A novel aryl amide-bridged ladderlike polymethylsiloxane synthesized by an amido H-bonding self-assembled template

Article abstract of DOI:10.1021/ja025650c

A novel soluble aryl amide-bridged ladderlike polymethylsiloxane (A-LPMS) was synthesized by stepwise coupling polymerization on the basis of amido H-bonding self-assembling template from monomer N, N′-bis(3-methyldiethoxylsilylpropyl)-[4,4′-oxybis(benzyl

Full text of DOI:10.1021/ja025650c

Published on Web 08/08/2002
A Novel Aryl Amide-Bridged Ladderlike Polymethylsiloxane
Synthesized by an Amido H-Bonding Self-Assembled
Template
Huadong Tang, Jin Sun, Jinqiang Jiang, Xiaoshu Zhou, Tengjiao Hu, Ping Xie, and
Rongben Zhang*
Contribution from the State Key Laboratory on Polymer Physics & Chemistry, Center for
Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received January 20, 2002
Abstract: A novel soluble aryl amide-bridged ladderlike polymethylsiloxane (A-LPMS) was synthesized
by stepwise coupling polymerization on the basis of amido H-bonding self-assembling template from
monomer N,N′-bis(3-methyldiethoxylsilylpropyl)-[4,4′-oxybis(benzyl amide)]. The monomer was prepared
in a high yield by the hydrosilylation reaction of template agent N,N′-diallyl-[4,4′-oxybis(benzyl amide)] with
methyldiethoxysilane in the presence of dicyclopentadienylplatinum dichloride (Cp2PtCl2) as catalyst. A
variety of techniques including 1H NMR, 13C NMR, 29Si NMR, FTIR, XRD, DSC, and, especially, static and
dynamic light scattering and viscosimetry were combined to confirm the presence of the ordered ladderlike
structure of polymer A-LPMS.
Fukuyama et al.4 again mentioned the preparation of so-called
PLPMS of an imaginary ladderlike structure, but no character-
Introduction
In general, ladderlike polysiloxane is divided into two
categories: oxygen-bridged polysiloxane (i.e., polysilsesqui-
oxane) and organo-bridged polysiloxane. As early as 1960,
Brown et al.1 first reported the synthesis of a soluble ladderlike
polyphenylsilsesquioxane (Ph-T) via “equilibration polymeri-
zation”. However, its structure was disputed 11 years later by
Frye and Klosowski. They concluded that the Ph-T is not a really
ladderlike polymer, but actually “partially opened polycyclic
cages”.2 The reason is that the reactive intermediates (phenyl-
silanetriols) form some randomly interlaced aggregates rather
than the ordered ladderlike ones due to the absence of template
assistance.
ization data were given. In recent years, the synthesis of
microstructure-controlled polymers based on a self-assembly
template of weak interactions (e.g., H-bonding, π-π stacking,
van der Waals force) has been investigated.5 Harata et al.6
synthesized a polymeric nanotube of cyclodextrins by linking
neighboring cyclodextrins prethreaded with poly(ethylene gly-
col) chain through H-bonding and van der Waals interactions.
Stupp et al.7 prepared two-dimensional polymers through
molecular recognition based on homochiral and π-π stacking
interactions. Most recently, Joe¨l et al.8 successfully prepared
long-range-ordered lamellar hybrid silica through an amido
H-bonding self-assembly.
Different from the oxygen-bridged polysilsesquioxane, or-
gano-bridged polysiloxane usually possesses better compatibility
with commercial polymers, improved mechanical properties, and
especially versatile structural adjustability. Thus, the synthesis
of this new kind of polymer has attracted great interest from
polymer chemists all over the world. Unfortunately, it has been
over 40 years since Andrianov et al.3 attempted to synthesize
1,4-phenylene-bridged ladderlike polymethylsiloxane (PLPMS)
in 1960. He obtained an insoluble cross-linked gel instead of
ladderlike polymer, because the reaction temperature (∼100 °C)
used is so high that the template effects of the π-π stacking of
bridged phenylene are entirely destroyed. Thirty years later,
In the light of these achievements, our group developed a
templating scheme to prepare oxygen-bridged ladderlike poly-
silsesquioxanes.9 It involves preaminolysis of trichlorosilane
with 1,4-phenylenediamine to form a coupled intermediate
capable of autoassociation to ladderlike superstructure through
H-bonding of amino and silanol groups, followed by hydrolysis
and polycondensation process. By this method, a series of
ladderlike polysilsesquioxanes (RSiO3/2)n (R ) alkyl, aryl, allyl,
(4) Oikawa, A.; Fukuyama, S.; Yoneda, Y. J. Electrochem. Soc. 1990, 137,
3223.
