L.-L. Chen et al.
of L-lactide in the absence of normal initiators (alkoxide, amide or
carboxylate), and the end groups of PLA are not from the catalysts,
but from the replacement of methyl groups when terminating the
polymerization with methanol containing 5% HCl. Thus, the initia-
tion may occur through the metal–alkoxide bond of the catalysts,
through the stable and selective acyl–oxygen bond cleavage of
the monomer, and the ring-opening polymerization also proceeds
[CuL(H2O)] (1)
To a stirred solution of H2L (0.165 g, 0.5 mmol) in absolute EtOH
(
5 ml), Cu(OAc)2·H2O (0.100 g, 0.5 mmol) was added and heated
under reflux for 5 h. The mixture was allowed to cool to room
temperature and filtered. The resultant clear brown solution was
left to stand at room temperature for several days to give a black
crystalline product of 1 in 72% yield. Calcd for C18H20N2O5Cu: C,
[
23]
via the typical ‘coordination-insertion’ mechanism.
5
3.26; H, 4.47; N, 6.90%; found: C, 53.38; H, 4.66; N, 6.88%. IR (KBr,
−
1
cm ): 3291b, 2923 w, 2828 w, 1646s, 1603 w, 1444 vs, 1240m,
1
3
215s, 728m, 697 w, 636 w, 582 w, 531 w, 463 w. ESI-MS (m/z):
90.90 [M − H2O + H] .
Conclusion
+
2
+
In conclusion, the monometallic Cu
complex 1 and the
2
+
3+
[Cu(L)Nd(NO3)3(DMF] (2)
bimetallic Cu –Nd
complex 2 were shown to efficiently
catalyze the bulk solvent-free melt ROP of L-lactide with moderate
molecularweightsandnarrowmolecularweightdistributions. The
correlation of molecular structure vs catalytic activity showed that
the different catalytic behaviors resulted from the different active
species; in particular, the involvement of rare ions effectively
passivated the catalytic behaviors on the ROP of L-lactide with
bigger polymeric molecular weights (Mw or Mn) and better
polymerization controllability. With these in mind, the design
of more active and multiple activity species of catalysts is now
underway.
To a solution of 1 (0.113 g, 0.3 mmol) in absolute EtOH (4 ml), a
solution of Nd(NO3)3 · 6H2O (0.132 g, 0.3 mmol) in absolute EtOH
(5 ml) was added, and the mixture was refluxed for 3 h. Then 1 ml
absolute DMF was added to give a clear green solution. Diethyl
ether was allowed to diffuse slowly into this solution at room
temperature and pale red single crystals of 2 were obtained in a
few weeks. Yield: 0.162 g (68%). Calcd for C21H25N6O14CuNd: C,
31.80; H, 3.18; N, 10.59%; found: C, 31.72; H, 3.32; N, 10.56%. IR
−
1
(KBr, cm ): 2987 w, 2846 w, 1629s, 1561 w, 1501s, 1470 vs, 1316s,
1279s, 1235s, 1167m, 1109 w, 1080m, 1027 w, 989 w, 956 w, 861 w,
8
6
11 w, 784 w, 740m, 685 w, 625 w, 582 w, 495 w. ESI-MS (m/z):
58.15 [M − DMF − NO3] .
+
Experimental Section
PLA
IR (KBr, cm 1): 3442 w, 3000 w, 2950 w, 1758s, 1245m, 1192m,
Materials and Methods
−
All chemicals of reagent grade were commercially available and
used without further purification. Element analyses were per-
formed on a Perkin-Elmer 240C element analyzer. Infrared spectra
were recorded on a Bruker Equinox55 FT-IR spectrophotometer in
132m, 1090m, 756m, 693 w. 1H NMR (400 MHz, CDCl3, ppm):
1
δ 5.18 (m, methine), 4.37 (d, methine), 3.60 (s, hydroxyl), 1.59 (s,
13
methyl), 1.57(s, methyl), 1.50(s, methyl). CNMR(400 MHz, CDCl3,
ppm): δ 169.62 (-C O), 69.01 (-CH), 16.64 (-CH3).
