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2a. 1H NMR (300 MHz, CDCl3) d ¼ 8.01(m, NCH 4H), 6.95(m,
ArH 4H), 6.72(m, ArH 4H), 3.71(m, CCH2N 8H), 2.14(s, ArCH3
24H), 0.86(m, AlCH2CH3 6H), ꢁ0.09(m, AlCH2 4H). 13C NMR
(100 MHz, CDCl3) d ¼ 170.53(NCH), all benzene ring: 163.54,
137.65, 130.48, 128.89, 124.58, 117.09; 63.60(NCH2),
41.95((CH2)4C), 20.12(ArCH3), 15.73(ArCH3), 9.64(AlCH2CH3),
1.11(AlCH2CH3). Elem. anal.: calcd C 70.29, H 7.08, N 7.29%;
found C 70.26, H 7.10, N 7.25%.
were purchased from Aldrich Inc. and used without further
purication. Toluene was distilled from Na-benzophenone.
Ethyl acetate and 2-propanol were distilled from CaH2 under
the protection of argon. rac-Lactide (Purac) was puried by
recrystallization from ethyl acetate and dried under vacuum at
room temperature (RT) before use. NMR spectra were recorded
ꢀ
on Bruker AV 300 M and Bruker AV 400 M in CDCl3 at 25 C.
Chemical shis are given in parts per million from tetrame-
thylsilane. Gel permeation chromatography (GPC) measure-
ments were conducted with a Waters 515 GPC with CHCl3 as the
eluent (ow rate: 1 mL minꢁ1, at 35 ꢀC). The molecular weights
were calibrated against polystyrene (PS) standards.
3a. 1H NMR (300 MHz, CDCl3) d ¼ 8.19(m, NCH 4H), 7.43(m,
ArH 4H), 6.91(m, ArH 4H), 3.80(m, CCH2N 8H), 1.50(m,
ArC(CH3)3 36H), 1.32(m, ArC(CH3)3 36H), 0.70(m, AlCH2CH3
6H), ꢁ0.19(m, AlCH2 4H). 13C NMR (100 MHz, CDCl3): d ¼
176.79(NCH), all benzene ring: 168.11, 157.96, 140.58, 136.57,
130.65, 127.00, 126.12, 117.81; 62.17(NCH2), 45.11((CH2)4C),
35.09, 33.84(ArC(CH3)3), 31.34(C(CH3)3), 29.35(C(CH3)3),
8.64(AlCH2CH3), 1.34(AlCH2CH3). Elem. anal.: calcd C 74.96, H
9.30, N 5.07%; found C 75.01, H 9.27, N 5.09%.
Synthesis of ligands (general procedure)
A solution of pentaerythrityl tetramine (1 mmol) in ethanol (50
mL) was added dropwise to a stirred solution of substituted
salicylaldehyde (4 mmol) in ethanol (50 mL). The reaction
mixture was reuxed for 12 h before cooling to RT. Aer removal
of the solvent under vacuum, a crystalline solid was produced
and puried by recrystallization in ethanol–CHCl3 mixture.
1. 1H NMR (300 MHz, CDCl3) d ¼ 13.20(s, ArOH 4H), 8.40(s,
NCH 4H), 7.33(m, ArH 4H), 7.23(m, ArH 4H), 6.97(m, ArH 4H),
6.89(m, ArH 4H), 3.75(s, CCH2N 8H). 13C NMR (100 MHz, CDCl3)
d ¼ 166.92(NCH), all benzene ring 160.80, 132.39, 131.33,
118.60, 118.21, 116.55; 60.93(NCH2), 43.53((CH2)4C). Elem.
anal.: calcd C 72.24, H 5.88, N 10.21%; found C 72.28, H 5.84, N
10.19%.
Polymerization of rac-LA
In a typical polymerization experiment, complexes 1a–3a (10
mmol), the required amount of rac-LA in toluene were loaded in
a ame-dried vessel containing a magnetic bar. The vessel was
placed in a thermostated oil bath. Conversion of the monomer
was determined on the basis of 1H NMR spectroscopic studies.