(5) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 5,
1154. (b) Bernt, I.; Bogge, H.; Carr, A. J.; Corbin, P. S.; Fujita, M. In
Molecular Self-assembly Organic Versus Inorganic Approaches; Fujita, M.,
Ed.; Springer-Verlag Berlin Heidelberg: New York, 2000.
(6) (a) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325. (b) Harada,
A.; Li, J.; Kamachi, M. Nature 1993, 364, 516. (c) Harada, A.; Li, J.;
Kamachi, M. Nature 1994, 370, 126.
(7) Stupp, S. I.; Son, S.; Lin, H. C.; Li, L. S. Science 1993, 259, 59.
(8) Moreau, J. J. E.; Vellutini, L.; Wong, M.; Man, C.; Bied, C.; Bantignies,
J.-L.; Dieudonne´, P.; Sauvajol, J.-L. J. Am. Chem. Soc. 2001, 123, 7957.
(9) (a) Zhang, R. B.; Dai, D. R.; Cui, L.; Xu, H.; Liu, C. Q.; Xie, P. Mater.
Sci. Eng. C 1999, 3, 13. (b) Liu, C.-Q.; Zhao, H.-T.; Zhang, Y.; Yuan, Q.;
Li, Z.; Xie, P.; Zhang, R.-B. Synth. Commun. 2000, 30, 1813.
* Corresponding author. Telephone: +86-10-62565612. Fax: +86-10-
62559373. E-mail: zhangrb@infoc3.icas.ac.cn.
(1) (a) Brown, J. F., Jr.; Vogt, L. H., Jr.; Katchman, A.; Eustance, K. W.;
Kiser, K. M.; Krantz, K. W. J. Am. Chem. Soc. 1960, 82, 6194. (b) Brown,
J. F., Jr. J Polym. Sci. Part C 1963, 1, 83.
(2) Frye, C. L.; Klosowski, J. M. J. Am. Chem. Soc. 1971, 93, 4599.
(3) (a) Andrianov, K. A.; Nikitenkov, V. E.; Kukharchuk, L. A.; Sokolv, N.
N. IzV Akad Nauk S S S R Otdel Khim Nauk 1958, 1004. (b) Andrianov,
K. A.; Nikimenkov, V. E. Vysokmol. Soeg. 1960, 2, 1099.
9
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J. AM. CHEM. SOC. 2002, 124, 10482-10488
10.1021/ja025650c CCC: $22.00 © 2002 American Chemical Society

A-LPMS Synthesized by a Self-Assembled Template
A R T I C L E S
Scheme 1. Synthetic Route to Template T and Monomer M
amino, and so on) have been synthesized.10 In recent years, we
also prepared some organobridged ladderlike polysiloxanes
including phenylene-bridged polysiloxane and its mesomorphic
polymers,11,12 diphenyl ether-bridged polysiloxane and aryl ester-
bridged polysiloxane via π-π stacking template.13,14 Although
these materials possess various interesting properties, their
structure imperfection and irregularity are noticeable due to the
weak template effect of phenylenediamine caused by fragile
amino H-bonding (N-H‚‚‚N) and weak π-π interactions. Most
recently, we reported the synthesis of ladderlike polysiloxane
based on a hydroquinone H-bonding self-assembling template.15
However, its regularity is still limited because of the easy
destruction of H-bonding via oxidation of hydroquinone. In
addition, its ladderlike structure was not fully confirmed, due
to lack of light scattering and viscosimetry investigation.
Obviously, efficient templates with strong secondary interactions
are extremely desirable for preparing highly ordered ladderlike
polysiloxanes.
