Syntheses and molecular structures of mono(guanidinate) lanthanide borohydrides and their catalytic activity for MMA-polymerization

Article abstract of DOI:10.1016/j.jorganchem.2006.04.022

The mono(guanidinate) lanthanide borohydride complexes of [(Me₃Si)₂NC(NCy)₂]Ln(BH₄) ₂(THF)₂ (Ln = Yb (1), Er (2)) have been synthesized by the reactions of corresponding Ln(BH₄)₃(THF)₃ with sodium guanidinate of [(Me₃Si)₂NC(NCy)₂]Na in a 1:1 molar ratio in THF. They were characterized by elemental analysis, infrared spectrum and X-ray diffraction analysis. 1 and 2 have similar structures. The lanthanide ion was bonded by an η²-guanidinate ligand, two η³-BH₄ ligands and two THF molecules as a distorted octahedron. The two BH₄ ligands in a complex are equivalent and cis to each other. The structure of solvated sodium guanidinate of {[(Me₃Si)₂NC(NCy)₂]Na(THF)}₂ (3) was also presented. In a dimeric molecule of 3, each Na atom is bound to three nitrogen atoms from two guanidinate groups and one oxygen atom from the THF molecule. 1 and 2 displayed moderate high catalytic activity for the polymerization of methyl methacrylate. The Er complex is more active than the Yb complex.

Full text of DOI:10.1016/j.jorganchem.2006.04.022

Journal of Organometallic Chemistry 691 (2006) 3377–3382
www.elsevier.com/locate/jorganchem
Syntheses and molecular structures of mono(guanidinate) lanthanide
borohydrides and their catalytic activity for MMA-polymerization
*
Fugen Yuan , Yan Zhu, Lifeng Xiong
Department of Chemistry, University of Science and Technology of Suzhou, Shang Fang Shan, Suzhou 215009, China
Received 4 September 2005; received in revised form 9 April 2006; accepted 20 April 2006
Available online 29 April 2006
Abstract
The mono(guanidinate) lanthanide borohydride complexes of [(Me3Si)2NC(NCy)2]Ln(BH4)2(THF)2 (Ln = Yb (1), Er (2)) have been
synthesized by the reactions of corresponding Ln(BH4)3(THF)3 with sodium guanidinate of [(Me3Si)2NC(NCy)2]Na in a 1:1 molar ratio
in THF. They were characterized by elemental analysis, infrared spectrum and X-ray diffraction analysis. 1 and 2 have similar structures.
The lanthanide ion was bonded by an g2-guanidinate ligand, two g3-BH4 ligands and two THF molecules as a distorted octahedron. The
two BH4 ligands in a complex are equivalent and cis to each other. The structure of solvated sodium guanidinate of {[(Me3Si)2NC(N-
Cy)2]Na(THF)}2 (3) was also presented. In a dimeric molecule of 3, each Na atom is bound to three nitrogen atoms from two guanidinate
groups and one oxygen atom from the THF molecule. 1 and 2 displayed moderate high catalytic activity for the polymerization of methyl
methacrylate. The Er complex is more active than the Yb complex.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Lanthanide; Borohydride; Guanidinate; Polymerization
1. Introduction
catalysts until recently. Guillaume and coworkers reported
the ring-opening polymerization of e-caprolactone by
Nd(BH4)3(THF)3 [6], and studied the polymerization
Covalent borohydride complexes of d-transition metals,
with the exception of those of group IV, are generally ther-
mally unstable and readily decompose into the correspond-
ing hydrides with elimination of diborane [1]. In contrast,
f-element borohydrides can be easily isolated, thus allowing
structural and chemical studies of the BH4 ligand [1,2].
Organolanthanide complexes containing Ln–C, Ln–H
and Ln–N bonds have been found to exhibit good catalytic
activity in various chemical transformations. Especially,
they are very effective single-component catalysts for the
polymerization of polar or non-polar monomers [3]. How-
ever, although the parent complexes Ln(BH4)3(THF)n were
prepared in 1960 [4] and structurally characterized for
Y(BH4)3(THF)3 about 20 years later [5], lanthanide boro-
hydrides have seldom been introduced as polymerization
mechanism with CpꢀSmðBH4ÞðTHFÞ [7]. Mountford and
2
coworkers found that methyl methacrylate (MMA) can
be polymerized by lanthanide borohydride complexes with
polydentate diamide–diamide ligands [8]. Very recently, the
trans-specific diene polymerization by Nd(BH4)3(THF)3 in
the presence of MgBu2 was also achieved [9]. These results
revealed that lanthanide borohydride complexes were new
types of polymerization catalysts, and their catalytic ability
has not fully been evaluated.
