Alternating ring-opening copolymerization of styrene oxide and maleic anhydride using asymmetrical bis-Schiff-base metal(iii) catalysts

Article abstract of DOI:10.1039/c4cy01064g

A ring-opening copolymerization of styrene oxide with maleic anhydride was performed by applying a series of asymmetrical bis-Schiff-base metal(iii) catalysts. The chromium catalyst exhibited the best performance and therefore the catalyst (salen)CrCl (sa

Full text of DOI:10.1039/c4cy01064g

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Alternating ring-opening copolymerization of
styrene oxide and maleic anhydride using
asymmetrical bis-Schiff-base metalIJIII) catalysts†
Cite this: Catal. Sci. Technol., 2015,
5, 562
ab
a
a
a
a
*
*
Xingmei Zhang, Luqun Zhu, Jing Wu and Xingqiang Lü
Dengfeng Liu,
A ring-opening copolymerization of styrene oxide with maleic anhydride was performed by applying a series
of asymmetrical bis-Schiff-base metalIJIII) catalysts. The chromium catalyst exhibited the best performance
and therefore the catalyst (salen)CrCl (salen = 4-((Z)-(2-((E)-3,5-dibromo-2-hydroxybenzylideneamino)
phenylimino)(phenyl)methyl)-1-(4-chlorophenyl)-3-methyl-1H-pyrazol-5(4H)-one) (7) was chosen for
further studies. The investigations on the effect of different co-catalysts on the copolymerization of styrene
oxide and maleic anhydride revealed that DMAP showed the highest activity, followed by PPN+Cl−, whereas
Ph3P showed the lowest activity. 1H NMR and MALDI-TOF-MS spectra of the copolymers formed confirmed
the alternating microstructures. The copolymerization of styrene oxide with maleic anhydride bearing
a double bond in its structure was shown to be highly dependent on the polymerization condition, type
of co-catalyst, monomer to catalyst ratio, temperature and time used in the copolymerization reaction.
Applying chain transfer agents, alcohols, resulted in a decrease in molecular weight.
Received 18th August 2014,
Accepted 23rd September 2014
DOI: 10.1039/c4cy01064g
www.rsc.org/catalysis
molecular weight alternating polyesters and the undesirable
side reactions of epoxide homo-polymerization prevented its
1. Introduction
Polyesters are generally produced by the step-growth conden-
sation polymerization from diols, diacids or diesters, which is
an energy intensive process requiring drastic conditions,
such as high temperatures and high vacuum removal of small
molecule byproducts to drive the reaction towards high
conversion, and they require very accurate co-monomer
stoichiometries to obtain high-molecular-weight polymers.1,2
Although the ring-opening polymerization of cyclic esters
takes place under mild reactions without any byproducts, the
availability of structurally diverse monomers restricts the
scope of the polymer architecture.3 Alternatively, the ring-
opening copolymerization of epoxides with cyclic anhydrides
has the potential to produce a wide range of polymer back-
bone structures.4
popularity for a considerable time. It was not until 2007 that
Coates7 employed zinc 2-cyano-β-diketiminato complexes as
catalysts for the solution copolymerization of vinyl cyclo-
hexene oxide and diglycolic anhydride, affording high molec-
ular weight copolymers (Mn = 55 000 g mol−1) with narrow
molecular weight distributions. This attracted the attention
of the scientists despite the lower catalytic activities and
significant amounts of ether linkages for unsaturated
anhydride–epoxide copolymerization. Since then, a series of
metal–porphyrinato,8 metal–salen9 and manganese–corrole10
complexes have been reported to catalyze this copolymeriza-
tion, several of which are capable of yielding high molecular
weight polymers with low molecular weight distributions.
However, for the copolymerization of unsaturated anhydride
maleic anhydride (MA) and styrene oxide (SO), these cata-
The catalytic coupling of epoxide and anhydride with inor-
ganic salts or tertiary amines as catalysts was earlier reported
by Inoue5 and Maeda,6 but the difficulty of obtaining high
lysts displayed low activity (53% of SO conversion, Mn
=
1420 g mol−1) and significant amount of ether linkages.11
As a matter of fact, the distinctive advantage of asymmetrical
salen-type Schiff-base scaffolds12 or analogies13,14 endows the
possibility of further fine-tuning their structures by introduc-
ing different electronic and steric effects. In the previous
studies, we have found that asymmetrical salen-type Schiff-
base catalysts are more efficient than symmetrical salen-type
Schiff-base catalysts in catalyzing cyclohexene oxide and MA
with regards to catalytic activity and selectivity.15–17 To the
best of our knowledge, research on the copolymerization of
electron-withdrawing SO and MA using the metal complexes
a School of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical
Material, Northwest University, Xi'an 710069, People's Republic of China.
E-mail: liudf78@xust.edu.cn, lvxing46@126.com; Fax: +86 29 88302312;
Tel: +86 29 88302312
b College of Chemistry and Chemical Engineering, Xi'an University of Science and
Technology, Xi'an 710054, People's Republic of China
† Electronic supplementary information (ESI) available: Crystallographic files of
complexes ij3IJH2O)]·H2O and [4(H2O)]·EtOH in CIF format, CCDC 1013801 for
[3(H2O)]·H2O, 1013802 for [4(H2O)]·EtOH. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4cy01064g
562 | Catal. Sci. Technol., 2015, 5, 562–571
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based on asymmetrical salen-type Schiff-base ligands as
the catalysts has not been reported. Herein, based on a
series of new asymmetrical Schiff-base ligands H2Ln (n = 1–5)
synthesized from the reaction of o-phenylenediamine,
1-(4-Cl-phenyl)-3-methyl-4-benzoyl-5-pyrazolone (Cl-PMBP),
and different aldehyde derivatives, a series of new catalysts
[Co(Ln)OAc] (n = 1, 1; n = 2, 2; n = 3, 3; n = 4, 4; n = 5, 5),
[Mn(L5)Cl] (6) and [Cr(L5)Cl] (7) have been obtained by a
two-step sequence. The ring-opening copolymerization behav-
iors of SO with MA in the presence of different co-catalysts
bis(triphenylphosphine)iminium chloride (PPN+Cl−), 4-N,N-
dimethylaminopyridine (DMAP) and triphenylphosphine
(Ph3P) with a stipulated reaction procedure have been examined.
and automated sample. Typical DSC experiments were
performed in crimped aluminum pans under nitrogen with a
heating rate of 10 °C min−1 from −70 °C to 200 °C.
