An energy efficient and facile synthesis of high molecular weight polyesters using ketenes

Article abstract of DOI:10.1039/c1cc14055h

A facile, ketene-based strategy for the synthesis of polyesters from stable Meldrum's acid monomers has been developed which overcomes many issues associated with traditional step-growth procedures. A significant increase in polymerization efficiency is o

Full text of DOI:10.1039/c1cc14055h

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COMMUNICATION
An energy efficient and facile synthesis of high molecular weight
polyesters using ketenesw
a
ab
Martin Wolffs,z Matthew J. Kadez and Craig J. Hawker*abc
Received 6th July 2011, Accepted 14th August 2011
DOI: 10.1039/c1cc14055h
A facile, ketene-based strategy for the synthesis of polyesters
from stable Meldrum’s acid monomers has been developed which
overcomes many issues associated with traditional step-growth
procedures. A significant increase in polymerization efficiency is
observed with only 10 min reaction time at 220 8C being needed
to obtain high molecular weight polymers.
polyesters by coupling with ketenes.11 Similarly, Garner has
described the copolymerization of sebacyl bisketene and
bisphenol-A12 which involves the generation of the reactive
bisketene by reaction of an acid chloride with base at low
temperature (ꢀ78 1C), under inert conditions and at high dilution.
To overcome the challenges with traditional polyester
synthesis and to further demonstrate the potential for ketenes
in materials chemistry, we report an efficient synthesis of high
molecular weight polyesters based on ketenes that are thermally
generated in situ from Meldrum’s acid derivatives.
Polyesters form one of the most intensely studied and com-
mercially successful class of commodity polymers with appli-
cations ranging from biomedical devices to beverage
containers.1–4 Synthetically, polyesters are typically obtained
by ring opening polymerization or via polycondensation stra-
tegies with poly(ethylene terephthalate) (PET) being a classic
example of the latter.1,5,6 In wide academic and industrial use,
condensation approaches to polyesters are often limited by the
efficient removal of a small molecule (typically methanol,
ethylene glycol or water) from the reaction mixture, and the
necessity to drive the polymerization to near completion to
obtain high molecular weight material.1,2 Stringent conditions,
such as high reaction temperature, high vacuum, long reaction
times, metal catalysts, and sequential polymerization steps are
required which makes the process of preparing high molecular
weight polyesters an extremely energy intensive and dedicated
procedure. In order to circumvent the rigorous and energy
demanding conditions typically employed for polyester synthesis,
a new strategy for the preparation of high molecular weight
polyesters is needed that is based on stable monomers, short
reaction times and non-demanding polymerization conditions.
In moving away from traditional esterification reactions,
our attention was drawn to the synthetic versatility and high
reactivity of ketenes. While well-established in small molecule
organic chemistry,7 ketenes have been exploited sparingly in
polymer synthesis. Examples include the synthesis of poly-
esters through the anionic polymerization of a ketene and an
aldehyde,8–10 and the chain extension of low molecular weight
Recently, our group has shown the applicability of Meldrum’s
acid derivatives as stable precursors to ketenes for the functio-
nalization and crosslinking of polymer thin films, processing of
commodity polymers, as well as for the stabilization of nanoscale
structures.13 Furthermore, the rich chemistry of Meldrum’s
acid allows a wide variety of monomeric building blocks to be
synthesized.14 At temperatures of ca. 200 1C, carbon dioxide
and acetone are liberated to produce the reactive ketene
intermediate with the overall process having a number of
attractive features. Quantitative generation of the ketene can
be performed in air and in the absence of solvent or catalysts in
a matter of minutes. For the specific application to polyester
formation, these advantages gain further importance as the
generation of small molecules (CO2 and acetone) during ketene
formation is non-reversible and is independent of the poly-
merization reaction. In turn, nucleophilic addition of alcohols
or phenols to the ketene is a very facile process, does not generate
by-products that require removal during polymerization and is a
non-reversible process.
