Alternating copolymerization of limonene oxide and carbon dioxide

Article abstract of DOI:10.1021/ja0472580

The alternating copolymerization of (R)- or (S)-limonene oxide and CO₂ using β-diiminate zinc acetate catalysts is reported. At 100 psi CO₂ and 25 °C, the catalyst exhibits a high selectivity for the trans isomer and produces regioregular polycarbonate. The copolymer contains >99% carbonate linkages, a narrow molecular weight distribution, and an M_n value consistent with the [epoxide]/[Zn] ratio. Copyright

Full text of DOI:10.1021/ja0472580

Published on Web 08/24/2004
Alternating Copolymerization of Limonene Oxide and Carbon Dioxide
Christopher M. Byrne, Scott D. Allen, Emil B. Lobkovsky, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received May 11, 2004; E-mail: gc39@cornell.edu
Scheme 1. Copolymerization of trans- (1a) and cis-(R)-Limonene
Oxide (1b) and CO2 using â-Diiminate Zinc Acetate Complexes
The discovery and use of abundant naturally occurring com-
pounds for chemical synthesis is an important strategy for reducing
our dependence on petroleum-derived raw materials.1,2 The use of
carbon dioxide as an inexpensive C1 feedstock is of particular note,
especially for the alternating copolymerization with epoxides.3
While the use of CO2 does carry several benefits, such as low
toxicity and general abundance, most epoxide/CO2 copolymerization
systems focus on petroleum derivatives such as propylene oxide
or cyclohexene oxide.3 While new substrates have been explored,4
the use of epoxides based on biorenewable resources has not been
reported. Given our previous work on the copolymerization of
epoxides and CO2 using â-diiminate (BDI) zinc complexes,5 we
focused on limonene oxide (1), derived from the naturally occurring
cyclic monoterpene, limonene.
Table 1. Effects of Temperature and Pressure on (R)-Limonene
Oxide and CO2 Copolymerization using Complex 2a
b
T
(°C)
PCO
(psi)
TOF
M c
(kg/mol)
% trans
in copolymerd
n
2
entry
(h-1
)
M /M c
w
n
Produced by more than 300 plants, limonene is the most common
terpene.6 The (R)-enantiomer constitutes 90-96% of citrus peel
oil,7 and its world production is estimated to be between 110 and
165 million pounds per year.8 Though the alicyclic epoxide is
commercially available as a mixture of the trans (1a) and cis (1b)
diastereomers (Scheme 1), kinetic resolution successfully affords
either 1a or 1b in >98% de.9 Its abundance, low cost, and structural
similarity to cyclohexene oxide make (R)-limonene oxide (LO) an
excellent choice as a biorenewable epoxide monomer for copoly-
merization with CO2. Herein, we describe the alternating copoly-
merization of LO and CO2 to give a new biodegradable polycar-
bonate from biorenewable resources, proceeding with highly
selective incorporation of 1a under mild conditions.
1e
2
3
4
5
6
7
8
9
50
35
25
10
0
25
25
25
25
25
100
100
100
100
100
50
250
400
550
700
1
33
32
4
4.5
6.9
9.3
NDf
NDf
4.0
6.2
6.0
4.5
3.7
1.34
1.24
1.13
NDf
NDf
1.14
1.12
1.12
1.14
1.13
86.9
96.8
98.3
NDf
NDf
98.4
98.6
98.5
98.7
99.1
0
17
28
27
21
16
10
a Conditions: 0.4 mol % 2, 1a:1b ) 1.2:1, 2 h. b TOF (turnover
frequency) ) mol LO‚mol Zn-1‚h-1, determined by 1H NMR spectroscopy.
c Determined by gel permeation chromatography (GPC). d Determined by
1H NMR spectroscopy; see Supporting Information. e Time ) 24 h. f Not
determined; polymer not recoverable.
On the basis of the conditions used for the cyclohexene oxide/
CO2 system, we performed initial copolymerizations using 0.4 mol
% 2 with commercially available 1 (1a:1b ) 1.2:1) at 50 °C and
100 psi CO2. Long reaction times (24 h) were required for
appreciable conversion (15%) with good selectivity for 1a (Table
mol % 2 over 24 h at 25 °C and 100 psi CO2. The effect of CO2
pressure was also studied for copolymerizations at 25 °C using
complex 2. Between 100 and 400 psi, the copolymerization is
essentially unaffected (entries 3, 7, 8), while pressures above (entries
9, 10) or below (entry 6) this range result in lower TOFs and Mn
values.
1
1, entry 1). After 2 h, polymer formation was not observed by H
NMR spectroscopy. Lowering the temperature to 35 °C resulted in
improved catalytic activity and greater selectivity for 1a (entry 2).