2.1 Preparation of
1-IJ4-Cl-phenyl)-3-methyl-4-benzoyl-2-pyrazolin-5-one
The 1,4-dioxane solution (25 mL) of 1-IJ4-Cl-phenyl)-3-
methyl-5-pyrazolone (1.04 g, 5 mmol) was stirred with
calcium hydroxide (0.9 g, 1 mmol) and barium hydroxide
(0.9 g, 1 mmol) under room temperature for 30 min. A solu-
tion of benzoyl chloride (0.70 mL, 5.5 mmol) in absolute
1,4-dioxane (5 mL) was slowly added, and the resulting mix-
ture was refluxed under nitrogen atmosphere for 24 h. After
cooling to room temperature, the reaction mixture was
poured into a 40 mL solution of ice-cold hydrochloric acid
[HCl : H2O = 3 : 7 (v/v)], and the resulting yellow precipitate
was filtered and recrystallized from acetone and distilled
water. Yield: 1.3 g (82%). 1H NMR (400 MHz, DMSO-d6, ppm):
δ 7.87 (d, 2H, J = 8.4 Hz, ArH), 7.63 (d, 2H, J = 7.6 Hz, ArH),
7.60 (d, 1H, J = 6.8 Hz, ArH), 7.54–7.51 (m, 2H, ArH), 7.44
(d, 2H, J = 8.4 Hz, ArH), 2.09 (s, 3H, –CH3). 13C NMR (400 MHz,
DMSO-d6, ppm): δ 195.3; 160.4; 148.4; 139.8; 137.1; 133.6;
132.0; 129.8; 129.7; 129.6; 129.5; 128.9; 128.8; 121.7; 121.6;
106.9; 16.0.
2. Experimental
General remarks: Tetrahydrofuran (THF) HPLC grade was
purchased from Fisher Scientific and purified over basic
alumina columns. Other solvents were used as received from
Sigma Aldrich and stored over 3 Å activated molecular sieves.
SO purchased from Sigma Aldrich, was dried over CaH2, dis-
tilled and stored under dry N2 prior to use. MA purchased
from Alfa Aesar, was dried overnight under vacuum and sub-
limed twice under dry N2 prior to use. DMAP, Ph3P and
PPN+Cl− were purchased from Fluka and used as received. All
the manipulations of air and water sensitive compounds
were carried out under dry N2 using the standard Schlenk
line techniques. Elemental analyses were performed on a
Perkin-Elmer 240C elemental analyzer. Infrared spectra were
recorded on a Nicolet Nagna-IR 550 spectrophotometer in the
region 4000–400 cm−1 using KBr pellets. 1H NMR spectra
were recorded on a JEOL EX 400 spectrometer in CDCl3 or
DMSO-d6 at room temperature. Gel permeation chromatogra-
phy (GPC) analyses of the molecular weights (Mn and Mw)
and molecular weight distribution (PDI = Mw/Mn) of the poly-
mers were performed on a Waters 150C instrument in HPLC
THF using polystyrene as standard. MALDI-TOF-MS analysis
was performed on a Voyager DE-STR from Applied Biosystems
equipped with a 337 nm nitrogen laser. An accelerating volt-
age of 25 kV was applied. Mass spectra of 1000 shots were
accumulated. The polymer samples were dissolved in HPLC
THF at a concentration of 1 mg mL−1. The cationization agent
used was KCF3COO (Alfa Aesar, >99%) dissolved in HPLC
THF at a concentration of 5 mg mL−1. The matrix used was
DCTB (Alfa Aesar, DCTB = trans-2-[3-(4-tert-butylphenyl)-2-
methyl-2-propenylidene] malononitrile) and was dissolved in
HPLC grade THF at a concentration of 40 mg mL−1. The solu-
tion of matrix, salt and polymer were mixed in a volume ratio
of 4 : 1 : 4, respectively. The mixed solution was transferred on
a stainless steel MALDI target and left to dry. The spectra were
recorded from the crude products. In-house developed soft-
ware was used to characterize the polymers in detail, which
allowed the elucidation of the individual chain structures, the
chemical composition and topology of copolymer.18 Differen-
tial scanning calorimetry of polymer samples was performed
on a TA Instruments Q1000 instrument equipped with a LNCS
2.2 Synthesis of the half-unit Schiff-base precursor HL0
(HL0 = 4-((2-aminophenylimino)(phenyl)methyl)-3-methyl-1-
(4-Cl-phenyl)-pyrazol-5-one)
To a solution of 1-IJ4-Cl-phenyl)-3-methyl-4-benzoyl-2-
pyrazolin-5-one (312.8 mg, 1 mmol) in absolute EtOH (7 mL),
a solution of 1,2-diaminobenzene (118.8 mg, 1.1 mmol) in
absolute EtOH (3 mL) was slowly added, and the resulted
mixture was refluxed for 6 h, After cooling to room tempera-
ture, the insoluble precipitate was filtered off, and recrystallized
with absolute EtOH to give an brown microcrystalline product.
Yield: 298.4 mg (74%). Anal. calcd for C23H19N4OCl: C, 68.57;
H, 4.75; N, 13.91. Found: C, 68.55; H, 4.80; N, 13.87. FT-IR
(KBr, cm−1): 3324 (w), 3054 (m), 1624 (s), 1578 (vs), 1535 (w),
1495 (s), 1387 (s), 1321 (w), 1207 (m), 1146 (w), 1090 (w),
1049 (w), 1007 (w), 835 (m), 745 (w), 702 (w), 596 (w).
1H NMR (400 MHz, DMSO-d6, ppm): δ 12.43 (s, 1H, –OH),
8.03 (d, 1H, J = 7.6 Hz, ArH), 7.37 (d, 6H, J = 6.0 Hz, ArH),
7.32 (d, 2H, J = 7.6 Hz, ArH), 6.96–6.92 (m, 1H, ArH), 6.68
(d, 1H, J = 8.4 Hz, ArH), 6.59 (d, 1H, J = 7.6 Hz, ArH), 6.45
(d, 1H, J = 8.0 Hz, ArH), 3.94 (s, 2H, –NH2), 2.09 (s, 3H, –CH3).
13C NMR (400 MHz, DMSO-d6, ppm): δ 170.6; 160.3; 150.1;
144.8; 142.6; 138.8; 132.9; 131.8; 131.1; 129.6; 129.5; 129.3;
129.2; 128.9; 128.8; 128.3; 128.2; 121.7; 121.6; 120.2; 118.6;
108.3; 16.0.