It is well-known that the amido H-bonding (N-H‚‚‚OdC)
existing in natural or synthetic polyamide is much stronger and
stabler compared with the π-π stacking interaction. For
example, poly(p-phenylene terephthalamide) can form H-
bonding-based lyotropic liquid crystal in concentrated sulfuric
acid or N-methylpyrolidone (NMP), which has been spun into
a high-strength fiber (Kevlar) for use in bullet proof vests by
Du Pont Co. since the early 1970s. So far, many 1-, 2-, and
3-dimensional supramolecular arrays aggregated through amido
H-bonding interaction have been reported.16 Herein, we report
the design and preparation of a new template, N,N′-diallyl-[4,4′-
oxybis(benzyl amide)] (T), which possesses much stronger
intermolecular amido H-bonding and its use as a powerful
template to synthesize highly ordered aryl amide-bridged
ladderlike polymethylsiloxane (A-LPMS).
made with a RM-102 differential refractometer (Otsuka Electronic Co.
Ltd.) at a wavelength of 632.8 nm to give a value of dn/dc ) 0.073.
The FTIR measurement was performed with a Perkin-Elmer 80
spectrometer. 1H NMR, 13C NMR, and 29Si NMR measurements were
carried out on Varian Unity 200 (USA) operating at 200 MHz, using
deuterated dimethyl sulfoxide (DMSO-d6) as solvent. Chemical shift
δ was given in ppm, referenced to an internal standard, tetramethylsilane
(TMS, δ ) 0 ppm), for 1H or 13C NMR and an external standard,
hexamethyldisiloxne (MM, δ ) 6.9 ppm), for 29Si NMR. Chromium-
(III) acetylacetonate was used as a relaxation reagent. The XRD analysis
was recorded on a Rigaku D/MAX 2400 diffractometer. DSC experi-
ments were performed on a Mettler Toledo Star-822 differential
scanning calorimeter at a heating rate of 20 °C/min. MS measurements
were carried out on a BIFLEX III MALDI-TOF (matrix-assisted laser
desorption/ionization time-of-flight) mass spectrometer (Bruker Analyti-
cal System, Inc) utilizing a 5-chlorosalicylic acid matrix containing
sodium ions. Element analysis was measured with a Heraeus CHN-
RAPID DATEL System instrument. It is noteworthy that all experi-
ments should be conducted in a fume hood due to low toxicity of the
solvents and reagents used.
Synthesis of Template N,N′-Diallyl-[4,4′-oxybis(benzyl amide)]
(T). The synthetic route to template T and monomer M is shown in
Scheme 1. To a 100 mL flask were added 20 mL of THF, 1.7 mL
(0.022 mol) of allylamine, and 3.0 mL (0.022 mol) of triethylamine.
Then, 3.0 g (0.01 mol) of 4,4′-oxybis(benzoyl chloride) dissolved in
40 mL of THF was added dropwise into the flask over 3 h under stirring
at 0 °C. The reaction mixture was stirred at room temperature for an
additional 2 h. After removing THF, residual allylamine, and trieth-
ylamine by vacuum distillation, a residual yellowish solid was obtained.
The solid was washed with water and recrystallized twice in acetone
to give a white needlelike crystal of template T in 71% yield. Mp:
153.7 °C. FTIR (KBr, cm-1): 3314 (s, ν N-H), 3069-2969 (ν C-H),
1638 (vs, ν CdO), 1597, 1494 (ν CdCarom), 1245 (ν C-N), 877 (γ
C-H). 1H NMR (DMSO-d6): 9.0 (s, 2H, NH); 7.8, 7.0 (d, d, 4H, 4H,
C6H4); 5.0-5.2, 5.7-5.9 (m, 6H, CH2dCH); 3.8 (m, 4H, NHCH2).
Synthesis of Monomer N,N′-Bis(3-methyldiethoxylsilylpropyl)-
[4,4′-oxybis(benzyl amide)] (M). To a 100 mL Schlenk flask were
added 3.0 g (8.9 mmol) of template T and 5 mg of catalyst Cp2PtCl2.
The reaction system was vacuumized and refilled with argon, and this
process was repeated three times. Under the argon atmosphere, 50 mL
of NMP and 3.1 mL (20 mmol) of methyldiethoxysilane were injected
into the system. The reaction mixture was stirred at 100 °C for 12 h.
Then after the NMP and residual methyldiethoxysilane were distilled
out under reduced pressure, monomer M was obtained in 93% yield
as a white, waxy solid. Characterization data for M are listed in Table
1.
