Kinetic studies of the alternating copolymerization of cyclic acid anhydrides and epoxides, and the terpolymerization of cyclic acid anhydrides, epoxides, and CO2 catalyzed by (salen)CrIIICl

Article abstract of DOI:10.1021/ma2026385

Copolymerization of a series of cyclic acid anhydrides with several epoxides using (salen)CrCl/onium salt catalysts has afforded polyesters with high molecular weights and narrow molecular weight distributions. The (salen)CrCl catalyst in the presence of

Full text of DOI:10.1021/ma2026385

Article
pubs.acs.org/Macromolecules
Kinetic Studies of the Alternating Copolymerization of Cyclic Acid
Anhydrides and Epoxides, and the Terpolymerization of Cyclic Acid
Anhydrides, Epoxides, and CO2 Catalyzed by (salen)CrIIICl
Donald J. Darensbourg,* Ross R. Poland, and Christina Escobedo
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: Copolymerization of a series of cyclic acid anhydrides
with several epoxides using (salen)CrCl/onium salt catalysts has
afforded polyesters with high molecular weights and narrow
molecular weight distributions. The (salen)CrCl catalyst in the
presence of the onium salts with formula PPNX (X = Cl−, N3 ) for
−
the copolymerization of the anhydrides, maleic (MA), succinic (SA),
phthalic (PA), cyclohexene (CHE), and cyclohexane (CHA) with the
epoxides, cyclohexene oxide (CHO), propylene oxide (PO), and
styrene oxide (SO) resulted in completely alternating enchainment of
monomers to provide pure polyesters. Temperature dependent studies of the ring-opening copolymerization of phthalic
anhydride and cyclohexene oxide monomers in toluene solution have yielded activation parameters of ΔH‡ = 67.5 kJ mol−1 and
ΔS‡ = −95.3 J mol−1, where the rate limiting step was ring-opening of the epoxide by the enchained anhydride. For the cyclic
acid anhydride (CHA), the relative order of reactivity with epoxides decreased PO > CHO ≥ SO, and for the epoxide (CHO)
the relative rate of copolymerization was CHA > PA > CHE. The (salen)CrCl/PPNN3 catalyst system was also shown to
effectively terpolymerize CHO/phthalic anhydride/CO2 to afford diblock copolymers, thereby producing in a one pot synthesis
poly(ester-co-carbonate). Tg values of the synthesized polyesters displayed a temperature range over 130 °C, from +95 °C to −39
°C.
(BDI = β-diiminate) complexes as active catalysts for the ring-
opening polymerization of epoxides and cyclic anhydrides.9 In
this manner, these researchers were able to synthesize new
aliphatic polyesters in a highly alternating copolymerization of
epoxides and cyclic anhydrides which afforded high molecular
weight copolymers with narrow molecular weight distributions.
Because (salen)CrCl complexes in the presence of onium
salts, like (BDI)ZnOAc complexes, have been very effective at
coupling CO2 and epoxides, it is anticipated that these
complexes will be productive catalysts for the copolymerization
of epoxides and cyclic anhydrides.10−13 Indeed, while this work
was in progress, DiCiccio and Coates presented results on the
ring-opening polymerization of maleic anhydrides with
epoxides catalyzed by (salen)CoO2CC6F5 or (salen)CrCl.14
Additional reports containing extensive MALDI−ToF−MS
studies of the ring-opening co- and terpolymerization of
cyclohexene oxide and a series of cyclic acid anhydrides and
CO2 catalyzed by chromium(III) porphyrinate and salen
complexes have been published by Duchateau and co-
workers.15 Herein, we report kinetic studies of the copoly-
merization of a variety of epoxides and cyclic anhydrides
catalyzed by this single-site chromium(III) catalyst. In addition
in situ infrared monitoring of the terpolymerization of phthalic
INTRODUCTION
■
It has become increasingly more apparent that there is a need
to develop the synthesis of new polymeric materials which
incorporate renewable resources.1 In this regard, the synthesis
of polyesters by way of ring-opening polymerization of cyclic
anhydrides and epoxides offers the possibility of providing a
wide variety of polyesters, including some from replenishable
reserves. For example, copolymers from succinic anhydride and
ethylene oxide or limonene oxide constitute polymeric
materials from such sources (Scheme 1).2,3 Furthermore,
Scheme 1
many of these aliphatic polyesters exhibit a wide range of
mechanical and thermal properties in addition to being
biodegradable.