2.3 Synthesis of series of asymmetric bis-Schiff-base ligand
H2Ln (n = 1–5)
For H2L1: to a solution of HL0 (80.6 mg, 0.2 mmol) in abso-
lute CHCl3 (5 mL), a solution of salicylaldehyde (21.0 μL,
0.2 mmol) in absolute EtOH (5 mL) was slowly added, and
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the resultant mixture was refluxed for 6 h. After cooling to
room temperature, the insoluble precipitate was filtered off,
and washed with cold absolute EtOH (10 mL) three times to
yield the bright yellow microcrystalline product of H2L1.
Yield: 80.8 mg (80%). Anal. calcd for C30H23ClN4O2: C, 71.01;
H 4.54; N, 11.05. Found: C, 70.97; H, 4.62; N, 11.01. FT-IR
(KBr, cm−1): 3329 (b), 1640 (m), 1614 (vs), 1570 (s), 1536 (m),
1491 (s), 1418 (w), 1385 (s), 1275 (m), 1233 (w), 1204 (m),
1171 (w), 1140 (w), 1086 (w), 1051 (m), 1007 (m), 937 (w), 914
(w), 895 (w), 829 (w), 756 (m), 700 (m), 681 (w), 627 (w), 586
(w), 557 (w), 519 (w), 503 (w), 440 (w). 1H NMR (400 MHz,
DMSO-d6, ppm): δ 12.88 (s, 1H, –OH), 12.02 (s, 1H, –OH),
8.95 (s, 1H, –CN), 8.06 (d, 2H, J = 8.8 Hz, ArH), 7.94 (d, 1H,
J = 6.0 Hz, ArH), 7.53–7.41 (m, 10H, ArH), 7.25 (t, 1H, J = 7.2 Hz,
ArH), 7.06–6.97 (m, 2H, ArH), 6.82 (d, 1H, J = 7.2 Hz, ArH),
1.44 (s, 3H, –CH3). 13C NMR (400 MHz, DMSO-d6, ppm): δ
167.1; 164.9; 163.6; 161.3; 157.2; 149.5; 149.4; 137.7; 134.0;
132.5; 132.1; 131.6; 130.9; 129.8; 129.6; 129.2; 129.0; 128.9;
128.8; 128.6; 128.5; 123.5; 123.4; 122.6; 122.3; 121.6; 118.5;
116.1; 107.8; 16.0.
6.86 (d, 1H, J = 8.0 Hz, ArH), 3.80 (s, 3H, –OCH3), 1.45 (s, 3H,
–CH3). 13C NMR (400 MHz, DMSO-d6, ppm): δ 167.8; 165.6;
160.3; 156.5; 152.4; 150.1; 145.8; 145.7; 139.5; 134.0; 131.1;
129.9; 129.4; 129.3; 129.2; 129.1; 128.9; 128.8; 128.7; 128.6; 123.7;
123.6; 123.3; 123.2; 122.9; 122.5; 119.6; 118.2; 101.3; 56.3; 16.1.
For H2L4: the bright yellow asymmetric bis-Schiff-base
ligand H2L4 solid product was prepared in the same way as
H2L1 except that 5-bromo-2-hydroxy-3-methoxy-benzaldehyde
(48.0 mg, 0.2 mmol) was used instead of salicylaldehyde
(21.0 μL, 0.2 mmol). Yield: 100.2 mg (81%). Anal. calcd for
C
31H24BrClN4O3: C, 60.39; H, 3.90; N, 9.09. Found: C, 60.35;
H, 3.97; N, 9.04. FT-IR (KBr, cm−1): 3354 (b), 1632 (m), 1613 (s),
1568 (s), 1535 (m), 1491 (vs), 1466 (m), 1445 (m), 1420 (m),
1377 (m), 1325 (m), 1273 (m), 1250 (m), 1190 (w), 1144 (w),
1090 (w), 1049 (m), 1009 (m), 976 (m), 864 (m), 833 (m),
800 (w), 772 (m), 748 (m), 706 (m), 679 (w), 611 (w), 584 (w),
1
559 (w), 546 (w), 507 (w), 480 (w), 457 (w), 424 (w). H NMR
(400 MHz, DMSO-d6, ppm): δ 13.04 (s, 1H, –OH), 11.39 (s, 1H,
–OH), 8.95 (s, 1H, –CN), 8.12 (d, 2H, J = 9.2 Hz, ArH), 7.91
(d, 1H, J = 2.4 Hz, ArH), 7.54–7.42 (m, 8H, ArH), 7.29 (d, 1H,
J = 2.4 Hz, ArH), 7.20 (t, 1H, J = 7.2 Hz, ArH), 6.99 (t, 1H, J =
7.6 Hz, ArH), 6.63 (t, 1H, J = 4.8 Hz, ArH), 3.86 (s, 3H, –OCH3),
1.43 (s, 3H, –CH3). 13C NMR (400 MHz, DMSO-d6, ppm): δ
167.3; 165.6; 160.2; 156.5; 154.7; 150.1; 145.8; 145.7; 139.5;
135.1; 133.2; 130.9; 129.4; 129.3; 129.2; 129.1; 128.9; 128.8;
128.7; 128.6; 127.4; 123.7; 123.6; 123.2; 123.1; 121.8; 121.4;
116.8; 107.3; 56.3; 16.2.
For H2L2: the bright yellow asymmetric bis-Schiff-base
ligand H2L2 solid product was prepared in the same way as
H2L1 except that 5-bromo-salicylaldehyde (42.0 mg, 0.2 mmol)
was used instead of salicylaldehyde (21.0 μL, 0.2 mmol). Yield:
91.7 mg (78%). Anal. calcd for C30H22BrClN4O2: C, 61.45; H, 3.76;
N, 9.56. Found: C, 61.42; H, 3.83; N, 9.54. FT-IR (KBr, cm−1):
3352 (b), 1634 (m), 1613 (m), 1566 (s), 1533 (m), 1491 (vs),
1464 (m), 1379 (s), 1313 (m), 1279 (m), 1254 (m), 1213 (m),
1142 (w), 1123 (w), 1088 (w), 1049 (w), 1007 (m), 972 (w),
864 (w), 831 (m), 777 (m), 754 (w), 733 (m), 704 (m), 631 (w),
613 (w), 588 (w), 548 (w), 505 (w), 482 (w), 455 (w), 426 (w).
