Modified poly(ε-caprolactone)s: An efficient and renewable access via thia-michael addition and Baeyer-Villiger oxidation

Article abstract of DOI:10.1021/ma500381n

The preparation of a novel class of ε-caprolactone (CL) monomers, modified at the β-position of the ester function, is described. The efficient thia-Michael addition to cyclohex-2-en-1-one and subsequent Baeyer-Villiger oxidation provided the regioselecti

Full text of DOI:10.1021/ma500381n

Article
pubs.acs.org/Macromolecules
Modified Poly(ε-caprolactone)s: An Efficient and Renewable Access
via Thia-Michael Addition and Baeyer−Villiger Oxidation
Matthias Winkler, Yasmin S. Raupp, Lenz A. M. Kohl, Hanna E. Wagner, and Michael A. R. Meier*
̈
Laboratory of Applied Chemistry, Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6,
76131 Karlsruhe, Germany
S
* Supporting Information
ABSTRACT: The preparation of a novel class of ε-caprolactone (CL)
monomers, modified at the β-position of the ester function, is described.
The efficient thia-Michael addition to cyclohex-2-en-1-one and subsequent
Baeyer−Villiger oxidation provided the regioselectively modified CL
monomers. To enable a sustainable Baeyer−Villiger oxidation, several
reaction procedures were investigated. In order to test a controlled ring-
opening polymerization of the prepared monomers, the kinetics were
studied and the monomer to initiator ratios were varied in order to
prepare poly(ε-caprolactone)s with different molecular weights and
different side groups.
oxidation procedures making use of, e.g., diverse metal or
enzyme catalysts and hydrogen peroxide as clean oxidant or
potassium peroxymonosulfate (KHSO5) as green oxidant are of
great interest for the synthesis of the desired CL monomers.
Besides the development of greener and more efficient
Baeyer−Villiger oxidation procedures there is a great interest in
CL monomers from renewable resources. In a very interesting
contribution by Heeres et al., the synthesis of CL was achieved
by catalytic transformation of 5-hydroxymethylfurfural, which
can be obtained from, e.g., D-fructose, starch, or cellulose.18
Hillmyer and colleagues described the synthesis of diverse CL
monomers from carvone or menthol.19,20 Interestingly, the use
of these renewable monomers enabled the synthesis of
functional PCLs with different material properties.
Generally, the attachment of functional groups along the
aliphatic chain of PCL (and other polymers) is highly attractive
in order to tailor important material properties such as
crystallinity, hydrophilicity, biodegradability, bioadhesion, and
mechanical properties. Furthermore, pendent functional groups
allow for a covalent attachment of molecules or probes of
biological interest.9 One common and important approach is to
directly attach pendent functional groups (e.g., initiators,
acrylates, alkynes, or halogens) to the lactone ring.21,22
However, usually multistep synthesis procedures and extensive
purification steps are needed to obtain the desired pure
monomers.
INTRODUCTION
■
Aliphatic polyesters are a class of interesting polymeric
materials. In particular, biodegradable and biocompatible
aliphatic polyesters are highly attractive and valuable bio-
materials, which can be used in a wide range of different
applications.1−7 Lactones and lactides are especially important
monomers used to prepare versatile aliphatic polyesters with
predictable molecular weights, narrow dispersities, and well-
defined end-groups. Among different polymerization techni-
ques, the controlled ring-opening polymerization utilizing
diverse catalysts is most widely applied.8 In order to promote
a controlled ring-opening polymerization of lactones or lactides,
a wide range of catalyst/initiators can be used, while tin and
aluminum alkoxides are most frequently applied.8 Considered
as environmentally friendly and valuable thermoplastics,
polycaprolactones (PCLs) have gained much interest since
they can be functionalized and tailored for special applications
in a straightforward manner.9−12 Because of its biodegradability
and biocompatibility, PCL is of great interests for tissue
engineering and drug delivery applications.7 Especially, PCL is
an excellent material for tissue engineering since it is of
nontoxic nature and revealed to be cyto-compatible with
diverse body tissues.7,13 Moreover, PCL can be used as drug
carrier enabling a homogeneous drug distribution and long-
term drug release.14,15
Industrially, the CL monomer is prepared by Baeyer−Villiger
oxidation of cyclohexanone using peracetic acid, whereas for lab
scale synthesis of CL monomers preferentially m-chloroper-
benzoic acid (mCPBA) is used as oxidant.16,17 Organic peracids
are usually not environmentally benign and are shock sensitive,
and the reactions are preferably carried out in chlorinated
solvents. Thus, green alternatives and environmentally benign
Considering the importance of functionalized PCLs and the
demand of CL monomers from renewable resources, we
Received: February 20, 2014
Revised: April 9, 2014
Published: April 22, 2014
© 2014 American Chemical Society
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searched for a very efficient and straightforward approach to
synthesize functionalized CL monomers, which can be derived
from renewable resources. Therefore, the efficient thia-Michael
addition to cyclohex-2-en-1-one is presented as universal
approach to synthesize functionalized CL precursors. A
subsequent Baeyer−Villiger oxidation of the cyclohexanone
derivatives was used to prepare the corresponding function-
alized CL monomers in a highly regio-selectivity manner.
