Journal of the American Chemical Society
Article
In Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
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its amorphous feature (Figure 3a). On the contrary, for (R)-
PCHC-98 sample, sharp diffraction peaks were observed at 2θ
values of 12.2, 17.9, 19.0, and 20.4°, demonstrating that the
isotactic PCHC is a typical semicrystalline polymer (Figure 3b).
CONCLUSIONS AND OUTLOOK
■
In summary, we have demonstrated that the dissymmetry of the
ligand concerning enantiopure salenCo(III) complexes as
catalysts is beneficial for enantioselective ring-opening of
meso-cyclohexene oxide during the asymmetric copolymeriza-
tion with CO2. With binary (1S,2S)-4d/PPNCl catalyst system,
a significant increase in enantioselective ring-opening of CHO
was observed in the presence of (S)-PO or (S)-2-methylte-
trahydrofuran, and the RR:SS ratio of the resultant polycar-
bonate was up to 98:2, the highest record in this asymmetric
polymerization catalysis. Also, we confirmed that the stereo-
chemistry in the CO2/CHO copolymerization concerning
chiral cobalt−salen catalyst systems did not correlate with poly-
mer chain-end control mechanism. In addition, the isotactic
poly(cyclohexene carbonate) (PCHC) is a typical semicrystal-
line thermoplastic, and possesses a high Tm of 216 °C. Further
studies on detailed crystallization behavior and kinetics are
currently underway in our laboratory.
The discovery of the first semicrystalline CO2-based
polycarbonate,26 highly isotactic poly(cyclohexene carbonate),
is a good start to develop a more environmentally benign route
for producing polycarbonates with excellent properties, though
production on a large scale has proven difficult nowadays, due
to relatively low catalyst efficiency and related process
limitations of low polymerization temperatures. Further efforts
will focus on designing highly stereoregular catalysts at enhanced
temperatures, and developing new CO2-based crystalline polymers
with improved thermal and mechanical properties comparable to
that of polyolefins.
ASSOCIATED CONTENT
* Supporting Information
■
S
(14) Xiao, Y.; Wang, Z.; Ding, K. Chem.Eur. J. 2005, 11, 3668−
General experimental procedures for the synthesis of various
unsymmetric enantiopure ligands and their cobalt(III) complexes,
as well as their characterizations. X-ray crystallographic file in CIF
format for the structure determination of one dissymmetric
salenAl(III) complex bearing one bulky adamantyl and one tert-
butyl at the phenolate ortho positions. Synthetic procedures of
various poly(cyclohexene carbonate)s. This material is available
3678.
(15) (a) Lu, X. -B.; Wang, Y. Angew. Chem., Int. Ed. 2004, 43, 3574−
3577. (b) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc.
2005, 127, 10869−10878.
(16) Shi, L.; Lu, X.-B.; Zhang, R.; Peng, X.-J.; Zhang, C.-Q.; Li, J.-F.;
Peng, X.-M. Macromolecules 2006, 39, 5679−5685.
(17) Ren, W.-M.; Zhang, W.-Z.; Lu, X.-B. Sci. China Chem. 2010, 53,
1646−1652.
(18) Although much effort was placed in attempts to clarify the
structure of these dissymmetric salenCo(III) complexes, we failed to
isolate their crystals. Fortunately, we succeeded in obtaining the X-ray
crystal structure (Figure S23 in Supporting Information) of one
dissymmetric salenAl(III) complex bearing one bulky adamantyl and
one tert-butyl at the phenolate ortho positions. The two N−Al−O
angles are 160° and 96°, indicating a distorted configuration. This is
very different from the symmetric SalenAl(III) complexes bearing
ethylene diamine backbone or/and cyclohexane diamine backbone
{See: (a) Darensbourg, D. J.; Billodeaux, D. R. Inorg. Chem. 2005, 44,
1433−1742. (b) Rutherford, D.; Atwood, D. A. Organometallics 1996,
15, 4417−4422 }.
AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (NSFC, Grant 21134002, 21104007), and
National Basic Research Program of China (973 Program:
2009CB825300).
(19) Coates, G. W. Chem. Rev. 2000, 100, 1223−1252.
(20) Li, B.; Zhang, R.; Lu, X.-B. Macromolecules 2007, 40, 2303−
2307.
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