1H NMR (400 MHz, DMSO-d6, ppm): δ 13.12 (s, 1H, –OH),
11.55 (s, 1H, –OH), 8.95 (s, 1H, –CN), 8.34 (s, 1H, ArH), 8.13
(d, 2H, J = 8.8 Hz, ArH), 7.58–7.46 (m, 8H, ArH), 7.41–7.39
(d, 1H, J = 8.0 Hz, ArH), 7.20 (t, 1H, J = 7.6 Hz, ArH), 6.98
(t, 2H, J = 8.4 Hz, ArH), 6.58 (d, 1H, J = 8.0 Hz, ArH), 1.44
(s, 3H, –CH3). 13C NMR (400 MHz, DMSO-d6, ppm): δ 167.4;
165.1; 163.1; 163.0; 156.5; 149.5; 149.4; 139.5; 135.8; 134.3;
134.2; 131.1; 129.3; 129.2; 129.1; 129.0; 128.9; 128.8; 128.7;
128.6; 128.5; 128.4; 123.5; 123.4; 123.3; 122.7; 118.5; 115.8;
107.7; 16.3.
For H2L5: the bright yellow asymmetric bis-Schiff-base
ligand H2L5 solid product was prepared in the same way
as H2L1 except that 3,5-dibromo-2-hydroxybenzaldehyde
(56.0 mg, 0.2 mmol) was used instead of salicylaldehyde
(21.0 μL, 0.2 mmol). Yield: 103.0 mg (78%). Anal. calcd for
C
30H21Br2ClN4O2: C, 54.15; H, 3.16; N, 8.42. Found: C, 54.13;
H, 3.24; N, 8.39. FT-IR (KBr, cm−1): 3337 (b), 1636 (m), 1613
(s), 1570 (s), 1535 (m), 1491 (vs), 1443 (m), 1383 (s), 1323 (m),
1296 (m), 1213 (m), 1150 (w), 1090 (w), 1049 (m), 1009 (m),
972 (w), 939 (w), 887 (w), 866 (w), 829 (m), 775 (w), 741 (m),
706 (w), 687 (w), 633 (w), 611 (w), 588 (w), 565 (w), 502 (w),
1
463 (w). H NMR (400 MHz, DMSO-d6, ppm): δ 13.04 (s, 1H,
–OH), 12.94 (s, 1H, –OH), 8.94 (s, 1H, –CN), 8.08–8.00
(m, 4H, ArH), 7.51–7.43 (m, 8H, ArH), 7.27 (t, 1H, J = 7.6 Hz,
ArH), 7.07 (t, 1H, J = 8.0 Hz, ArH), 6.78 (d, 1H, J = 8.0 Hz,
ArH), 1.46 (s, 3H, –CH3). 13C NMR (400 MHz, DMSO-d6,
ppm): δ 170.6; 160.3; 160.1; 159.3; 149.4; 144.8; 144.7; 139.5;
138.8; 133.9; 133.0; 131.8; 131.0; 129.6; 129.5; 129.3; 129.2;
128.9; 128.8; 128.7; 128.6; 124.5; 124.4; 123.9; 122.6; 122.5;
118.3; 116.2; 108.4; 16.3.
2.4.1 General procedure for the preparation of the
corresponding CoIJIII)-bis-Schiff-base complexes [Co(Ln)(OAc)]
(n = 1–5, 1–5). The synthetic procedures and characterizations
for complexes 1–5 can be found in the ESI.†
2.4.2 General procedure for the preparation of the
corresponding MnIJIII)-bis-Schiff-base complexes [Mn(L5)Cl]
(6). For [Mn(L5)(Cl)] (6): solid H2L5 (99.7 mg, 0.15 mmol) and
anhydride Mn(OAc)2 (76.1 mg, 0.15 mmol) were added to a
flame dried Schlenk flask charged with a Teflon-coated stir
For H2L3: the orange asymmetric bis-Schiff-base ligand
H2L3 solid product was prepared in the same way as H2L1
except that o-vanillin (30.5 mg, 0.2 mmol) was used instead
of salicylaldehyde (21.0 μL, 0.2 mmol). Yield: 82.4 mg (77%).
Anal. calcd for C31H25ClN4O3: C, 69.27; H, 4.66; N 10.43.
Found: C, 69.25; H, 4.72; N, 10.41. FT-IR (KBr, cm−1): 3356 (b),
1634 (m), 1613 (s), 1568 (s), 1537 (m), 1491 (vs), 1462 (m),
1418 (m), 1379 (m), 1325 (m), 1281 (m), 1254 (m), 1142 (w),
1123 (w), 1088 (w), 1049 (m), 1007 (m), 972 (m), 934 (w),
866 (w), 831 (m), 777 (m), 735 (m), 704 (m), 613 (w), 586 (w),
546 (w), 505 (w), 482 (w), 457 (w), 430 (w). 1H NMR (400 MHz,
DMSO-d6, ppm): δ 12.81 (s, 1H, –OH), 12.04 (s, 1H, –OH),
8.94 (s, 1H, –CN), 8.06 (d, 2H, J = 8.4 Hz, ArH), 7.51–7.41
(m, 9H, ArH), 7.25 (t, 1H, J = 7.2 Hz, ArH), 7.16 (d, 1H, J = 7.6 Hz,
ArH), 7.05 (t, 1H, J = 7.6 Hz, ArH), 6.95 (t, 1H, J = 8.0 Hz, ArH),
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bar. Under an atmosphere of dry N2, absolute CHCl3 (5 ml)
and MeOH (5 ml) were injected and the resultant mixture
was stirred at ambient temperature for 6 h. The stopper of
the flask was removed, lithium chloride (38.0 mg, 0.90 mmol)
was added and the resultant reddish brown solution was
stirred at room temperature for 10 h while exposed to dry air.
The final solution was filtered and the clear filtrate was left
to stand at room temperature for several days to give the dark
brown polycrystalline solid product of 6. Yield: 129.0 mg,
57%. Anal. calc for C30H19Cl2Br2MnN4O2: C, 47.80; H, 2.52;
N, 7.44. Found: C, 47.76; H, 2.64; N, 7.39. FT-IR (KBr, cm−1):
3443 (b), 2928 (w), 1634 (vs), 1597 (s), 1559 (s), 1524 (m),
1497 (m), 1464 (s), 1427 (s), 1381 (m), 1355 (w), 1339 (w),
1313 (w), 1285 (w), 1215 (s), 1090 (m), 1061 (m), 1013 (m),
873 (w), 854 (w), 829 (w), 789 (w), 756 (w), 721 (m), 604 (m),
582 (w), 550 (m), 519 (w), 490 (w). ESI-MS (in THF) m/z
754.15 [M − H]+; 717.71 [M − Cl]+.
2.4.3 General procedure for the preparation of the
corresponding CrIJIII)-bis-Schiff-base complexes [Cr(L5)Cl] (7).