Aer a certain reaction time, the polymer was isolated by
precipitation with cold methanol. The precipitate was collected
and dried under vacuum at RT for 36 h.
2. 1H NMR (300 MHz, CDCl3) d ¼ 13.21(s, ArOH 4H), 8.34(s,
NCH 4H), 7.05(s, ArH 4H), 6.88(s, ArH 4H), 3.78(s, CCH2N 8H),
2.29(s, ArCH3 24H). 13C NMR (100 MHz, CDCl3)
d
Results and discussion
¼
167.03(NCH), 156.58, 134.58, 128.91, 126.98, 125.18, 117.19 all
benzene ring; 61.08(NCH2), 43.29((CH2)4C), 20.09(ArCH3),
15.02(ArCH3). Elem. anal.: calcd C 74.52, H 7.32, N 8.48%;
found C 74.50, H 7.38, N 8.51%.
Ligand synthesis
Pentaerythrityl tetramine was synthesized by a three-step
procedure as shown in Scheme 2. The versatile ligand family
was synthesized from pentaerythrityl tetramine with
substituted salicylaldehyde. Ligands 1–3 had identical aliphatic
backbones but the different salicylaldehyde substituents: H for
1, CH3 for 2 and C(CH3)3 for 3. The 1H NMR spectra of ligand 2
shows signals at d 8.34 and 3.78 which were attributed to the
N]CH protons and NCH2 protons of the pentaerythrityl tetra-
mine, respectively. The intensity ratio of the signals at 8.34 and
3.78 ppm was 1 : 2, which conrmed the structure of ligand 2.
3. 1H NMR (300 MHz, CDCl3) d ¼ 13.71(s, ArOH 4H), 8.55(s,
NCH 4H), 7.46(s, ArH 4H), 7.18(s, ArH 4H), 3.84(s, CCH2N 8H),
1.53(s, ArC(CH3)3 36H), 1.35(s, ArC(CH3)3 36H). 13C NMR (100
MHz, CDCl3) d ¼ 167.87(NCH), all benzene ring 157.70, 139.96,
136.34, 126.93, 125.86, 117.32; 61.16(NCH2), 43.79((CH2)4C),
34.74, 33.78(ArC(CH3)3), 31.11(C(CH3)3), 29.11(C(CH3)3). Elem.
anal.: calcd C 78.27, H 9.70, N 5.62%; found C 78.30, H 9.68, N
5.67%.
Synthesis of complexes
AlEt3 (0.2 mmol) in toluene (5 mL) was added to the stirred 1 mL
toluene solution of ligands 1–3 (0.1 mmol) at RT. The reaction
was maintained at 80 ꢀC for 12 h, and the reaction mixture was
then slowly cooled to RT. The toluene was removed under
vacuum.
1a. 1H NMR (300 MHz, CDCl3) d ¼ 8.05(m, NCH 4H), 7.36(m,
ArH 4H), 7.01(m, ArH 4H), 6.80(m, ArH 4H), 6.54(m, ArH 4H),
3.75(m, CCH2N 8H), 0.62(m, AlCH2CH3 6H), ꢁ0.12(m, AlCH2 4H).
13C NMR (100 MHz, d8-THF) d ¼ 173.13(NCH), all benzene ring:
168.38, 136.92, 134.60, 123.35, 120.69, 117.07; 64.70(NCH2),
44.14((CH2)4C), 10.21(AlCH2CH3), 2.19(AlCH2CH3). Elem. anal.:
calcd C 67.67, H 5.83, N 8.53%; found C 67.71, H 5.85, N 8.55%. Scheme 2 Synthetic pathway for the preparation of ligands.
22562 | RSC Adv., 2014, 4, 22561–22566
This journal is © The Royal Society of Chemistry 2014