Scalable synthesis and polymerisation of a β-angelica lactone derived monomer

Article abstract of DOI:10.1039/d0gc00338g

Bio-based levulinic acid is easily ring-closed to α-angelica lactone (α-AL). α-AL can be isomerized to the conjugated β-AL under the influence of base, but since this is an equilibrium mixture it is very hard to devise a scalable process that would give pure β-AL. This problem was circumvented by distilling the equilibrium mixture to obtain a 90?:?10 mixture of β-and α-AL in 88% yield. This mixture was used for Diels-Alder reactions on 3 terpenes and on cyclopentadiene in up to 100 g scale. The latter DA adduct was subjected to a ROMP reaction catalysed by the Grubbs II catalyst. The resulting polymer has some similarities to poly-norbornene but is more polar. The polymer can be processed into films with very good transparency.

Full text of DOI:10.1039/d0gc00338g

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DOI: 10.1039/D0GC00338G
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Scalable synthesis and polymerisation of a β-angelica lactone
derived monomer
Received 00th January 20xx,
Accepted 00th January 20xx
Andrea Dell’Acqua,a Bernhard M. Stadler,a, Sarah Kirchhecker, Sergey Tin and Johannes G. de
a
a
,†
†
Vriesa,*
DOI: 10.1039/x0xx00000x
Bio-based levulinic acid is easily ring-closed to -angelica lactone (-AL). -AL can be isomerized to the conjugated -AL
under the influence of base, but since this is an equilibrium mixture it is very hard to devise a scalable process that would
give pure -AL. This problem was circumvented by distilling the equilibrium mixture to obtain a 90 : 10 mixture of -
and -AL in 88% yield. This mixture was used for Diels-Alder reactions on 3 terpenes and on cyclopentadiene in up to 100g
scale. The latter DA adduct was subjected to a ROMP reaction catalysed by the Grubbs II catalyst. The resulting polymer
has some similarities to poly-norbornene but is more polar. The polymer can be processed into films with very good
transparency.
(solvent), ester derivatives (plasticisers, fragrances and
fuels),28-30 and the nylon intermediates adipic acid and
caprolactam.31-37 LA can be
Introduction
The debate about when fossil fuels will run out has been
ongoing for many years. Technological advancements have
made the exploitation of an increasing number of reservoirs
profitable. However, there is a consensus that all the natural
reservoirs may eventually become depleted, and thus a
gradual switch to renewable feedstocks will be necessary. And
although it will be impossible to fulfil the total global demand
for fuel and energy with biomass-based analogues, there is
more than enough biomass available on a yearly basis to serve
as the raw material for all of the chemicals we need.1
a)
O
O
O
O
O
O
AL
AL
AL
b)
O
O
O
H2
C7-C10
O
Alkanes
Ir-ReOx
K2CO3
Platform chemicals are small molecules that can be
produced in good yields from bio-based raw materials such as
O
O
lignocellulose or sugars by fermentation or by using
3
thermocatalytic reactions.2,
Further (preferably catalytic)
c)
O
R1
conversions of these chemicals allows the synthesis of fine
O
5 mol-% NEt3
Diene
15
chemicals,4-8 monomers8-13
,
and fuels.14,
Especially
O
O
R2
100°C
1.5h
renewable polymers16-18 and adhesives19 based on them, often
have novel properties which are considered an advantage
beyond renewability. One very prominent example of a bio-
based platform chemical is levulinic acid (LA) which can be
efficiently obtained by the acid-catalysed decomposition of the
C-6 sugars in lignocellulose.2, 20-23 LA itself can be converted
into a wide range of useful compounds.24 The most studied
O
O
106 g
90% Content
88% Yield
Dienes:
Myrcene - bio based
Farnesene - bio based
Cyclopentadiene - Fossil based
and used ones are aminolevulinic acid (herbicide), 2-methyl
26
tetrahydrofuran (solvent and fuel),25,
γ-valerolactone27
Isoprene - bio based
route possible
a. Leibniz Institut für Katalyse e. V.