For [Cr(L1)Cl] (7): solid H2L5 (199.4 mg, 0.3 mmol) and
anhydrous CrCl2 (37.0 mg, 0.3 mmol) were added to a flame
dried Schlenk flask charged with a Teflon-coated stir bar.
Under an atmosphere of dry N2, absolute THF (10 mL) was
injected and the resultant mixture was stirred at ambient
temperature for 24 h. The stopper of the flask was removed,
and the resultant brown solution was stirred at room temper-
ature for another 24 h while exposed to dry air. After the reac-
tion solution was poured into diethyl ether (50 mL), the organic
layer was washed with aqueous saturated NH4Cl (3 × 30 mL)
and brine (3 × 30 mL) followed by drying with anhydrous
Na2SO4. After filtration to remove solid impurities and drying
agent, toluene (20 mL) was added into the filtrate. The final
clear solution was left to stand at room temperature for several
days to obtain the brown polycrystalline solid product of 7.
Yield: 136.0 mg (60%). Anal. calcd for C30H19Cl2Br2CrN4O2: C,
47.99; H, 2.53; N, 7.46. Found: C, 47.89; H, 2.64; N, 7.40. FT-IR
(KBr, cm−1): 3057 (w), 2924 (m), 2854 (m), 1627 (s), 1563 (s),
1528 (m), 1462 (vs), 1447 (s), 1427 (s), 1381 (m), 1313 (m),
1216 (w), 1152 (m), 1055 (m), 1014 (w), 843 (w), 790 (w),
750 (s), 712 (w), 690 (w), 594 (w), 548 (w), 514 (w). ESI-MS
(in THF) m/z 751.21 [M − H]+; 714.77 [M − Cl]+.
2.6 Copolymerization procedures of SO and MA
The copolymerization between SO and MA was performed in
both bulk and solution (toluene) with a stipulated molar ratio
of SO : MA : catalyst : cocatalyst for the selected reaction time.
2.6.1 Representative copolymerization procedure in bulk.
In the standard Schlenk vacuum line system, SO (2.5 mmol),
MA (2.5 mmol), catalyst 7 (0.01 mmol) and co-catalyst DMAP
(0.01 mmol) were placed in a vial equipped with a small stir
bar. The vial was sealed with a Teflon lined cap, and placed
in an aluminum heat block preheated to the desired temper-
ature of 110 °C. After the allotted reaction time of 150 min,
the vial was removed from the heat block and a small aliquot
1
was removed for crude H NMR analysis to determine mono-
mer conversion. The viscous reaction mixture was then
dissolved in a minimum amount of CH2Cl2, and precipitated
with an excess of anhydrous diethyl ether. The polymer was
collected and dried in vacuum to give a white solid product.
2.6.2 Representative copolymerization procedure in
solution. In the standard Schlenk vacuum line system, SO
(2.5 mmol), MA (2.5 mmol), catalyst 7 (0.01 mmol) and
co-catalyst DMAP (0.01 mmol) were placed in a vial equipped
with a small stir bar. Then absolute toluene (1 ml) was
injected into the vial. The vial was sealed with a Teflon lined
cap, and placed in an aluminum heat block preheated to the
desired temperature of 110 °C. After the allotted reaction
time of 300 min, the vial was removed from the heat block
1
and a small aliquot was removed for crude H NMR analysis
to determine monomer conversion. The viscous reaction mix-
ture was then dissolved in a minimum amount of CH2Cl2,
and precipitated with an excess of anhydrous diethyl ether.
The polymer was collected and dried in vacuum to give a
white solid product.
2.6.3 Representative characterization of copolymers from
SO and MA. Copolymer (SO–MA): FT-IR (KBr, cm−1): 3316 (w),
2956 (w), 2874 (w), 2649 (w), 1962 (w), 1732 (vs), 1649 (m),
1574 (w), 1535 (w), 1496 (w), 1453 (m), 1401 (m), 1350 (w),
1207 (s), 1162 (s), 1078 (w), 1026 (w), 1001 (w), 915 (w),
818 (w), 760 (m), 699 (m), 637 (w), 525 (w). 1H NMR (400 MHz,
DMSO-d6): 7.32 (s, 5H, ArH), 6.44 (d, 2H, J = 7.2 Hz, –CHCH–),
6.03–5.94 (m, 1H, –CH–), 4.40–4.10 (m, 2H, –CH2–).
3. Results and discussion
2.5 Structure determination
3.1 Synthesis of the novel asymmetrical bis-Schiff-base
ligands and corresponding complexes
The single crystals of complexes ijCoIJL3)IJOAc)] (3) and [Co(L4)
(OAc)] (4) of suitable dimensions were mounted onto thin
glass fibers. All the intensity data were collected on a Bruker
SMART CCD diffractometer (Mo-Kα radiation and λ = 0.71073 Å)
in Φ and ω scan modes. Structures were solved by Direct
methods followed by difference Fourier syntheses, and then
refined by full-matrix least-squares techniques against F2
using SHELXTL-97.19 All other non-hydrogen atoms were
refined with anisotropic thermal parameters. Absorption cor-
rections were applied using SADABS.20 All hydrogen atoms
were placed in calculated positions and isotropically refined
using a riding model.
As shown in Scheme 1, we synthesized five novel asymmetri-
cal bis-Schiff-base ligands H2Ln (n = 1–5). The asymmetrical bis-
Schiff-base ligands were synthesized by a three-step procedure,
wherein we first prepared 1-IJ4-Cl-phenyl)-3-methyl-4-benzoyl-2-
pyrazolin-5-one by the reaction of 1-(4-Cl-phenyl)-3-methyl-5-
pyrazolone with benzoyl chloride in 1 : 1 molar ratio in abso-
lute 1,4-dioxane. Then HL0 was obtained by the reaction
of 1-(4-Cl-phenyl)-3-methyl-4-benzoyl-5-pyrazolone with
o-phenylenediamine in either 1 : 1 or 2 : 1 molar ratio in abso-
lute EtOH. When the monoamine precursor HL0 was allowed
to react with one of the aldehyde derivatives (salicylaldehyde,
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Scheme 1 Synthesis of ligands H2Ln and complexes 1–7.
