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Discrete Metal-Based Catalysts for the Copolymerization of CO2 and Epoxides Discovery Reactivity Optimization and Mechanism.

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Reviews
G. W. Coates and D. R. Moore
Polymerization Catalysts
Discrete Metal-Based Catalysts for the
Copolymerization of CO2 and Epoxides: Discovery,
Reactivity, Optimization, and Mechanism
Geoffrey W. Coates* and David R. Moore
Keywords:
carbon dioxide · epoxides ·
homogeneous catalysis ·
polymers · renewable
resources
Angewandte
Chemie
6618
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460442
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Angewandte
Chemie
CO2–Epoxide Copolymerization
Most synthetic polymers are made from petroleum feedstocks.
Given the non-renewable nature of these materials, there is
increasing interest in developing routes to polymeric materials
from renewable resources. In addition, there is a growing demand
for biodegradable polymeric materials. Polycarbonates made
from CO2 and epoxides have the potential to meet these goals.
Since the discovery of catalysts for the copolymerization of CO2
and epoxides in the late 1960-s by Inoue, a significant amount of
research has been directed toward the development of catalysts of
improved activity and selectivity. Reviewed here are well-defined
catalysts for epoxide–CO2 copolymerization and related reactions.
From the Contents
1. Introduction
6619
2. Early Discoveries and Background
6620
3. Aluminum and Manganese Catalysts 6622
4. Chromium Catalysts
6624
5. Cobalt Catalysts
6627
6. Lanthanide-Based Catalysts
6627
7. Zinc and Cadmium Catalysts
6628
8. Summary and Outlook
6636
1. Introduction
Since petroleum resources are predicted to be exhausted
within the next century at the current rate of consumption,[1]
there is a growing effort to develop new chemical processes
using biorenewable resources.[2–4] One such resource of
particular interest is CO2, a nontoxic, nonflammable, naturally abundant C1 feedstock.[5–8] The reaction of CO2 with
metal complexes has been extensively studied, revealing
potential pathways for catalytic reactions.[9–15] However the
thermodynamic stability of CO2 has hampered its utility as a
reagent for chemical synthesis; in fact its high stability makes
it an ideal medium for many chemical processes.[7, 16, 17] To
overcome this limitation, reactions employing CO2 with
highly reactive reagents have been explored. In particular,
the catalytic coupling of CO2 with heterocycles has received
considerable attention over the past 35 years.[18–23] A majority
of these publications involve the reaction of CO2 with
epoxides to generate polycarbonates and/or cyclic carbonates
(1,3-dioxolan-2-ones) (Scheme 1).
Scheme 1. Alternating copolymerization of cyclohexene oxide (CHO)
and propylene oxide (PO) with CO2.
Aliphatic polycarbonates have potential applications as
packaging materials, as well as in the synthesis of engineering
thermoplastics and resins, pyrotechnics, and interliners for
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
safety glass.[24–26] Poly(propylene carbonate) (PPC)[+] decomposes uniformly and controllably to cyclic propylene carbonate below 250 8C,[27, 28] making it particularly useful as a binder
for ceramics, adhesives, and propellants.[29] The glass transition temperature (Tg) of PPC is 35–40 8C, which hinders its
broad utility as a bulk material.[28] Therefore, efforts are being
directed to employ aliphatic polycarbonates as additives and
pore formers. Clearly one of the most promising applications
of PPC is as a mid-segment of polyurethanes.[25] PPCs with
high ether linkage content (80 % ether) have been reported to
exhibit excellent solubility in supercritical CO2, a rare
property for non-fluorinated polymers.[30] Alicyclic polycarbonates, such as poly(cyclohexene carbonate) (PCHC),
typically have much higher Tg7s (115 8C for PCHC) resulting
in materials with properties very similar to poly(styrene).[31]
PCHC also has a higher decomposition temperature
( 300 8C), which allows melt-processing.[26] Alicyclic polycarbonates are finding applications in lithographic processes
for the construction of microfluidic devices.[32–35] Cyclic
carbonates are utilized industrially as polar aprotic solvents,
substrates for small molecule synthesis, additives, antifoam
agents for antifreeze, and plasticizers.[36, 37] The five-membered ring cyclic carbonates (1,3-dioxolan-2-ones) are generally incapable of ring-opening polymerization due to their
thermodynamic stability, but do undergo polymerization with
partial loss of CO2 to yield macromolecules with both ether
and carbonate linkages.[38] Due to such uses, a number of
syntheses of cyclic carbonates have been described over the
[*] Prof. G. W. Coates, Dr. D. R. Moore
Department of Chemistry and Chemical Biology
Baker Laboratory
Cornell University
Ithaca, NY 14853-1301 (USA)
Fax: (+ 1) 607-255-4137
E-mail: gc39@cornell.edu
[+] A list of frequent abbreviations is given at the end of this Review.
DOI: 10.1002/anie.200460442
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6619
Reviews
G. W. Coates and D. R. Moore
last 30 years. For example, tetraalkylammonium salts, phosphanes, main-group and transition-metal complexes, and
alkali metal salts convert epoxides and CO2 to cyclic
carbonates.[20]
The moderate thermal stability and low temperature of
thermal deformation of aliphatic polycarbonates, coupled
with their high cost (ca. $100 per lb) hamper their widespread
use as bulk polymeric materials. More economically viable
processes and the synthesis of new types of improved aliphatic
polycarbonates would clearly increase the number of applications for these polymers, as well as lower their cost. A
significant contributor to the cost of these materials is the low
activity of the industrial zinc/dicarboxylic acid catalysts used
to polymerize epoxides and CO2. As a result, a significant
amount of recent research has focused on the discovery and
development of new catalysts for this process.
There are many parallels between the development of
alkene polymerization catalysts and those for CO2/epoxide
polymerization. In each field, the catalysts initially discovered
were heterogeneous; subsequent work focused on the empirical optimization to provide higher activity and selectivity for
the polymerizations. Eventually, discrete, homogeneous
metal complexes were explored in academic laboratories as
a way to probe reaction mechanisms. It was envisaged that a
detailed understanding of the polymerizations at the molecular level could be applied to the design of improved catalytic
systems. In some cases, these new homogeneous catalysts
have significant advantages over their heterogeneous counterparts.
Heterogeneous catalysts are the workhorse of many
industrial processes. They have many processing advantages
over their soluble counterparts, but often contain multiple
active sites that result in polymers with broad polydispersity
indices (PDIs) and composition distributions. In many cases,
only a small percentage of the metal sites are active, and
residual catalyst remains in the polymeric product. As a result
of these drawbacks, a significant amount of research has been
directed toward the development of well-defined, single-site
homogeneous catalysts. Homogeneous catalysts are typically
of the form LnMR, where Ln is a set of permanently bound
ligands, M is a metal center, and R is an efficient initiation
group. These homogeneous catalysts are discrete species,
rendering them amenable to precise modification as well as
detailed mechanistic studies. Most of the major advances in
metal-catalyzed polymerization, including stereoselective[39]
and living[40] alkene polymerization, lactide and lactone
polymerization,[41] olefin metathesis,[42] and alkene/CO
copolymerization,[43] are the result of progress in homogeneous catalyst design. Homogeneous catalysts are being used
to develop unique polymer architectures that lead to new,
industrially relevant materials. However, it should be noted
that the vast majority of industrial polymerization catalysts
are still of the heterogeneous variety.
1.1. Scope of Review
The purpose of this review is to give a thorough account of
the CO2/epoxide polymerization literature,[18–22] with a strong
emphasis on single-site homogeneous catalysts and their
mechanisms of operation. The review is organized according
to the active metal center of the catalyst and although
polymerization is the focus, the production of cyclic carbonates is discussed when appropriate. Activities in the form of
turnover frequencies (TOFs) are given in mole epoxide
converted to product per mole metal per hour (assuming all
metal centers are active). The activity of a system is defined
on the basis of TOFs as follows: low (< 5 TO h 1), moderate
(5–200 TO h 1), and high (> 200 TO h 1). For conformity, CO2
pressures are reported in atm (1 atm = 14.7 psi = 1.013 bar =
1.013 B 105 Pa).
2. Early Discoveries and Background
In 1969, Inoue and co-workers made the remarkable
discovery that a mixture of ZnEt2 and H2O was active for
catalyzing the alternating copolymerization of propylene
oxide (PO) and CO2 (Scheme 1), marking the advent of
epoxide–CO2 coupling chemistry.[44, 45] An optimum 1:1 ratio
of ZnEt2/H2O gave the best yields of methanol-insoluble PPC
with an activity of 0.12 h 1 (mol of PO converted to polymer
per mol Zn per h) at 80 8C and 20–50 atm CO2 (Table 1). On
the basis of elemental analysis, the copolymer contained 88 %
carbonate linkages. Notably, a 1:1 mixture of ZnEt2 and
MeOH did not generate an active catalytic species for
polycarbonate synthesis. Following this initial lead, Inoue
investigated the use of dihydric sources, including resorcinol,[46, 47] dicarboxylic acids,[48] and primary amines,[49] in
mixtures with ZnEt2 for PO–CO2 copolymerization. These
Geoffrey W. Coates, born in Evansville, Indiana in 1966, obtained a B.A. degree in
chemistry from Wabash College in 1989 and
a Ph.D. in organic chemistry from Stanford
University in 1994. In his thesis work, under
the direction of Robert M. Waymouth, he
investigated the stereoselectivity of metallocene-based Ziegler–Natta catalysts. Following his doctoral studies, he was an NSF
Postdoctoral Fellow with Robert H. Grubbs
at the California Institute of Technology. In
1997, he joined the Department of Chemistry at Cornell University where he is currently
Professor of Chemistry.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
David R. Moore, born in Hamilton, New
Jersey in 1976, graduated from Drew University in 1998 with a B.A. degree in chemistry
and subsequently studied organic chemistry
under the supervision of Geoffrey W. Coates
at Cornell University. His doctoral research
focused on the mechanism of alternating
copolymerization of epoxides and carbon
dioxide using b-diiminate–zinc catalysts.