The potentially superior control over metal-centered
reactivity has promoted the search for non-cyclopentadie-
nyl ligand environments for the lanthanide metals. This is
especially relevant for the design of new polymerization
catalysts. Cyclopentadienyl-alternative ligands such as ami-
dinates [10], b-diketiminates [11] and guanidinates [12]
have been developed to understand their lanthanide chem-
istry. Of those, guanidinate ligand is one of the most attrac-
tive. For example, bis(guanidinate) lanthanide methyl
*
Corresponding author.
E-mail address: yuanbox@szcatv.com.cn (F. Yuan).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.04.022

3378
F. Yuan et al. / Journal of Organometallic Chemistry 691 (2006) 3377–3382
complexes were found to be effective catalysts not only for
the polymerization of e-caprolactone and MMA [13], but
also for the polymerization of styrene [14], which is nor-
mally quite difficult to achieve with lanthanocene catalysts.
Bis(guanidinate) lanthanide diisopropylamide complexes
exhibited comparable activity with the corresponding lant-
hanocene complexes for MMA-polymerization [15]. These
fascinating results should be relevant with the open coordi-
nation environment and the strong electronic donor ability
of the guanidinate ligand.
Under these background, we started to prepare lantha-
nide borohydride complexes with guanidinate ligand and
investigated their catalytic activity for the polymerization
of MMA. We found that the guanidinate lanthanide boro-
hydride complexes could be readily prepared from sodium
guanidinate and homoleptic Ln(BH4)3(THF)3 in THF.
These complexes displayed moderate high catalytic activity
for the polymerization of MMA. To the best of our knowl-
edge, it is the first example with respect to the structure and
catalytic activity of guanidinate lanthanide borohydride
complexes.
Fig. 1. The molecular structure of 1.
2. Results and discussion
2.1. Syntheses and structures of mono(guanidinate)
lanthanide borohydrides
The reaction of Ln(BH4)3(THF)x with alkali anionic
reagents is a convenient synthetic route to some lanthanide
borohydride complexes such as [(COT)Nd(BH4)(THF)]2
[16], (THF)(BH4)2Nd(l-g7:g7-C7H7)Nd(BH4)(THF)2 [17]
and
[K(18-crown-6){(C13H8)CPh2(C5H4)Nd(BH4)2}]2 Æ
C4H8O2 [18]. We wish to synthesize guanidinate lanthanide
borohydrides similarly by the reactions of Ln(BH4)3(THF)x
with [(Me3Si)2NC(NCy)2]Na. As an equivalent of [(Me3-
Si)2NC(NCy)2]Na was added into the suspension of
Yb(BH4)3(THF)3 in THF, the color of the mixture changed
from nearly colorless to orange, indicating the reaction pro-
ceeding. After workup, red crystals of [(Me3Si)2NC(N-
Cy)2]Yb(BH4)2(THF)2 (1) were successfully obtained from
the THF-Et2O solution. The reaction was also applicable
for Er element. Similar reaction of Er(BH4)3(THF)3 and
[(Me3Si)2NC(NCy)2]Na in a 1:1 molar ratio in THF affor-
ded the analogous [(Me3Si)2NC(NCy)2]Er(BH4)2(THF)2
(2).