To demonstrate the wide applicability of this approach for
polyester synthesis, two polymerization strategies, AA-BB
and A-B, were investigated. In both cases, the stability of mono-
mers derived from Meldrum’s acid greatly facilitates structural
diversity. For the AA-BB strategy, monomer synthesis starts from
commercially available 1,4-bis(chloromethyl)benzene, 1, and
2,2,5-trimethyl-1,3-dioxane-4,6-dione (monomethyl Meldrum’s
acid), 2. Reaction of a mixture of 1 and 2 under basic con-
ditions in N,N-dimethylformamide (DMF) at 50 1C gave the
desired bis(Meldrum’s acid) derivative, 3, in 71% yield
(Scheme 1). Significantly, 3 proved to be very stable and could
be obtained as a crystalline solid allowing access to high purity
monomer.
a Materials Research Laboratory, USA. E-mail: hawker@mrl.ucsb.edu
b Department of Chemistry and Biochemistry, USA
c Department of Materials, University of California, Santa Barbara,
CA 93106, California, USA
w Electronic supplementary information (ESI) available: Synthetic
and characterization data. CCDC 833163. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1cc14055h
Similarly, synthesis of the A-B type monomers starts from
commercially available 2. For the benzylic alcohol A-B
z These authors contributed equally to the work.
c
10572 Chem. Commun., 2011, 47, 10572–10574
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Scheme 1 Synthesis of the bis(Meldrum’s acid) derivative 3 (K2CO3,
DMF, 50 1C, 17 h, 71%) and the corresponding crystal structure of 3.
Scheme 3 Synthesis of the Meldrum’s acid derivative 12 (a) Water,
monomer, 6, tetrahydropyranyl (THP)-protected 4-chloro-
methylbenzyl alcohol15 is first reacted with 2 yielding 5 in
74% yield (Scheme 2). The THP group could then be removed
by stirring over Dowex acidic resin to provide 6 in 96% yield.
The synthetic availability of phenol-containing benzyl
halides and the desire to demonstrate alternative deprotection
conditions, prompted a modified synthetic strategy for the
phenolic A-B monomer 12 (Scheme 3). Starting from Meldrum’s
acid 7, condensation with 4-(benzyloxy)benzaldehyde 8 gives
916 which on reduction with sodium borohydride (80%) yields
10. Subsequent methylation with methyl iodide in DMF (95%)
then gives the benzyl protected derivative, 11, with removal of
the benzyl ether using hydrogen gas and Pd/C as a catalyst
affording the desired phenolic monomer, 12, in 90% yield.
To investigate the temperature at which ketene formation
occurs, the Meldrum’s acid derivatives 3, 6, and 12 were
analyzed by thermogravimetric analysis-mass spectrometry
(TGA-MS). In all cases, the measured mass loss corresponds
to the calculated mass loss of one equivalent of carbon dioxide
and acetone per Meldrum’s acid unit for compounds 6 and 12,
while for compound 3 the experimental value slightly deviates
from the theoretical value due to evaporation. The bis(Meldrum’s
acid) 3 shows a clear onset of ketene formation at ca. 210 1C,
which is comparable to previous reports.13 Remarkably, the
alcohol-containing derivatives 6 and 12 display significantly
decreased ketene formation temperatures, ca. 160 1C for 6
and 150 1C for 12. We attribute this difference to the presence
of alcohol/phenol groups and postulate that hydrogen bond-
ing to the carbonyl group of the Meldrum’s acid unit facilitates
formation of the ketene. Intrigued by the reduction in
temperature for ketene formation in the A-B systems, the
AA-monomer 3 was physically mixed in the solid state with
three different diols, hydroquinone, 1,4-benzenedimethanol
and 1,6-hexanediol, and the thermal behavior of the mixtures
75 1C, 2 h, 37%; (b) NaBH4, THF, 80%; (c) Methyl iodide, DMF,
K2CO3, 50 1C, 24 h, 95%; (d) H2, Pd/C, THF, RT, 12 h, 90%.
are examined by TGA-MS. Similar to compounds 6 and 12,
the addition of alcohol-based comonomers to 3 significantly
decreases the temperature for ketene formation (ca. 150 1C for
hydroquinone, 160 1C for 1,4-benzenedimethanol and 130 1C for
1,6-hexanediol), providing further evidence for the influence of
H-bonding groups on the decomposition of Meldrum’s acid
derivatives and subsequent ketene formation.