After graduating with a Ph.D. in 2003, he
became a research scientist at the General
Electric Global Research Center in Niskayuna, New York.
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Angewandte
Chemie
CO2–Epoxide Copolymerization
systems showed TOFs of 0.17, 0.43, and 0.06 h 1, respectively
(see Table 1).
Following Inoue7s discoveries, Kuran and co-workers
developed a copolymerization system using ZnEt2 and
trihydric phenols, including pyrogallol and 4-bromopyrogal-
coordination–insertion mechanism (Scheme 2). The mechanism includes several prevailing principles:
1) Mechanism: The alternating copolymerization of epoxides and CO2 is a two-step process; the insertion of CO2
into a metal alkoxide is followed by insertion of epoxide
into a metal carbonate. Hence,
most catalysts (polymerization
Table 1: Selected heterogeneous catalysts for PO–CO2 copolymerization.[a]
initiators) are metal-alkoxide or
Complex
p(CO2) [atm]
t [h]
T [8C]
TON[b]
TOF [h 1][c]
Ref.
metal-carboxylate species that
are similar to the putative cataZnEt2/H2O[d]
20–50
48
80
5.9
0.12
[44]
lytic intermediates.
30
48
35
8.1
0.17
[46]
ZnEt2/resorcinol[d]
ZnEt2/isophthalic acid[d]
40
44
35
19.1
0.43
[48]
2) Regiochemistry: In the copoly40
44
35
19.6
0.45
[48]
ZnEt2/m-hydroxybenzoic acid[d]
merization of CO2 and aliphatic
ZnEt2/a-phenethylamine[d]
40
68
40
3.9
0.06
[49]
epoxides
(propylene oxide,
60
44
35
12.0
0.27
[50]
ZnEt2/pyrogallol[e]
etc.),
epoxide
ring-opening is
ZnEt2/4-bromopyrogallol[e]
60
45
35
13.8
0.31
[51]
[f ]
typically
favored
at the leastZn(OH)2/glutaric acid
30
40
60
44.3
1.1
[53]
hindered C O bond, although
25
40
60
134
3.4
[108]
ZnO/glutaric acid[f ]
cleavage is normally observed at
[a] All polymerizations result in PPC, which is collected as the MeOH-insoluble fraction. [b] Moles of PO
both C O bonds, giving regioirconsumed per mole of zinc. [c] Moles of PO consumed per mole of zinc per hour. [d] 1:1 ratio of Zn to
regular polymers.
protic source, reaction in dioxane. [e] 2:1 ratio of ZnEt2 to protic source, reaction in dioxane. [f] Reaction
3) Stereochemistry: In the copolyrun in neat PO.
merization of CO2 and alicyclic
epoxides, such as cyclohexene
oxide (CHO), C O bond cleavage typically occurs with
lol, that produced PPC with TOFs up to 0.3 h 1 at 35 8C and
inversion of configuration at the site of attack (SN2-type
60 atm CO2.[50, 51] In general, di- or tri-protic sources and
ZnEt2 produced PPC, while monoprotic sources, such as
mechanism) to give the trans ring-opened product.[52, 59] To
alcohols and secondary amines, only gave propylene carbondate, there are no reports of catalysts that generate tactic
ate (PC) (Schemes 1 and 2).[52] In an effort to develop more
polycarbonates by chain-end control mechanisms, presumably due to the distance between the stereogenic
active catalysts, Hattori and co-workers synthesized a heterocenter of the chain end and the active metal center. There
geneous catalyst from Zn(OH)2 and glutaric acid. Under
are examples of stereocontrol by site-control mechanisms
30 atm CO2 and 60 8C, the Zn(OH)2/glutaric acid mixture
using chiral metal catalysts (see Chapters 5, 7).
yielded PPC with a TOF of 1.1 h 1 (Mn = 12 000 g mol 1).[53]
4) Polymer/cyclic product selectivity: Cyclic species are a
While the discoveries of ZnEt2/R(OH)x and Zn(OH)2/
common by-product of the copolymerization of CO2 and
glutaric acid catalysts for epoxide–CO2 coupling marked
salient scientific findings, the active species responsible for
aliphatic epoxides. Many systems produce predominantly
polymer and cyclic formation remain unknown. Nevertheless,
cyclic species,[20] which are thermodynamically more
several mechanistic studies indirectly support the theory that
stable than polycarbonates. The percentage of polymer
multi-site or polymeric catalysts are operative in the altertypically increases at lower reaction temperature. Systems
nating copolymerization of CO2 and epoxides.[18, 19, 52, 54–58] In
can be tuned to favor cyclic-species or polymer formation
depending on the catalyst, additives, CO2 pressure,
the absence of multiple metal sites (i.e., 1 ZnEt2 + 2 equivalents monohydric source), cyclic compounds are the preepoxide concentration, and temperature. Formation of
dominant product of epoxide–CO2 coupling. Epoxide–CO2
cyclic species results from degradation of the growing
polycarbonate chain by depolymerization or backbitcopolymerization is generally accepted to proceed by a
ing.[51] In most cases, cyclic carbonates are
thought to be generated by the backbiting of
a metal-alkoxide into an adjacent carbonate
linkage (Scheme 2).[57]
5) Ether and dicarbonate linkages: The presence
of ether linkages as a result of consecutive
epoxide enchainment can be observed in
some aliphatic polycarbonates. Most systems
can be tuned to favor CO2 incorporation by
catalyst selection, CO2 pressure, epoxide
concentration, and polymerization temperature. The enthalpically disfavored consecutive insertion of two molecules of CO2 to
give dicarbonate linkages has not been
reported.
Scheme 2. The basic mechanism of epoxide–CO2 copolymerization and the formation
of cyclic carbonates (Ln = ligand set, M = metal, P = polymer chain).
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6621
Reviews
G. W. Coates and D. R. Moore
Scheme 3. Qualitative, ideal free-energy profile depicting alternating copolymerization of propylene oxide and CO2, as well as potential side-reactions.
These prevailing principles are depicted in a qualitative
free-energy profile for the copolymerization of epoxides and
CO2 (Scheme 3).
As observed in early studies, only a few metals are active
for the coupling of epoxides and CO2, including Al, Cr, Co,
Mg, Li, Zn, Cu, and Cd.[18, 19] Studies have shown that large
differences in catalytic efficacy result from the organic
frameworks surrounding these metals, especially in the case
of zinc. Accordingly, subsequent studies have largely focused
on empirical modification of ligands to generate improved
catalysts.
Figure 1. Aluminum and manganese porphyrins for the homopolymerization of epoxides and copolymerization of epoxides and CO2
(R = alkyl, oligomer of PPO).
3. Aluminum and Manganese
Catalysts
In 1978, Inoue developed the first
single-site catalysts for epoxide–CO2
copolymerization based on a tetraphenylporphyrin (tpp) ligand framework, 1 a–d.[60] [(tpp)AlCl] (1 a) and
[(tpp)AlOMe] (1 b) (Figure 1) were
found to be living initiators for the
homopolymerization of PO and of
lactones, including lactide, b-butyrolactone, and e-caprolactone, as well as
for the copolymerization of CO2 and
epoxides and of PO and phthalic
anhydrides.[61–67] 1 a and 1 b reacted
with PO to form poly(propylene
oxide) (PPO) in a living polymerization with PDIs of 1.07–1.15
(Scheme 4). The chloride initiator
ring-opened the least hindered C O
bond and generated a regioregular
6622
Scheme 4. Reactivity of aluminum–porphyrin complexes.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Angewandte
Chemie
CO2–Epoxide Copolymerization
PPO. In addition, 1 b copolymerized PO and CO2 at 20 8C and
8 atm CO2, giving PPC (Mn = 3900 g mol 1; Mw/Mn = 1.15)
with 40 % carbonate linkages over the course of 19 days.[61, 64]
Although molecular weights were low and reaction times
were long, this reaction marked the first example of monodisperse polycarbonates having a narrow PDI.
Similarly to 1 a, [(tpp)AlOR] (1 d), where R is an oligomer
of PO, did not react with CO2. However, upon addition of 1methylimidazole (MeIm), reversible CO2 insertion was
observed. On the basis of 1H NMR studies, MeIm was
proposed to bind to aluminum in a trans fashion and activate
the metal toward CO2 insertion. The addition of ethylene
oxide (EO) and CO2 to 1 d catalytically produced cyclic
ethylene carbonate (EC).[65] The copolymer of PO and CO2
was synthesized upon addition of ammonium or phosphonium salts. 1 a and 1 equivalent of EtPh3PBr gave PPC (Mn =
3500 g mol 1, PDI = 1.09) with a TOF of 0.18 h 1 at 20 8C and
48 atm CO2. Both EtPh3PBr and Et4NBr were efficient
cocatalysts, drastically increasing the percentage of carbonate
linkages to > 99 %. Additionally, cyclic propylene carbonate
was produced as a by-product in approximately 20 % yield
relative to converted PO. 1 a/EtPh3PBr was also active for
EO–CO2 and CHO–CO2 alternating copolymerizations producing poly(ethylene carbonate) (PEC) with 70 % carbonate
linkages, a Mn of 5500 g mol 1, and a PDI of 1.14. Poly(cyclohexene carbonate) was produced with > 99 % carbonate
linkages, a Mn of 6200 g mol 1, and a PDI of 1.06 (TOF =
0.30 h 1; Table 2). Even though this system yields PC, neither
EC nor cyclohexene carbonate (CHC) was observed.
Finally, Inoue and co-workers synthesized several ABand ABA-type block copolymers, incorporating PPO, PPC,
and/or poly(PO-alt-phthalic anhydride), illustrating the
“living” nature of the polymerizations.[67] In 1999, Ree and
co-workers also explored metalloporphyrins for PO–CO2
copolymerization. In contrast to Inoue7s results, Ree found
that 1 a and Et4NBr cocatalyst gave PPC (Mn = 1900 g mol 1;
Mw/Mn = 1.10) with only 75 % carbonate linkages at 20 8C and
52 atm CO2.[68] Aluminum porphyrins have also been utilized
in the exclusive production of cyclic carbonates.[69]
The low molecular weights of polymers produced by
{(tpp)Al} catalysts suggest chain transfer, which supports
Inoue7s proposal of an “immortal” type polymerization.[60] An
immortal polymerization allows for multiple chains to propagate from one metal center, whereas a living polymerization
grows only one chain per metal center. Protic sources
facilitate chain swapping such that there are more polymer
chains than active catalytic sites (Scheme 5). Free chains are
Scheme 5. “Immortal” polymerization of PO using aluminum–porphyrin complexes.
dormant, but continue to grow polymer when exchanged onto
the active site. If the chain swapping is more rapid than
propagation, polymer chains with narrow PDIs are produced.