Several early publications have reported upon catalytic
systems utilizing a range of metal initiators which are able to
accomplish these copolymerization reactions.4 However, these
processes generally display low catalytic activities and provide
copolymers of relatively low molecular weights.5−8 More
recently, Coates and co-workers have employed (BDI)ZnOAc
Received: December 5, 2011
Revised: February 10, 2012
Published: February 22, 2012
© 2012 American Chemical Society
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Scheme 2
overnight. Excess anhydride and catalyst were removed by dissolution
in 1.0 M HCl in methanol.
anhydride, cyclohexene oxide, and CO2 to provide a diblock
copolymer of polyester and polycarbonate, i.e., poly(ester-co-
carbonate) is examined.
RESULTS AND DISCUSSION
■
The various epoxide and cyclic anhydride monomers examined
in these studies are listed in Scheme 2. Initially we investigated
the copolymerization of phthalic anhydride (PA) and cyclo-
hexene oxide (CHO) employing the (salen)CrCl catalyst
(Figure 1) in the presence of several onium salts to optimize
EXPERIMENTAL SECTION
■
Reagents and Methods. Unless otherwise specified, all syntheses
and manipulations were carried out on a double-manifold Schlenk
vacuum line under an atmosphere of argon or in an argon filled
glovebox. Propylene oxide, cyclohexene oxide, and styrene oxide were
purchased from VWR and either distilled from CaH2 or used as
received. Phthalic anhydride, succinic anhydride, cyclohexane anhy-
dride, cyclohexene anhydride, and maleic anhydride were purchased
from VWR and used as received. Oxetane (Alfa Aesar) was freshly
distilled from CaH2 and stored in the freezer of the glovebox.
(Salen)CrIIICl was purchased from Strem. PPNCl was purchased from
Aldrich and recrystallized from diethyl ether and acetonitrile. PPNN3
was synthesized by mixing molar equivalents of NaN3, followed by
recrystallization from diethyl ether and acetonitrile. n-Bu4NCl was
purchased from VWR and recrystallized before use. n-Bu4NX, where X
Figure 1. N,N′-Bis(3,5-di-tert-butylsalicylidine)-1,2-cyclohexane
diaminochromium(III) chloride.
−
= Br−, N3 , or I− was synthesized from n-Bu4NCl and the appropriate
salt, followed by recrystallization.
the catalytic activity. As illustrated in the infrared traces of
polymer formation in Figure 2, the chromium(III) catalyst
alone was ineffective and the onium salt alone was less effective
than a combination of the two species. This is to be contrasted
with the previous literature report where (salen)CrCl did not
require a cocatalyst for the copolymerization of maleic
anhydride and propylene oxide.14 On the other hand, in the
Duchateau studies a DMAP (4-N,N-dimethylamino-pyridine)
cocatalyst was necessary in the presence of (Salophen)CrCl to
impart catalytic activity.15 In both instances employed in this
study the polyesters afforded exhibited narrow molecular
weight distributions, with PDIs ranging from 1.07 to 1.13.
Utilizing the two monomers (PA and CHO) in the
accompaniment of (salen)CrCl and various anions derived
from PPN+ and n-Bu4N+ salts we endeavored to determine the
optimal anionic initiator.