−
1
the region 4000–400 cm in KBr pellets. ESI-MS was performed
on a Finnigan LCQ
DECA
n
XP HPLC-MS mass spectrometer with a
Crystallography
mass to charge (m/z) range of 2000 using a standard electrospray
1
13
ion source and toluene as solvent. H NMR and C NMR spectra
were measured on a Varian Unity INOVA 400NB instrument using
DMSO-δ6 or CDCl3 as solvent and TMS as internal standard at
room temperature. Electronic absorption spectra in the UV–vis
region were recorded with a Hewlett Packard 8453 UV–vis spec-
trophotometer. Thermogravimetric analyses were carried out on
a Netzsch TG 209 Instrument under flowing nitrogen by heating
Single crystals of [Cu(L)(H O)] (1) and [Cu(L)Nd(NO ) (DMF)] (2),
2
3 3
of suitable dimensions, were mounted onto glass fibers for
crystallographic analyses. The structure of 1 was similar to that
previously reported in the literature.[ For 2, all the intensity data
were collected at 293(2) K on a Bruker SMART CCD diffractometer
15]
(Mo-K radiation,λ = 0.71073 Å)inꢀandω scanmodes.Structure
α
was solved by direct methods followed by difference Fourier
◦
the samples from 25 to 600 C.
syntheses, and refined by full-matrix least-squares techniques
2
[24]
against F using SHELXTL.
All the non-hydrogen atoms
were refined with anisotropic thermal parameters. Absorption
Preparations
[25]
corrections were applied using SADABS.
All hydrogen atoms
ꢁ
were placed in calculated positions and refined isotropically using
a riding model. Crystallographic data and refinement parameters
forthecomplex2arepresentedinTable 2.Selectedbonddistances
and bond angles for 1 and 2 are given in Table 3.
N,N -Bis(3-methoxy-salicylidene)ethylene-1,2-diamine (H2L)
The Salen-type Schiff-base ligand H2L was synthesized by the
typicalprocedure[ bycondensationofo-vanillin(6.3 g, 40 mmol)
and 1,2-diaminoethane (1.4 ml, 20 mmol) in absolute EtOH under
reflux for about 10 h. After cooling to room temperature, the
insoluble precipitate was filtered and was re-crystallized using
absolute EtOH to give the yellow polycrystalline solid. Yield: 5.0 g,
6%. Calcd for C18H20N2O4: C 65.84, H 6.14, N 8.53%; found: C,
5.58, H, 6.06, N, 8.63%; IR (KBr, cm ): 3444b, 3001 w, 2930 w,
840 w, 1632s, 1468s, 1410m, 1249s, 1080m, 958m, 736m, 695 w,
11]
Polymerization Experiments
L-Lactidewaspreparedfrom L-lacticacidaspreviouslyreported.[26]
The crude product was further purified by re-crystallization three
times from dried ethyl acetate, then dried for 24 h in vacuum
7
6
2
6
−
1
◦
at 30 C. Under nitrogen, 1.000 g (6.94 mmmol) of the freshly
1
42 w, 564 w, 525 w. H NMR (400 MHz, [D6]-DMSO): δ 13.54 (s, 2H,
OH), 8.55 (s, 2H, -CH N), 6.99 (m, 4H, -Ph), 6.77 (t, 2H, Ph), 3.91 (s,
H, -CH2), 3.74 (s, 6H, -MeO).
re-crystallized L-lactide monomer and the catalyst 1 or 2, in a
stipulated molar ratio ([M] : [C]), were charged in an ampoule
inside a glove box. The ampoule was put under high vacuum
-
4
wileyonlinelibrary.com/journal/aoc
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 310–316