For example, addition of HCl does not quench the polymerization. Instead, it yields [(tpp)AlCl] which allows for new
polymer chains to be initiated (see Scheme 5). [(tpp)AlCl]
reinitiates polymerization and grows a new polymer chain in
the same “immortal” manner, eventually giving a bimodal
polymer distribution. Although no rate studies were reported
for the alternating copolymerization of CO2 and epoxides,
Inoue showed a second-order dependence on the catalyst for
the homopolymerization of d-valerolactone[70] and proposed a
Table 2: Selected homogeneous catalysts for epoxide–CO2 copolymerization.[a]
Complex
Epoxide
p(CO2) [atm]
t [h]
T [8C]
TON[b]
TOF [h 1][c]
Carbonate
linkages [%][d]
Mn [kg mol 1][e]
Mw/Mn
Ref.
1 a + EtPh3PBr (1:1)
8 + DMAP (1:1)
10 + MeIm (1:5)
11 + DMAP (1:1)
13 a
14 a
17 a
36 a
39 b
46
CHO
CHO
CHO
PO
PO
CHO
CHO
CHO
CHO
PO
48
225
60
35
55
135
55
7
7
7
336
18
24
4
3
24
48
0.5
0.17
2
20
110
80
75
25
100
80
50
50
25
100
3120
774
640
243
210
364
180
380
470
0.30
173
32.2
226[g]
81[h]
8.8
7.6
360
2290
235[i]
> 99
97
> 99
98
95
93
> 99
95
90
> 99
6.2
3.9
8.9[f ]
16.7
15.3
17.0
42.0
15.8
22.9
36.7
1.06
1.16
1.2[f ]
1.38
1.22
6.4
6.0
1.11
1.09
1.13
[67]
[81]
[87]
[90]
[93]
[117]
[125]
[151]
[149]
[150]
[a] Polymerizations using CHO and CO2 result in PCHC, whereas those incorporating PO and CO2 yield PPC. [b] Moles of epoxide consumed per mole
of metal. [c] Moles of epoxide consumed per mole of metal per hour. [d] Calculated by integration of methine resonances in the 1H NMR spectrum of
the polymer. [e] Determined by gel-permeation chromatography, calibrated with polystyrene standards. [f] Molecular weight and molecular weight
distribution were taken from a run without MeIm. [g] A 71:29 ratio of PPC/PC was observed by 1H NMR spectroscopy. [h] > 99 % PPC/PC was observed
by 1H NMR spectroscopy. [i] A 75:25 ratio of PPC/PC was observed by 1H NMR spectroscopy.
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
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G. W. Coates and D. R. Moore
linear transition-state mechanism incorporating two aluminum–porphyrin complexes for the ring-opening of epoxides.[71]
In 2003, Inoue and co-workers developed a related
porphyrin system utilizing manganese as the active metal
center.[72] At 80 8C, [(tpp)MnOAc] (1 e; Figure 1) reacted with
CHO and 50 atm CO2 to produce PCHC (99 % carbonate
linkages; Mn = 6700 g mol 1; Mw/Mn = 1.3) with a moderate
TOF of 16.3 h 1. In this system, additives such as PPh3,
pyridine, or MeIm compromised polymerization rates and
decreased the percentage of carbonate linkages. At 80 8C and
only 1 atm of CO2, 1 e catalyzed PCHC formation providing
activities of up to 3.3 h 1 (95 % carbonate linkages; Mn =
3000 g mol 1; Mw/Mn = 1.6). Finally, the coupling of PO and
CO2 yielded PC, but gave no PPC.
Recently, a salicylaldimine(salen)–aluminum complex,
2 a, was found to be highly active for the cyclization of EO
and CO2 to ethylene carbonate (Figure 2).[73, 74] Lewis bases or
Figure 2. Salen catalyst systems (cocatalyst = tetrabutylammonium
halide, pyridine, or MeIm) for the synthesis of ethylene carbonate.
quaternary ammonium salts, including pyridine, MeIm, and
nBu4NX (X = Cl, Br, I), were utilized as cocatalysts, enhancing rates by up to a factor of five. At 110 8C and in
supercritical CO2 (ca. 150 atm CO2), a 1:1 mixture of 2 a/
nBu4NBr catalyzed the conversion of EO to EC with a TOF of
2220 h 1. Salen-chromium (2 b) and -cobalt (2 c) analogs also
promoted cyclic-species formation showing rates of 2140 and
1320 h 1, respectively (see Chapters 4, 5). Darensbourg and
co-workers have reported AlCl4 -based complexes that
exhibit TOFs up to 50 h 1 for the synthesis of propylene
carbonate from PO and CO2.[75]
In 1998, Kuran and co-workers reported an aluminum
calix[4]arene, 3, derived from 25,27-dimethoxy-26,28-dihydroxy-p-tert-butylcalix[4]arene and diethylaluminum chloride
(Figure 3).[76] Complex 3 was found to be active for the
alternating copolymerization of CO2 with PO or CHO. At
60 atm CO2 and 35 8C, PPC (Mn = 5620 g mol 1) was formed
with a TOF of 0.11 h 1. The polycarbonate contained low
levels of carbonate linkages, comparable to the PPC made
from [(tpp)AlCl] (1 a) in the absence of quaternary salts.
Under the same reaction conditions, 3 converted CHO and
CO2 to PCHC (Mn = 1930 g mol 1) with a TOF of only
0.05 h 1. Additionally, cyclic PC and CHC were produced as
by-products in 14 % and 4 % yield with respect to epoxide.
Kuran et al. proposed a mechanism involving participation of
two aluminum complexes in the alternating copolymerization, although no mechanistic studies were reported. Finally, 3
was active for the homopolymerization of PO and CHO,
giving poly(alkene oxides) with fairly narrow PDIs (Mw/Mn =
1.36–1.51).
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Aluminum complexes for the homopolymerization of epoxides and copolymerization of epoxides and CO2.
Aluminum alkoxides have also been shown to convert
epoxides and CO2 to polycarbonates. Beckman and coworkers reported several aluminum complexes, including 4,
5, and 6 (Figure 3), that reacted with CHO and CO2 to give
PCHC with a maximum TOF of 2.7 h 1.[30, 77, 78] At 80 atm CO2
and 60 8C, 4 produced PPC (Mn = 5000 g mol 1; Mw/Mn = 2.89)
with only 22 % carbonate linkages and a TOF of 2.0 h 1. These
low-carbonate content polymers have shown promise as
solubilizers in supercritical CO2 (scCO2).[30] As expected,
these complexes are also active for the homopolymerization
of CHO.
Aluminum complexes are indeed active for the copolymerization of epoxides and CO2 ; however, they are plagued
by low activities and yield polycarbonates with high percentages of ether linkages. It appears that without additives,
current aluminum catalysts do not cleanly generate alternating copolymer. Nevertheless, the “immortal” polymerization
of [(tpp)AlX] compounds shows promise for the synthesis of a
wealth of unique copolymers with varying levels of carbonate
linkages provided the activities can be improved.
4. Chromium Catalysts
Kruper and Dellar discovered that [(tpp)CrX] (7 a, b;
Figure 4) in mixtures with 4–10 equivalents of a Lewis-basic
amine cocatalyst (such as MeIm or (4-dimethylamino)pyridine (DMAP)) are moderately active for the cyclization of
epoxides and CO2.[79, 80] A wide range of epoxides, including
PO, trans-2-butene oxide, epichlorohydrin, CHO, and cyclopentene oxide (CPO), were rapidly converted to the corresponding cyclic carbonates. For instance, under 50 atm CO2
and at 80 8C, 7 b and MeIm converted PO to PC affording a
TOF of 158 h 1. 7 a and DMAP catalyzed CHC formation at
50 atm CO2 and 95 8C, exhibiting activities of 103 h 1. In this
case, PCHC was isolated as the major product. Following
thermolysis, a 95:5 ratio of trans and cis CHC was observed,
suggesting the possibility of dual mechanisms. Unexpectedly,
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CO2–Epoxide Copolymerization
Jacobsen and co-workers found [(salen)CrCl] complexes to be highly active in the asymmetric ringopening of epoxides.[84] This elegant work has since led
to many crucial discoveries in the coupling of epoxides
with CO2 ; in fact the first report of (salen)chromiummediated epoxide–CO2 polymerization appeared in a
2000 patent by Jacobsen and co-workers.[85] Nguyen and
Paddock reported highly active [(salen)CrCl]/DMAPbased systems, 9 a–c and 10 (Figure 5), for the cycloaddition of CO2 and a variety of terminal aliphatic
epoxides, including PO, epichlorohydrin, butadiene
monoepoxide, and styrene oxide (SO).[86] Cis[(salen)CrCl] (9 a) was the most efficient catalyst and
Figure 4. Chromium–porphyrin complexes for the coupling of epoxides
and CO2.
the conversion of CPO to cyclopentene carbonate gave the cis
isomer as the exclusive product. To account for this, two
mechanistic pathways were suggested: 1) double inversion
leading to retention of configuration; and 2) inversion of
configuration due to backbiting into a polymer chain. A
possible mechanism to explain the double inversion is shown
in Scheme 6.
Scheme 6. Proposed mechanism of dioxolanone synthesis using
[(tpp)CrCl].