catalyst or washing with water, while preforming the reaction
under mild and solvent-free conditions.31
Hence, we first used different thiols to prepare the
cyclohexanones 1−4 by thia-Michael addition. To prepare the
modified CL monomers 5−8, the cyclohexanones 1−4 were
subsequently transformed by Baeyer−Villiger oxidations. Thia-
Michael additions were performed under solvent-free con-
ditions using triethylamine as catalyst. Full conversion of the
substrates was observed by GC−MS after 16h at 30 °C. The
obtained products were purified by simply applying high-
vacuum to remove the catalyst.
RESULTS AND DISCUSSION
■
One aspect of this work was to synthesize the desired CL
monomers from renewable resources. In the range of renewable
resources given by nature, plant oils appeared to be very useful
substrates to produce a variety of valuable chemicals.23 The use
of olefin metathesis appeared to be a very interesting
oleochemical process to transform plant oils into valuable
chemicals.23 If polyunsaturated fatty acid derivatives are used
for self-metathesis reactions, an intramolecular ring closing
metathesis (RCM) to form 1,4-cyclohexadiene (CHD) as a
byproduct occurs.24 Our group also reported the self-metathesis
of polyunsaturated fatty acid methyl esters to prepare long-
chain diesters, wherein CHD is obtained as a byproduct (Figure
1).25 Moreover, the group of Cole-Hamilton reported the
Without further purification steps, the cyclohexanone
derivatives were used in Baeyer−Villiger oxidations to prepare
the desired monomers (Figure 2). For the Baeyer−Villiger
Figure 2. Thia-Michael addition of cyloehex-2-enone using different
thiols.
oxidations, we first used conventional reaction conditions,
utilizing mCPBA as oxidant and dichloromethane as solvent in
order to have the modified CL monomers in hands and to
study their polymerization and material properties. Further
studies of more sustainable Baeyer−Villiger procedures will be
discussed below. It has to be noted that during the Baeyer−
Villiger oxidation, the sulfide moiety was oxidized to the
sulfone. Reaction studies utilizing mCPBA as oxidant revealed
that the oxidation of the ketone and the sulfide proceed
simultaneously. Therefore, to ensure full oxidation of the sulfide
to the sulfone and the oxidation of the ketone to the ester an
excess of mCPBA was used (see also the experimental part in
the Supporting Information). Using ice bath cooling, mCPBA
was added and full conversion was observed after stirring the
reaction mixture for 24 h at room temperature. The desired
monomers could be isolated in yields up to 85%. Very
interestingly, detailed NMR analysis revealed the formation of
only one regio-isomer, namely 3-(substituted-sulfonyl)-ε-
caprolactone, during the Baeyer−Villiger oxidation allowing
the preparation of well-defined modified PCLs (see Supporting
Information). Generally, the synthetic strategy using cyclohex-
2-en-1-one as an efficient Michael acceptor to prepare modified
cyclohexanones as CL precursors and subsequent monomer
synthesis by Baeyer−Villiger oxidations is of outstanding
simplicity (Figure 3). Interestingly, the functionalization at
the β-position of CL is rarely described, which will lead to a
novel class of modified CL monomers having sulfonyl moieties
for the preparation of poly(3-sulfonyl-ε-caprolactone)s.