Experimental Section
Materials. All the reagents and solvents were commercially available
and of analytical grade. Tetrahydrofuran (THF) was distilled from
sodium benzophenone complex. Methyldiethoxysilane and N-meth-
ylpyrrolidone (NMP) were dried with zeolite overnight and then distilled
twice prior to use. The catalyst of hydrosilation reaction, dicyclopen-
tadienyldichloroplatinum (Cp2PtCl2), was prepared according to a
literature report.17
Techniques. Light scattering measurements (SLS and DLS) were
performed with a DLS-700 apparatus (Otsuka Electronic Co. Ltd.)
equipped with He-Ne laser source (10 mW, λ ) 632.8 nm).
Determination of differential refractive index increment (dn/dc) was
(10) (a) Zhang, L.; Dai, D.; Zhang, R. Polym. AdV. Technol. 1997, 8, 662. (b)
Xie, Z.; Zhang, R. Chin. J. Polym. Sci. 1991, 9, 266. (c) Li, Z.; Cao, X.
Y.; Xu, H.; Xie, P.; Cao, M.; Zhang, R. B. React. Funct. Polym. 1999, 39,
1. (d) Liu, C. Q.; Liu, Y.; Shen, Z. R.; Xie, P.; Zhang, R. B.; Yang, J. L.;
Bai, F. L. Macromol. Chem. Phys. 2001, 202, 1581. (e) Liu, C. Q.; Liu,
Y.; Shen, Z. R.; Xie, P.; Dai, D. R.; Zhang, R. B.; He, C. B.; Chung, T. S.
Macromol. Chem. Phys. 2001, 202, 1576.
(11) Cao, X. Y.; Wang, L. T.; Li, B.; Zhang, R. B. Polym. AdV. Technol. 1997,
8, 657.
(12) Liu, C. Q.; Liu, Y.; Kong, B.; Xie, P.; He, C. B.; Zhang, R. B. Liq. Cryst.
2001, 28, 35.
Synthesis of N,N′-Dipropyl-[4,4′-oxybis(benzyl amide)]-Bridged
Ladderlike Polymethylsiloxane (A-LPMS). Monomer M (3.0 g, 5
mmol) was dissolved in 30 mL of NMP. A mixed solution of 0.45 mL
(25 mmol) of water and 20 mL of NMP was added dropwise into the
monomer solution over 4 h under ice-water bath. The reaction mixture
was stirred at room temperature for several hours and was slowly
warmed to 40 °C for another 10 h to complete the hydrolysis reaction.
Then, a drop of concentrated H2SO4 was added into the mixture to
catalyze the condensation reaction. The reaction system was stirred at
(13) Liu, C. Q.; Zhao, H. T.; Xie, P.; Zhang, R. B. J. Polym. Sci. Part A: Polym.
Chem. 2000, 38, 2702.
(14) Liu, C.; Liu, Z.; Liu, Y.; Xie, P.; Zhang, R. Polym. Int. 2000, 49, 1658.
(15) Guo, G.; Zhang, Y.; Li, H.; Xie, P.; Zhang, R. Macromol. Rapid Commun.
2002, 23, 366.
(16) (a) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem.,
Int. Ed. 2001, 40, 988. (b) Lawrence, D. S.; Jiang, T.; Levett, M. Chem.
ReV. 1995, 95, 2229.
(17) Apfel, M. A.; Finkelann, H.; Janini, G. M.; Laub, R. J.; Luˆhmann, B.-H.;
Price, A.; Roberts, W. L.; Shaw, T. J.; Smith, C. A. Anal. Chem. 1985, 57,
651.
9
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A R T I C L E S
Tang et al.
Scheme 2. A Proposed Mechanism for the Synthesis of Polymer A-LPMS
Table 1. Characterization Data of Monomer M
(100 °C) to destroy this coordination, and consequently, the
monomer M was obtained in a high yield of 93%.