1
Measurements. All H and 13C NMR spectra were performed in
1
CDCl3. H NMR spectra were recorded at 295 K using an Inova
Varian spectrometer at 500 MHz, and 13C NMR spectra were
recorded at 295 K at 125 MHz. Chemical shifts are given in ppm
relative to TMS and coupling constants (J) in hertz. High-pressure
reaction kinetic measurements were performed using an ASI ReactIR
1000 reaction analyses system with stainless steel Parr autoclave
modified with a permanently mounted ATR crystal (SiComp) at the
bottom of the reactor (purchased from Mettler Toledo). In situ
infrared experiments were performed using a ReactIR ic10 with a
SiComp ATR crystal. Tg values were measured using a Mettler Toledo
polymer DSC equipped with a liquid nitrogen cooling system and 50
mL/min purge of nitrogen gas. Samples (∼10 mg) were weighed into
40 μL aluminum pans and subjected to two heating cycles, first cycle
was at 10 °C/min from −100 to +200 °C, second was at 5 °C/min
over the same range of temperatures. Molecular weight determination
was performed using a Viscotek GPC instrument with a low angle light
scattering (LALS), right angle light scattering (RALS), and refractive
index (RI) detectors with THF as eluent.
The rates of copolymerization initiated by the anions of
several PPN+ (bis(triphenylphosphine)iminium) salts in the
presence of (salen)CrCl did not vary significantly with the
anion or its concentration. For example, the second-order rate
Representative ReactIR Monitored Copolymerization. A
jacketed reaction vessel was charged with 64 mg (1 equiv) of
(salen)CrIIICl, 57 mg PPNN3 (1 equiv), and 3 g of phthalic anhydride
(200 equiv) followed by purging with argon. Then 23 mL of toluene
was added to the flask, at which point the flask was heated to 80 °C
and allowed to equilibrate for 15 min. Once it became apparent that all
the phthalic anhydride had dissolved, 2 mL of cyclohexene oxide (200
equiv) was injected into the flask, at which point the FTIR monitoring
was started. Reactions were allowed to proceed until 100% conversion
was achieved.
Representative Copolymerization. A vial was charged with a
stirring bar, 2 g of phthalic anhydride (533 equiv), 16 mg of
(salen)CrIIICl (1 equiv), and 7.5 mg of n-Bu4NCl (1 equiv). The vial
was purged for several minute with argon, then 10 mL of toluene and
1 mL of cyclohexene oxide (400 equiv) were injected into the vial. The
vial was inserted into an oil bath heated to 80 °C and allowed to react
constants employing one equivalent of Cl−, N3 , and DNP−
−
(dinitrophenoxide) spanned the range 1.44−1.76 × 10−2 M−1
s−1 at 80 °C and were not altered upon carrying out the process
using two equivalents of onium salts. The use of n-Bu4NX (X =
Br or I) salts resulted in a modest decrease in the reaction rate.
The lack of copolymerization rate on [onium salt] in the
presence of the (salen)CrCl catalyst indicates that the process
proceeds solely via a coordination insertion mechanism in these
instances. Further confirmation of this mechanistic pathway is
seen in the terpolymerization of PA/CHO/CO2, where the
coupling of CHO and CO2 only occurs with the aid of the
(salen)CrCl complex (vide infra).
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Figure 2. Growth of polyester peak at 1738 cm−1 as monitored by in situ ATR-FTIR spectroscopy for various combinations of catalyst and
cocatalyst: (green line) [Cr]:[Cl]:[PA]:[CHO] = 1:1:200:200, (red line) [Cr]:[Cl]:[PA]:[CHO] = 1:0:200:200, and (blue line) [Cr]:[Cl]:[PA]:
[CHO] = 0:1:200:200. Cr = (salen)CrCl, Cl = PPNCl, PA = phthalic anhydride, and CHO = cyclohexene oxide. Inset shows the linear plot of
ln(A∞/A∞ − At) vs time, where A∞ and At are the absorbances of the band at 1738 cm−1 at t∞ = infinity and t = time, respectively.
Figure 3. 1H NMR spectrum of PA/CHO. PA phenyl appears at 7.65 ppm. Encircled region represents where ether linkages should appear, hence
copolymer is >99% completely alternating.
