Sustainable terpene-based polyamides: Via anionic polymerization of a pinene-derived lactam

Article abstract of DOI:10.1039/c7cc08266e

A sustainable lactam, which is derived from the renewable terpene β-pinene, is converted to polyamides via a convenient anionic ring-opening polymerization (ROP), which can be easily handled without the use of a costly catalyst. The resulting polyamides have prosperous thermal properties, which enable their future use as high-performance polymers.

Full text of DOI:10.1039/c7cc08266e

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DOI: 10.1039/C7CC08266E
Authors
Malte Winnacker*a,b and Jacob Saga,b
(Both authors contributed equally to this work.)
ORCID-ID of Dr. Malte Winnacker: https://orcid.org/0000-0001-7003-2671
ORCID-ID of M.Sc. Jacob Sag: http://orcid.org/0000-0003-2979-3916
Affiliations
a WACKER-Chair of Macromolecular Chemistry, Lichtenbergstraße 4, Technische Universität
München, 85747 Garching bei München, Germany
b Catalysis Research Center, Ernst-Otto-Fischer-Straße 1, Technische Universität München, 85748
Garching, Germany
Acknowledgements
---

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DOI: 10.1039/C7CC08266E
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COMMUNICATION
Sustainable Terpene-Based Polyamides via Anionic Polymerization of a Pinene-
Derived Lactam
Malte Winnacker* and Jacob Sag
A sustainable lactam, which is derived from the renewable
terpene β-pinene, is converted to polyamides via a convenient
anionic ring-opening polymerization (ROP), which can be easily
handled without the use of a costly catalyst. The resulting
polyamides have prosperous thermal properties, which enable
their future use as high-performance polymers.
Polyamides (PAs) are very important polymers with applications
from commodities, engineering thermoplasts and fibers up to
functional materials in biomedicine, and their impact is
continuously growing.26,27,28 They are mainly synthesized by
polycondensation reactions of dicarboxylic acids and diamines
or of amino acids, or via ROP of lactams.29,30 Therefore, the
preparation of PAs from renewable resources is a major task in
polymer chemistry. In addition to the “classical” polyamides like
Nature can provide many building blocks for the synthesis of
sustainable polymers.1,2 This fact has gained increasing impact
within the past decades for the preparation of advanced and
functional materials, and the natural pool of compounds is
interesting in terms of green chemistry and alternative
feedstock in general,3,4,5 and especially with regards to the novel
very significant molecules and structures which can thus be
obtained for the synthesis of innovative polymers for special
applications in material science.6,7,8 There is a “polymer
approach”, where natural polymers and/or their modifications
are utilized (e.g. cellulose derivatives9 or microbial
polyhydroxyalkanoates10), and a “monomer approach”, where
sustainable monomers are isolated and transformed to
polymers (e.g. polylactide from lactic acid11). Due to their
sustainability and (maybe even more) their variety of structures
and functions, terpenes and their derivatives are very interesting
for the synthesis of sustainable polymers.12,13,14,15 For instance,
cationic polymerizations of myrcene16 and pinene17 have been
described, as well as the utilization of (epoxidized) limonene
(limonene oxide) for the synthesis of innovative sustainable
polycarbonates,18,19 polyesters20 and polyurethanes.21 Another
strategy is the utilization of terpenoid ketones: L-menthone and
L-carvone have been transformed to the corresponding lactones
and polymerized via ring-opening polymerization (ROP) to afford
biorenewable polyesters with very significant structures.22,23, 24,25
polyamide 66 (Nylon 6,6) or polyamide
6 (Nylon 6,
polycaprolactam, Perlon®), several interesting biobased
polyamides have been developed within the last decades, for
instance based on sebacic acid,31 succinic acid,32 brassylic acid,33
abietic acid34 furan dicarboxylic acid35 and also on
carbohydrates.36,37
Cyclic terpenoid ketones, which can be considered as
substituted cyclohexanone derivatives, can be transformed to
their corresponding lactams and polymerized via ROP to
polyamides (in analogy to the established synthesis of Nylon 6
from cyclohexanone via ε-caprolactam). This has been
successfully demonstrated e.g. for menthone-based polyamides,
38, 39, 40
and it has also briefly been described for a camphor-
based lactam.41 In this context, β-Pinene (1) is one of the most
abundant components of turpentine oil,12,42 the main resource
for terpenes, and it is thus predestined as building block for
sustainable polymers.43 For instance, it has recently been used in
a very elegant way for the synthesis of polyesters.44 Recently,
also a procedure has been described for the transformation of
β-pinene (1) to nopinone (2), oxime 3 and lactam 4, followed by
cationic acid-induced (e.g. with aqueous HCl, H3PO4)
polymerization of 4 to pinene-based polyamides poly-4.45 Now,
we describe in this study here the polymerization of 4 via a
completely different and convenient anionic procedure (Scheme
1). This strategy provides a significant progress and two
important advantages compared to the previously described
aqueous cationic procedure45, which makes it different and
superior to it: First, the use of powdery bases enables a much
better dosage and reaction control, which thus also facilitates
upscaling. Second, the use of benzoylated lactam Bz-4 as a co-
initiator, which is more electrophilic and which can be
Both authors contributed equally to this work.
Dr. Malte Winnacker and M. Sc. Jacob Sag, WACKER-Chair of
Macromolecular Chemistry, Lichtenbergstraße 4, Technische Universität
München, 85747 Garching bei München, Germany
Catalysis Research Center, Ernst-Otto-Fischer-Straße 1, Technische
Universität München, 85748 Garching bei München, Germany
Electronic Supporting Information (ESI) is available.
nucleophilically opened much more easily than
a non-
benzoylated lactam, equips the polyamides with characteristic
1