To demonstrate the simplicity of polyester formation, poly-
merizations were performed in the melt, under atmospheric
conditions without stirring or extensive drying of the monomers.
In the case of the A-B systems, where no mixing of reagents is
necessary, the monomer was heated at 220 1C for only 10 min
to achieve polymerization. For the AA-BB polymerizations, equi-
molar amounts of 3 and either hydroquinone, 1,6-hexanediol
or 1,4-benzenedimethanol were initially heated at 190 1C for
10 min to ensure mixing and then at 220 1C for an additional
10 min. Even with the reduced reaction times and simple
polymerization set-up, both the A-B and AA-BB procedures
gave polymers with molecular weights (Mw) from 25 kg molꢀ1
to 110 kg molꢀ1 (Scheme 4).17 This is in marked contrast to
traditional condensation strategies for polyester synthesis
where a catalyst is required in addition to extended reaction
conditions (B250 1C for multiple hours).
Structural characterization of the polyesters, P1-5, was by a
1
combination of IR, H and 13C NMR spectroscopy14 which
showed only peaks corresponding to ester formation by
nucleophilic addition to the ketene. This is especially clear
from the carbon NMR, since the characteristic peak for the
carbonyl of the cyclobutadione group ([2+2] cycloaddition
reaction) normally observed at 214 ppm is absent. This
suggests that the polymerization conditions lead to exclusive
addition to the ketene with no evidence of dimerization side
reactions. The high molecular weights obtained also provides
evidence for facile ester formation as dimerization would lead
to an imbalance of reactive groups in the AA-BB polymerization
systems and low molecular weight materials.
The thermal properties of the different polyesters were eval-
uated using TGA and differential scanning calorimetry (DSC).
TGA reveal degradation temperatures (Td) for P2–P5 that are
well above 350 1C with P1 having a reduced Td of ca. 240 1C.
As expected, DSC analysis shows that the glass transition
temperature, Tg, is the lowest for P3, (Tg = ꢀ13 1C), which
Scheme 2 Synthesis of the Meldrum’s acid derivative 6 (a) K2CO3,
DMF, 50 1C, 17 h, 74%; (b) DOWEX acid resin, THF/Methanol, 6 h,
96%.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10572–10574 10573

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Scheme 4 Synthesis of the different polyesters P1–P5 and their molecular weight Mw (PDI) and thermal characterization data such as the
degradation temperature Td, glass-transition temperature Tg and if applicable the crystallization temperature Tcr.
possesses the highest degree of aliphatic character in its
molecular structure, with the Tg increasing to 6 1C for P4
and to 13 1C for P1, both of which contain benzyl ester
groups. For the phenyl esters, the Tg increases even further
to 61 1C and 63 1C for P5 and P2 respectively. Additionally,
the increased aromatic character leads to the appearance of
crystallinity for P2 and P5, with both polymers show mutliple
crystallization events (Fig. 1).
R.A.A. Bovee is gratefully acknowledged for the high
temperature SEC measurements in o-dichlorobenzene and
Dr N. Lynd for helpful discussions. MW thanks the Netherlands
Organization for Scientific Research (NWO) for funding through
a Rubicon fellowship. This work was supported by the chemistry
(CHE-0957492) and MRSEC (DMR-1121053) Programs of the
National Science Foundation.