Albert-Einstein-Strasse 29a, 18059 Rostock
E-mail: johannes.devries@catalysis.de
† These authors contributed equally
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1 a) Different isomers of angelica lactone b) The pioneering work of Mascal
and co-workers regarding the conversion of Al to fuels c) Our approach to upgrade α-AL
to functional monomers for ROMP
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converted into methyl vinyl ketone (monomer, vitamin A
precursor) under oxidative or via
conditions,38
decarbonylation of the intermediate angelica lactone.39, 40
View Article Online
O
O
DOI: 10.1039/D0GC00338G
Cat.
O
O
Temperature
There are three isomers of angelica lactone (α-AL, β-AL,
and γ-AL) which are shown in Scheme 1a. α-AL is
conveniently obtained by reactive distillation of LA in high
yields with water being the sole side product.41-43 This makes
α-AL an interesting bio-based building block, which has been
used as a monomer in UV-light induced, cationic and ring-
opening polymerisations.44 The UV-light induced and cationic
polymerisations tend to afford only sticky colourful
oligomers.45,46 The product obtained by anionic ring-opening
polymerisation has a higher molecular weight but consists of
a mixture of C-C and ester connected monomer units.47 Such
an irregular structure would make it rather challenging to use
these polymers in applications where an exact control of the
polymer structure is necessary. For instance, the oligomers
that are typically used in the production of thermoplastic
polyurethanes require well-defined end groups.48 Higher
molecular weight polyesters on the other hand are typically
used in extrusion and spinning processes49 for which high
crystallinity and melting points are desirable traits, something
poly α-AL may not provide due to the high stereo- and regio-
irregularity. Other studies which were initiated by Mascal and
co-workers focused on the utilisation of α-AL as a precursor
for jet-fuels. Here α-AL is converted in the presence of K2CO3
to its di- and trimers, which can then be hydrodeoxygenated to
the branched alkanes typically used in gasoline (Scheme 1b).50,
51 The dimerisation step caught our attention as this reaction is
CPD
90/10
-Al

Eq.
(CPD)
T
t
Yielda
Entry
Catalyst
Endo/Exoa
[%]
[°C]
[h]
5 mol%
Al(Otf)3
1
2
3
RT
16
2
-
-
-
-
5 mol%
Al(Otf)3
3
100
3
4
5
-
-
-
3
3
3
80
0.5
0.5
0.5
19
91/9
70/30
95/5
100
60
40b
25
6
7
8
5 mol% ZnCl2
5 mol% ZnCl2
3
3
80
RT
RT
0.5
16
16
40
84/16
89/11
90/10
63
5 mol% ZnCl2 10
90 (86)b
General conditions: Reactions were carried out in closed reaction tubes and
heated with microwave irradiation. aDetermined by 1HNMR spectroscopy;
bIsolated by column chromatography
known to occur via the β-isomer which we deem to be an We wanted to use these Diels-Alder adducts in ring-opening
interesting building block. -AL itself has been dimerised to jet- metathesis reactions (ROMP) to obtain bio-based norbornene
fuels precursor and, for the first time, polymerized to acrylic- polymers. They can be easily functionalised as it is known that
type polymers by the group of Hong.52
the -valerolactone moiety readily undergoes reactions with
We have explored the synthesis of bio-based monomers primary amines.63
via Diels-Alder reactions (Scheme 1c) to obtain lactone
functionalised derivatives. Diels-Alder reactions for the
O
O
preparation of bio-based monomers recently received much
attention53-56 as they are very atom-efficient and usually can
be conducted in the absence of a solvent. Such functionalised
norbornenes can then be used for example as monomer for
adhesives,57 shape memory materials58 and polymer based
electrolytes in batteries.59 Adducts containing the angelica
lactone moiety have been prepared using this approach, but
thus far the AL that was used in these reactions was not
made from αAL, likely due to the necessity to have pure
AL.60-62
O
5 mol-% NEt3
O
O
O
100°C
1.5h
10%
-Al
90%
-Al
-Al



88% Yield
106 g
Scheme 2 Synthesis of a mixture enriched with -AL.