Following this lead, Holmes and co-workers developed
[(tfpp)CrCl] (8; Figure 4), which showed activities of up to
173 h 1 for the alternating copolymerization of CHO and CO2
at 225 atm CO2 (scCO2) and 110 8C (Table 2).[81, 82] As with
complexes 7 a and 7 b, the copolymerization only yielded
polycarbonate when 8 was combined with a cocatalyst such as
DMAP. The fluorinated aromatic moieties improved catalyst
solubility in scCO2, and consequently increased the yields of
PCHC. Similar to aluminum–porphyrin catalysts for epoxide–
CO2 copolymerization, these chromium analogs yielded
polycarbonates with narrow PDIs (Mw/Mn = 1.08–1.50) and
low molecular weights (Mn = 1500–9400 g mol 1). Furthermore, the resultant PCHC contained high percentages of
carbonate linkages (97 %). More recently, polymer supported
chromium porphyrins have been found to be active for PCHC
production.[83]
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Figure 5. Salen–chromium and salen–cobalt complexes for the coupling of epoxides and CO2.
was approximately twice as active as trans-[(salen)CrCl] (9 b).
At 100 8C and 7 atm CO2, 9 a and 1 equivalent of DMAP
rapidly converted PO to PC in 1 h, exhibiting a high TOF of
916 h 1. Furthermore, activities were largely dependent on
DMAP concentration. Cyclization activities increased when
rising DMAP concentrations up to 2 equivalents, but trailed
off significantly at higher cocatalyst loadings. Nguyen also
reported that cycloaliphatic epoxides such as CHO are
copolymerized with CO2 in the presence of [(salen)CrCl]
complexes. Holmes and Mang also reported the conversion of
glycidol derivatives to cyclic carbonates using 10/DMAP.[82]
More recently, He and co-workers reported the synthesis of
ethylene carbonate using 2 b/cocatalyst mixtures.[73, 74]
Darensbourg and Yarbrough reported that the relatively
air-stable complex 10 (Figure 5) is an effective catalyst for the
alternating copolymerization of CHO and CO2.[87] At 80 8C
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with a TOF of 602 h 1. Interestingly, cocatalyst was not
and 60 atm CO2, compound 10 converted CHO to PCHC with
1
a moderate TOF of 10.4 h . Analysis of the polycarbonate
essential for copolymerization using 10.[87]
1
showed nearly 100 % carbonate linkages, a Mn of 8900 g mol
The proposed CO2–epoxide coupling mechanisms using
seemingly similar catalysts 9 a/DMAP, 10/MeIm, and 11/
and a Mw/Mn of 1.2. Based on the TON and lack of cyclic byDMAP differ considerably. At the current time, there is a lack
product, the PCHC should exhibit a theoretical molecular
of agreement regarding the mode of operation of these
weight of approximately 35 000 g mol 1. Like the alumicatalysts. Jacobsen proposed a bimetallic mechanism for the
num[60, 67] and chromium[79] porphyrin systems, activities
asymmetric ring-opening of epoxides based on the observaincreased upon addition of MeIm, such that 5 equivalents
tion that bimetallic catalysts enhanced rates and enantiomeric
MeIm tripled the copolymerization rates to 32.2 h 1 (see
excesses.[84] Nguyen suggested a bimetallic mechanism for the
Table 2). Although complex 10 is chiral, the resultant polymer
13
was completely atactic, as determined by C NMR. Addiformation of cyclic species.[86] The trans coordination of
tionally, complex 10 catalyzed the coupling of PO and CO2 to
DMAP to 9 a is proposed to activate CO2 to give a
PC and PPC; activities were not specified. At 80 8C, cyclic PC
[(salen)Cr{C(C=O)O }] species. This intermediate is then
is the predominant product, but as the temperature is reduced
proposed to attack PO bound to another salen–chromium
to 40 8C, PPC production becomes a competitive pathway.
complex in a cooperative bimetallic ring-opening process,
Finally, silylated aliphatic epoxides, such as 2-(3,4-epoxycyfollowed by elimination of the cyclic product.
clohexyl)ethyl-trimethoxysilane, and CO2 can also be copolyDarensbourg and co-workers offered a dual CHO–CO2
merized by salen–chromium complexes and a MeIm cocatacopolymerization mechanism for the 10/MeIm system. Here,
lyst.[88] At 80 8C and 55 atm CO2, 9 d (Figure 5) and 2.5 equivinitiation occurs by a bimetallic process and propagation
operates by monometallic enchainment of epoxide.[87, 89]
alents MeIm catalyzed the formation of the silylated polycarbonate with a TOF of 12.0 h 1.
Initiation is accelerated by MeIm, which facilitates chloroSubsequent work detailed the intricate energetics of
ligand attack on a CHO monomer bound to a second salen–
polymer versus cyclic-species formation using compound
chromium complex (Scheme 7). Subsequent CO2 insertion
10.[89] In CHO–CO2 coupling, the
activation energies (Ea) for CHC
and PCHC formation are 31.8 and
11.2 kcal mol 1, illustrating markedly higher activation barriers for
formation of the cyclic species. The
activation barriers for PC and PPC
in PO–CO2 coupling were determined to be 24.0 and 16.2 kcal mol 1, respectively. The significantly larger Ea for CHC versus
PCHC is consistent with the exclusive formation of PCHC, while the
slightly larger Ea for PC versus
PPC is consistent with the formation of PC during PO–CO2 copolymerization.
Recently, Rieger and co-workers found that a slightly modified
complex, [(salen)CrCl] (11), and
DMAP cocatalyst rapidly copolymerize PO and CO2 (TOFs
approaching 226 h 1; see Table 2)
at 75 8C and 35 atm CO2.[90] Anal- Scheme 7. Proposed CO2–CHO copolymerization mechanism using 10/MeIm (P = polymer chain).
ysis of the PPC revealed molecular
weights up to 16 700 g mol 1 (lower
than predicted assuming the lack of chain transfer reactions),
into the newly-generated chromium alkoxide generates a
PDIs as low as 1.38, and carbonate linkages as high as 98 %.
chromium carbonate. Because rate studies showed a firstThe DMAP/11 ratio drastically affected the product distriorder dependence on both CHO and catalyst, chain propbution in the coupling process. Without DMAP, no converagation was proposed to occur by a concerted epoxide ringsion to PC or PPC was observed. At 0.5 equivalents DMAP,
opening that proceeds through a four-membered transition
the maximum ratio of PPC to PC formation (154:34) was
state.
observed. Higher DMAP/11 ratios decreased the proportion
Alternatively, in PO–CO2 copolymerization using 11/
of PPC to PC until only PC was observed. For example, when
DMAP, Rieger and co-workers proposed that DMAP coor2 equivalents of DMAP were added, only PC was observed
dinates strongly to Cr and facilitates dissociation of the
polymer-chain alcoholate and carbonate (Scheme 8).[90]
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CO2–Epoxide Copolymerization
Scheme 8. Proposed PO–CO2 copolymerization mechanism for 11/DMAP catalyst
system.
Under sufficient pressures of CO2, the dissociated carbonate
attacks PO-bound 11/DMAP in a monometallic fashion.
Subsequent CO2 insertion into the newly-formed chromium
alkoxide propagates the polycarbonate chain. The anionic
nature of the chain-end promotes the degradative backbiting
to cyclic propylene carbonate. Therefore, increased stoichiometric amounts of DMAP enhance formation of cyclic
product and eventually exterminate copolymer formation.
Owing to similar catalysts and coupling processes, it is
unlikely that all of the epoxide ring-opening steps discussed
above are occurring simultaneously. Furthermore, no mechanism has accounted for the retention of configuration as
reported by Kruper, or the low polymer molecular weights
that are indicative of chain transfer or the formation of
macrocycles. Studies generally agree that CO2 reacts with a
metal alkoxide and cyclic products are formed by backbiting
of a metal alkoxide into an adjacent carbonate linkage.
Indeed, detailed mechanistic studies must be performed to
delineate the various intermediates and epoxide ring-opening
steps involved in these highly active salen–chromium catalysts.
5. Cobalt Catalysts
In 1979, Co(OAc)2 was reported to copolymerize PO and
CO2 with an extremely low TOF (0.06 h 1).[91] Since this
report, few examples of cobalt-catalyzed coupling of epoxides
and CO2 have been found. He and co-workers reported the
synthesis of ethylene carbonate using 2 c/cocatalyst mixtures.[73, 74] Shi et al. reported that related salen-cobalt comAngew. Chem. Int. Ed. 2004, 43, 6618 – 6639
plexes such as 12 (Figure 5) can be activated with
Lewis-basic amines for the synthesis of propylene
carbonate (see Section 7.3).[92] Recently, our group
published that salen–cobalt complexes 13 a–c
(Figure 5) exhibited moderate activities (up to
81 h 1 with 13 a) for the copolymerization of PO
and CO2.[93] At 25 8C and 55 atm CO2, 13 a catalyzed
the copolymerization to yield PPC with no observable cyclic by-products, 95 % carbonate linkages, a
Mn of 15 300 g mol 1, and a Mw/Mn of 1.22. Pressures
of 55 atm were essential for polymerization activity
as lower pressures (40 atm) significantly hindered the
copolymerization. In contrast to copolymerizations
using the related salen–chromium catalysts, no
heterocyclic additives were necessary. In addition,
13 a–c all showed unprecedented selectivities for
PPC formation (> 99 % PPC versus PC). (S)-PO–
CO2 copolymerization using enantiomerically-pure
13 c yielded isotactic (S)-PPC (TOF = 71 h 1, > 99 %
PPC, 99 % carbonate linkages, Mn = 6900 g mol 1,
Mw/Mn = 1.58) with the highest reported level of
head-to-tail linkages (93 %). Finally, 13 c exhibited a
modest level of selectivity (krel = 2.8) in the kinetic
resolution of PO. More recently, a catalyst system
comprised of [(tpp)CoCl]/DMAP has been used for
the synthesis of a range of dioxolanones from
epoxides and CO2,[94] and 13 c/(nBu)4NX has been
used to resolve racemic propylene oxide by forming
PPC with krel values ranging from 1.1 to 9.0.[95]
6. Lanthanide-Based Catalysts
Yttrium, aluminum, rare-earth metals, and combinations
of multiple metal reagents have shown activity for the
copolymerization of epoxides and CO2. For example, PO–
CO2 copolymerization was effected by a rare-earth metal
system comprised of yttrium tris[bis(2-ethylhexyl)phosphate], AliBu3, and glycerol. The PPC produced contained
only 10–30 % carbonate linkages, although molecular weights
of up to 476 000 g mol 1 were achieved.[96] This system also
exhibited activity for the alternating copolymerization of CO2
with epichlorohydrin[97] and glycidyl ether monomers.[98]
Other rare-earth metal systems consisted of yttrium carboxylates [Y(CO2CF3)3 or Y(CO2RC6H4)3 where R is H, OH, Me,
or NO2], ZnEt2, and glycerine.[99–101] The alternating copolymerization of CO2 and PO yielded PPC with up to 98.5 %
carbonate linkages, turnover frequencies up to 2.5 h 1, and
molecular weights reaching 100 000 g mol 1. CHO–CO2
copolymerization produced PCHC with 100 % carbonate
linkages, molecular weights of 19 000–330 000 g mol 1, and
Mw/Mn7s of 3.5–12.5. It must be noted that control experiments indicated that ZnEt2, but not Y(CO2CF3)3, was
essential for polymerization. Finally, ternary catalysts composed of Nd(CO2CCl3)3, ZnEt2, and glycerol were also
reported to copolymerize PO and CO2.[102]
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7. Zinc and Cadmium Catalysts
A variety of metal-based catalysts has shown activity for
the coupling of epoxides and CO2 ; however, few have
exhibited the success associated with zinc-based complexes.
