Poly(propylene succinate): A new polymer stereocomplex

Article abstract of DOI:10.1021/ja509440g

Herein we show the formation of a polymer stereocomplex by mixing isotactic, regioregular chains of poly(propylene succinate) synthesized via the copolymerization of cyclic anhydrides and epoxides. The stereocomplex exhibits significantly improved thermal

Full text of DOI:10.1021/ja509440g

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Poly(propylene succinate): A New Polymer Stereocomplex
Julie M. Longo, Angela M. DiCiccio, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
S
* Supporting Information
biotechnology.10 In contrast, polyesters synthesized via the
ABSTRACT: Herein we show the formation of a polymer
stereocomplex by mixing isotactic, regioregular chains of
poly(propylene succinate) synthesized via the copolymer-
ization of cyclic anhydrides and epoxides. The stereo-
complex exhibits significantly improved thermal properties
in comparison to the enantiopure parent polymers. We
demonstrate that stereocomplexation is a route to a new
class of semicrystalline polyesters with improved proper-
ties, produced from readily accessible starting materials.
ring-opening alternating copolymerization (ROAC) of epoxides
and cyclic anhydrides can be made by purely chemical means
with a wide array of commercially available starting materials. A
variety of metal-based complexes have been reported to
catalyze this reaction.6 We recently discovered that cobalt
salcy complexes (salcy = N,N′-bis(salicylidene)-
cyclohexanediimine) with a nucleophilic cocatalyst such as
[PPN][NO3] ([PPN] = [Ph3P+−NPPh3]) can be used for
the regioselective ROAC of various cyclic anhydrides and
epoxides (Scheme 1a).6m
olyethylene and isotactic polypropylene are the two most
Scheme 1. (a) Regioselective Polymerization of Cyclic
Anhydrides and Epoxides and (b) Stereocomplex Formation
by Crystallization of Enantiomeric Polymer Chains
P
widely produced polymers in the world, accounting for
over half of the approximately 100 million pounds of plastic
produced annually in the United States alone.1 These
semicrystalline polyolefins are ubiquitous because of their low
cost and impressive physical and thermal properties, allowing
for their use in a wide array of applications. Since the discovery
of these materials, relatively few new classes of semicrystalline
polymers have been commercialized, and even fewer where the
monomers are nearly as readily available and inexpensive as
ethylene and propylene.2 One potential class includes polymers
made from epoxides, isotactic versions of which were first
synthesized around the same time as the first isotactic
polyolefins.3 As with olefins, epoxides are inexpensive and
readily available, making them attractive monomers for large-
scale polymers. However, unlike simple α-olefins, their
corresponding epoxides contain stereogenic centers, which
can be both an asset and a challenge for synthesizing polymers
of controlled tacticity.4
Epoxides have also been studied in alternating copolymeriza-
tions with CO2 to form polycarbonates5 and with cyclic
anhydrides to form polyesters.6 In the case of the
copolymerization of propylene oxide (PO) with CO2, even
the highly regio- and stereoregular poly(propylene carbonate)
is amorphous.7 However, we hypothesized that the copoly-
merization of PO with cyclic anhydrides could result in the
formation of a new class of semicrystalline polyesters given the
wide array of anhydride comonomers available. In general,
polyesters are functionally diverse and bioresponsive, giving
them advantages over polyolefins in the range of applications to
which they can be applied.8 Additionally, many commercial
aliphatic polyesters, including polylactic acid (PLA), polyhy-
droxybutyrate (PHB), and polycaprolactone (PCL), can be
decomposed in aerobic composting environments, and in vivo
in the case of biomedical devices.9 However, commercializing
many of the current biodegradable polyesters for large-scale use
challenges the existing industrial infrastructure because the
processes required to produce them rely partially or fully on
Catalytic epoxide/anhydride copolymerization not only
utilizes a large substrate scope, but also allows control over
polymer microstructure. Additionally, by employing the
regioselective ROAC of enantiopure epoxides with cyclic
anhydrides, highly tactic polyesters can be synthesized, which
increases the probability for crystallinity in the resulting
materials. Because these polyesters are chiral, using enantiopure
epoxides results in enantiomerically pure forms of each
polymer. When these handed chains are mixed, they can result
in an amorphous mixture, a semicrystalline material with
segregated homocrystallites, or a semicrystalline material with
stereocomplexed crystallites (Scheme 1b). Although we have
observed many examples of the former two situations, we are
Received: September 12, 2014
© XXXX American Chemical Society
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unaware of an example of stereocomplexation in polyesters
formed from epoxides and anhydrides. Stereocomplexation is
an attractive route to polymeric materials with improved
thermal and mechanical properties compared to those of the
individual enantiopure polymers.11
Mn of 9.2 kDa, a PDI of 1.22, and a specific optical rotation of
20
[α]D −3.2°. Likewise, polymerization of (R)-PO proceeded
using (S,S)-(Cl-salcy)CoNO3 to give polymer with an Mn of 9.0
20
kDa, a PDI of 1.47, and a specific optical rotation of [α]D
+3.3°. These molecular weights are likely low because of the
formation of cyclic polymer chains or transesterification of
polymer chains by the catalyst.