Results and Discussion
α-AL and β-AL are in equilibrium with each other in the
presence of base but also undergo dimerisation under the
reaction conditions necessary for isomerisation, a fact that
makes the highly selective synthesis of pure β-AL from α-AL at
high conversion a very challenging reaction that has not been
Table 1 Screening of the reaction conditions for the DA-reaction between -AL and CPD
2 | J. Name., 2012, 00, 1-3
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achieved up to this date. Usually yields between 40-60% of β-
AL can be achieved.64-66 This and the fact that in the existing
protocols product separation needs to be carried out by
column chromatography negatively affects the sustainability
and scalability. Additionally, the formation of various
azeotropes (see ESI) of the isomers under vacuum distillation
conditions results in a further yield penalty.
However, as shown in Scheme 2, we found that it is
possible to obtain a high content of β-AL in the product
fraction (90 mol-%), if the reaction time is kept rather short
and no solvent is used. The usage of triethylamine allows its
easy separation during vacuum distillation preventing further
dimerisation. The
View Article Online
DOI: 10.1039/D0GC00338G
O
O
O
R
200 °C
16 h
+
+
O
O
O
R
R
10 eq.
Entry
Diene
Yielda [%]
1
Isoprene (R = Me)
Myrcene (R =
60
)
2
3
54
18
1) 5 mol% ZnCl2, 30 min.
2) 2 eq, CPD fed over 10h
O
O
O
O
+
O
O
10 h, 70°C
-Farnesene (R =
)
11%
89%
90 mol%
-Al
82% Yield
Table 2 Screening of dienes in the Diels-Alder reaction with -AL
Scheme 3 Synthesis of the DA-adduct with CPD on 50 g scale
General conditions: reactions were carried out in 4 ml vials equipped with a
septum and magnetic stirring bar placed into a stainless steel 300 ml autoclave
pressurised with 20 bar of nitrogen. aIsolated yield.
mixture of the two angelica lactones was thus obtained in 88%
yield after distillation on a 100 g scale. A small forerun was
obtained, containing mostly -AL, which can be reused. The
only side products that remain are the di- and trimers which
O
O
O
Grubbs II (0.5 mol%)
O
might be interesting raw materials for fuels. 50, 67
.
DCM
1 min
n
Next the solvent free Diels-Alder reaction of the AL-mixture
containing 90% β-AL with cyclopentadiene (CPD) was
investigated in the presence and absence of a catalyst (Table
1). When aluminium triflate was used as the catalyst (entries
1+2) the contents were converted to a black charred residue
and no desired product formation was observed. When no
catalyst was used it was possible to obtain the product in 19 to
25% at 80°C to 90°C respectively (Table 1, entries 3 and 5). At a
higher temperature (100°C) it was possible to isolate 40% of
the DA-adduct. A similar yield was obtained at 80°C in 30
minutes when anhydrous zinc chloride was used as catalyst.
Increasing the reaction time from 30 minutes to 16 hours only
led to a yield of 63%. As it was apparent that the low yields are
caused by the competing di- and trimerisation of the CPD with
itself, the equivalents of CPD with respect to β-AL were
increased up to 10. This resulted in 90% yield of the desired
product. Since a process that uses 10 eq. of CPD is not very
efficient and sustainable, we decided to perform the reaction
under semi-batch conditions on a 50 g scale (See Scheme 3).
After dissolving zinc chloride in the β-AL enriched mixture 2
equivalents of CPD were slowly added over 10 hours. The ZnCl2
could be easily separated by adding acetone and any formed
di- or trimers of CPD could be precipitated by simple
dissolution of the product mixture in methanol. No column
chromatography was necessary to obtain the product as a
mixture of isomers in 82%
Mn=171 kg/mol
Scheme 4 Initial ROMP experiment with Cp/-AL
yield. This is very fortunate as purification by distillation of the
adduct is not an option since the retro-Diels-Alder reaction to
β-AL commences at 136°C (See ESI for a DSC/TGA). Aiming for
higher renewable atom content, dienes other than
cyclopentadiene have also been investigated. Use of furans
a
N
N
Mes
Ph
Mes
Cl
Ru
Cl
PCy3
Figure 1 Grubbs II
metathesis catalyst
was not successful under both catalysed and thermal
conditions, due to rapid formation of humines (See Supporting
Information). On the other hand, use of isoprene and terpenes
(-myrcene and farnesene) were more promising (Table 2).