Therefore, the majority of the work reported in this field has
been performed using complexes with zinc as the active metal
center. These catalysts have undergone a renaissance over the
past ten years, shifting focus from heterogeneous mixtures to
discrete and single-site catalysts which exhibit unprecedented
reaction rates and selectivities.
7.1. Heterogeneous Zinc Catalysts
As described in Section 2, the first active species for the
alternating copolymerization of epoxides and CO2 were based
on mixtures of: 1) ZnEt2 with an assortment of di- and
trihydric sources and 2) carboxylic acids and Zn(OH)2.
Subsequent to these groundbreaking discoveries, several
companies pursued the commercialization of PPC using
these and related catalysts.[103–106] In 1999, Ree et al. reported
a variant of the Zn(OH)2/glutaric acid (zinc glutarate)
system[53] using ZnO as the zinc source. An optimal activity
of 3.4 TO h 1 was achieved for PO–CO2 copolymerization at
60 8C and 25 atm CO2, which at the time was the highest
activity reported for zinc carboxylates (Table 1). Analysis of
the PPC revealed a Mn of 210 000 g mol 1, a PDI of 1.3, and a
Tg of 38 8C.[107–109] In addition, PO–CO2 copolymerizations
using zinc glutarate have been run in scCO2, a suitable
replacement for organic solvents.[110] More recently, polycrystalline[111] and single-crystal[112] zinc glutarate have been
studied using X-ray diffraction, which could play an important
role in determining the mechanism of this catalyst. Ethylsulfinate capped zinc glutarates have also been reported.[113]
They have the potential to reduce the complexity inherent in
these systems. Furthermore, terpolymerizations of CO2, PO,
and e-caprolactone have been performed using zinc glutarate
catalysts.[114] PPC has also been synthesized using zinc
glutarate produced by an ultrasonic method, giving TOFs
up to 7.7 h 1.[29, 111] Finally, carboxy-containing polymers, such
as styrene and acrylic acid copolymers,[115] and g-alumina[116]
have been used as supports in zinc-catalyzed copolymerization of PO and CO2.
A soluble counterpart to these systems was reported by
Beckman et al., who observed that zinc oxide and a highly
fluorinated carboxylic acid derived from a monoester of
maleic acid[105] (14 a; Figure 6) is active for the alternating
copolymerization of CHO with supercritical CO2.[117] The
fluorination increased solubility of the catalyst mixture and
facilitated PCHC formation with 8.8 TO h 1 at 135 atm CO2
and 100 8C (Table 2). Additionally, 14 b (Figure 6), a soluble
ZnII-based compound, produced PCHC (Mn = 2150, Mw/Mn =
4.4) with a TOF of only 1.2 h 1 at 90 8C and 110 atm CO2.[77] In
1999, Darensbourg and Zimmer reported that zinc crotonate
is a soluble catalyst precursor for this system.[118] This catalyst
afforded TOFs of approximately 16 h 1 at 80 8C and 55 atm
CO2, yielding PCHC with 84 % carbonate linkages. A
promising set of heterometallic catalysts are the double
metal cyanide (DMC) catalysts originally reported by Kruper
and Smart.[119] Heterogeneous compounds of the form
M1a(M2(CN)x)b (a and b: 1, 2, 3; x: 4, 5, 6) were active for
epoxide–CO2 copolymerization. DMC catalysts such as zinc
hexacyanoferrate(iii) converted epoxides, including EO, PO,
1-butene oxide and CHO, to polycarbonates (50–95 %
carbonate linkages and PDIs of 2–6) with TOFs of approximately 4 h 1. Chen later reported similar DMC catalysts that
exhibited better activities for PO–CO2 copolymerization.[120]
(PEO)aZn(Fe(CN)6)bCl2–3b(H2O)c(KCl)d (PEO = polyethylene oxide, a (mole ratio chelating atoms/Zn) = 2.2, b = 0.50,
c = 0.76, d = 0.20) copolymerized PO and CO2 at 60 8C and
50 atm CO2 to give PPC (Mn = 20 000 g mol 1) with 9.0 TO per
mol zinc per h. To shed light on the origin of activity in DMC
catalysts, Darensbourg et al. designed the related homogeneous complex 15, which was prepared by the reaction of
[{KCpFe(CN)2PPh2(CH2)1.5}2] (Cp = cyclopentadienyl) with
ZnI2 (Figure 7).[121, 122] Surprisingly, the coupling of CHO
Figure 7. Soluble double metal cyanide complex.
and CO2 gave predominantly cyclic cis-cyclohexene carbonate, a small percentage of low molecular weight polycarbonate was also isolated. Unfortunately, the intrinsic complexity
of heterometallic catalysts hampers mechanistic studies, and
currently the active species are unknown. Despite these
difficulties, the ease of synthesis and inexpensive nature of
these materials makes them highly attractive.
7.2. Zinc and Cadmium Phenoxides for Epoxide–CO2 Coupling
Figure 6. Zinc catalysts for the alternating copolymerization of CHO
and CO2.
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Heterogeneous systems are often marred by poor reproducibility and the production of non-uniform polymers,
caused by the presence of many different types of active
sites that produce polymer with different activities and
selectivities. To address these issues, Darensbourg and
Holtcamp reported in 1995 the first discrete zinc complexes
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CO2–Epoxide Copolymerization
for the alternating copolymerization of epoxides and CO2
(Figure 8).[123] This discovery marks an important step in the
development of catalysts for the copolymerization of CO2 and
epoxides. Compound 16 a, which was synthesized from 2,6diphenylphenol and Zn[N(SiMe3)2]2, crystallized as a bis((2,6-
Figure 8. Zinc–bis(phenoxide) compounds for the alternating copolymerization of CHO and CO2.
diphenyl)phenoxy)zinc complex containing two diethyl ether
solvent molecules coordinated to a tetrahedral zinc center.
Under 55 atm CO2 and at 80 8C, PCHC (91 % carbonate
linkages, Mn = 38 000 g mol 1, Mw/Mn = 4.5) was produced
with a TOF of 2.4 h 1. Additionally, 16 a catalyzed the
random terpolymerization of CHO, PO, and CO2, yielding
polycarbonate with approximately 20 % propylene carbonate
linkages, 70 % cyclohexene carbonate linkages, and 10 %
ether linkages. Approximately the same ratios of PO and
CHO incorporation were observed regardless of feedstock
composition.
Subsequent work investigated steric influences of N-aryl
substituents, including 2,4,6-tri-tert-butyl (16 b), 2,6-di-tertbutyl (16 c), and 2,4,6-trimethyl (16 d), on CHO–CO2 copolymerization (see Figure 8).[124] Complex 16 d displayed the
highest activities (TOF = 9.6 h 1), thus illustrating that bulky
ortho substituents were not essential for high copolymerization rates. In the case of 16 b, only 50 % carbonate linkages
were observed in the resultant PCHC. In line with this result,
the zinc phenoxides were also efficient catalysts for the
homopolymerization of CHO. Electronic perturbations of the
N-aryl ortho substituents revealed that electron-withdrawing
groups resulted in higher activities for CHO–CO2 copolymerization: F > Cl > Br.[125] Addition of 2,6-dihalophenols to
Zn[N(SiMe3)2]2 gave four-coordinate, dimeric zinc phenoxides with coordinated THF solvent molecules. Compound 17 a
showed a moderate TOF of 7.6 h 1. Analysis of the PCHC
revealed PDIs of 6.0, molecular weights of 42 000 g mol 1, a Tg
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
of 115 8C, and > 99 % carbonate linkages.[31, 125] In general,
zinc–bis(phenoxide) compounds catalyzed PO–CO2 copolymerization at 40 8C and PO–CO2 cyclization at 80 8C in
unspecified yields. Finally, zinc–bis(phenoxide) catalysts were
active for CHO homopolymerization, CHO–CO2 copolymerization, and CHO–PO–CO2 terpolymerization, but did not
readily react with bulky alicyclic epoxides such as a-pinene
and exo-2,3-epoxynorbornane. Although no copolymerization activity was observed, X-ray analysis of 18 showed two
molecules of exo-2,3-epoxynorbornane coordinated to
zinc,[126] providing a potential model compound for epoxidebound intermediates in the polymerization.
Darensbourg et al. proposed that two coordination sites
were required for polyether formation, while only one was
necessary for copolymer formation.[125, 127] To probe this
theory, phosphane (PCy3, PMe3, etc.) adducts of the zinc
phenoxides were synthesized; these have only one open
coordination site.[127] Compounds 19 a and b (Figure 8) were
found to be three-coordinate zinc compounds with a distorted
trigonal planar geometry around zinc. While 16 b generated
PCHC with only 50 % carbonate linkages, 19 b facilitated the
formation of PCHC with 100 % carbonate linkages, without
loss of catalytic activity. Furthermore, 17 a, which possesses
only one open coordination site, produced PCHC with
essentially 100 % carbonate linkages whereas 17 b (the PCy3
adduct of 17 a) was not active for the copolymerization of
CHO and CO2.