Notes and references
Crystallization is observed at 96 and 184 1C for P2, while at
153 and 204 1C in the case of P5. We attribute the presence of
multiple crystallization temperatures to polymorphisms as a
result of the chiral center in the repeat unit. The difference in
crystallization temperature could be a result of the crystalli-
zation rate and its relation to the molecular weight of the
polymers. It is interesting to note that while P2 is obtained
from an AA-BB strategy and P5 from an A-B strategy, the
similarity in molecular structure is reflected in the comparable
thermal properties for both polymers.
´
1 O. Coulembier, P. Degee, J. L. Hedrick and P. Dubois, Prog.
Polym. Sci., 2006, 31, 723.
2 G. I. Ozturk, A. J. Pasquale and T. E. Long, J. Adhes., 2010, 86,
395–408.
3 Y. Liu and S. R. Turner, J. Polym. Sci., Part A: Polym. Chem.,
2010, 48, 2162.
4 (a) D. Quinzler and S. Mecking, Angew. Chem., Int. Ed., 2010,
49, 4306; (b) H. Middleton, S. Tempelaar, D. M. Haddleton and
A. P. Dove, Polym. Chem., 2011, 2, 595–600.
5 R. C. Jeske, A. M. DiCiccio and G. W. Coates, J. Am. Chem. Soc.,
2007, 129, 11330.
6 C.-Y. Ko, M. Chen, H.-C. Wang and I.-M. Tseng, Polymer, 2005,
46, 8752.
7 T. T. Tidwell, Eur. J. Org. Chem., 2006, 563.
8 Y. Yamashita, A. Uchikawa, K. Yoshida and T. Kobayashi,
Makromol. Chem., 1967, 292.
9 A. Sudo, S. Uchino and T. Endo, J. Polym. Sci., Part A: Polym.
Chem., 2001, 39, 2093.
10 D. Nagai, A. Sudo and T. Endo, Macromolecules, 2006, 39, 8898.
11 P. Zarras and O. Vogl, Prog. Polym. Sci., 1991, 16, 173.
12 P. Garner, J. Polym. Sci., Part A: Polym. Chem., 1982, 20, 2979.
13 (a) F. A. Leibfarth, M. Kang, M. Ham, J. Kim, L. M. Campos,
N. Gupta, B. Moon and C. J. Hawker, Nat. Chem., 2010, 2, 207;
(b) F. A. Leibfarth, Y. Schneider, N. A. Lynd, A. Schulz, B. Moon,
E. J. Kramer, G. C. Bazan and C. J. Hawker, J. Am. Chem. Soc.,
2011, 132, 14706; (c) Y. Miyamura, P. Chiyoung, K. Kinbara,
F. A. Leibfarth, C. J. Hawker and T. Aida, J. Am. Chem. Soc.,
2011, 133, 2840.
In conclusion, through the use of stable Meldrum’s acid
precursors, reactive ketene monomers could be generated
in situ in the presence of alcohols or phenols with subsequent
rapid polymerization to give a library of polyester derivatives.
The absence of solvent, catalysts, or high vacuum coupled with
the dramatically reduced reaction times make this an attractive
platform for future energy efficient approaches to polyesters
and other condensation systems.
14 A. M. Dumas and E. Fillion, Acc. Chem. Res., 2010, 43, 440.
15 M. E. Pierce, G. D. Harris, Q. Islam, L. A. Radesca, L. Storace,
R. E. Waltermine, E. Wat, P. K. Jadhav and G. C. Emmet, J. Org.
Chem., 1996, 61, 444.
16 C. G. Frost and B. C. Harley, J. Org. Chem., 2009, 74, 3599.
17 The high polydispersity index for P5 is likely caused by the reaction
temperature being below the crystallization temperature leading to
a near, solid-state reaction.
Fig. 1 Differential scanning calorimetry of P2 (a) and P5 (b) showing
the multiple melting and crystallization transitions.
c
10574 Chem. Commun., 2011, 47, 10572–10574
This journal is The Royal Society of Chemistry 2011