Upon reaction with AL at high temperature, full conversion
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was achieved. The diene oligomers formed as by-product were strain that is present in the norbornene-type struVciteuwrAert.icOlenOntlhinee
DOI: 10.1039/D0GC00338G
removed by a simple filtration through a short silica path, other hand, the polymerization of the Cp/-AL adduct was
affording the Diels-Alder products in yields of up to 60%.
successful. Interestingly, at the initial monomer concentration
The ring-opening metathesis polymerisation (ROMP) of the of 1 mol/l after the addition of the initiator (0.5 mol-% w.r.t.
DA adducts was next investigated.68-70 Grubbs’ 2nd generation Cp/-AL) the reaction mixture formed a gel within one minute
catalyst was chosen for this. Dichloromethane (DCM) is the indicating a great reactivity of this monomer in ROMP
solvent of choice for this reaction, since the polymeric reactions (See Scheme 4). To further fine tune the
products normally dissolve in it, thus allowing the formation of polymerisation conditions different solvents, commonly
higher molecular weight polymers. The fully bio-based adducts considered as “greener” or safer choices, beside DCM were
shown in Table 2 were not polymerizable under these investigated at a lower catalyst loading. These solvents and
conditions, probably due to the absence of the ring
their influence on the ROMP of the CP adduct are shown in
Table 3. 2-Methyl-tetrahydrofuran was chosen due to its
similar polarity to DCM but lower toxicity71 and potential
renewability. Ethyl acetate (EtOAc) was included as it is one of
Table 3 Influence of different solvents in the ROMP of Cp/-AL
O
O
O
73
the safest solvents regarding flammability and toxicity,72,
Grubbs II (0.25 mol%)
O
which is also applicable for methyl isobutyl ketone (MIBK).
Although not a “green” compound in general we also included
tert-butyl methyl ether (MTBE) since it remains still a common
solvent in industry and does not form peroxides.
Notably, in all solvents expect DCM a white precipitate
appeared after a few minutes of reaction time. In all the
attempts the DA adduct did polymerise, yielding white gummy
materials after precipitation with cold methanol.
As expected, the polymer obtained in DCM had a
substantially higher molecular weight than the one predicted
assuming a living polymerization (Table 3, entry 1). When using
2-MeTHF the obtained molecular weight was reduced to 80.6
kg/mol. Ethyl acetate (EtOAc) and MTBE on the other hand
afforded molecular weights of 64.1 kg/mol and 67.8 kg/mol
respectively which are relatively close to the theoretical
molecular weight of 65.6 kg/mol. These findings indicate that
the very safe and potentially renewable solvent EtOAc does
not lead to catalyst deactivation/inhibition compared to DCM.
Aiming to assess the relation between substrate to
catalyst ratio and molecular weight of the polymer, different
catalyst loadings were investigated. A substantial linearity in
the range between 100 : 1 and 600 : 1 (mol of substrate / mol
of catalyst) was achieved, showing that the final molecular
weight can be controlled by varying the amount of catalyst
(Fig. 2).
Solvent, r.t.
16 h
n
b
Mn
b
Yielda
[%]
Mw
Entry
Solvent
[kg/
mol]
Đb
[kg/mol]
-
Calc.
DCM
-
65.6
122
-
-
1
78
264
2.17
2-
MeTHF
2
69
80.6
154
1.91
3
4
5
EtOAc
MIBK
MTBE
61
52
70
64.1
70.9
67.8
118
1.46
138
1.85
2.06
2.04
The obtained polymers tend to decompose at about 378°C
in a nitrogen atmosphere. No melting points were detected in
the DSC analysis (as expected) confirming the amorphous
nature of the polymer. In addition, a glass transition could also
not be observed, which is an indication that the Tg occurs at or
above the decomposition temperature. (Figures S8 and S9). In
contrast, poly-norbornene has a Tg at 35 °C. It is not easy to
account for the huge difference in Tg between the two
polymers. The high Tg of our polymer can possible be
explained by the stereoregular nature. For steric reasons, the
polymer has to be all-trans.
aIsolated yield; bDetermined by GPC (DMF/LiBr);
Contact angle measurement revealed that the presence of
the lactone moiety in poly-Cp/-AL is increasing the
hydrophilicity compared to poly-norbornene (θ(Cp/-AL) =
75.7±1.9°; θ(Cp/poly-norbornene) = 83.9±2.3°).
