C O M M U N I C A T I O N S
Table 1. CO2/(Propylene Oxide) Copolymerization Resultsa
than one chain-growing unit in 1 (see Supporting Information). The
molecular weights (Mn) reported with the binary catalytic system
such as 2 have rarely exceeded 30 000 because it shows high activity
only at a low [PO]/[catalyst] ratio.1a Complex 3 provides only oily
oligomers at 80 °C (entry 8), even thought it was able to give a
high molecular-weight polymer (Mn ) 84 000) at 25 °C.8
d
cat;
T
t
TOFb
Mn
1
1
3
The H NMR spectra of the polymers obtained with 1 indicate
entry
[PO]/[cat]
(
°
C)
(h)
(h-
)
TONb
selectivityc
(
×
10-
)
Mw/Mn
perfectly alternating copolymers not containing any ether linkage.
The 13C NMR spectra indicate mostly head-to-tail linkage (94-
83%).
1
2
3
4
5
1; 25000 50 7.0
650
4600
2400
3300
3500
100
94
94
90
84
88
0
97
60
90
75
61
71
80
53
95
1.23
1.19
1.25
1.32
1.35
1.46
1; 25000 70 1.0 2400
1; 25000 80 1.0 3300
1; 25000 90 1.0 3500
Conclusively, a catalytic system containing a Lewis acidic metal
center and a quaternary ammonium salt unit in a molecule is
prepared, which shows activity even at a condition of high
temperature and high [epoxide]/[catalyst] ratio. A very high TOF
(up to 3500 h-1) and TON (up to 14 500) are achieved, which have
never been attained with other catalytic systems. The high selectivity
and the high molecular weight of the obtained polymers are the
other merits of the catalyst.
1; 50000 80 4.5f 3200 14500
6e 1; 25000 80 6.0f 1400
8300
0
980
1100i
610
7g 2; 25000 80 1.0
0
8h 2; 2000
9
45 0.7 1400
3; 25000 80 3.0
26
ndi
7
1.01
nd
1.22
370
610
10j 3; 2000
60 1.0
a Polymerization condition: Propylene oxide (10.0 g), CO2 (initial
pressure, 2.0 MPa). b Calculated based on the weight of the isolated polymer
not including the cyclic carbonate. c Selectivity for the polycarbonate over
the cyclic carbonate in unit of % determined by the 1H NMR spectroscopy
of the crude product. d Determined on GPC using the polystyrene standard.
e Scale-up reaction: Propylene oxide (200 g), CO2 pressure (2.0-1.7 MPa).
f Enough time is given to maximize TON. g Only the cyclic carbonate was
Acknowledgment. This work was supported by Korea Research
Foundation Grant funded by Korea Government (MOEHRD, Basic
Research Promotion Fund) (KRF-2005-015-C00233)), and E.K.N.
and S.J.N. are grateful for financial support through the BK21
program.
obtained with a TOF of 1950 h-1 h Data from ref 1a. i Isolation of the
.
polymer by precipitating in methanol failed by nature of its low molecular
weight, and the TOF and the TON were calculated from the 1H NMR spectra
of the crude product. j Data from ref 8.
Supporting Information Available: Details of the syntheses and
characterization of the compounds, polymerization procedure, and
polymer characterization. This material is available free of charge via
polymer at all by the increase of both the ratio to 25 000 and the
temperature to 80 °C (entry 6).
The high selectivity (90%) achieved with 3 at the condition of
low [PO]/[catalyst] ratio of 2000 and low temperature of 60 °C
(entry 10) is not preserved by the increase of the [PO]/[catalyst]
ratio and the temperature (60% at 80 °C and [PO]/[catalyst]
)25 000, entry 9). Furthermore, the TOF (370 h-1) observed for 3
is significantly lower than those (∼3000 h-1) observed for 1 at a
condition of high [PO]/[catalyst] ratio of 25 000.
High selectivity (>90%) for the copolymer over the cyclic
carbonate even at a high [PO]/[catalyst] ratio and a high temperature
is another merit of 1 in addition to the high TOF, high TON, and
thermal robustness. The comparison catalytic system 2 gives only
the cyclic carbonate (entry 7), and complex 3 shows very poor
selectivity (60%) at our standard condition ([PO]/[catalyst] )
25 000, 80 °C).
Producing high molecular-weight polymers is another advantage
of 1. Because most of the catalysts for this copolymerization show
some living characteristics, the molecular weight is basically
dependent on the attained TON. Complex 1 is capable of providing
high molecular-weight polymers (Mn ) 53 000-95 000) because
it is able to provide high TONs. The average molecular weight
increases as the TON increases in case the polymerization time is
the same (entries 2-4), but the increase of TON by increasing the
polymerization time in the diluted condition is not accompanied
by an increase of the molecular weight (entries 3 and 5). This
indicates that some chain transfer reactions operate when the
polymerization reaction is carried out for a long time in a diluted
condition.10 Fairly narrow molecular-weight distributions (Mw/Mn
) 1.19-1.35) are observed for the copolymers obtained with 1,
even though the GPC traces of the polymers exhibit a narrow
bimodal shape, which might be attributed to the presence of more
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J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8083