It can be found from H NMR data of monomer M (Table
characterization items
results
1
appearance
solubility
white waxy solid
highly soluble in polar solvents:
1) that the above-mentioned hydrosilylation reaction leads to
the almost exclusive formation of â-adduct product. The reason
for the absence of R-adduct is possibly the use of highly polar
solvent (NMP) and the higher steric hindrance of CH3SiH(OEt)2
in the formation of R-adduct than â-adduct.19 In addition, 13C
NMR data listed in Table 1 show similar results that there is
only the â-adduct resonated at 18.4 (SiCH2CH2-), 11.0
(SiCH2CH2-), and -4.8 ppm (SiCH3) without any other
byproducts. Moreover, it is shown from the 29Si NMR data
(Table 1) that only â-adduct product CH3(OEt)2SiCH2CH2-
(δ ) -4.114 ppm) is produced. The same conclusions can also
be obtained from FTIR spectrum, MALDI-TOF measurement,
and element analysis of monomer M, as shown in Table 1. There
is no diffraction peak for monomer M in XRD measurement,
suggesting that the monomer is an amorphous solid. In a word,
it is demonstrated from FTIR,1H NMR, 13C NMR, 29Si NMR,
XRD, element analysis, and MALDI-TOF results that the
monomer M obtained is the expected pure template compound.
Synthesis of the Titled Polymer A-LPMS. A proposed
mechanism for the formation of the polymer A-LPMS is shown
in Scheme 2. It should be emphasized that keeping the
H-bonding template in play throughout the whole synthetic
process is absolutely essential to the highly ordered, ladderlike
structure of polymer A-LPMS. So in the hydrolysis step, water
should be added very slowly at low temperature to gradually
hydrolyze the monomer M, resulting in the formation of
intermediate 1, whose silanols groups (tSi-OH) can form the
stable square-planar H-bonding proved by Kakudo.20 Then, via
strong H-bonding interactions of CdO‚‚‚H-N and Si-OH
themselves, a speculative extended H-bonding self-assembled
ladderlike supramolecular structure 2 is formed, which was
condensed with concentrated H2SO4 as catalyst to form lad-
acetone, THF, NMP, etc.
FTIR (cm-1
(KBr)
)
3315 (s, ν NH); 2972, 2879 (ν CH);
1638 (vs, ν CdO); 1597, 1495 (ν CdCarom);
1245 (ν CN); 1170, 1079 (ν SiOEt);
878 (γ CHarom); 799 (ν SiMe)
1H NMR
(δ in ppm)
9.0 (s, 2H, NH); 7.8, 7.0 (d, d, 4H, 4H, C6H4);
3.8 (q, 8H, OCH2); 3.4 (m, 4H, NHCH2);
1.6 (m, 4H, SiCH2CH2); 1.1 (t, 12H, OCH2CH3);
0.7 (t, 4H, SiCH2); 0.1 (S, 6H, SiCH3)
165.9 (CdO), 158.4 (Ph), 130.3 (Ph), 129.6 (Ph),
118.3 (Ph), 57.5 (OCH2), 42.3 (NHCH2),
22.9 (OCH2CH3), 18.4 (SiCH2CH2),
11.0 (SiCH2), -4.8 (SiCH3)
13C NMR
(δ in ppm)
29Si NMR
(δ in ppm)
XRD
-4.114 CH3(OEt)2SiCH2CH2
no diffraction peaks
MALDI-TOF MS calcd for C30H48N2O7Si2 (M+) 604.3,
found 627.5 (M + Na)+
elemental
analysis
Calcd for C30H48N2O7Si2: C, 59.57; H, 8.00;
N, 4.63. Found: C, 59.48; H, 7.86; N, 4.72.
40 °C for 72 h under reduced pressure to remove the water released.
Occasionally, a small amount of gel would appear during the condensa-
tion process, which could be removed by filtering or centrifugation.
For end-capping the terminal silanol of the polymer, 40 mg (0.5 mmol)
of pyridine and 55 mg (0.5 mmol) of trimethylchlorosilane mixed with
10 mL of NMP were added dropwise into the flask under stirring. The
mixture was stirred at room temperature for a further 24 h, followed
by the addition of 10 mL of water as precipitating agent. Finally, some
solids gradually emerged that were dried in a vacuum oven at 50 °C
for 24 h to give the titled polymer A-LPMS as a white solid in 57%
yield.