Fig. 2. The molecular structure of 2.
and two THF molecules. The N(1)–N(1A)–B(1)–B(1A)
four atoms are coplanar with the Ln atom. The overall
geometry around the Ln atom can be described as a dis-
torted octahedron, with N(1)–N(1A)–B(1)–B(1A) atoms
defining the equatorial plane and O(1) and O(1A) atoms
being in axial positions. The two THF rings were bended
away from the bulky guanidinate ligand. The four-mem-
ber-ring formed by the guanidinate ligand and the lantha-
nide atom is a complete plane. The two C–N distances
within a chelating guanidinate ligand are equivalent. They
Ln(BH4)3(THF)3+[(Me3Si)2NC(NCy)2]Na
THF
[(Me3Si)2NC(NCy)2]Ln(BH4)2(THF)2+NaBH4
(Ln=Yb(1), Er(2))
˚
are shorter than the C–N single bond length (1.47 A), but
significantly longer than the C@N double bond length
˚
The molecular structures of 1 and 2 are shown in Figs. 1
and 2, respectively. The selected bond parameters are listed
in Table 1. 1 is isostructural with 2. The lanthanide ion is
bonded by an anionic guanidinate ligand, two BHꢁ4 ligands
(1.29 A), and consistent with a p-electron-delocalized gua-
˚
nidinate ligand. The Yb–N bond lengths (2.3044(18) A) in
complex 1 are consistent with those in [(SiMe3)2NC(N-
˚
Cy)2]2Yb[N(SiMe3)2] (2.301(15) to 2.329(13) A) [12a] and

F. Yuan et al. / Journal of Organometallic Chemistry 691 (2006) 3377–3382
3379
Table 1
˚
Selected bond lengths (A) and angles (ꢁ) for 1 and 2
˚
˚
Bond length (A)
Bond length (A)
Ln(1)–N(1)
Ln(1)–B(1)
1
2
1
2
2.3044(18)
2.534(3)
1.465(3)
2.328(2)
2.559(4)
1.461(3)
Ln(1)–O(1)
N(1)–C(1)
N(2)–C(1)
2.3745(17)
1.337(2)
1.434(4)
2.3864(19)
1.336(3)
1.443(5)
N(1)–C(2)
Angle (ꢁ)
Angle (ꢁ)
N(1)–Ln(1)–N(1A)
O(1)–Ln(1)–N(1)
O(1)–Ln(1)–B(1)
O(1)–Ln(1)–O(1A)
N(1)–C(1)–N(2)
57.82(9)
87.71(6)
89.76(9)
173.78(8)
123.56(13)
57.41(10)
87.61(7)
89.67(10)
173.31(9)
123.17(16)
B(1)–Ln(1)–B(1A)
O(1)–Ln(1)–N(1A)
O(1)–Ln(1)–B(1A)
N(1)–C(1)–N(1A)
N(1A)–C(1)–N(2)
102.78(14)
86.85(6)
94.12(9)
112.9(3)
123.56(13)
102.97(17)
86.53(7)
94.49(11)
113.7(3)
123.17(16)
[(SiMe3)2NC(NiPr)2]2Yb(l-Cl)2Li(THF)2] (2.295(3) to
of sodium bis(trimethylsilyl)amide and N,N0-dicyclohexyl-
carbodiimide in THF afforded the THF-solvated sodium
guanidinate of {[(Me3Si)2NC(NCy)2]Na(THF)}2 (3). The
molecular structure of 3 is shown in Fig. 3. The selected
bond lengths and angles are listed in Table 2. In contrast
to the trimeric {[(Me3Si)2NC(NCy)2]Na}3, complex 3 is
dimeric in the solid state. Each Na atom is bound to three
nitrogen atoms from two guanidinate groups and one
oxygen atom from the THF molecule as formally four-
coordinate. The two planes of N(1)–Na(1)–N(2) and
N(1A)–Na(1A)–N(2A) are approximately parallel. The
N(1)–Na(1)–N(1A)–Na(1A) four atoms construct a plane.
This quadrilateral is nearly vertical to the planes of N(1)–
Na(1)–N(2) and N(1A)–Na(1A)–N(2A). The overall geom-
etry of 3 is like two steps of a flight. Different from those in
˚
2.332(3) A) [13]. The Ln–N bond lengths in 1 and 2 are
˚
very consistent, considering there is 0.02 A ionic radius dif-
ference between Er3+ and Yb3+
.