Differential scanning calorimetry (DSC) was used to
investigate the thermal properties of the enantiopure,
regioregular polymers and of the mixed specimens. The DSC
thermograms of both poly(S)- and poly((R)-propylene
succinate) show no crystallinity, and only a glass transition
(Tg) of −4 °C is observed (Figure 1a). Figure 1b shows that the
Stereocomplexation occurs when a stereoselective interaction
takes place between two stereoregular complementary poly-
mers in the crystalline state.11a Most commonly, stereo-
complexes are formed between enantiomerically complemen-
tary polymer strands. One of the most widely studied
stereocomplexes is a 1:1 mixture of poly(L-lactic acid)
(PLLA) and poly(D-lactic acid) (PDLA).11b,12 From solution,
PLLA and PDLA form helices that preferentially crystallize
together when combined, creating a stereocomplex with a
melting transition (Tm) approximately 50 °C higher than that
of the parent polymers (230 vs 180 °C).12 Stereocomplexation
improves and allows for tunability of polymer properties,
including crystallinity as well as biodegradability.11
In this report, we utilize cobalt salcy complexes with ionic
cocatalysts for the synthesis of regioregular poly(propylene
succinate). By polymerizing both (R)- and (S)-PO separately
with succinic anhydride and subsequently mixing the resulting
polyesters, we were able to investigate stereocomplex
formation. This strategy resulted in the formation of a
stereocomplex from enantiomeric strands of isotactic poly-
(propylene succinate), producing a material with an increased
Tm and a greater level of crystallinity. Not only is poly-
(propylene succinate) formed from readily available monomers
and likely to be biodegradable, but also the stereocomplex
exhibits a Tm comparable to low density polyethylene.2 In
addition, succinic anhydride (SA) is an attractive monomer
because its synthesis from biorenewable resources has expanded
widely in recent years.13
To form the polyester stereocomplex, we first synthesized
poly(S)- and poly((R)-propylene succinate) from (S)- and (R)-
PO, respectively (Scheme 2). Poly(propylene succinate)
Scheme 2. Regioselective Alternating Copolymerization of
Enantiopure Propylene Oxide and Succinic Anhydride with
“Matched” and “Mismatched” Catalysts
Figure 1. DSC thermograms of (a) isotactic poly((S)-propylene
succinate) and poly((R)-propylene succinate), (b) crystallization of
poly((R)-propylene succinate) over time, and (c) the stereocomplexed
mixture of both polymers.