Table
1
Crystal data and structure refinement for complexes
5-bromo-salicylaldehyde, o-vanillin or 5-bromo-2-hydroxy-3-
methoxy-benzaldehyde, 3,5-dibromo-2-hydroxy-benzaldehyde)
in 1 : 1 molar ratio, a series of new asymmetrical bis-Schiff-base
ligands H2Ln (n = 1–5) were obtained in good yields, respec-
tively. In addition to these ligands, the corresponding Co(III)
complexes (1–5), Mn(III) complex 6 and Cr(III) complex 7 were
synthesized. All of the synthesized ligands and complexes
were well characterized by elemental analyses, FT-IR and
1H NMR (ESI†).
ij3IJH2O)]·H2O and [4(H2O)]·EtOH
Compound
ij3IJH2O)]·H2O
ij4IJH2O)]·EtOH
Formula
Fw
C33H30ClCoN4O7
688.99
C35H33BrCoClN4O7
795.94
Cryst syst
Space group
a, Å
b, Å
c, Å
Triclinic
P1
Triclinic
¯
¯
P1
10.418(12)
12.073(14)
13.296(16)
90.683(2)
110.550(2)
96.916(2)
1551.7(3)
2
1.475
0.35 × 0.27 × 0.20
296(2)
712
0.695
12.326(15)
12.739(15)
13.021(16)
62.534(2)
82.794(2)
79.008(2)
1779.1(4)
2
1.486
0.29 × 0.23 × 0.21
296(2)
812
1.733
α, deg
β, deg
γ, deg
3.2 X-ray single-crystal structures of complexes 3 and 4
V, Å3
The molecular structures of complexes ijCoIJL3)IJOAc)IJH2O)]·H2O
([3(H2O)]·H2O) and [Co(L4)(OAc)(H2O)]·EtOH ([4(H2O)]·EtOH)
were determined by X-ray single-crystal diffraction analyses.
Crystallographic data for the two complexes are presented in
Table 1, and selected lengths and angles are given in Table 2.
Complex ij3IJH2O)]·H2O crystallizes in the triclinic space
Z
Dcalc, g cm−3
Cryst size, mm3
Temp, K
FIJ000)
μ, mm−1
θ range, deg
Reflns measd
Reflns used
Params
R (I > 2σIJI))
2.11–27.21
8839
6387
1.82–26.06
9352
6701
¯
group of P1, and its structural unit is composed of one
mononuclear [Co(L3)(OAc)(H2O)] framework and one solvated
H2O molecule. As shown in Fig. 1, the central Co3+ ion (Co1)
lies in a six-coordinated environment and adopts a slightly
distorted octahedral geometry, where two N atoms (N3 and
N4) and two O atoms (O1 and O2) from the asymmetrical bis-
Schiff-base ligand H2L3 form an equatorial plane, two other
O atoms (O5 from the coordinated OAc− anion and O6 from
the coordinated H2O) occupy the two axial positions of the
coordination polyhedron. The distortion to the octahedron is
primarily caused by the non-planar conformation of the
equatorial planes with the mean deviations of 0.08–0.16 Å
and the dihedral angle of 29.7 between the two stable
delocalized six-membered (CoNCCCO) chelate rings and two
417
443
R1 = 0.0512
WR2 = 0.1486
R1 = 0.0708
WR2 = 0.1700
1.054
R1 = 0.0553
WR2 = 0.1547
R1 = 0.0923
WR2 = 0.1834
0.878
R (all data)
S
N–Co–O bite angles (N3–Co1–O1 and N4–Co1–O2) of
94.02(10) ° and 94.14(10) °, respectively. The existence of
coordinated OAc− anion at the axial position suggested the
formation of the oxidized Co(III) complex 3. The solvated H2O
molecule is not bound to the monomer framework and no
interaction with the host structure is observed.
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Table 2 Selected bond lengths (Å) and bond angles (°) for ij3IJH2O)]·H2O
and [4(H2O)]·EtOH
ij3IJH2O)]·IJH2O)
ij4IJH2O)]·EtOH
CoIJ1)–NIJ3) 1.930(3)
CoIJ1)–NIJ3) 1.920(4)
CoIJ1)–NIJ4) 1.883(3)
CoIJ1)–NIJ4) 1.880(3)
CoIJ1)–OIJ1) 1.955(2)
CoIJ1)–OIJ1) 1.925(3)
CoIJ1)–OIJ2) 1.904(2)
CoIJ1)–OIJ2) 1.872(3)
CoIJ1)–OIJ5) 1.885(2)
CoIJ1)–OIJ3) 1.882(3)
CoIJ1)–OIJ6) 1.971(2)
CoIJ1)–OIJ6) 1.956(3)
NIJ3)–CoIJ1)–NIJ4) 84.69(10)
NIJ3)–CoIJ1)–OIJ1) 94.02(10)
NIJ3)–CoIJ1)–OIJ2) 178.46(10)
NIJ3)–CoIJ1)–OIJ5) 93.00(10)
NIJ3)–CoIJ1)–OIJ6) 91.18(10)
NIJ4)–CoIJ1)–OIJ2) 94.14(10)
NIJ3)–CoIJ1)–NIJ4) 85.71(15)
NIJ3)–CoIJ1)–OIJ1) 95.00(13)
NIJ3)–CoIJ1)–OIJ2) 178.00(14)
NIJ3)–CoIJ1)–OIJ3) 92.71(14)
NIJ3)–CoIJ1)–OIJ6) 89.57(14)
NIJ4)–CoIJ1)–OIJ2) 94.50(14)
Fig. 2 Perspective drawing of ij4IJH2O)]·EtOH. H atoms and solvated
molecules are omitted for clarity.
formation of the oxidized Co(III) complex 4. The solvated
EtOH molecular is not bound to the monomer framework
and no interaction with the host structure is observed.
3.3 Alternating copolymerization studies of SO and MA
catalyzed by complexes 1–7
As shown in Scheme 2, the bulk or solution copolymerization
behavior of SO and MA monomers employing complexes 1–7
as catalysts in the presence of co-catalyst (DMAP, Ph3P or
PPN+Cl−) are examined, and the results are summarized in
Table 3. In comparison, the blank experiments in bulk copoly-
merization without catalyst and co-catalyst (entry 1 in Table 3)
show no copolymers obtained. Simultaneously, co-catalyst
Fig. 1 Perspective drawing of ij3IJH2O)]·H2O. H atoms and solvated
molecules are omitted for clarity.