In a related study, Dinger and Scott reported that zinc–
phenoxide cluster compounds showed activity for the alternating copolymerization of CHO and CO2.[128] A variety of
solvent-dependent tri-, tetra-, penta-, and hexanuclear compounds were synthesized from tris(3,5-dialkyl-2-hydroxyphenyl)methane derivatives and ZnEt2. For example, compound
20 (Figure 9) catalyzed the copolymerization of CHO and
CO2 to give PCHC with 81 % carbonate linkages and a TOF
of 1.3 h 1.
Figure 9. Trinuclear zinc–alkyl catalyst for CHO–CO2 copolymerization.
Cadmium complexes have not shown significant activities
for the coupling of epoxides and CO2. Nevertheless, cadmium
compounds can exhibit organometallic reactivity similar to
their zinc analogs and therefore serve as good structural
models (Figure 10). Perhaps the most intriguing insight
derived from these models came from Darensbourg7s discovery of tris(pyrazolylhydroborate)cadmium acetate complexes
[(tp)CdOAc] (21 a–c).[129, 130] Complexes 21 a and b contain
bound PO and CHO and were proposed as possible models
for initiation in epoxide–CO2 polymerization. Darensbourg
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Figure 10. Tris(pyrazolylborate)cadmium acetates and cadmium phenoxides as model compounds for CO2–epoxide copolymerization.
and co-workers have also synthesized bis(phenoxy)
cadmium
derivatives
as
model Figure 11. Benzoate, bis(salicylaldiminato), and (dialkylamino)ethylcyclopentadienyl
reagents.[125, 126, 131–133] Compounds 22 a–c exhibited zinc catalysts for CHO–CO2 copolymerization.
a distorted tetrahedral geometry around cadmium
and illustrated the coordination of epoxides such
as CHO and exo-2,3-epoxynorbornane. In the absence of
tionally, under 55 atm CO2 at 80 8C, PCHC was generated
coordinating solvents, dimeric three-coordinate cadmium
with essentially 100 % carbonate linkages. A number of
phenoxides were synthesized (23 a, b). Addition of coordinatbis(salicylaldiminato)–zinc complexes (26 a–d) were syntheing solvents, such as THF, regenerated monomeric cadmium
sized by reaction of Zn[N(SiMe3)2]2 with 2 equivalents
phenoxides. Finally, bis(phenoxy) cadmium and [(tp)CdOAc]
salicylaldimines.[135] Complex 26 a was the most active catalyst
complexes were not active for epoxide–CO2 copolymerizacopolymerizing CHO and CO2 to give PCHC (> 99 %
tion; however, they did catalytically convert PO and CO2 to
carbonate linkages, Mn = 41 000 g mol 1, Mw/Mn = 10.3) with
PC at an unspecified rate.
a TOF of 6.9 h 1. Related zinc–bis(trimethylsilylamido) and
Although the discrete catalysts above represent an
zinc–phenoxide complexes have been reported by Chisholm
important advance in catalyst design, the active species for
et al.; however, no copolymerization activity was
the copolymerization remain unclear. Regarding these phenobserved.[137] The single-site catalyst [(dec)ZnOAc] (27) was
oxide–zinc complexes, one or more ligands are likely to act as
synthesized by deprotonation of the dec ligands followed by
polymerization initiators, and thus become the chain-end of
reaction with Zn(OAc)2.[136] X-ray analysis of 27 revealed a
the growing polymer chain.
dimeric compound featuring bridging acetates and h1-coordination of the cyclopentadienyl moiety to zinc. At 30 8C and
40 atm CO2, 27 exhibited low activity (1.2 TO h 1), and the
7.3. Discrete Zinc Complexes for Epoxide–CO2 Copolymerization
resultant PCHC possessed 15–20 % ether linkages.
Recently, Hampel et al. reported that quinoxaline-derived
zinc alkoxide complexes, 28 and 29, exhibited low activities
Given the success of zinc phenoxide compounds for
for CHO–CO2 copolymerization (Figure 12).[138] At 80 8C and
copolymerization of CHO and CO2, Darensbourg and co[134]
workers investigated zinc benzoate (24 and 25),
80 atm CO2, compounds 28 and 29 showed TOFs of 4.9 and
bis(salicylaldiminato)zinc (26 a–d),[135] and (dialkylamino)ethyltetra3.6 h 1, respectively. The PCHC yielded from 28 possessed
methylcyclopentadienyl (dec) zinc derivatives (27)[136]
97 % carbonate linkages, a Mn of 13 500 g mol 1, and a PDI of
4.59.
(Figure 11). Zinc benzoate aggregates including compound
Binaphthyldiamino (binap) salen-type metal complexes
24 were synthesized by reaction of Zn[N(SiMe3)2]2 with 2,6(12, 30 a, b) in combination with Lewis basic cocatalysts,
disubstituted benzoic acids.[134] Dimeric 24 was converted with
including NEt3, DMAP, and pyridine, were reported by Shi
pyridine to generate a monomeric complex with three
coordinated pyridines (25). 24 displayed moderate CHO–
and co-workers for the cyclization of terminal epoxides and
CO2 copolymerization activities (TOF = 7.7 h 1) comparable
CO2 (Scheme 9).[92] At 35 atm CO2 and 100 8C, 30 a/NEt3
to those observed with zinc phenoxide compounds. Addi(2 equiv) converts PO to PC with approximately 57 TO h 1.
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CO2–Epoxide Copolymerization
7.4. Pyridine–Zinc Halide Complexes for Epoxide–CO2 Coupling
Despite extensive research on catalysts for cyclic carbonate synthesis, there are limited reports on catalyst intermediates or reaction mechanisms. Notably, Kim and co-workers
addressed both of these issues using pyridinium alkoxy zinc
dibromides.[139, 140] [(Pyridine)2ZnBr2] (31 a) was found to
couple PO and CO2 to PC with a TOF of 308 h 1 at 100 8C
and 35 atm CO2. [(2-R-pyridine)2ZnBr2] (31 a R = H, 31 b R =
Me) reacted with excess PO to provide dimeric pyridinium
alkoxy zinc dibromides (32 a R = H, 32 b R = Me;
Scheme 10). Complex 32 a crystallized as a dimeric compound
Figure 12. Quinoxaline-derived zinc phenoxides for CHO–CO2 copolycontaining a distorted tetrahedral geometry around both zinc
merization.
centers. Interestingly, PO ring-opening occurred exclusively
at the least hindered carbon and only the meso dimer
crystallized. At 100 8C and 35 atm CO2, 32 a
converted PO and CO2 to PC with a TOF of
340 h 1, indicating the presence of a slight
initiation time as 31 a reacts to form 32 a. A
series of [(2-R-pyridine)2ZnBr2] (31 a–c) with
varying electronic properties were examined for
activity. Electron-donating substituents promoted activity, while electron-withdrawing
groups extinguished reactivity (Me > H @ Cl),
signifying the necessity for Lewis basic pyridine
ligands. At 100 8C and 35 atm CO2, 32 b exhibited
an activity for PO–CO2 cyclization of 530 TO h 1.
In addition to PO–CO2 cyclization, 32 a and
32 b were active for cyclic ethylene carbonate
(EC) production with TOFs of 1200 h 1 and
1450 h 1, respectively. Surprisingly, the reaction
of EO with 31 a and b resulted in trimeric
compounds (33 a and b) containing an alternating
zinc and oxygen six-membered ring at the core of
the molecule (Scheme 10). Apparently, the sterics of the epoxide determine whether dimeric or
trimeric intermediates are formed. Complex 33 a
demonstrated activities almost identical to those
of 32 a for both PC (327 TO h 1) and EC
(1180 h 1), suggesting the same catalytic species.
Scheme 9. Proposed catalytic cycle for the cyclization of terminal epoxides and CO2
Kim proposed that the dimeric form was the
using chiral binap salen-type metal catalysts.
active species for epoxide–CO2 coupling
(Scheme 11). Insertion of CO2 in 32 a gave zinc
Copper complex 30 b and NEt3 also catalyze the
formation of PC with a TOF of 32 h 1. Moreover,
catalysts 30 a, b were discovered to transform a
variety of terminal epoxides, such as butylene
oxide, SO, and epichlorohydrin, to their corresponding cyclic carbonates. Isotope-labeling experiments using trans-deuterioethylene oxide derivatives indicated that epoxide ring-opening occurred
by nucleophilic attack of a Lewis base (e.g., NEt3)
followed by CO2 insertion (Scheme 9). Subsequent
ring closure produced the trans-cyclic carbonate
with overall retention of configuration. Although
30 a, b are chiral catalysts, virtually no asymmetric
induction was observed.
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Scheme 10. Syntheses of dimeric and trimeric zinc pyridinium alkoxides.
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G. W. Coates and D. R. Moore
7.5. Single-Site b-Diiminate Zinc Catalysts for Epoxide–CO2
Coupling
Our research group discovered a highly active, living
epoxide–CO2 copolymerization system using bulky b-diiminate (bdi) zinc catalysts, such as 36–39, under low pressures
(7 atm CO2) and temperatures (50 8C) (Figure 14).[143–153]
Scheme 11. Proposed mechanism for the synthesis of propylene carbonate using dimeric zinc pyridinium alkoxides.
alkoxide carbonate dimer containing a six-membered ring,
that subsequently eliminated PC. Species 32 a was regenerated upon reaction with PO.
Shortly thereafter, Darensbourg et al. illustrated that
related pyridine–zinc halide adducts (Figure 13) were catalytically active for CHO–CO2 copolymerization and cycliza-
Figure 14. b-Diiminate–zinc catalysts for epoxide–CO2 copolymerization.