The important structural feature of complexes 1 and 2 is
the mode of attachment of the tetrahedral BH4 ligands. In
each complex, the two BH4 ligands involved are equivalent
and cis to each other. The Yb–BH4 distances in 1
˚
(2.534(3) A) are comparable with the Er–BH4 distances in
˚
˚
2 (2.559(4) A), if allowing 0.02 A for the difference of ionic
radii of Er3+ and Yb3+. These Ln–BH4 distances are
significantly shorter than the corresponding Ln–g2-BH4
˚
distances in Y(BH4)3(THF)3 (2.68(2) A) [5] and (CH3O-
CH2CH2C9H6)2Y(BH4) (2.693(8) A) [19]. The short dis-
˚
tances are characteristics of tridentate BH4 ligands. Hence,
all the boron atoms in the two complexes are linked to the
metal center via three bridging hydrogen atoms.
˚
{[(Me3Si)2NC(NCy)2]Na}3 (2.335(7) to 2.726(7) A), the
Na–N bond lengths in 3 change little (2.400(3) to
˚
2.463(3) A). The C(1) atom deviates a little from the
2.2. Structure of the solvated sodium guanidinate complex
N(1)–Na(1)–N(2) plane, resulting in the inclination of the
Me3Si(1) group toward the Na(1) atom. But the orientation
of (Me3Si)2N group relative to the corresponding N–Na–N
plane is still vertical. The two C–N bond distances (1.329(3)
Anionic guanidinate ligand, ðMe3SiÞ NCðNCyÞꢁ, has
2
2
long been introduced into the lanthanide complexes [12a].
The structures of solvent-base-free alkali-metal guanidi-
nates have also been reported [20]. We found that reaction
˚
and 1.315(3) A, respectively) in a guanidinate ligand are a
little shorter than those in 1 and 2 (1.337(2) and
˚
1.336(3) A, respectively).
(Me3Si)2NNa+CyN=C=NCy
THF
{[(Me Si) NC(NCy) ]Na(THF)}
2
0.5
3
2
2
(3)
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for 3
˚
Bond length (A)
Na(1)–O(1)
Na(1)–N(2)
N(1)–C(1)
N(2)–C(1)
N(3)–C(1)
2.310(2)
2.400(3)
1.329(3)
1.315(3)
1.469(3)
Na(1)–N(1)
Na(1)–N(1A)
N(1)–C(2)
2.463(3)
2.423(3)
1.458(3)
1.453(3)
N(2)–C(8)
Angle (ꢁ)
O(1)–Na(1)–N(1)
O(1)–Na(1)–N(1A)
N(1)–Na(1)–N(1A)
C(1)–N(1)–Na(1)
C(1)–N(2)–Na(1)
N(1)–C(1)–N(3)
124.45(10)
124.16(10)
110.75(7)
89.39(16)
92.50(16)
121.3(2)
O(1)–Na(1)–N(2)
N(1)–Na(1)–N(2)
N(2)–Na(1)–N(1A)
Na(1A)–N(1)–Na(1)
N(1)–C(1)–N(2)
111.87(10)
55.20(8)
105.35(9)
69.25(7)
116.9(2)
121.8(2)
N(2)–C(1)–N(3)
Fig. 3. The molecular structure of 3.

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F. Yuan et al. / Journal of Organometallic Chemistry 691 (2006) 3377–3382
Table 3
MMA-polymerization with complexes 1 and 2a
Entry
Catalyst
[MMA]/[Cat]
Temperature (ꢁC)
Time (h)
Yield (%)
Mn · 10ꢁ4
Mw/Mn
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
1
1
200
200
300
300
300
300
400
600
300
300
0
0
0
15
30
45
0
0
0
15
1.0
1.5
1.5
1.5
1.5
2.5
1.5
2.5
1.5
1.5
47.8
60.5
47.4
41.6
33.4
20.4
34.4
27.5
40.9
36.9
2.59
2.58
2.89
2.37
2.16
1.71
3.33
3.77
1.51
1.36
2.66
2.54
2.75
2.89
2.42
1.50
2.65
2.15
2.21
1.88
10
a
Polymerization condition: in toluene; [MMA] 4.644 mol Æ Lꢁ1
.
tal analyzer. Lanthanide metal analyses were carried out by
complexometric titration. The IR spectra were recorded on
a Magna 550 spectrometer. MMA was dried over CaH2
and distilled. 1H NMR spectra were obtained using a Unity
Inova-400 spectrometer. The molecular weights and molec-
ular weight distributions of polymers were determined by a
Waters 1515 gel permeation chromatography (GPC).