Results and Discussion
Synthesis of Monomer M. Monomer M was synthesized
via a hydrosilylation reaction of template T with methyldi-
ethoxysilane. It is known that an amido group can retard the
hydrosilylation reaction because it can slightly poison the
catalyst Cp2PtCl2 via coordination.18 Hence, as shown in Scheme
1, the reaction should be conducted at higher temperature
(18) (a) Michel, U.; Radecker, J. US Patent 4 530 989, 1985. (b) Sato, Y.;
Shiobara, T. US Patent 4 077 937, 1978.
(19) Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Pergamon
Press: Oxford, 1992.
(20) (a) Kakudo, M.; Watase, T. J. Chem. Phys. 1953, 21, 167. (b) Kakudo,
M.; Kasai, N.; Watase, T. J. Chem. Phys. 1953, 21, 1864. (c) Kasai, N.;
Kakudo, M. Bull. Chem. Soc. Jpn. 1954, 27, 605.
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A-LPMS Synthesized by a Self-Assembled Template
A R T I C L E S
Table 2. Light Scattering Data of the Polymer A-LPMS
no.
M
w
R
g
A
2
R
h
R /R
[η]
g
h
1
2
3
4
5
2.0 × 104
6.5 × 104
9.4 × 104
2.1 × 105
1.2 × 106
18.1
29.2
41.0
53.3
67.0
2.8 × 10-4
1.3 × 10-4
1.3 × 10-4
5.6 × 10-5
2.1 × 10-6
10.4
16.7
24.5
33.1
49.7
1.74
1.75
1.68
1.60
1.35
3.50
14.1
23.9
50.1
67.0
derlike oligomer and further condensed to ladderlike polymer
A-LPMS. To prevent the polymer from cross-linking, trimeth-
ylsilylation of terminal silanols was performed using trimeth-
ylchlorosilane as end-capper and pyridine as HCl absorbent.
The main factors affecting the regularity of the ladderlike
structure of polymer A-LPMS are discussed below.
Effect of Temperature on the Synthesis of Polymer
A-LPMS. It was found that the regularity of polysiloxane is
strongly affected by the reaction temperature. If the hydrolysis
reaction was carried out at high temperature (>60 °C) or water
was added too rapidly, some insoluble gel immediately emerged.
The reason is that the H-bonding is broken at high temperature,
resulting in destruction of the ladderlike superstructure 2 and
formation of randomly interlaced derivatives. The latter are
rapidly condensed into an insoluble three-dimensional network
of gels. For the same reason, although the ordered ladderlike
superstructure 2 has been formed, insoluble gels would be
generated if the final polycondensation reaction was conducted
at high temperature. Therefore, it is necessary to strictly control
the reaction temperature (usually not more than 50 °C) to
guarantee the H-bonding template effect throughout the whole
synthetic process.
Figure 1. The relationship between Rg/Rh and Mw of the polymer A-LPMS.
Effect of Catalyst on the Synthesis of the Polymer
A-LPMS. As we all know, the condensation reaction of silanols
can be promoted in the presence of acid or base as catalyst. As
mentioned above, the H-bonding of aryl amide groups is stable
under acidic conditions. Moreover, acidic catalyst usually has
stronger condensing ability than the basic one for the silanols
having electrodonating groups. Hence, to prevent the amido
group from decomposition and the H-bonding from breaking
by alkali catalyst, the sulfuric acid is chosen as the condensation
catalyst. The preferred proton concentration is found to be
10-2-10-3 mol L-1. In addition, since the polycondensation
reaction is a balance process between monomer and polymer,
the water of condensation reaction byproduct must be removed
timely from the reaction system in order to obtain a high
molecular weight polymer. Therefore, the reaction mixture must
be continuously stirred and evaporated under reduced pressure.
Figure 2. Molecular weight dependence of intrinsic viscosity [η] of the
polymer A-LPMS.
details of light scattering instrumentation were described
elsewhere.21 All samples for determination were carefully treated
by precipitation of polymer from NMP solution with distilled
water and further purified by reprecipitation from dimethyl
sulfoxide (DMSO) solution with methanol for the purpose of
narrowing the molecular weight distribution. Dusts and other
impurities were removed by pressure filtration [sintered glass
filter S2 for viscometric measurement and 0.2-µm microfilter
(Millipore) for light scattering measurement]. The Mw of the
polymer A-LPMS is measured by SLS instead of the usual GPC
method, because the polymer A-LPMS is a relatively stiff
wormlike macromolecule of a double-chained skeleton; never-
theless, the standard compound (polystyrene) usually used for
calibrating a GPC instrument is a flexible linear polymer that
takes a coil-like conformation in THF. The results of the light
scattering experiment are summarized in Table 2.