Figure 2 Obtained Mn values via ROMP of Cp/-AL performed at different
substrate to catalyst ratios (S/C)
As shown in Figure 2 the polymer obtained via the ROMP
of Cp/-AL can be cast into clear transparent films.
4 | J. Name., 2012, 00, 1-3
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Comparison with poly-norbornene (prepared with the same (wet) surfaces in coating applications. The lactone moiety does
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catalyst in DCM) shows there is no negative effect of the not only increase the bio-based caDrbOoI:n10.c10o3n9t/eDn0tGCo0f033t8hGe
lactone group on transparency. Therefore it might be possible materials, but it also enables further derivatization which gives
to find similar applications in optical wave guides,74 easy access to a plethora of different polymers.
75
transparent coatings
or other applications where high
transparency in a large range of wavelengths is necessary.76
Experimental section
Preparation of a mixture of angelica lactones with 90 mol-%
content of the β-isomer.
-Angelica lactone (98%, 120 g, 1.2 mol) was added to a 250
ml two neck flask equipped with a condenser and a magnetic
stirrer followed by the addition of triethyl amine (5 mol%, 8.5
ml). The mixture was heated to 100 °C under an argon
atmosphere and monitored by 1H-NMR. After 1.5 hours a ratio
of β/α-angelica lactone of 90-95/10-5 was reached and the
condenser exchanged with a distillation head. Subsequent
vacuum distillation at 6 x10-2 mbar yielded two fractions: 38-
42°C containing mainly the α-isomer (this fraction can be
reused for the next isomerisation run) and a fraction at 45-
50°C containing a mixture of angelica lactones with 90 mol-%
content of the β-isomer (106 g, 88% of theory).
1H NMR (400 MHz, CD2Cl2, signals of β-isomer are reported) δ
7.46 (dd, J = 5.7, 1.5 Hz, 1H), 6.04 (dd, J = 5.7, 2.0 Hz, 1H), 5.11
(qt, J = 6.9, 2.0, 1.5 Hz, 1H), 1.41 (d, J = 6.9 Hz, 3H). 13C NMR
(101 MHz, CD2Cl2) δ 173.3, 158.0, 121.3, 80.0, 19.0.
Diels-Alder reaction between -angelica lactone and
cyclopentadiene.
In a dry and argon purged 10 ml reaction tube the angelica
lactone mixture (-AL=90%, 10 mmol, 900 µl) and the desired
catalyst were mixed. Freshly prepared CPD (3.0-10 eq. 2.5-8.3
ml) was added, the tube was sealed and heated with a
microwave oven to the desired reaction temperature where it
is kept for the indicated time. The 2 diasteroisomers of the
product were separated by flash column chromatography
(heptane/ethyl acetate 8:2), affording the endo and exo
isomers (each of them is a racemic mixture of 4 stereoisomers)
adduct as colourless oils (endo: 949 mg, 5.9 mmol, 30%; exo:
119 mg, 0.7 mmol, 7%; endo/exo 89/11).
Figure 3 Films obtained from a) poly-Cp/-AL and b) polynorbornene obtained by
solution castings. Below: Absorbance spectra of these films in the range from 150-1000
nm. E=Extinction.