regioregularity was analyzed by gas chromatography of the
diol products that resulted from degrading the enantiopure
polymers under basic conditions.14 High regioregularity (96−
97% head-to-tail linkages) was observed when a chloro group
was present in the para position of the salicylidene moiety of
the catalyst, (Cl-salcy)CoNO3, and when enantiopure PO was
polymerized using the catalyst of the opposite absolute
stereochemistry (Scheme 2).14 Copolymerization of (S)-PO
with SA proceeded to full conversion in 36 h at 30 °C using
enantiopure (R,R)-(Cl-salcy)CoNO3 to give polymer with an
parent polymers do slowly crystallize from the melt, with
crystallinity first appearing after about 1 week and gradually
increasing over time.14 For poly((R)-propylene succinate), a
relatively large amount of a low melting polymorph with a Tm
around 50 °C and a ΔH value of 48 J/g is observed after 19
days, along with a very small amount of a higher melting
polymorph (Tm around 70 °C). Some degree of crystallinity is
B
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Journal of the American Chemical Society
Communication
observed from precipitation of the enantiopure polymers with
methanol, and a Tm of 79 °C along with ΔH values of 68−78 J/
g can be obtained, although this method is not ideal for
industrial applications as the resulting polymer is a fine powder
and crystallinity disappears completely upon melting, and no
Tm is seen on the second heat.14
To test for stereocomplex formation, dissolved solutions of
poly((S)-propylene succinate) and poly((R)-propylene succi-
nate) in dichloromethane were mixed and the solvent was
allowed to slowly evaporate. The sample was then dissolved
again in dichloromethane and precipitated by adding methanol
dropwise with vigorous stirring. After drying, the resulting
polymer was analyzed by DSC and the thermal properties of
the poly(propylene succinate) stereocomplex were compared
to the enantiopure polymers (Figure 1c). For the stereo-
complex, not only does the melting point increase by
approximately 40 °C (to 120 °C) on the first heat relative to
the enantiopure polymers, but also recrystallization occurs
upon cooling to approximately 92 °C, and the same Tm and
ΔH value of 88 J/g are observed on the second heat. Both the
rate of heating and cooling and the maximum temperature of
heating affected the recrystallization of the stereocomplex, but
had no effect on the enantiopure polymers. All polymers shown
in Figure 1 were heated to a maximum temperature of 130 °C,
but while the enantiopure polymers in Figure 1a,b were heated
and cooled at a rate of 10 °C/min, the stereocomplex was
heated at 2 °C/min to optimize the recrystallization temper-
ature.14
The immediate stereocomplex crystallization upon cooling
shows a vast improvement over any other polyester we have
synthesized by the ROAC of propylene oxide and cyclic
anhydrides. While we have observed the formation of other
semicrystalline polyesters using this synthetic method, they
only show crystallinity following isolation from solution, not
from the melt after initial heating above the Tm.6m Additionally,
none of these polyesters to date have shown stereocomplex-
ation upon mixing of both enantiopure polymers. The t1/2 of
recrystallization at ambient temperature was calculated for both
the enantiopure polymers and the racemate, and was found to
be approximately 3 orders of magnitude faster for the
stereocomplex than for the parent polymers.14
Powder X-ray diffraction (XRD) was used to confirm the
formation of a stereocomplex via the presence of different
crystalline diffraction peaks in comparison to the diffraction
profiles of the parent polymers. Samples of both the
enantiomerically pure, isotactic polymers and the stereo-
complex were prepared by precipitation via dropwise addition
of methanol into a dichloromethane solution of the polymer or
polymer mixture to induce crystallization, especially important
in the case of the parent polymers. Figure 2 shows the powder
XRD profiles of both enantiopure forms of poly(propylene
succinate) and of the poly(propylene succinate) stereocomplex.
Both poly(S)- and poly((R)-propylene succinate) show five
large crystalline diffraction signals at 10.2°, 18.5°, 19.4°, 21.8°,
and 24.7° along with four smaller peaks at 9.2°, 15.4°, 16.8°,
and 25.7°. In contrast, the polymer stereocomplex shows three
large peaks at 17.9°, 20.8°, and 24.1° that are not present in the
enantiopure polymer samples. Additionally, the stereocomplex
shows one small shoulder at 16.2°.
Figure 2. Powder XRD profiles of (a) poly((S)-propylene succinate),
(b) poly((R)-propylene succinate), and (c) stereocomplexed poly-
(propylene succinate).
ring-opening alternating copolymerization of succinic anhy-
dride with enantiopure propylene oxide and subsequent
stereocomplex formation, a new class of semicrystalline
biodegradable polyesters has been established. In addition, we
have shown that a relatively high melting semicrystalline
material with a Tm similar to low density polyethylene can be
created from two very slow-crystallizing, low Tm aliphatic
polyesters. With the wide range of cyclic anhydrides and
epoxides available commercially, the current focus of our
research is to widen the scope of biodegradable and
biocompatible materials that can be improved via stereo-
complexation. In addition, we are working to increase the
molecular weights of polyesters formed via ring-opening
alternating copolymerization, in particular poly(propylene
succinate), so that we can investigate the relationship between
molecular weight and stereocomplex formation. Finally, we are
developing catalysts for the isoselective polymerization of
racemic epoxides toward the goal of in situ stereocomplex
formation so that we can further improve this process by
eliminating the need for enantiopure starting materials.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data and spectra of
all new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
coates@cornell.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-1413862). J.M.L. gratefully acknowledges an NSF
Graduate Research Fellowship.
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