2.3. Polymerization of MMA with 1 and 2
The polymerization results are listed in Table 3. Both 1
and 2 afford PMMA under mild conditions. The Er complex
is more active than the Yb complex. Under similar polymer-
ization conditions, the catalytic activities of complex 2 are
comparable with those of [(Me3Si)2NC(NiPr)2]2YN(iPr)2
[15] and [(N2NNTMS)Sm(BH4)]2 [N2NNTMS = (2-C5H4N)-
CH2N(CH2CH2NSiMe3)2] [8]. For example, using
[(Me3Si)2NC(NiPr)2]2YN(iPr)2 as the catalyst at [MMA]/
[Ln] = 200 at 0 ꢁC for 2 h, 58.3% of yield of PMMA was
obtained, while complex 2 can produce the PMMA in a
60.5% yield in 1.5 h (Table 3 entry 2). As [MMA]/[Ln] was
kept at 400, [(N2NNTMS)Sm(BH4)]2 gives a 50% yield of
PMMA at 0 ꢁC for 3 h, while complex 2 achieves a 34.4%
yield in 1.5 h (Table 3 entry 7). It is noteworthy that homo-
leptic guanidinate complexes are completely inert to the
polymerization. This demonstrates the presence of BH4
group in the catalyst is crucial for the polymerization. More-
over, the BH4 groups in 1 and 2 are located at terminal posi-
tions, while they link two samarium atoms as two bridges in
[(N2NNTMS)Sm(BH4)]2.
3.1. Synthesis of {[(Me3Si)2NC(NCy)2]Na(THF)}2 Æ
THF (3 Æ THF)
A flask was charged with N,N0-dicyclohexylcarbodii-
mide (18.06 g, 0.0875 mol), and THF (about 100 mL) was
condensed in. It was stirred to become a clear solution.
To the solution was added the THF solution (about
80 mL) of sodium bis(trimethylsilyl)amide (0.0875 mol).
The mixture was stirred at room temperature for 2 h, and
the product of 3 Æ THF was precipitated due to the solubil-
ity limit. It was collected by filtration and dried under vac-
uum (36.54 g, 0.0367 mol, 83.9%). The single crystals of
3 Æ THF were produced from the THF solution at room
1
temperature. H NMR (C6D6, d): 3.61 (s, 8H, THF), 3.43
The effect of temperature on polymerization is great. As
the temperature increases, the polymerization activity
decreases and the molecular weight of PMMA decreases
too. The lower catalytic activity and molecular weight at
higher reaction temperatures is due to more facile catalyst
deactivation processes at higher temperatures (e.g. back-
biting), which is normally found in MMA-polymerization
cases by organolanthanide complexes [3].
(d, 4H, unique Cy H), 1.80–1.00 (m, 40H + 8H, C6H10
and THF), 0.50–0.14 (m, 36H, SiMe3) ppm.
3.2. Synthesis of [(Me3Si)2NC(NCy)2]Yb(BH4)2(THF)2
(1)
A flask was charged with Yb(BH4)3(THF)3 (0.833 g,
1.92 mmol) and 3 Æ THF (0.956 g, 0.96 mmol), and THF
(about 25 mL) was condensed in. The reaction mixture was
stirred at 50 ꢁC and turned orange quickly. After being stir-
red for 12 h, the mixture was centrifugalized. The clear THF
solution was evaporated off under vacuum, and the residue
was extracted with diethyl ether. The red extract solution
was concentrated and kept at ꢁ20 ꢁC. Red crystals of 1
(0.75 g, 1.05 mmol, 54.7%) were produced. Anal. Calc. for
C27H64B2N3O2Si2Yb: C, 45.44; H, 9.04; N, 5.89; Yb, 24.25.
Found: C, 45.36; H, 8.98; N, 5.76; Yb, 24.18%. IR (KBr pel-
let, cmꢁ1): 2387 (w), 2293 (s), 2227 (m), 1633 (w), 1450 (s),
1350 (s), 1252 (s), 1184 (s), 1137 (s), 1071 (m), 1004 (s), 965
(s), 937 (s), 863 (s), 838 (s), 756 (m), 665 (m), 642 (m).