Effect of Medium Polarity on the Synthesis of Polymer
A-LPMS. To maintain a strong H-bonding template effect in
polymerization, nonpolar or low-polar solvent should be pre-
ferred. However, these solvents normally have a very poor
solvating power for the high polar reaction intermediates (1 and
2) and the final polymer A-LPMS. So a suitable solvent needs
to have a balance between the polarity and solvating power.
Since H-bonding-based poly(p-phenylene terephthalamide) can
keep strong lyotropic liquid crystallinity in NMP or concentrated
sulfuric acid, the polar aprotic NMP was chosen as the reaction
solvent.
More meaningfully, the molecular shape of A-LPMS in
solution can be described by a combination of the light scattering
and viscosimetry measurement. As shown in Table 2, both the
radius of gyration (Rg) and the hydrodynamic radius (Rh)
increase with increasing Mw, but the second virial coefficient
A2 decreases with increasing Mw. The Rg/Rh ratio is plotted
Characterization of Polymer A-LPMS. Investigation of
solution properties of the polymer A-LPMS was conducted by
light scattering (LLS and DLS) and viscosimetry methods. The
(21) (a) Chu, B. Laser Light Scattering; Academic Press: New York, 1991. (b)
Wang, X.; Xu, Z.; Wan, Y.; Huang, T.; Pispas, S.; Mays, J. W.; Wu, C.
Macromolecules 1997, 30, 7202.
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A R T I C L E S
Tang et al.
Figure 3. FTIR spectrum of the polymer A-LPMS.
against Mw in Figure 1. It has been proved that, for a given
polymer/solvent system, the Rg/Rh ratio mainly depends on the
chain architecture, conformation, and polydispersity.22 For
example, Rg/Rh values of 0.78, 1.24, and >2 stand for a hard
sphere,23 a Gaussian coil,24 and a rigid rodlike polymer,25
respectively. The comparatively large values of Rg/Rh ) 1.6-
1.8 indicate that the polymer A-LPMS has a rather extended
conformation in NMP solution with rare branches. Namely, the
chain of polymer A-LPMS is not like a flexible coil but like a
semirigid soft ladder. Bohidar et al.26 also found analogous
results in investigation on gelatin-glutaraldehyde supramo-
lecular structures, where the relatively high value of Rg/Rh )
1.5-1.6 indicates that the supramolecule mostly possesses
“double-strand” ladderlike structure, rather than a branched
random architecture. It was found that the Rg/Rh value becomes
smaller as Mw increases, showing that the A-LPMS molecule
has a tendency of contract with an increase of Mw. This is
consistent with the conclusion drawn by Josef27 as well as Fang28
that the rigidity of the ladderlike polysiloxane decreases as Mw
increases.
Similar results can be found in the viscosity investigation.
The viscosity of the titled polymer A-LPMS was determined
with Ubbelohde viscometers using NMP as solvent (concentra-
tion: 2-7 g L-1). The flow time of pure solvent was maintained
at least 100 s and thus the kinetic correction could be negligible.
The intrinsic viscosity of A-LPMS was determined by Heller’s
method.29 Figure 2 demonstrates the molecular weight depen-
dence of intrinsic viscosity, [η]. The obtained Mark-Houwink
equation is [η] ) 4.27 × 10-5 M1.15. It is well-known that the
conformation index (R) in the Mark-Houwink equation mainly
depends on the chain conformation in solution. For example,
under θ-conditions, R ) 0.5; for an extremely rigid rodlike
polymer, R ) 2.0; and for a flexible polymer, R ) 0.5-0.9.
Clearly, the higher value R ) 1.15 for the polymer A-LPMS is
an indication of the stiffness of the main chain. That is consistent
with the results of above-mentioned light scattering investiga-
tion. In a word, based on the investigation of solution properties
of the polymer A-LPMS, one can conclude that the A-LPMS
is a linear, semirigid polymer rather than a flexible, branched
macromolecule.