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ChemComm
aromatic end-groups. These are very useful for polymer analysis
isomer); d) NaH or KOtBu, Bz-4; Box: analogy to the established
via NMR and MS in terms of determination of the polymer
masses/lengths (via end-group analysis) and of the mechanism,
and they improve solubility of poly-4 in organic solvents. It is a
very important knowledge-profit that biobased 4 can be
polymerized via anionic ROP in analogy to the industrial
petroleum-based Nylon 6 synthesis. These findings present thus
a significant advance in terms of the utilization of terpene-based
building blocks for sustainable high performance polymers.
Although this reaction works also with other acylated lactams
(e.g. benzoylated ε-caprolactam), we have chosen Bz-4 as the
coinitiator in order to keep the analogy to the synthesis of Nylon
6, where also acylated monomer (ε-caprolactam) is applied.29,38
synthesis of Nylon 6 (Perlon®). R = lactam cycle; B) mechanism of
View Article Online
DOI: 10.1039/C7CC08266E
step d) (ROP).
For polymerization, lactam and initiator(s) were heated in a
sealed vial under argon. After reaction, the residue was
dissolved and polymer was precipitated (details see ESI).
Polymerizations were performed at different temperatures and
variable ratios of monomer and initiator(s) (M:I and M:CoI).
Table 1. Anionic polymerization of 4 to poly-4. Polymerizations
were performed for 4h at 200 °C or at 250 °C.
Entry Initiator(s)
[M]/[I]
[M]/[CoI]
10:1
10:1
25:1
25:1
50:1
50:1
25:1
25:1
7:1
10:1
18:1
25:1
35:1
50:1
18:1
T / °C
200
200
200
250
200
200
200
250
Mn/Mw
PDI Yield
(%)
For the reaction, β-Pinene (1) was oxidized to 2, which was
transformed to 3 and then via Beckmann rearrangement to
lactam 4. For anionic ring opening polymerization (ROP) to poly-
4, sodium hydride and KOtBu were used as the initiator, as well
as Bz-4 as the coinitiator (Scheme 1A). The analogy to the Nylon-
6 synthesis is shown in the box, and its transfer to the new
monomer is thus the basis of this new work. The mechanism of
the polymerization is thus also analogous to the Nylon-6
synthesis:29 4 is deprotonated and attacks Bz-4 nucleophilically
at its amide group, and thus Bz-4 is opened. Thus, 4 remains an
acylated cycle, which can again be nucleophilically attacked by
another deprotonated lactam. This sequence repeats together
with deprotonation and reprotonation steps for every growth
step, until the monomer is used up or the reaction is quenched,
and poly-4 is formed (Scheme 1B). This mechanism indicates
that the Bz-group stays as the end group of the polymer (as also
confirmed by NMR data, see later).
1
2
3
4
5
6
7
8
KOtBu +
Bz-4
KOtBu +
Bz-4
KOtBu +
Bz-4
KOtBu +
Bz-4
NaH +
Bz-4
NaH +
Bz-4
NaH +
Bz-4
NaH +
Bz-4
2700/6400 2.4
2700/4800 1.7
1800/3300 1.8
2200/4900 2.2
2500/7100 2.8
3200/7600 2.4
1700/2900 1.7
3200/8100 2.5
58
44
4
44
81
53
9
75
25:1
First, 200 °C was chosen, as it is a standard temperature for
successful anionic lactam polymerization, and polymerization at
150 °C failed. Slight differences of the applied M/I ratios
comparing the experiments with NaH and KOtBu are based on
the simplicity of weighting small amounts in the mg-scale: NaH it
is more difficult to be exactly dosed compared to KOtBu due to
its much smaller MW. Poly-4 was obtained in different yields
dependent on the applied conditions (table 1). For instance,
polymerization of 4 by means of KOtBu and Bz-4 in a ratio 10:1
afforded polymer with Mn/Mw=2700/6400, as determined by gel
permeation chromatography/size exclusion chromatography
(GPC/SEC), with a yield of 58% (entry 1). Increasing the ratio
(more lactam relative to initiator) did not improve the yields – in
contrast, slight decrease of masses and yields was observed
(entries 2-3). Increasing T results in similar yields (entry 4).
Applying similar amounts of NaH afforded also polymer poly-4,
with a slight increase when increasing the ratio, indicating a
partial character of a living anionic polymerization (entries 5-6).
Higher ratios resulted in lower masses and yields (entries 7),
OH
O
A)
N
O
a)
b)
c)
HN
1
2
3
4
O
O
N
O
Bz-4
d)
R
N
H
O
n
poly-4
O
O
NOH
N
H
HN
O
n
CL
Nylon 6
Cyclohexanone
O
O
B)
N
O
O
O
O
Base
O
Bz-4
N
HN
N
while increasing the
T improves the results (entry 8).
N
Furthermore, it was shown that omitting coinitiator Bz-4 yields
polymer with similar masses but with remarkable reduced yield
(table S1, entries 1-6). Therefore, the use of coinitiator Bz-4 is a
prerequisite for good yields in anionic polymerizations of 4.
Reactions without coinitiator Bz-4 and at only 150 °C did not
lead to polymers, similar to heating of the lactam without any
initiator. In further experiments it could also be shown that a
further increase of the ratio does not improve the results – in
contrast, very little initiator does not leads to successful
polymerization (table S1, entries 7-10), indicating that further
variations of this ratio do not make sense at this stage. It can
thus not be excluded that – though working under an inert gas
atmosphere – small amounts of initiator are more prone to
premature quenching. Therefore, obtaining higher masses
O
4
deprotonation/
reprotonation
O
N
N
N
H
O
O
O
HN
n
O
4
Base
N
H
O
n
Scheme 1. A) Synthesis of lactam 4 and anionic polymerization
of 4 to poly-4. a) KMnO4, Al2O3, H2O/CH2Cl2, 63%;42,45 b)
NH4OHHCl, NaHCO3, MeOH/H2O, 98% (two isomers); c)
polyphosphoric acid, 73% (two isomers; 60% for the depicted
2

