diester 4b via the two-step, one-pot process described above.
The desired ultra long-chain diester could be obtained in 92% in
a purity greater than 99%, as determined by GC analysis (cf.
Supporting Information).
accessible in pure form on a multigram scale, we prepared
polyesters-32,32 and -48,48 in a melt polycondensation using
titanium(IV) tetrabutoxide ([Ti(OnBu)4]) as a transesterification
catalyst (cf. SI Table S3). The influence of the long aliphatic
polymethylene segment of the monomers is reflected in the
thermal properties of the polyesters and unprecedented melting
temperatures were observed. Whereas polyester-32,32 exhibits
a peak melting point of Tm = 113 °C, the longer analog polyester-
48,48 shows a transition at Tm = 120 °C. Accordingly, polyester-
48,48 is the first aliphatic polyester that clearly surpasses the
melting point of low density polyethylene (LDPE) which
represents an important benchmark value for many
applications.[33]
Chain Doubling
products 4a
A well crystallizable hydrocarbon segment is also crucial for
physical cross-linking of aliphatic thermoplastic elastomers
(TPEs).[34] Polycondensation of C48 diester 7b and
poly(trimethyleneglycol) (PPDO, Mn = 3000 g mol-1) using
[Ti(OnBu)4] as transesterification catalyst yielded an elastomeric
material (TPE-48, 80 wt% soft segment) with a narrow melting
transition (Tm = 80 °C). Tensile test on injection molded samples
1.0
0.5
α,β-unsaturated
starting material 3a
revealed
a good recovery and strength (Young modulus
undesired chain
Hexenolysis
products 5a
E = 9 MPa and elongation at break εb = 780% cf. SI for complete
data). By strong contrast, a material based on C23 dicarboxylic
acid and a correspondingly shorter PPDO as a soft phase with an
overall similar composition (73 wt% soft segment) possesses a
broad melting transition with a peak at only Tm = 39 °C, which
could not even be injection molded on account of the low
crystallization temperature and crystallinity.[35] This underlines the
ability of the ultra long-chain units to function as a crystallizable
block in block copolymers. The aforementioned mechanical
properties of TPE-48 also compare favorably to those of
commercial thermoplastic polyester elastomers.[36]
lengths
0.0
5
10
15
retention time [min]
Figure 1. GC trace of crude product after ‘Chain Doubling’ of α,β-unsaturated
diester 3a with 2-/3-hexenes.
This observations of a high crystalline order encourage a study of
the self-assembly properties of the ultra long-chain compounds.
To this end, we synthesized C48 dicarboxylic acid 9 with two more
hydrophilic carboxylate moieties by saponification of diester 7b
with KOH in THF. Notably, WAXS of 9 as a bulk material already
revealed the characteristic orthorhombic polyethylene structure.
This differs from the behaviour of shorter analogs, as for example
C23 dicarboxylic acid 10 (available from isomerizing
alkoxycarbonylation of erucic acid)[37] which pack
As a further demonstration of the utility, reactivity and purity of the
ultra long-chain α,ω-diesters, saturated analogs, corresponding
diols and a polyester were generated (Scheme 3). Saturated
diesters 7a and 7b were prepared by hydrogenation of 4a and 4b
using Pd/C and hydrogen. The diesters were obtained in 95 and
93% respectively, in polymerization grade purity as determined by
GC (cf. SI Figures S26 and S27). Corresponding diols 8a and 8b
were synthesized by reduction of the ester moieties using LiAlH4
in 92% and 85% yield, respectively. With these compounds
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Scheme 3. Preparation of polymeric materials composed of aliphatic ultra long-chain monomers.
Dynamic light scattering (DLS) revealed the presence of particle
with sizes around 6-7 nm with a rather narrow size distribution of
PDI = 0.22.
We believe that the precise ultra-long polymethylene sequence
acts as a strong driving force for crystallization leading to the
formation of crystalline nanoparticles. These particles are
stabilized by the hydrophilic interaction of the ordered -COOH
groups with the surrounding medium. Astoundingly, transmission
electron microscopy (TEM) revealed anisotropic shapes
resembling the hexagonal structure of typical crystalline
polyethylene nanoparticles (Figure 3).[38] The crystal thickness
was determined from particles oriented parallel to the electron
beam, yielding a value of 6 nm. This thickness corresponds to the
length of the fully extended C48 dicarboxylic acid and supports the
single layer, extended chain nature of the formed nanocrystals (cf.
Figure 3d, 3e).
In summary, we show how the methylene chains of two fatty acid
molecules can be linked to form an ultra long-chain product. This
provides access to α,ω-difunctional products with chain lengths
that bear potential in many instances, but have not been
accessible to date. Readily available unsaturated fatty acids serve
as a starting material. Our approach comprises catalytic steps
exclusively, namely olefin isomerization, crystallization and olefin
metathesis, which are all provenly suited for large scales.[21,39] As
the only co-reagents, short chain olefins which are waste products
of the petroleum industry and furthermore can be recycled are
required. Almost the entire starting material is incorporated in the
product, and the small amount of short-chain esters formed can
Figure 2. WAXS of semi crystalline polyethylene (black), C48 dicarboxylic acid
9 (red), and C23 dicarboxylic acid (blue); self-assembly behaviour of ultra long-
chain dicarboxylic acid 9, and C23 dicarboxylic acid for comparison 10, in
aqueous CsOH solution.
into a different crystal structure (Figure 5).
To study the self-assembly to nanostructures, dicarboxylic acids
9 and the C23 analog for comparison were dissolved in hot THF
and injected in a 0.01 M aqueous CsOH solution under
ultrasonication. In contrast to C23 diacid 10, which formed
macroscopic precipitates directly upon addition, C48 diacid 9 leads
to the formation of a clear transparent dispersion (Figure 2).