Figure 1. Generation of 1,4-cyclohexadiene (CHD) as byproduct of
the olefin metathesis of polyunsaturated fatty acid methyl esters
(FAMEs) and preparation of cyclohex-2-en-1-one from 1,4-CHD.
metathesis of cardanol with ethylene, also yielding CHD as a
byproduct.26 The described synthetic procedures offer a green
alternative to the conventionally produced CHD from fossil
resources. In particular, the valorization of CHD as byproduct
is beneficial in a biorefinary concept, where all products of a
feedstock should be utilized.
CHD cannot be directly converted to the desired modified
CL monomers. A possibility to synthesize CL monomers is the
Baeyer−Villiger oxidation of cyclohexanones. In order to
prepare chemically modified CL, we thus used cyclohex-2-en-
1-one as key substrate for Michael additions (Figure 1).
Interestingly, cyclohex-2-en-1-one can be obtained from CHD
in a sustainable way by selective two-phase hydrogenation and
subsequent oxidation or direct liquid-phase oxidation of
CHD.27−30 As these reactions are described in the literature
in detail, we did not further investigate this transformation and
directly used commercially available cyclohex-2-en-1-one as
starting material. Cyclohex-2-en-1-one is an ideal substrate for
efficient Michael addition reactions. The reaction of such
enones with thiols is also classified as click-reaction, allowing
for an equimolar use of the reagents, enabling a very simple
purification of the products by evaporation of the amine
To enable a more sustainable Baeyer−Villiger oxidation of
the cyclohexanone derivatives, we studied several alternative
oxidation procedures (Table 1). It has to be noted that the
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Figure 3. Baeyer−Villiger oxidation of the cyclohexanone substrates 1−4 and the ring-opening polymerization of the ε-caprolactone monomers 5−8.
the monomers, the best results were obtained if the
polymerizations were performed at 150 °C under solvent-free
conditions using tin(II)octanoate as catalyst and 1-octanol as
initiator (Figure 3). The kinetics of the polymerization were
studied by SEC analysis by monitoring the monomer
conversion (Figure 4).
Table 1. Results Obtained from the Different Baeyer−
Villiger Oxidation Procedures
a
b
procedure
conversion [%]
yield [%]
A (Novozyme 435/ethyl acetate)
B (Novozyme 435/octanoic acid)
C (urea hydrogen peroxide/TFAA)
D (oxone)
≤10
∼5
100
100
100
n.d.
n.d.
n.d.
55−70
77−85
E (mCPBA)
a
b
Determined via GC-MS. Isolated yield, n.d. = not determined.
Baeyer−Villiger oxidation should be performed under mild
reaction conditions to avoid an elimination of the sulfide (more
precisely the intermediately formed sulfoxide). At first, with
regard to a reaction optimization, the produced m-chloroben-
zoic acid was isolated from the crude reaction mixture in order
to enable a recycling of the used mCPBA as already described
by, e.g., Jain and co-workers.32
In Baeyer−Villiger oxidations using octanoic acid (Table 1,
entry A) or ethyl acetate (entry B) as peracid source and
Novozyme 435 as catalyst, only low conversions were obtained
after a reaction time of 7 days. Thus, contrary to the conversion
of, e.g., cyclohexanone to CL via Novozyme 435 catalyzed
Baeyer−Villiger oxidation,33,34 the thiol functionalized sub-
strates are seemed incompatible with the enzyme catalyst and/
or the applied reaction conditions. Baeyer−Villiger oxidations
preformed with urea hydrogen peroxide as oxidant as reported
by Mecking and co-workers35 yielded full conversions of the
substrates, but low yields and regio-selectivity were obtained
(entry C). Most promising, the utilization of oxone as green
oxidant for Baeyer−Villiger oxidations, as, e.g., described by
Hillmyer et al.,19 yielded full conversion of the substrates
performing the reaction at room temperature (entry D).