The optimal balance of high catalytic activity and selectivity was
achieved at 25 °C, yielding regioregular polycarbonate 11 (entry
3). Overall, copolymerization activities decline drastically at any
temperature above 35 °C or below 25 °C (entries 4, 5), while
selectivity is compromised beyond 35 °C. At 50 °C, the copoly-
merization yields 12, which exhibits a broadened molecular weight
distribution (MWD ) Mw/Mn ) 1.34).
Using this optimized set of conditions, we sought to identify
the most active catalyst for the copolymerization. In previous work,
systematic variation of [(BDI)ZnOAc] complexes revealed im-
proved activity with electron-withdrawing groups on the ligand
backbone and moderate steric bulk present at the ortho positions
of the N-aryl rings.5c-e Given that structurally similar catalysts
displayed a broad range of activities, we synthesized and screened
a collection of [(BDI)ZnOAc] complexes (2-10) for the copoly-
merization of LO and CO2 (Table 2). Entries 1-3 emphasize the
sensitivity of the copolymerization to ligand sterics. Complex 3,
with bulky isopropyl groups in the ortho positions, yields little
polymer, most likely due to the inability of the complex to attain
the necessary arrangement for epoxide enchainment. Decreasing
steric bulk at the R1 and R2 positions to ethyl substituents (complex
2) increases catalytic activity, while the use of methyl groups
(complex 4) results in no activity.
1
The copolymer regiochemistry can be determined by H NMR
spectroscopy. Copolymer 11 exhibits a single resonance in the
polycarbonate methine region (δ 4.9 to 5.2 ppm), while 12 displays
a pair of resonances, representing head-to-tail and tail-to-tail
linkages.10 Lower temperature copolymerizations give polycarbonate
11 through repeated regio- and stereoregular ring opening of 1a.
Additionally, the regioregular polymers have narrow MWDs and
Mn values in agreement with [epoxide]/[Zn] ratios.11 A sample of
higher molecular weight polymer (Mn ) 25 kg/mol; MWD ) 1.16)
was readily obtained using a cis/trans mixture of (S)-LO and 0.2
An electron-withdrawing trifluoromethyl group on the ligand
backbone has a varying effect on catalytic activity as well (entries
9
11404
J. AM. CHEM. SOC. 2004, 126, 11404-11405
10.1021/ja0472580 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Effect of Ligand Variation on (R)-Limonene Oxide and
Scheme 2. Ring Opening of 1a and 1b during Copolymerization
and Hydrolytic Cleavage to Give Diaxial Diol 13
CO2 Copolymerizationa
M d
(kg/mol)
% trans
in copolymere
b,c
TOF
n
entry
complex
(h-1
)
M /M d
w
n
1
2
3
4
5
2
3
4
5
6
7
8
8
9
32
10
0
16
29
20
37
19
9
9.3
NDf
NDf
4.8
7.8
5.5
1.13
NDf
NDf
1.13
1.12
1.15
1.12
1.14
NDf
NDf
98.3
NDf
NDf
99.2
99.5
99.3
98.9
98.5
NDf
NDf
6
7
10.8
8.1
8g
9g
10g
NDf
NDf
10
3
a Conditions: 0.4 mol % complex, 100 psi CO2, 25 °C, 1a:1b ) 1.2:1,
2 h. b TOF ) mol LO‚mol Zn-1‚h-1 c Determined by 1H NMR spectro-
.
polymer can be produced using longer reaction times and higher
[epoxide]/[Zn] ratios.
scopy. d Determined by GPC. e Determined by 1H NMR spectroscopy; see
Supporting Information. f Not determined; polymer not recoverable. g Per-
formed with 1 mL of CH2Cl2, 4 h.
Acknowledgment. We thank Dr. John Alexander for prelimi-
nary copolymerization results, Jessica Myers for kinetic resolution
work, and Prof. David B. Collum for helpful discussions. G.W.C.
gratefully acknowledges a Packard Foundation Fellowship in
Science and Engineering and funding from the NSF (CHE-0243605)
and the Cornell University Center for Biotechnology, a New York
State Center for Advanced Technology. This research made use of
the Cornell Center for Materials Research Shared Experimental
Facilities supported through the NSF MRSEC program (DMR-
0079992).
6-8). A comparison of complexes 2 and 6 offers essentially
identical activities; however, for 5 and 8, the electron-withdrawing
group at the R4 position doubles the TOF. The location of the CF3
group also dictates reactivity: complex 8 shows almost twice the
activity of 7. Substitution of a cyano group at the R3 position of
the ligand resulted in complexes that were insoluble in neat epoxide.
Copolymerizations with these complexes were run with 1 mL of
CH2Cl2 and resulted in moderate TOFs after 4 h (entries 9, 10).
Under the same conditions, complex 8 still gave superior reactivity.