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10.1002/anie.201702796
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COMMUNICATION
yield) in a purity greater than 99%, as determined by 1H NMR spectroscopy
and GC analysis.
also be useful materials. In principle, the procedure can be
iterative as the ‘chain-doubled’ product can serve as the starting
material for another cycle of isomerization/crystallization and
metathesis. The potential and unique properties of these new
building block are highlighted by
Synthesis of Dimethyl 1,32-Dotricont-16-enedioate (4a). In a typical
experiment, 10.50 g (30.8 mmol) of α,β-unsaturated diester 3a were
degassed in a Schlenk tube. The diester was suspended in 50 mL of dry
heptane and 39.25 mL (26.30 g, 312 mmol) of 2-/3-hexene. 96.6 mg
(154 µmol) of Cat1 and 33.8 mg (0.31 mmol) of 1,4-benzoquinone were
dissolved in 1.0 mL of dry dichloromethane and added to the mixture via
syringe. The mixture immediately turned dark green and was stirred at
40 °C for 20 minutes. At 30 °C, the solvent was removed in vacuo, another
50 mL of dry heptane were added, and the slurry was sonicated for 1 min.
Afterwards, the solvent was removed in vacuo, and the mixture was kept
at 30 °C for 15 minutes in vacuo. The reaction was quenched by the
addition of 10 mL of ethyl vinyl ether and was stirred at room temperature
for 5 min. After addition of 20 mL of dichloromethane, the reaction was
allowed to stir for further 10 min. The dark green solution was filtered
through a short silica plug and washed with dichloromethane. The solvent
was removed by rotary evaporation, and the colorless solid was dried in
vacuo. After recrystallization from methanol and from heptane ultra long-
chain diester 4a was obtained as colorless crystals in 78% (6.47 g,
12.1 mmol) yield in a purity greater than 99%, determined by GC analysis.
a
b
d
6 nm
c
e
~ 20 nm
Synthesis of Dimethyl 1,48-Octatetracont-24-enedioate (4b). In a
typical experiment, a Schenk flask was charged with 5.00 g (11.0 mmol)
of diester 3b and degassed. 5 mL of dry dichloromethane and 13.87 mL
(9.29 g, 110 mmol) of 2-/3-hexene were added and the mixture was heated
to 40 °C. After the diester was dissolved, 34.6 mg (55 µmol) of Cat1 and
11.9 mg (110 µmol) of 1,4-benzoquinone were added in 0.5 mL of dry
dichloromethane. The solution was stirred for 15 min. at 40 °C, cooled
down to room temperature, and 10 mL of dry heptane were added.
Subsequently the volatiles were removed slowly in vacuo. The resulting
solid was dissolved in dichloromethane and ethyl vinyl ether was added to
quench the reaction. After stirring for 15 min. at room temperature the
solution was diluted with dichloromethane and filtered through a short silica
plug to remove residual catalyst. After recrystallization from heptane, ultra
long-chain diester 4b was obtained as colorless crystals in 92% yield in a
purity greater than 99%, determined by GC analysis.
Figure 3. TEM images of stained (1% poly tungstic acid) C48-nanoparticles; (a)
overview; section with particles (b) parallel and (c) perpendicular to the electron
beam; (d,e) schematic representations of (b,c). The blue arrows indicate the
direction of the electron beam.
derived high melting polyesters, thermoplastic elastomeric block
copolymers and self-assembled nanocrystals. Particularly the
latter two cases reveal the particular ability of the ultra long C48
entity to crystallize in a polyethylene-like fashion. This clearly
differs from shorter analogs, corresponding to only the length of
one fatty acid chain. In this sense, the compounds accessible
through the ‘Chain Doubling’ approach can be regarded as
polyethylene telechelics[19,40] with an absolutely precise chain
length and end group fidelity.
Acknowledgements
We gratefully acknowledge financial support by the Stiftung
Baden-Württemberg. We thank Lars Bolk for GPC and DSC
measurements and Dr. Marina Krumova for TEM measurements.
Experimental Section
General Procedure for the Synthesis of α,β-Unsaturated Diesters 3a
and 3b. In a typical experiment, a 1 L round bottom flask with a nitrogen
inlet was charged with 25.00 g of diester 2a or 2b that were degassed and
subsequently dissolved in 900 mL of dry alcohol. The solution was cannula
transferred into a 1 L double-walled glass reactor with a magnetic stirring
bar attached to a cryostat. 0.5 mol% of isomerization catalyst Cat2,
dissolved in 5 mL of the corresponding alcohol were added to the solution,
and the reactor was closed and stirred at 250 rpm. By means of the
cryostat, the reactor was cooled (0.25 K min-1) to a temperature where the
solution became cloudy due to the formation of initial crystals (typical
clouding temperature for 2a: 6 – 10 °C; for 2b: 26 – 30 °C). Subsequently,
the reaction was cooled to the final temperature over the course of the
designated time (cf. SI Figures S2 and S9). Upon reaching this
temperature, the isomerization was quenched by the addition of 5 mL of
triethylamine and the slightly yellow precipitate was filtered off. The
precipitate was dissolved in a minimal amount of dichloromethane and
filtered through a short plug of silica to remove residual catalyst, followed
by recrystallization from methanol (in case of 2a) or heptane (in case of
2b), respectively, yielding the pure diesters 3a (70% yield) and 3b (57%
Keywords: olefin metathesis • selective double bond
isomerization • fatty acids • α,ω-difunctional building blocks •
precise polyethylene telechelics
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