Thereby, the utilized solvent is of crucial importance. In
oxidation reactions performed in dichloromethane or meth-
anol/water, low yields were obtained. However, if the reaction
was performed in N,N-dimethylformamide full conversion and
yields up to 70% were achieved. Although the conventionally
performed Baeyer−Villiger oxidation afforded higher yields, the
applied procedure making use of oxone appeared to be a
promising green alternative.
Figure 4. Kinetic studies of the ring-opening polymerization of the
modified caprolactone monomers 5−8 and CL.
Compared to CL, the polymerizations of the modified CLs
proceeded slower, except for monomer 5 having the n-
octylsulfonyl moiety. Although 5 seemed to be sterically
more hindered than monomer 6, the polymerization was
significantly faster (Figure 4). This might be due to a difference
of the viscosity of the reaction mixture containing the monomer
and growing polymer, resulting in different diffusion
coefficients. This assumption is supported by the result of the
kinetic measurement of monomer 7 having a cyclohexylsulfonyl
moiety. Monomer 7 displayed the highest melting point (136
°C) and the polymerization mixture was hardly stirrable after
45 min of reaction time, resulting in a slower polymerization
rate. However, the kinetic measurements revealed a living
character of all performed ring-opening polymerizations (linear
correlation of −ln([Mt]/[M0]) vs reaction time and Mn vs
conversion; Figure 4 and Figure S30 in the Supporting
Subsequently, the prepared CL monomers were studied in
ring-opening polymerizations. Because of the poor solubility of
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Information). It has to be noted that the polymerization
displayed a short induction time, but in this time range, the
determination of the monomer conversion was difficult by SEC
due to overlapping peaks. It should also be noted here that a
determination of conversions by NMR and/or GC was not
possible due to indistinguishable peaks in NMR and impossible
detection of the monomers in GC.
It should be also noted that the molecular weights
determined by SEC analysis differ from the theoretically
expected values due to an inaccuracy of SEC analysis since the
prepared polyesters with the sulfonyl moieties are very different
to the PMMA standards used for the calibration. However, the
SEC analysis of all polyesters revealed polymers with narrow
dispersities of 1.10−1.28 and homogeneous molecular weight
distributions (Figure 5, Table 1 and 2). P4 and P5 showed
The resulting polyesters were obtained as transparent
materials in nearly quantitative yields after a reprecipitation
and their thermal properties were studied by DSC analysis. All
polyesters containing the sulfonyl moieties displayed no melt
transitions and no thermal degradation up to 180 °C. In
contrast to conventional CL, the sterically demanding sulfonyl
moieties prevent interactions of the polymer chains and
formation of crystalline segments. The DSC measurements
showed glass transition temperatures of the polyesters between
0−52 °C (Table 2). Polymer P10 showed the highest Tg with
52 °C, whereas polymer P9 having a long dangling side chain
showed the lowest Tg of 0 °C. Generally, the Tg decreases with
a longer dangling sulfonyl moiety, which prevents chain−chain
interactions. In case of P10 the polymer shows a higher Tg than
the other polymers due to increased chain interaction by π−π
interaction of the aromatic moiety. The obtained results prove
the successful use of the modified CL monomers for the
preparation of defined polyesters with narrow dispersities and
varying pendent side groups.
Table 3. Results of the SEC and DSC Analysis of the
Polyesters P3, P8−P10 Using a Monomer to Initiator Ratio
of [M]/[I] = 40:1
polymer
Mn,SEC [Da]
Đ
Tg [°C]
P3 (n-butyl)
P8 (cyclohexyl)
P9 (n-octyl)
P10 (benzyl)
7400
7680
8840
6100
1.10
1.17
1.12
1.15
7
40
0
52
Depending on the thiol used in the thia-Michael reaction to
modify cyclohex-2-en-1-one, the thermal properties of the
corresponding polyester are significantly different. Moreover,
the presented synthesis strategy offers an efficient and very
straightforward possibility for the design of diverse modified CL
monomers. Additionally, the method can be extended to other
Michael donors making this strategy a universal approach to
prepare versatile CL monomers and polyesters with novel
material properties.
Figure 5. SEC analysis of the polyesters P1−P5 prepared from
monomer 6 using different monomer to initiator ratios.