Figure 13. Zinc–pyridine complexes for CHO–CO2 copolymerization.
tion with TOFs as high as 13.5 h 1.[141] 2,6-Dimethoxypyridine
reacted with zinc halides to give [Zn(2,6-dimethoxypyridine)4]2+[Zn2X6]2 (34 a–c) while 3-trifluoromethylpyridine
reacted with ZnBr2 to generate (3-CF3-pyridine)2ZnBr2
(31 d). The order of halide reactivity was Cl Br > I and the
complex with 2,6-dimethoxypyridine was more active than
that with 3-trifluoromethylpyridine. At 55 atm CO2 and 80 8C,
PCHC was produced with 80–91 % carbonate linkages, Mn up
to 44 000 g mol 1, and broad PDIs. In situ IR studies revealed
a first-order dependence on catalyst concentration for
polymerization, but an unusual fractional order of 1.5 for
CHC formation. It was suggested that both dimeric and
monomeric zinc active species accounted for the mixed order
in zinc. Kim and co-workers recently reported that pyridine
alkoxide ligated zinc acetates 35 a–d (Figure 13) catalyze the
copolymerization of CHO and CO2. Complex 35 c produces
PCHC (Mn = 9500 g mol 1, Mw/Mn = 2.5) with a TOF of
153 h 1. The polycarbonate is unusual in that it has only
63 % carbonate linkages.[142]
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Several key design features, including initiating groups,
sterics, and electronics, drastically altered the efficacy of the
catalysts. To model the growing polycarbonate chain, zinc–
acetate (36), zinc–methoxide (37 a, b), and zinc–isopropoxide
(37 c, d) complexes were synthesized as mimics for zinc
carbonates and zinc alkoxides (Scheme 12). Zinc acetates
were produced from the deprotonated (bdi)Li adducts and
Zn(OAc)2[143, 146] or by addition of acetic acid to [(bdi)ZnEt]
compounds.[150, 151] (bdi)ZnEt was generated by addition of
ZnEt2 to (bdi)H ligands.[143] [(bdi)ZnOAc] compounds crystallized as dimeric compounds with bridging m,h2-acetates.
The reaction of [(bdi)ZnEt] with MeOH provided dimeric
[(bdi)ZnOMe] compounds (37 a, b; 39 a, b).[143, 146] Deprotonation of (bdi)H using Zn[N(SiMe3)2]2 gave monomeric, 3coordinate [(bdi)Zn{N(SiMe3)2}] (38 a, b).[146, 154] Subsequent
alcoholysis with iPrOH yielded [(bdi)Zn(OiPr)] complexes
(37 c, d) that are dimeric in the solid state. (bdi)Zn acetate,
methoxide, isopropoxide, and bis(trimethylsilyl)amido complexes were all active for the alternating copolymerization of
CHO and CO2.[143, 145, 146, 149–151] In addition, [(bdi)Zn(OiPr)]
complexes were highly active for the living polymerization of
lactide,[154, 155] e-caprolactone, and b-butyrolactone.[156] While
X-ray crystallography revealed that [(bdi)ZnOAc], [(bdi)Zn(OiPr)], and [(bdi)ZnOMe] compounds were dimeric,
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Scheme 12. Synthesis of b-diiminate–zinc complexes.
1
H NMR spectroscopy revealed that the solution states were
highly dependent upon sterics.[143, 146, 151, 154] For instance, it was
demonstrated that compounds 36 a and 37 c exhibited a
monomerQdimer equilibrium under appropriate temperatures and concentrations.[143, 156] On the other hand, 36 b, 37 b,
and 37 d were exclusively dimeric in solution, even at elevated
temperatures.[146, 156] Concurrently, subtle modifications on the
N-aryl ortho positions highly influenced polymerization
activity. Sterically unencumbered methyl substituents exhibited no polymerization activity, whereas sterically congested
ethyl and isopropyl substituents promoted the copolymerization giving TOFs of 431 h 1 and 360 h 1, respectively
(Table 2).[151] Furthermore, the unsymmetrical 36 c yielded
PCHC (99 % carbonate linkages, Mn = 23 300 g mol 1, Mw/
Mn = 1.15) with a TOF of 729 h 1. Electronics also played a
dramatic role in activity, such that electron-withdrawing
cyano substituents enhanced polymerization rates. Complex
36 d produced PCHC (90 % carbonate linkages, Mn =
17 900 g mol 1, Mw/Mn = 1.15) with a TOF of 917 h 1 in only
20 minutes. The combination of the unsymmetrical ligand
geometries and the electron-withdrawing cyano substituent
yielded the most active catalysts reported to date.[149] At 50 8C
and in only 10 minutes, 39 a and 39 b catalyzed the copolymerization of 1000 equivalents CHO and 7 atm CO2 to give
high molecular weight polymers (Mn 22 000 g mol 1),
narrow PDIs (Mw/Mn = 1.09–1.11), and extremely high
TOFs of 2170 and 2290 h 1, respectively (see Table 2).
In an attempt to isolate monomeric b-diiminate complexes, Chisholm et al. investigated bulky initiators including
tBuOH and Ph3SiOH.[157, 158] As expected, monomeric compounds 40 and 41 (Figure 15) were active for CHO–CO2
alternating copolymerization. Compound 41 also coupled
PO and CO2 to give propylene carbonate in unspecified yield.
Unexpectedly, Chisholm7s [(bdi)ZnNiPr2], an analog of our
compound 38 a, was not active for the copolymerization,
although it readily reacted with CO2 to generate 42. Rieger
et al. recently showed that an ethyl sulfinate is a viable
initiator for the copolymerization.[159] Compound 43
(Figure 15), synthesized by bubbling SO2 through a [(bdi)ZnEt] solution, was also active for CHO–CO2 copolymerization. Catalytic activities were comparable to 36 a, indicating
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
Figure 15. Monomeric and dimeric [(bdi)ZnOR] complexes.
similar active species. From bimodal gel permeation chromatography traces, monomeric and dimeric bdi–zinc species
were believed to be active, although no mechanistic studies
were conducted. Recently, oligomeric bdi ligands have been
generated from 4,4’-methylenedianiline and 2,4-pentanedione. The zinc complexes have shown modest activity for
CHO–CO2 copolymerization (TOF = 11.3 h 1).[160–162]
Yu and Jones recently reported that b-diiminate–zinc
catalysts immobilized on silica exhibited moderate activities
for CHO–CO2 copolymerization.[163] Compounds 44, 45 a, and
45 b (Figure 16) are precursors to the silica-immobilized
Figure 16. Precursors to silica-immobilized bdi–zinc catalysts.
complexes and show activities of 110, 60, and 65 h 1,
respectively, under 7 atm CO2 at 50 8C. The resultant polycarbonates exhibited > 92 % carbonate linkages, molecular
weights of 8700–13 300 g mol 1, and PDIs of 1.03–1.28. Complexes 45 a and 45 b were supported on mesoporous SBA-15
and controlled-pore glass, but the rates of PCHC formation
(TOFs = 5–21 h 1) and percent carbonate linkages (33–78 %)
decreased. A corresponding drop in molecular weights was
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not observed indicating fewer active zinc centers. Finally,
based on the activity observed with the silica-supported zinc
catalysts, Yu and Jones suggested that the copolymerization
proceeded via a monometallic mechanism.[163] High-throughput equipment has been used to screen CHO–CO2 copolymerization in the presence of b-diiminate–zinc catalysts, but
so far this research has been limited to known compounds
(38 a, b).[164]
Morokuma and co-workers performed theoretical studies
using the hybrid molecular orbital method ONIOM on the
alternating copolymerization of epoxides and CO2 catalyzed
by monomeric 37 a.[165] The rate-determining step of the
copolymerization was proposed to be epoxide insertion into a
zinc carbonate bond. Furthermore, in alicyclic epoxides such
as CHO, the activation barrier for epoxide ring-opening by a
zinc carbonate is drastically reduced due to ring, torsion, and
angle strain. By contrast, the study predicted that aliphatic
epoxides such as EO could not be copolymerized due to high
activation barriers for epoxide ring-opening.
Subtle electronic and steric perturbations to [(bdi)ZnOR]
(R = alkyl or acyl) complexes resulted in drastic enhancements in activity for CHO–CO2 copolymerization.[146, 149, 151]
During the course of these studies, catalysts for PO–CO2
coupling were also discovered by our research group.[150]
PO–CO2 coupling proved to be highly sensitive on reaction
conditions. Both temperature and pressure had a profound
effect on product formation. At 50 8C and 20 atm CO2,
unsymmetrical 36 e yielded PC with a TOF of 50 h 1. By
simply reducing the temperature to 25 8C, PC formation was
suppressed, and the selectivity for PPC/PC was 85:15 with a
TOF of 47 h 1. The polycarbonate exhibited > 99 % carbonate linkages, a Tg of 38 8C, a Mn of 43 300 g mol 1, and a PDI of
1.09. Furthermore, 13C NMR spectroscopy revealed a nearly
regiorandom copolymer,[166, 167] indicating that ring-opening
occurred at both C O bonds. Further modifications of the
ligand architecture generated complex 46 (Scheme 13), the
highly active for PC formation at high temperatures. At 75 8C,
PC is produced with TOFs over 1000 h 1.[168]
Recently, our research group performed mechanistic
studies on the [(bdi)ZnOR]-catalyzed copolymerization of
CHO and CO2.[151] Stoichiometric reactions of the copolymerization initiation steps showed that zinc alkoxides insert
CO2, while zinc acetates react with CHO. For example, 37 c,
which is a monomeric zinc alkoxide at room temperature
([Zn] = 0.01m in [D6]benzene) instantaneously reacted with
CO2 to give 47 (Scheme 14). X-ray analysis revealed that 47
Scheme 14. Insertion reaction of CO2 with [(bdi)Zn(OiPr)].
crystallized as a m,h2-carbonate-bridged dimer. Complex 47 is
virtually isostructural to 36 a, suggesting similar reactivity in
the copolymerization. Complex 36 d inserted CHO to give 48
over the course of days (Scheme 15). X-ray analysis showed
Scheme 15. Insertion reaction of CHO with [(bdi)ZnOAc].