Complex ij4IJH2O)]·EtOH crystallizes in the triclinic space
¯
group of P1, and its structural unit is composed of one
mononuclear [Co(L4)(OAc)(H2O)] framework and one solvated
EtOH molecule. As shown in Fig. 2, the central Co3+ ion
(Co1) lies in a six-coordinated environment and adopts a
slightly distorted octahedral geometry, where two N atoms
(N3 and N4) and two O atoms (O1 and O2) from the asym-
metrical bis-Schiff-base ligand H2L4 form an equatorial plane,
two other O atoms (O3 from the coordinated OAc− anion and
O6 from the coordinated H2O) occupy the two axial positions
of the coordination polyhedron. The distortion to the octahe-
dron is primarily caused by the no-planar conformation of
the equatorial planes with the mean deviations of 0.08–0.14 Å
and the dihedral angle of 25.9° between the two stable
delocalized six-membered (CoNCCCO) chelate rings and
two N–Co–O bite angles (N3–Co1–O1 and N4–Co1–O2) of
95.00(13)° and 94.50(14)°, respectively. The existence of
coordinated OAc− anion at the axial position suggested the
Scheme 2 Synthesis of polyesters from SO and MA using 1–7 as
catalysts.
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Table 3 Copolymerization of SO and MA catalyzed by catalysts 1–5
Conv.a
%
Ethera
%
Mn
PDIb
Tg
b
c
Entry
Monomer
M1 + M2
Method
Cat
Co-cat
Molar ratio
Time
min
T
M1 : M2 : Cat : Co-cat
°C
1
2
3
4
5
6
7
8
9
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
SO + MA
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
—
—
7
1
2
3
4
5
6
—
DMAP
—
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
250 : 250 : 0 : 0
250 : 250 : 0 : 1
250 : 250 : 1 : 0
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
150
150
150
150
150
150
150
150
150
150
110
110
110
110
110
110
110
110
110
110
—
14
11
89
92
83
90
92
95
98
—
55
63
6
11
<1
<1
5
—
—
—
1000
1600
2600
3000
3300
3400
3800
4000
4700
1.56
1.37
1.11
1.28
1.24
1.26
1.15
1.12
1.09
25
27
22
27
29
32
35
35
35
<1
<1
10
7
a
Determined by 1H NMR. b Determined by GPC. c Determined by DSC.
DMAP alone (entry 2 in Table 3) or catalyst 7 alone (entry 3 in
Table 3) are not very active (SO conversions 11–14%); thus,
lower activity can be observed, and the lower molecular
weight of copolymers (Mn = 1000–1600 g mol−1) obtained con-
tain a high percentage of ether linkages (55–63%). In contrast,
the incorporation of one of complexes 1–7 as the catalyst and
DMAP as the co-catalyst (other entries in Table 3) not only
accelerates the copolymerization, but also significantly stimu-
lates the formation of ester linkages. Based on the experi-
ments, we propose that a copolymer was generated via the
mechanism proposed in Scheme 3. The cobalt(III) complex
acts as a Lewis acid and activates an epoxide for ring-opening
to form a cobalt(III) alkoxide, then the cobalt(III) alkoxide
reacts with anhydride to form a polymer.
asymmetrical bis-Schiff-base complexes ijCoIJLn)IJOAc)] (n = 1–5,
1–5) on the catalytic performance, the copolymerization
(entries 4–8 in Table 3) of SO and MA are carried out in bulk
with DMAP as the co-catalyst under the stipulated molar ratio
(250 : 250 : 1 : 1) of SO : MA : catalyst : co-catalyst at polymeriza-
tion temperature of 110 °C for 150 min. As expected, catalyst 1
gives significantly higher activity of SO (89% of SO conversion)
and better selectivity (6% of ether content) in the presence of
co-catalyst DMAP. Interestingly, the utilization of the electron-
withdrawing Br substituent para to the phenoxide group in
catalyst 2 (R1 = Br, R2 = H), when compared to catalyst 1 (R1 =
H, R2 = H), gives relatively higher reactivity (92% of SO conver-
sion), which is possibly due to the increase of electrophilicity
of Co3+ ion in 2 due to the delocalization of electron conjuga-
tion. Conversely, the electron-donating MeO substituent ortho
to the phenoxide group in catalyst 3 (R1 = H, R2 = MeO) has a
negative effect on the catalytic activity (83% of SO conversion).
With regards to the effect of both the electron-withdrawing Br
substituent at the para orientation and the electron-donating
MeO substituent at the ortho orientation to the phenoxide
group in catalyst 4 (R1 = Br, R2 = MeO) or two electron-
withdrawing Br substituents at the para and ortho orientations
to the phenoxide group in catalyst 5 (R1 = Br, R2 = Br), a simi-
lar trend in catalytic activity is observed, in which catalyst 5
proves to be more active (92% of SO conversion) for SO–MA
copolymerization, while the activity (90% of SO conversion) of
catalyst 4 is almost identical to that of catalyst 1. To study the
effect of the metal of the catalyst 5 on the catalytic perfor-
mance, we decided to vary the metal in the catalyst 5. Of the
three catalysts, the chromium-based catalyst 7 exhibited the
best performance under the applied conditions, followed by
the manganese-based catalyst 6. The cobalt-based catalyst 5
showed the least activity under the applied conditions.
The polymer produced by 7/DMAP appears to be an almost
perfectly alternating CHO–MA copolymer (<1% of the ether
content shown in Fig. 3), which is in agreement with the
MALDI-TOF-MS spectrum (Fig. 4) of the product, where an m/z
interval of 218 between the consecutive peaks corresponding
to the addition of a [SO + MA] repeating unit (entry 10 in
Table 3) is observed. These are obscured in the MALDI-TOF-MS
3.3.1 Effect of catalyst structure of complexes 1–7 on
catalytic copolymerization of SO and MA. To study the effect
of the electronic and steric environment of a series of
Scheme 3 Proposed mechanism for the copolymerization of SO and
MA (R = alkyl or polymer).