Table 2. Results of the SEC Analysis of the Prepared
Polyesters P1−P5 Using Monomer 6 and Different
Monomer to Initiator ([M]/[I]) Ratios
polymer
[M]/[I]
Mn,theo [Da]
Mn,SEC [Da]
Đ
CONCLUSION
P1
P2
P3
P4
P5
20:1
30:1
40:1
50:1
60:1
4812
7153
2740
6320
1.28
1.13
1.10
1.16
1.17
■
The preparation of a novel class of CL monomers modified
with different sulfonyl moieties at the β-position of the carbonyl
function is described. To achieve this, the thia-Michael addition
to cyclohex-2-en-1-one is revealed to be an excellent strategy to
prepare modified CL precursor since complete atom-efficiency,
simple purification, and performance under very mild and
solvent-free conditions makes this approach highly attractive.
The modified CL monomers were prepared by Baeyer−Villiger
oxidation of the cyclohexanone derivatives. As an alternative
and more sustainable Baeyer−Villiger reaction procedure, the
use of oxone as green oxidant revealed to be very promising.
Interestingly, the performed Baeyer−Villiger oxidations are
highly regioselective, allowing the synthesis of well-defined
polyesters. The kinetics of the ring-opening polymerizations of
the modified CL monomers were studied and compared to CL,
revealing a controlled ring-opening polymerization of these
novel monomers. Furthermore, by changing the monomer to
initiator ratio, the molecular weights of the PCL bearing
sulfonyl groups could be well adjusted. The resulting polymers
were transparent and showed variable glass transitions
depending on the sulfonyl moiety attached to the polyester.
All in all, the presented synthetic strategy is of high simplicity
9494
7400
11 835
14 176
12 540
18 420
small shoulders at higher molecular weights, which can be
explained by a slow intermolecular transesterification process
occurring after complete conversion.20,36,37 For further studies
(n-butylsulfonyl)cyclohexane was used to investigate the
stability of the sulfone moiety (see Supporting Information).
The results demonstrated that the sulfone is stable under the
applied polymerization conditions and no side reactions
occurred. Moreover, 1H NMR analysis of the crude and
precipitated polymers revealed defined polyesters without the
occurrence of side reactions, such as elimination, also
demonstrating the compatibility of the monomers with the
applied polymerization conditions.
By varying the monomer to initiator ratio, the molecular
weight of the polymers could be well adjusted, also clearly
demonstrating a good control of the ring-opening polymer-
ization (Figure 5, Table 1; see also Supporting Information).
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and the desired modified CL monomers, which were used to
prepare PCLs having different sulfonyl groups, are obtained in
only two very straightforward and efficient synthesis steps.
(23) Biermann, U.; Bornscheuer, U.; Meier, M. A. R.; Metzger, J. O.;
ASSOCIATED CONTENT
■
Schafer, H. J. Angew. Chem., Int. Ed. 2011, 50 (17), 3854−3871.
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S
* Supporting Information
(24) Mathers, R. T. Plant based monomers and polymers, U.S.
Provisional Application No. 61/654,589, WO2013180837 A1, 2013.
(25) Mutlu, H.; Hofsa; Montenegro, R. E.; Meier, M. A. R. RSC Adv.
2013, 3 (15), 4927−4934.
Detailed description of all experimental procedures and 1H/13C
NMR and mass data. This material is available free of charge via
the Internet at http://pubs.acs.org.
(26) Mmongoyo, J. A.; Mgani, Q. A.; Mdachi, S. J. M.; Pogorzelec, P.
J.; Cole-Hamilton, D. J. Eur. J. Lipid Sci. Technol. 2012, 114 (10),
1183−1192.
AUTHOR INFORMATION
Corresponding Author
*(M.A.R.M.) E-mail: m.a.r.meier@kit.edu.
■
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Braz. Chem. Soc. 2000, 11, 293−297.
(28) Lv, X.-M.; Kong, L.-J.; Lin, Q.; Liu, X.-F.; Zhou, Y.-M.; Jia, Y.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors would like to thank Pavleta Tzvetkova (KIT) for
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