Since the common PPN+ salts of chloride and azide provide
similar rates of initiation of polymerization, we have chosen to
use the azide anion in our comprehensive studies because of its
selected samples were subjected to molecular weight analysis by
gel permeation chromatography. Infrared analysis of the
purified polyester confirmed the location of the νCO stretching
frequency at 1738 cm−1 for all of the polyesters described
herein. The νCO vibration is significantly different from that of
the cyclic anhydrides, located at approximately 1770 cm−1,
which allows for the monitoring of polyester formation via in
situ ATR-FTIR analysis. The 1H NMR spectra of the polyesters
displayed chemical shifts in the 4−6 ppm range indicative of
hydrogens bound to the carbons α to the ester group, both
strong νN absorption band in the infrared. For these
3
investigations we have synthesized a series of polyesters
employing the combination of anhydride and epoxide
monomers listed in Scheme 2. The appropriate monomers
were dissolved in toluene and heated at 80 °C in the presence
of (salen)CrCl and 1 equiv of PPNN3. The resultant polyesters
were characterized by infrared spectroscopy, 1H NMR
spectroscopy, differential scanning calorimetry (DSC), and
1
from the anhydride and epoxide units (Figure 3). H NMR
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Scheme 3
Figure 4. Copolymerization of equimolar quantities of cyclohexene oxide and phthalic anhydride in the presence of 4.05 mM (salen)CrCl and one
equivalent of PPNN3 in 2.5 mL of toluene at 80 °C. (A) Absorbance of the growth of the 1738 cm−1 band of the polyester vs time. (B) Plot of
ln(A∞/[A∞ − At]) vs time.
spectra of all other polyester synthesized in this report may be
found in the Supporting Information. Furthermore, analysis of
the H NMR spectra revealed that the copolymerization of
epoxide and anhydride takes place in an almost exclusive
(>99%) alternating fashion.
Kinetic Studies. The suggested reaction pathways for the
formation of alternating copolymers from epoxides and
anhydrides are summarized in Scheme 3. As indicated, the
rate of epoxide and CO2 coupling to provide polycarbonates is
slow relative to polyester formation (vide infra). This latter
observation provides a one-step production of diblock
copolymers of polyesters and polycarbonates as previously
demonstrated by Coates and co-workers employing β-diiminate
zinc catalysts,9 and Duchateau and co-workers employing
(salophen)CrCl/DMAP as catalyst.15 The rate of the alkoxide
anion reacting with an anhydride monomer (k2) is generally
much faster than a carboxylate anion ring-opening an epoxide
(k3), hence the latter process is rate-determining. At constant
initiator concentration, the copolymerization is first-order with
respect to the epoxide monomer (Figure 4) and to the initiator
at equimolar ratios of monomers. An increase in the cyclic
anhydride concentration by 50% lead to only a slight change in
reaction rate.16 In the absence of the (salen)CrCl catalyst, a
slower copolymerization pathway similar to that described in
Scheme 3 presumably takes place initiated by ring-opening of
the epoxide by the free anion from the onium salt followed by
1
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successive alternating ring-opening of anhydride and epoxide.
Such a reaction scheme is included in the Supporting
Information.
The copolymerization reactions of equimolar quantities of
phthalic anhydride and cyclohexene oxide at a constant catalyst
concentration were monitored by in situ infrared spectroscopy
over a temperature range of 40 °C. Table 1 lists the rate
constants obtained from the linear plots in Figure 5 as a
Table 1. Temperature Dependent Rate Constants for the
a
Copolymerization of PA and CHO.
b
temperature (oC)
k3 × 103 M‑1 s‑1
Mn (PDI)
70.0
80.0
4.42
7.06
17.8
29.6
13 000 (1.12)
−
18 000 (1.13)
Figure 6. Eyring plot of the PA and CHO copolymerization reaction,
with R2 = 0.980.