3. Experimental
All manipulations were carried out under an atmosphere
of argon using Schlenk techniques. Solvents were distilled
from sodium/benzophenone ketyl prior to use. Sodium
bis(trimethylsilyl)amide and N,N0-dicyclohexylcarbodii-
mide were purchased from Acros and used as received
without further purification. Ln(BH4)3(THF)3 (Ln = Yb,
Er) were prepared according to the literature procedures
[4,5]. Carbon, hydrogen and nitrogen analyses were carried
out by direct combustion on an EA1110-CHNSO elemen-

F. Yuan et al. / Journal of Organometallic Chemistry 691 (2006) 3377–3382
3381
Table 4
Details of the crystallographic data collection and refinement for 1–3
1
2
3 Æ THF
Empirical formula
Molecular weight
Temperature (K)
Wavelength (A)
Size (mm)
C27H64B2N3O2Si2Yb
713.65
193(2)
0.71070
0.49 · 0.40 · 0.30
Monoclinic
p2/n
C27H64B2N3O2Si2Er
707.87
193(2)
0.71070
0.30 · 0.20 · 0.18
Monoclinic
p2/n
C50H104N6Na2O3Si4
995.73
193(2)
0.71070
0.51 · 0.48 · 0.39
Monoclinic
c2/c
˚
Crystal system
Space group
˚
a (A)
9.2922(8)
11.2692(12)
17.594(2)
90
103.461(3)
90
1791.7(3)
2
1.323
9.2956(12)
11.2863(12)
17.618(2)
90
103.535(4)
90
1797.0(4)
2
1.308
24.665(4)
15.670(2)
17.111(3)
90
113.395(4)
90
6070.0(15)
4
1.090
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalcd. (g/cm3)
Absorption coefficient (mmꢁ1
F(000)
)
2.702
742
2.474
738
0.153
2192
h Range for collection (ꢁ)
Reflections collected
Independent reflections
Variables
Goodness-of-fit on F2
R, Rw [I > 2r(I)]
R, Rw (all data)
3.32–25.35
17,265
3290
189
1.075
3.32–25.35
16,904
3295
189
1.108
3.32–25.35
29,567
5554
288
1.116
0.0194, 0.0484
0.0198, 0.0487
0.0239, 0.0509
0.0256, 0.0517
0.0666, 0.1790
0.0737, 0.1848
3.3. Synthesis of [(Me3Si)2NC(NCy)2]Er(BH4)2(THF)2
(2)
by Fourier techniques. Atomic coordinates and thermal
parameters were refined by full-matrix least-squares analy-
sis on F2. All non-hydrogen atoms were refined anisotrop-
ically. The hydroborate hydrogen atoms were refined
isotropically and the other hydrogen atoms were intro-
duced in calculated positions. CCDC-273907 (for 1),
-274215 (for 2) and -274214 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk or from the
Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: (internat.) +44 1223 336
033; e-mail: deposit@ccdc.cam.ac.uk].
The procedure followed was similar to that for 1. Com-
plex 2 was obtained as pink crystals in a 58.6% yield. Anal.
Calc. for C27H64B2N3O2Si2Er: C, 45.81; H, 9.11; N, 5.94;
Er, 23.63. Found: C, 45.74; H, 8.86; N, 5.85; Er, 23.48%.
IR (KBr pellet, cmꢁ1): 2293 (s), 2227 (m), 1636 (w), 1444
(s), 1344 (s), 1298 (m), 1250 (s), 1174 (s), 1137 (s), 1061 (m),
1004 (s), 965 (s), 941 (s), 863 (s), 804 (s), 756 (m), 642 (m).
3.4. Typical procedure for the polymerization reaction
A flask equipped with a magnetic stirring bar, to which
were added the catalyst and toluene solvent, was then
placed in a thermostatic bath. After some time, the mono-
mer was added into the flask using a syringe. The contents
of the flask were stirred for a determined time. The reaction
was quenched by addition of ethanol containing hydro-
chloric acid. The polymer was washed with ethanol con-
taining acid, dried under vacuum at 45 ꢁC and weighed.
Acknowledgements
We thank the Qinglan Foundation of Jiangsu Province
(HB 2001-27) and the Foundation of Educational Commis-
sion of Jiangsu Province (04KJD560171) for financial
support.
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