1
For all catalysts, the polycarbonate produced was 11 (by H and
Supporting Information Available: General experimental proce-
dures, selectivity data, TGA, DSC, and NMR data for polycarbonate
polymers, and X-ray data for 13 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
13C NMR analysis), which displayed narrow MWDs.
The selectivity demonstrated by these catalysts was confirmed
by the copolymerization of pure 1a with 0.4 mol % 2 at 100 psi
CO2. The reaction proceeds smoothly at 25 and 50 °C to give
regioregular polycarbonate 11. Attempted copolymerization of 1b
with CO2 at both 25 and 50 °C after 24 h resulted in no observable
reaction. Structural analysis of 11 using one- (1H, 13C) and two-
dimensional (HMBC, HSQC) NMR experiments revealed the
polymer repeat unit to be consistent with the copolymer 11; there
is no indication of polyether linkages.5 Hydrolysis of copolymers
11 and 12 each yielded a single diol product, (1S,2S,4R)-1-methyl-
4-(1-methylethenyl)-1,2-cyclohexanediol (13), matching an inde-
pendently synthesized authentic sample and further characterized
by single-crystal X-ray diffraction (Scheme 2). Given that copoly-
mer 12 comprises both 1a and 1b and that hydrolysis yields a single
repeat unit, the regiochemistry of the epoxide ring-opening can be
explained. Nucleophilic attack on the epoxides occurs at different
sites for each diastereomer, resulting from a preference for axial
attack (Scheme 2).12 The attack on 1a occurs at the less hindered
carbon, while attack on 1b occurs at the tertiary carbon of the
oxirane,13 both of which result in the (1S,2S,4R)-1-methyl-4-(1-
methylethenyl)-1,2-cyclohexanediol (13). Since attack on a tertiary
center (as in 1b) is less favorable than on a secondary center (as in
1a), 1a is consumed more readily than 1b at lower temperature.
In conclusion, we have demonstrated the alternating copoly-
merization of limonene oxide with CO2. Complex 8 exhibits the
best catalytic activity (TOF ) 37 h-1) and maintains superior
selectivity for 1a (% trans in copolymer ) 98.9%). The alternating
polycarbonate copolymer produced from a cis/trans mixture of
epoxide is highly regio- and stereoregular. High molecular weight
References
(1) (a) Poliakoff, M.; Anastas, P. Nature 2001, 413, 257. (b) Poliakoff, M.;
Fitzpatrick, J. M.; Farren, T. R.; Anastas, P. T. Science 2002, 297, 807-
810. (c) Anastas, P.; Warner, J. Green Chemistry: Theory and Practice;
Oxford University Press: Oxford, 1998.
(2) Aresta, M. In Utilization of Greenhouse Gases; Liu, C. J., Mullinson, R.
G., Aresta, M., Eds.; ACS Symposium Series 852; American Chemical
Society: Washington, DC, 2003; pp 2-39.
(3) (a) Super, M. S.; Beckman, E. J. Trends Polym. Sci. 1997, 5, 236-240.
(b) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. ReV. 1996, 153,
155-174. (c) Rokicki, A.; Kuran, W. J. Macromol. Sci., ReV. Macromol.
Chem. Phys. 1981, C21, 135-186.
(4) Darensbourg, D. J.; Fang, C. C.; Rodgers, J. L. Organometallics 2004,
23, 924-927.
(5) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1998,
120, 11018-11019. (b) Cheng, M.; Moore, D. R.; Reczek, J. J.;
Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
2001, 123, 8738-8749. (c) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.;
Coates, G. W. Angew. Chem., Int. Ed. 2002, 41, 2599-2602. (d) Moore,
D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
2003, 125, 11911-11924. (e) Allen, S. D.; Moore, D. R.; Lobkovsky, E.
B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 14284-14285.
(6) Burdock, G. A. Fenaroli’s Handbook of FlaVour Ingredients, 3rd ed.;
CRC Press: Boca Raton, FL, 1995.
(7) Nonino, E. A. Perfum. FlaVor. 1997, 22, 53-58.
(8) Braddock, R. J. Handbook of Citrus By-Products and Processing
Technology; John Wiley & Sons: New York, 1999.
(9) Steiner, D.; Ivison, L.; Goralski, C. T.; Appell, R. B.; Gojkovic, J. R.;
Singaram, B. Tetrahedron: Asymmetry 2002, 13, 2359-2363.
(10) See Supporting Information.
(11) Similar results (TOFs, MWDs, Mn values) were achieved using a cis/
trans mixture of (S)-(-)-limonene oxide.
(12) Parker, R. E.; Isaacs, N. S. Chem. ReV. 1959, 59, 737-799.
(13) Nucleophilic attack on the tertiary center occurs more rapidly at higher
temperatures, perhaps resulting from a cationic ring-opening mechanism.
JA0472580
9
J. AM. CHEM. SOC. VOL. 126, NO. 37, 2004 11405