90.0
100.0
17 000 (1.12)
a
Reaction conditions: (salen)CrCl/PPNN3/PA/CHO = 1:1:200:200,
where the [(salen)CrCl] = 4.05 × 10−3 M in toluene solution.
resulting polymer revealed it to be a well-defined copolymer
with two distinct Tg values (48 °C for PA/CHO and 115 °C
PCHC).
b
Determined by GPC in tetrahydrofuran using monodisperse
polystyrene standards.
The reaction rates for the copolymerization of cyclohexene
oxide with various cyclic anhydrides observed under similar
reaction conditions provided relative reactivities of CHA > PA
> CHE. Additionally, employing the CHA monomer while
varying the epoxide under identical reaction conditions afforded
a reactivity trend of PO > CHO ≥ SO (Figure 8). The percent
1
conversions were determined by integration of monomer H
1
NMR peaks vs polymer peaks. A representative series of H
NMR spectra for the CHA/PO copolymerization process is
included in the Supporting Information. However, the four-
membered cyclic ether, oxetane, was essentially unreactive
under these reaction conditions.
Thermal Properties of Copolymers. The glass transition
temperatures of the various polyesters synthesized from the
monomers listed in Scheme 2 utilizing a (salen)CrCl/PPNN3
catalyst system in toluene at 80 °C were determined by
differential scanning calorimetry (DSC) analysis. These data,
along with the corresponding molecular weight data for
selected copolymers obtained from GPC measurements, are
summarized in Table 2. As anticipated, the steric bulk of the
pendant groups and rigidity of the monomeric structure have a
direct effect on the Tg of the resultant polyester. That is, in
general the Tg values increased with epoxides in the order CHO
> SO > PO, and increased with cyclic anhydrides in the order
CHE > PA ≥ CHA > MA > SA. Specifically, the glass transition
temperature found for the polyesters formed from propylene
oxide and CHE, PA, MA, and SA were +36, +41, −23, and −39
°C, respectively. Similarly, the copolymer provided by the
alternative coupling of SA with CHO or SO displayed
corresponding Tg values of 34 and 4.4 °C.
The range of Tg values achievable from the alternating ring-
opening copolymerization of the studied epoxides and cyclic
anhydrides is over 130 °C, with a high Tg of +95 °C for the
copolymer from CHE and CHO and a low Tg of −39 °C for
that produced from SA and PO. Furthermore, it has been
shown that only a small percentage of cross-linking can
drastically increase the Tg of resultant polyesters. In this regard,
we have observed two vastly different Tg parameters (−23 and
+28 °C) for maleic anhydride/propylene oxide copolymer
samples obtained from very similar synthetic procedures. We
speculate here that this is due to a spontaneous photo-
dimerization reaction. Consistent with the suggestions is the
Figure 5. First-order plots for the copolymerization of PA and CHO
as a function of temperature.
function of temperature, with an Eyring plot of these data
shown in Figure 6. The ΔH‡ and ΔS‡ values derived from
Figure 6 for the copolymerization of PA and CHO were
determined to be 67.5 and −95.3 J/mol, respectively, with ΔG‡
being 101 kJ/mol at 80 °C.
Consistent with earlier studies, the terpolymerization of
cyclohexene oxide, phthalic anhydride, and CO2 catalyzed by
(salen)CrCl and PPNN3 afforded a one-pot synthesis of a
diblock copolymer of polyester and polycarbonate (Figure 7).15
As noted in Scheme 3 this observation requires that the route
governed by k2 and k3 to be much faster that the pathway
proceeding via CO2 insertion and subsequent polycarbonate
formation. This has been interpreted as resulting from a slower
rate of CO2 insertion vs cyclic anhydride insertion into the
metal alkoxide intermediate.17 Alternatively, since CO2
insertion into metal-alkoxides is highly reversible and generally
thought to be nonrate-limiting, this behavior may be the result
of a much slower ring-opening step of the metal−carbonate
intermediate with the epoxide monomer, a process driven by k4
in Scheme 4. Differential scanning calorimetry analysis of the
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Figure 7. In situ FTIR analysis of block copolymerization process. Reaction conditions used were (salen)CrCl/PPNCl/PA/CHO = 1:1:200:600
under 500 psi CO2, 80 °C with 19 mL toluene (solvent, volume totaling 25 mL). Deconvolution was performed by ReactIR ic10 software. Green line
= polyester growth (1738 cm−1), red line = anhydride consumption (1770 cm−1), and blue line = polycarbonate growth (1750 cm−1). The three-
dimensional surface of the reaction is shown in the inset on the right.