Scheme 13. Unsymmetrical, electron-deficient bdi–zinc complex for the
copolymerization of PO and CO2.
most potent catalyst reported to date for PO–CO2 copolymerization. Complex 46 copolymerized PO and CO2 at 25 8C
and 7 atm CO2 to give PPC (> 99 % carbonate linkages, Mn =
36 700 g mol 1, Mw/Mn = 1.13) with a TOF of 235 h 1 (Table 2).
However, the selectivity for polymer was only 75 %. Increasing the CO2 pressure to 35 atm favored polymer formation
with a selectivity of 93 %, while only moderately attenuating
catalytic activity (TOF = 138 h 1). Finally, compound 46 is
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
that 48 is a m,h2-acetate-m,h1-cyclohexyloxide acetate-bridged
dimer in the solid state. Both 47 and 48 serve as model
compounds for presumed intermediates in the copolymerization. Due to the expeditious reaction of CO2 with 37 c, CHO
insertion was predicted to be the rate-determining step. To
monitor propagation, rate studies were performed on compounds 36 a–c using in situ FT-IR. The kinetic studies revealed
a zeroth-order dependence in CO2 and a first-order dependence in CHO. Hence, insertion of CHO into a zinc carbonate
was indeed the rate-determining step. The copolymerization
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CO2–Epoxide Copolymerization
of CHO (1.98 m in toluene) and 20 atm CO2 at 50 8C using
sterically-unhindered, dimeric 36 b resulted in an order of
1.02 0.03 in [(bdi)ZnOR] concentration (R = alkyl, acyl, or
polymer chain). However, under the same conditions, an
order in [(bdi)ZnOR] concentration of 1.83 0.04 was
determined for 36 c. Therefore, the copolymerization of
CHO and CO2 using 36 c at 50 8C exhibited the following
overall rate law: d[P]/dt = k[epoxide]1.0[Zn]1.83. At lower
temperature (30 8C), a decrease in the order in [(bdi)ZnOR]
concentration to 1.37 0.02 was observed for 36 c. On the
basis of [(bdi)ZnOR] solution studies (R = alkyl or acyl),
stoichiometric insertion reactions, and rate studies, a bimetallic mechanism was proposed (Scheme 16 a). Sterically
building blocks. CHO, a meso molecule, is an ideal substrate
for desymmetrization using chiral catalysts.[170] In 1999,
Nozaki et al. reported that a 1:1 mixture of ZnEt2 and (S)a,a-diphenylpyrrolidine-2-yl-methanol (49) was active for
stereoselective CHO–CO2 copolymerization at 40 8C and
30 atm CO2 (Schemes 17 and 18).[171, 172] The polycarbonate
Scheme 17. Asymmetric copolymerization of CHO and CO2.
Scheme 18. Chiral zinc catalysts for the
asymmetric, alternating copolymerization of
CHO and CO2.
Scheme 16. a) Proposed copolymerization mechanism using bdi–zinc
complexes; b) epoxide ring-opening transition state (P = polymer
chain).
encumbered bdi–zinc complexes (36 a and 36 c) ring-open
CHO in a bimetallic transition state (Scheme 16 b) with a
predominantly monomeric ground state. Conversely, sterically unhindered bdi–zinc complexes (36 b) insert CHO in a
bimetallic transition state with a completely dimeric ground
state.
7.6. Zinc Catalysts for Asymmetric CHO–CO2 Copolymerization
There is significant interest in controlling the absolute
stereochemistry of ring-opening in epoxide–CO2 copolymerization for several reasons. First, microstructure directly
affects polymer properties.[169] Second, the kinetic resolution
of racemic epoxides or desymmetrization of meso epoxides by
copolymerization is a potential route to valuable chiral
Angew. Chem. Int. Ed. 2004, 43, 6618 – 6639
exhibited 100 % carbonate linkages, a Mn of 8400 g mol 1, and
a PDI of 2.2. Hydrolysis of PCHC with base produced the
corresponding trans-cyclohexane-1,2-diol with 73 % enantiomeric excess. 13C NMR spectroscopy studies of model polycarbonate oligomers afforded spectral assignments for isotactic dyads (153.7 ppm) and syndiotactic dyads (153.3–
153.1 ppm).[173] Finally, ring-opening proceeded by complete
inversion of configuration (SN2 mechanism), hence no ciscyclohexane-1,2-diol was observed.
In a recent report, Nozaki and co-workers isolated
presumed intermediates in the asymmetric alternating
copolymerization.[174] Reaction of ZnEt2 and (S)-a,a-diphenylpyrrolidine-2-yl-methanol yielded dimeric 50, which was
structurally characterized by X-ray diffraction studies
(Scheme 18). At 40 8C and 30 atm CO2, 50 catalyzed the
formation of isotactic PCHC (Mn = 11 800 g mol 1, Mw/Mn =
15.7, TOF = 0.6 h 1) with a slightly lower enantiomeric excess
of 49 %. When the copolymerization was attempted with 50
and 0.2 to 1.0 equivalents of EtOH, enantioselectivities
increased up to 80 % ee, and better control of molecular
weights and PDIs resulted. Compound 51 was proposed to be
the active initiating species (see Scheme 18). End-group
analysis by MALDI-TOF mass spectrometry revealed in the
absence of EtOH signals assignable to the aminoalcoholinitiated polymerization. However, as EtOH addition was
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. W. Coates and D. R. Moore
increased from 0.2 to 1.0 equivalents, signals corresponding to
the aminoalcohol-derived polycarbonate disappeared as
peaks for the EtOH-initiated PCHC emerged. This was
further confirmed by end-group analysis using 1H NMR
spectroscopy. Finally, mechanistic studies suggest that the
dimeric form of the catalyst is in fact the active species.
In 2000, our research group developed C1-symmetric
imine-oxazoline ligated zinc bis(trimethylsilyl)amido compounds for the stereoselective, alternating copolymerization
of CHO and CO2 (Scheme 18).[145] Through multiple electronic and steric manipulations, compound 52 was found to
exhibit the highest enantioselectivity (RR/SS ratio 86:14,
72 % ee) in the copolymerization. The resultant PCHC exhibited
100 % carbonate linkages, a Mn of 14 700 g mol 1, a PDI of 1.35,
and a Tg and Tm of 120 and 220 8C, respectively. Furthermore,
stereocontrol was also achieved in the alternating copolymerization of CPO and CO2, producing poly(cyclopentene
carbonate) with a RR/SS ratio of 88:12 (76 % ee). As revealed
by 13C NMR spectroscopy, the experimental carbonyl tetrad
concentrations matched the predicted tetrad concentrations
for an enantiomorphic-site control mechanism.[145] The contributions described above represent an important step forward in the production of well-defined, tactic polycarbonates.
Alternative comonomers, including lactones, isocyanates,
aziridines, as well as new epoxides, such as limonene oxide
and styrene oxide, could be promising reagents in the
production of unique copolymers. By utilizing homogeneous
living catalysts, there are infinite possibilities of new, exciting
copolymer microstructures. Living systems allow for precise
control of molecular weights, low PDIs, copolymer synthesis
by sequential monomer addition, and the ability to functionalize chain ends. Because only one chain is produced by each
metal center in homogeneous catalyst systems, the development of strategies related to Inoue7s “immortal polymerization” may lead to more productive systems. Although the
enantioselective copolymerization of CO2 and epoxides has
been accomplished, much higher levels of stereocontrol as
well as regiocontrol are required for improved physical
properties.
Undoubtedly, the future will witness continued research at
this exciting interface of inorganic and polymer chemistry to
the benefit of both fields. Homogeneous catalysts will lead to
epoxide–CO2 copolymers with unprecedented levels of architectural control, providing many new opportunities in polymer science.
Abbreviations
8. Summary and Outlook
Over the last decade, significant advances have been
achieved in CO2–epoxide coupling chemistry. Homogeneous
catalysts for CO2–epoxide copolymerization offer significant
increases in rate as well as selectivity versus their heterogeneous counterparts. Well-defined, homogeneous catalysts
have given new momentum to the field of CO2 utilization,
and have made possible a much deeper mechanistic understanding of these systems. Current catalysts provide reasonable routes to a variety of polycarbonates from inexpensive
epoxides, including poly(cyclohexene carbonate), poly(propylene carbonate), and terpolymers of CHO, PO, and CO2.
Moreover, living catalyst systems were found that provide
block copolymers and polymers with molecular weights
predetermined by monomer/initiator ratios. In addition,
both polycarbonates and cyclic carbonates can be synthesized
with high selectivities and rates, owing to subtle modification
of the catalyst7s architecture. Finally, precise control of cyclic
carbonates and/or polycarbonate production is achieved by
utilizing homogeneous catalysts with appropriate reaction
conditions such as pressure, temperature, and cocatalyst
additives.
Despite the recent breakthroughs, the future challenges
are many. At the current time there are limited applications of
epoxide–CO2 copolymers due to their high cost and low
thermal stability. Therefore it will be necessary to develop
catalyst systems that are capable of producing these polymers
economically. In addition, methods for producing polymers
with increased thermal stability are needed. Surprisingly, only
Cr, Co, and Zn compounds have shown significant activity for
the copolymerization of CO2 and epoxides. The unusual
reactivity of these metals must be further exploited, and other
metals such as Mg, Mn, Fe, and Ni should be explored.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
bdi
CHC
CHO
CPO
EC
EO
PC
PCHC
PEC
PO
PPC
PPO
SO
tpp
b-diiminate
cyclohexene carbonate
cyclohexene oxide
cyclopentene oxide
ethylene carbonate
ethylene oxide
propylene carbonate
poly(cyclohexene carbonate)
poly(ethylene carbonate)
propylene oxide
poly(propylene carbonate)
poly(propylene oxide)
styrene oxide
tetraphenylporphyrin
We gratefully acknowledge support from the NSF (CHE9875261, CHE-0243605, DMR-0079992), the Cornell University Center for Biotechnology, Eastman Chemical and Sumitomo Chemicals, and the Packard, Sloan, and Beckman
Foundations for support of our research involving CO2
utilization. D.R.M. is grateful for a Corning Foundation
Science Fellowship.
Received: April 26, 2004
Published Online: November 23, 2004
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