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RSC Advances
174.1 (1C), 52.3 (1C), 51.4 (1C), 47.0 (1C), 34.0 (1C), 32.5 (1C), The Zn/Co molar ratio of this catalyst was 2.0 based on the
29.4–29.1 (4C), 26.0 (1C), 25.0 ppm (1C).
elemental analysis.17b The catalytic center of the catalyst was
conrmed to be a Zn–OH bond on the surface of the catalyst, as
shown in Fig. S1.†19 For Zn–Co(III) DMCC-catalyzed CO2–
epoxide copolymerization, it is still a big challenge to obtain a
fully alternating copolymer. The best reported alternation of the
styrene oxide–CO2 copolymer is 99.4% from Zn–Co(III) DMCC,
in our recent study.20 This kind of Zn–Co(III) DMCC catalyst
exhibited nanolamellar structures with thicknesses of 20–80 nm
and high surface areas of ca. 600 m2 gꢀ1, which could provide
more active sites than the conventional catalyst.20 Herein, we
used this nanolamellar Zn–Co(III) DMCC catalyst directly for the
copolymerization of CO2 with EMU. A polycarbonate with a
perfectly alternating structure was expected because of the long
bulky side groups in EMU (Scheme 1).
The bio-based epoxide, EMU, was synthesized from 10-
undecenoic acid by two methods21 that are oen used in the
laboratory, as shown in Scheme 1. Firstly, the esterication of
10-undecenoic acid with methanol was carried out using sul-
phuric acid as the catalyst.21a Then, the methyl 10-undecenoate
was oxidized by mCPBA at 25 ꢁC for 12 h using DCM as the
solvent. The FT-IR (Fig. S2†) and 1H NMR spectra (Fig. S3†)
proved the successful synthesis of EMU, with a yield of 93%.21b
Note that this epoxy monomer could be synthesized by
sustainable methods, for example, direct pyrolysis of methyl
ricinoleate could produce methyl 10-undecenoate4,5 and an
environmentally friendly oxidant, such as hydrogen peroxide,
could be used for the epoxidation.22
Preparation of the nanolamellar Zn–Co(III) DMCC catalyst
The catalyst was synthesized according to our reported
method.20 Elemental analysis of the catalyst: Co: 12.48; Zn:
27.29; N: 16.57; C: 23.34; H: 2.27; Cl: 9.50.
Representative copolymerization of CO2 and epoxy methyl
10-undecenoate
A 10 mL autoclave with a small magnetic stirrer was dried at 120
ꢁC for 3 h, and cooled to room temperature in a closed desic-
cator. The Zn–Co(III) DMCC catalyst (16.0 mg) and EMU (2.0 mL)
were transferred into the autoclave. The autoclave was sealed
and lled with CO2, then heated to the target temperature in a
pre-heated oil bath. The system was adjusted to the set pressure
and stirred for the set time. Aer the copolymerization, the
autoclave was cooled with an ice-water bath and the pressure
was slowly vented. A small amount of crude product was
collected for 1H NMR spectroscopy. The remaining products
were dissolved with small amounts of DCM and precipitated by
excess methanol, dried at 60 ꢁC under vacuum to a constant
weight. The obtained copolymer was colorless and viscous.
FT-IR: 3450 cmꢀ1 (OH), 1741 cmꢀ1 (COOCH3); 1H NMR
(CDCl3, 500 MHz, d ppm): 1.43–1.16 (m, 10H), 1.70–1.49 (m,
4H), 2.33–2.21 (t, 2H), 3.64 (s, 3H), 4.42–3.95 (m, 2H), 4.86 (s,
1H); 13C NMR (CDCl3, 100 MHz, d ppm): 174.3 (1C), 154.7 (1C),
75.8 (1C), 68.2 (1C), 51.5 (1C), 34.1 (1C), 30.5 (1C), 29.2 (4C), 25.0
ppm (2C).
A series of EMU–CO2 copolymerizations were successfully
catalyzed by nanolamellar Zn–Co(III) DMCC. The experimental
conditions and the results are summarized in Table 1. No
products were collected when the reaction temperature was
30 ꢁC (entry 1, Table 1) for 24 h. Increasing the temperature
Representative procedure for the synthesis of triblock
copolymers from L-lactide
ꢁ
Polycarbonate (Mn ¼ 4400 g molꢀ1, 0.1645 g) and L-lactide
(5.586 mmol, 0.805 g) were placed in a 25 mL Schlenk ask
under a nitrogen atmosphere and dissolved in THF (3 mL). DBU
(45 mL) in DCM (2 mL) was added to prepare the triblock poly-
mer, and the mixture was stirred for 3 h. Benzoic acid was added
to quench the reaction. The crude polymer was precipitated
from methanol three times, and the white polymer was dried
under vacuum. 1H NMR (CDCl3, 500 MHz, d ppm): 1.26 (s), 1.55
(m), 2.27 (t), 3.64 (s, 3H), 4.42–3.95 (m), 4.86 (s), 5.13 (q); 13C
NMR (CDCl3, 100 MHz, d ppm): 174.3 (1C), 169.5 (1C), 154.7
(1C), 75.8 (1C), 68.2 (1C), 68.9 (1C), 51.5 (1C), 34.1 (1C), 30.5
(1C), 29.2 (4C), 25.0 (2C), 16.6 (3C).
from 40 to 100 C (entries 2–8) resulted in an increase in the
production of cyclic carbonate from 4.8 to 20.0 wt% and a
decrease in the Mn of the resultant copolymer from 18.6 to 7.1
kg molꢀ1. For obtaining copolymers with relative low Mn, water
was used as the chain transfer agent in the copolymerization
system.15b As shown in entries 9–11, when the water content in
the monomer was 400 ppm, the Mns of the copolymer was
decreased to 3.7–4.4 kg molꢀ1 without reducing the monomer's
ꢁ
conversion at 50 C. In this case, increasing the CO2 pressure
from 3.0 to 5.0 MPa (entries 9–11) resulted in a clear increase in
the catalyst productivity and a slight increase in the weight
percentage of the cyclic carbonate in the total product (Wcc)
from 1.9 to 5.7 wt%. Except from entries 1 and 2 in Table 1, EMU
in entries 3–11 was completely converted to the product within
12 h, according to the 1H NMR spectra of the crude copolymers
Results and discussion
Zn–Co(III) DMCC is a typical catalyst that has been developed as (Fig. S5–S13†). The optimized reaction temperature and CO2
a highly efficient catalyst for CO2–epoxide copolymerization.17 pressure were 50 ꢁC and 3.0 MPa, respectively, because a small
In Zn–Co(III) DMCC, the zinc and cobalt atoms are associated amount of the cyclic product was produced, as shown in
with cyanide bridges, affording
a
three-dimensional entries 3, 9–11.
backbone. The empirical formula of the catalyst prepared at
The formation of the ether units is usually thermodynami-
below 40 ꢁC in the presence of tert-butanol (t-BuOH) is cally favourable during Zn–Co(III) DMCC catalysis;15b however,
Zn3[Co(CN)6]2$xZnCl2$yt-BuOH$zH2O (x, y, and z vary based on in this case, the alternating degree (FCO ) of all the resultant
2
the preparation conditions).17f,18 We have previously reported a copolymers obtained at 40–80 ꢁC were estimated as >99%
1
nanolamellar Zn–Co(III) DMCC catalyst synthesized at 75 ꢁC. because no ether unit signals were observed in their H NMR
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RSC Adv., 2014, 4, 36183–36188 | 36185