Scheme 4
Table 2. GPC and DSC Data for Selected Polyesters
polyester (cyclic anhydride/
epoxide)
Mn
Mw
Tg
(°C)
c
(×10−3
)
(×10−3
)
PDI
CHE/CHO
CHA/CHO
CHE/SO
MA/CHO
PA/SO
10
9.2
5.7
8.5
19
18
14
14
9.7
6.5
20
24
20
15
1.34
1.06
1.14
2.3
95
85
53
53
46
48
−
1.27
1.13
1.07
a
PA/CHO
b
PA/CHO
a
Sample was synthesized using both PPNN3 and (salen)CrCl.
Sample was synthesized using only PPNCl as initiator. Tg values
b
c
represent the midpoint temperature during the second heating cycle.
synthesized in this report. That is, the other polyester
synthesized displayed narrow polydispersity indices (<1.30)
with Mw values of about 10 000 or greater.
CONCLUSIONS
■
Herein, we have reported kinetic studies of the alternating ring-
opening copolymerization of various epoxides with cyclic acid
anhydrides to afford polyesters of high molecular weights and
narrow molecular weight distributions using a (salen)CrCl
catalyst in the presence of onium salts. The activation
parameters for the process involving phthalic anhydride/
cyclohexene oxide monomers in toluene solution were
determined to be ΔH‡ = 67.5 kJ mol−1 and ΔS‡ = −95.3 J
mol−1, where the rate-determining step was ring-opening of the
epoxide by the enchained anhydride. For a given cyclic
anhydride, cyclohexane anhydride, the relative rate of coupling
with epoxide decreased in the order PO > CHO ≥ SO,
whereas, for cyclohexene oxide the relative reactivity order with
anhydrides was CHA > PA > CHE. This catalytic system was
also shown to terpolymerize epoxide/cyclic anhydride/CO2 to
afford diblock copolymers with very little tapering, thereby
providing a one-step synthesis of poly(ester-co-carbonate). In
the polyester product resulting from the ring-opening polymer-
Figure 8. Comparison of epoxides: polymer growth vs time. Reactions
were performed in sealed NMR tubes using 2.0 mL of toluene-d8 as
solvent. Then 200 equiv of the appropriate epoxide was added to tube,
which was then dissolved in a stock solution of 1.8 mL of toluene-d8,
300 mg of CHA (200 equiv), 6.3 mg of (salen)CrCl (1 equiv), and 5.7
mg of PPNN3 (1 equiv). The tubes were then heated to 80 °C and
allowed to react until 100% conversion was achieved.
observation that copolymers from MA and epoxides have the
broadest polydispersity indices (∼2.3) of all polyester
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ization of maleic anhydride and propylene oxide two vastly
different Tgs were observed at −23 and +28 °C. We have
proposed the higher Tg value to be the result of some degree of
cross-linking by photoinduced dimerization. Further efforts are
being directed at providing a more definitive assessment of this
possibility.
ASSOCIATED CONTENT
* Supporting Information
Reaction scheme for cyclic anhydride/epoxide copolymeriza-
■
S
1
tion initiated by onium salts alone, H NMR spectra of all
polyesters, and 1H NMR monitoring of the copolymerization of
cyclohexane anhydride and propylene oxide. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE
0543133 and CHE 1057743) and the Robert A.Welch
Foundation (A-0923) for financial support for this work.
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