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of 110 °C for 300 min also produces the expected perfectly
alternating SO–MA copolymer, and also leads to both higher SO
conversion (95%) and the largest copolymer Mn (5200 g mol−1
,
PDI = 1.07) than those in bulk as the general feature.12a It is
evident that the asymmetrical catalyst 7 is better than similar
catalyst described in the literature for SO–MA copolymeriza-
tion (Mn = 1420 g mol−1, PDI = 1.53).11 To study the effect
of co-catalysts on catalytic behavior, two other types of
co-catalysts including Ph3P and PPN+Cl− are compared in com-
bination with the catalyst 7 for the solution ring-opening copo-
lymerization of SO–MA (2–3 in Table 4). Compared with the
effective catalysis of DMAP, no copolymer was obtained with
Ph3P as co-catalyst, and this can be attributed to the large
cone angle of three steric phosphines in Ph3P considerably
lowering its nucleophilicity. Onium salt PPN+Cl− had shown
considerable higher activity as a co-catalyst than Ph3P. In
agreement with the common feature of the variation of cata-
lyst and co-catalyst concentration on the catalytic activity,12,13
for DMAP, there is a slight decrease in the reactivity with
increasing or decreasing catalyst and co-catalyst concentra-
tion (entries 4–5 in Table 4). The polymer molecular weight
(2900–3700 g mol−1) also decreased due to the chain transfer
effect of excess of DMAP or the insufficient reaction of less
DMAP.13b Considering the effect of the reaction temperature
on the polymerization behavior (entries 6–7 in Table 4), a
lower temperature (90 °C) results in lower polymerization rate
(56% of CHO conversion) and lower polymer molecular
weight (Mn = 2400 g mol−1). It indicates that a lower reaction
temperature shows negative influence on the reaction rate
constant under the consistent conditions. According to
Arrhenius equation, an elevated temperature favors most of
the reactions to a certain degree because their activation
energy is usually positive.21 It appears that higher tempera-
ture is more desirable for ring-opening copolymerization of
SO–MA, but the active decline of the catalyst 7 is caused by
higher temperatures (130 °C). The effect of polymerization
time on the catalytic activity is similar to that of the polymeri-
zation temperature. The catalytic activity increases at first from
150 min to 300 min, then decreases from 300 min to 450 min,
and the peak appears at 300 min. The relatively lower polymer
molecular weight (Mn = 2400 g mol−1) and a larger PDI (1.33)
than those (Mn = 5200 g mol−1 and PDI = 1.07) at the reaction
time of 300 min from the same 7/DMAP system along with the
higher ether content (20%) of poly(ester-co-ether) are observed
for the longest reaction time of 450 min, which should also be
due to the chain transfer effect of DMAP with the longest
reaction time. Compared to the run without chain transfer
agents (Table 4, entry 1), the addition of benzyl alcohol
(Table 4, entry 10) or n-butyl alcohol (Table 4, entry 11) as
chain transfer agents in all the cases resulted in the expected
reduction of molecular weight (Mn = 3000–3100 g mol−1 and
PDI = 1.53–1.56), whereas the conversions remained compa-
rable. We assumed that the chelating effects of the chain
transfer agents on the copolymerization of SO and MA might
deactivate the catalyst, but it plays only a limited role in this
catalyst system.
Fig.
3
Representative 1H NMR spectrum of SO–MA copolymer,
Table 3, entry 10.
Fig. 4 Representative MALDI-TOF-MS spectrum of SO–MA copolymer,
Table 3, entry 10.
spectrum by the intrinsically charged DMAP-end capped
chains. The glass transition temperatures of the copolymers
were determined by differential scanning calorimetry analysis.
These data are summarized in Table 3. Specifically, the glass
transition temperature found for the polyesters formed from
SO and MA with catalysts 1–7 were 22, 27, 29, 32, 35, 35, and
35 °C, respectively. There is a slight fluctuation in the Tgs of
the copolymers, which is most probably the result of the low
molecular weight of the copolymers and possibly the inclu-
sion of the traces of monomer.
3.3.2 Effect of copolymerization procedures of catalyst 7
on the catalytic copolymerization of SO and MA. Because the
chromium-based catalyst 7 proved to be the most active of
all, catalyst 7 was chosen for further studies with different
copolymerization procedures. The results are listed in Table 4.
The typical solution polymerization (entry 1 in Table 4) from
7/DMAP under the stipulated molar ratio (250 : 250 : 1 : 1) of
SO : MA : catalyst : co-catalyst at the polymerization temperature
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Table 4 Copolymerization of SO and MA catalyzed by catalyst 7 from different polymerization procedures
Molar ratio
Time
min
T
Conv.a
%
Ethera
%
PDIb
Tg
b
c
Entry
Method
Co-cat
M1 : M2 : Cat : Co-cat
°C
Mn
1
2
3
4
5
6
7
8
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
DMAP
Ph3P
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
150 : 150 : 1 : 1
500 : 500 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
250 : 250 : 1 : 1
300
300
300
300
300
300
300
150
450
300
300
110
110
110
110
110
90
130
110
110
110
110
95
—
86
90
72
56
71
82
97
84
82
<1
—
7
11
13
18
12
6
5200
—
1.07
—
35
—
34
33
32
20
33
33
33
23
31
PPN+Cl−
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
4300
3700
2900
2400
4500
2700
2400
3100
3000
1.16
1.38
1.24
1.14
1.33
1.41
1.33
1.56
1.53
9
20
15
19
10d
11e
a
Determined by 1H NMR. b Determined by GPC. c Determined by DSC. d Reaction conditions: solution, [SO] : [MA] : [7] : [DMAP] : [benzyl alcohol] =
250 : 250 : 1 : 1 : 10. e Reaction conditions: solution, [SO] : [MA] : [7] : [DMAP] : [n-butyl alcohol] = 250 : 250 : 1 : 1 : 10.
3 J. W. Kramer, D. S. Treitler, E. W. Dunn, P. M. Castro,
4. Conclusion
T. Roisnel, C. M. Thomas and G. W. Coates, J. Am. Chem.
In conclusion, we have significantly demonstrated the higher
activity of asymmetrical Schiff-base catalysts 1–7 compared to
the similar catalysts described in the literature for the ring-
opening copolymerization of SO with MA. Among the seven
catalysts applied, the chromium catalyst 7 proved to be the
most active. The MALDI-TOF-MS spectrum of the copolymer
formed by 7/DMAP showed a completely alternating micro-
structure. Of three co-catalysts tested, DMAP exhibited the
highest activity, PPN+Cl− showed a relatively higher activity
than Ph3P, which exhibited a considerably low activity. The
variation of the monomer to catalyst ratio, polymerization
time and polymerization temperature affected the epoxide–
anhydride copolymerization in a similar way as observed for
the corresponding SO–MA copolymerization. Upon investigat-
ing the role of chain transfer agents in the alternating ring-
opening copolymerization of SO with MA, it was found that
chain transfer agents resulted in the reduction of molecular
weight, whereas the conversions remained comparable.
Further studies that are focused on increasing the efficiency
and expanding the monomer scope with the asymmetrical
Schiff-base metal catalysts are currently in progress.
Soc., 2009, 131, 16042–16044.
4 C. Robert, F. de Montigny and C. M. Thomas, Nat. Commun.,
2011, 2(586), DOI: 10.1038/ncomms1596.
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1985, 18, 1049–1055; (c) S. Asano, T. Aida and S. Inoue,
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Acknowledgements
We gratefully acknowledge the financial support from the
National Natural Science Foundation (21373160), the
Scientific Research Program Foundation of Shaanxi Provincial
Education Department (2013JK0701) and the Scientific
Research Culture Foundation of Xi'an University of Science
and Technology (201117).
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