Conclusions
It is still challenging to obtain pure β-AL from renewable α-AL
in a scalable and sustainable procedure. In this work we show
that a 90 : 10 mixture of the two can be prepared in a scalable
procedure. This mixture is rich enough in β-AL to be used
efficiently as dienophile in Diels-Alder reactions with
cyclopentadiene, isoprene, myrcene and -farnesene. The
adduct obtained with cyclopentadiene can be efficiently
polymerized in a ROMP reaction catalysed by the Grubbs II
catalyst, either in DCM or alternatively using the renewable
solvents 2-MeTHF or ethyl acetate. A linear relationship
between Mn and the amount of catalyst is observed between
a monomer to catalyst ratio of 100 to 600, thus allowing
control of the molecular weight. The resulting polymers can be
processed into films that have high transparency over a wide
range of wavelengths. Compared with poly-norbornene the
lactone moiety increases (surface) hydrophilicity, which could
give rise to various benefits; for instance, higher miscibility
with polar additives or better interaction with certain
Exo: 1H NMR (300 MHz, CD2Cl2) δ 6.21 – 6.04 (m, 2H), 4.17 (qd,
J = 6.4, 3.2 Hz, 1H), 3.24 – 3.03 (m, 1H), 2.82 (dtq, J = 3.1, 1.5,
0.8 Hz, 1H), 2.59 (dt, J = 8.3, 1.3 Hz, 1H), 2.08 – 1.97 (m, 1H),
1.50 – 1.35 (m, 7H), 1.30 (d, J = 6.4 Hz, 3H). 13C NMR (75 MHz,
CD2Cl2) δ 177.4, 136.4, 134.9, 78.9, 51.6, 48.5, 48.2, 46.0, 45.6,
22.8.
1
Endo: H NMR (300 MHz, CD2Cl2) δ 6.22 – 6.09 (m, 2H), 3.95
(qd, J = 6.5, 3.1 Hz, 1H), 3.26 – 2.97 (m, 3H), 2.67 – 2.54 (m,
1H), 1.57 – 1.46 (m, 1H), 1.39 – 1.29 (m, 1H), 1.25 (d, J = 6.5 Hz,
3H). 13C NMR (75 MHz, CDCl3) δ 177.3, 137.6, 137.5, 80.7, 50.0,
48.9, 47.6, 46.3, 43.1, 23.2.
Semi continuous synthesis of the -Angelica lactone CPD adduct.
A 500 mL 2-neck round bottom flask was filled with
dicyclopentadiene (100 mL) and iron(0) powder and equipped
with a distillation setup. Cyclopentadiene was obtained by
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thermal cracking of its corresponding dimer at 180 °C and then
condensed into a dropping funnel. 2 equivalents (84 mL, 67 g,
1.0 mol) were dropped over 10 hours into a 1 liter 3-necks
round bottom flask containing the angelica lactone mixture (-
AL=90%; 50 g, 0.5 mol) and zinc(II) chloride (3.5 g, 0.03 mol,
0.05 eq) while heating up to 70 °C. Once the addition was
complete, the reaction was stirred for another 10 hours and
monitored by GC. After cooling down to room temperature,
acetone (80-150 ml) was added to precipitate the Lewis acid
catalyst. The remaining cyclopentadiene and -Angelica
lactone were removed by vacuum distillation (50 °C/0.06
mbar) and the remaining cyclopentadiene dimers were
precipitated by addition of ice-cold methanol (100 ml)
followed by filtration. Solvent removal afforded the product as
an orange liquid (69 g, 0.4 mol, 82% yield, endo:exo 89/11).
Notes and references
1.
View Article Online
P. N. R. Vennestrøm, C. M. OsmuDnOdsI:e1n0,.1C0.3H9/.DC0hGrCis0t0e3n3s8eGn
and E. Taarning, Angew. Chem. Int. Ed., 2011, 50, 10502-
10509.
2.
Top Value-Added Chemicals from Biomass Vol. I—Results
of Screening for Potential Candidates from Sugars and
Synthesis Gas, ed. T. Werpy and G. Petersen, U. S.
Department of Energy (DOE) by the National Renewable
Energy Laboratory a DOE national Laboratory, 2004.
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The authors declare no conflict of interest
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The authors greatly appreciate the help of C. Wulf with the
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and TGA measurements. AD and SK are highly thankful to
Henkel AG & Co for the financial support. The authors are
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grateful to Andreas Taden, Kenji Ito, Adrian Brandt and Horst
Beck (Henkel AG & Co) for the useful discussions.
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This research was funded by the Bio-Based Industries Joint
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Undertaking under the European Union’s Horizon 2020
research and innovation program, grant agreement No 720695
(GreenSolRes).
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A new bio-based norbornene polymer was prepared starting from -angelica lactone, which can be
easily obtained from the platform chemical levulinic acid. The synthetic procedure is highly atom
efficient and scalable. The new material displays transparency comparable to pure, petrochemical-
derived polynorbornenes.