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would obviously require another synthesis strategy, which could
Measurements of the optical activity of poly-4 show defined
involve e.g. a (previously isolated) lactamate salt or carbenes as
the initiator.29,30 GPC-measurements show pure polymers and
thus a successful separation of the remaining monomers from
the polymers via precipitation from solution (Figure 1A, S1, S2).
1H-NMR spectra show characteristic signals for all protons,
including the benzoyl end group in the entries where Bz-4 was
used (Figure 1B; for 13C NMR spectra see ESI). This enables a
determination of the polymer lengths and masses also via NMR.
As expected, aromatic signals are not visible in those
experiments that were performed without coinitiator Bz-4 (ESI).
values (Figure S19) and confirm the chirality of these polymers,
View Article Online
and thus the successful transformation of the stereocenters of
the lactam monomer into the polymers. DOI: 10.1039/C7CC08266E
Conclusions
We have developed a convenient procedure for the anionic
polymerization of terpene-based lactam 4, which results in
polyamides (poly-4) with prosperous thermal properties. This
procedure makes the polymerization controllable and easily
scalable, also in view of copolymerizations. Accordingly, further
studies in terms of obtaining higher polymer masses will be
performed, which seems to require other procedures and/or
other catalysts (e.g. lactamate salts or carbenes).30 The
preparation of (block-)copolymers, e.g. with other lactams or
with the more hydrophilic polyethylene glycol (PEG), are also on
the way. Another issue is SAXS/WAXS measurements in terms of
the crystallinity in these polyamides, which are currently
ongoing. The investigation of the distance parameters in
46
comparison to conventional nylon 6
and the effect of the
alkyne side chains on the unit cell is thus under further
investigation, as well as the crystallinity of these polymers in
relation to their processing. Polymer processing is tested also
with regards to the preparation of components. In this context,
the mechanical properties are further investigated in detail,
which is topic of ongoing investigations. The preparation of
surfaces, films and layers of these PAs for interactions of these
materials with living cells can also be envisioned in terms of their
utilization as biomaterials.
There are no conflicts of interest to declare.
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