It can be seen from the FTIR spectrum of polymer A-LPMS
(Figure 3) that instead of two absorption peaks at 1107 and
1079 cm-1 assigned to the Si-OEt group of monomer M, a
very strong and broad adsorption at 1077.15 cm-1, correspond-
ing to vibration of the Si-O-Si linkage, was observed, which
indicates that the monomer M has been completely hydrolyzed
and condensed to a high polymer containing Si-O-Si bonds.
In addition, no vibration peak of silanol (3400-3600 cm-1) is
observed, thus indicating that the silylation end-capping reaction
is completely finished. It is noteworthy that the strong peaks at
3312.54 and 1639.95 cm-1assigned to an amido group confirm
the existence of strong interactions between the amido groups
located at the ladder rungs of the polymer A-LPMS.
As shown in the 1H NMR spectra of polymer A-LPMS
(Figure 4), the resonance peaks corresponding to Si-OEt groups
have disappeared, suggesting that the monomer M has been
completely hydrolyzed. The widened humplike peaks of aro-
matic hydrogen and other groups, arising from the high viscosity
of A-LPMS in deuterated DMSO, implies that the monomer
has been condensed to a relatively high molecular weight
polymer. Unfortunately, the coupling constants are not observed,
due to the limits of the NMR instrument used.
(22) Kirkwood, J. G.; Riseman, J. J. Chem. Phys. 1948, 16, 565.
(23) (a) Fixman, M. J. J. Chem. Phys. 1986, 84, 4085. (b) Sun, S. T.; Nishio,
T.; Swislow, G.; Tanaka, T. J. Chem. Phys. 1980, 73, 5971.
(24) (a) Freed, K. F.; Wang, S. Q.; Roovers, J.; Douglas, J. Macromolecules
1988, 21, 2219. (b) Pyun, C. W.; Fixman, M. J. J. Chem. Phys. 1964, 41,
937.
(25) (a) Huglin, M. B. Light Scattering from Polymer Solutions; Academic
Press: New York, 1972. (b) Riseman, J.; Kirkwood, J. G. J. Chem. Phys.
1950, 18, 512.
(26) Sharma, J.; Bohidar, H. B. Eur. Polym. J. 2000, 36, 1409.
(27) Kova´_r, J.; Mrkvie`kova´-Vaculova´, L.; Bohdanecky´, M. Makromol. Chem.
1975, 176, 1829.
(28) Zhengping, F.; De, L.; Anwei, Q.; Tongyin, Y. Chin. J. Polym. Sci. 1987,
4, 332.
Three important structural messages can be deduced from
Figure 5. First, there are only two distinct kinds of silicon atom
(29) Heller, W. J. Colloid Sci. 1963, 1, 83.
9
10486 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

A-LPMS Synthesized by a Self-Assembled Template
A R T I C L E S
Figure 4. 1H NMR spectrum of the polymer A-LPMS.
Figure 5. 29Si NMR spectrum of the polymer A-LPMS.
in the polymer A-LPMS (the peak at δ ) 6.900 ppm stands for
the resonance of the external standard hexamethyldisiloxne).
One kind of Si-atom at δ ) -18.881 ppm represents the internal
silicon atom in the backbone, CH3(CH2)SiO2/2, and the other
one at δ ) 12.4 ppm represents the terminal silicon atom
(CH3)3SiO1/2 of the trimethylsiloxyl end-capping group. This
result indicates that the polymer A-LPMS consists of two types
of structural units, CH3(CH2)SiO2/2 and (CH3)3SiO1/2, without
noticeable branched moieties. That is consistent with the
expected ladderlike structure of A-LPMS. Second, according
to the ratio value (R ) 42.1) of integrated areas of two peaks,
the number-averaged molecular weight (Mn) of A-LPMS can
be approximately estimated30 to be 4.2 × 104, which is close
to the value (Mn ) 5.1 × 104) calculated from weight-averaged
macromolecule. Third, it is known that the higher the regularity
of polymer, the narrower the resonance peak is. Obviously, the
narrow half-peak width (less than 1 ppm) at δ ) -18.881 ppm
of the main-chain unit (CH3(CH2)SiO2/2) of A-LPMS indicates
that A-LPMS has high structural regularity, excluding the
presence of the branched structures.
There are two distinct peaks in the XRD spectrum of polymer
A-LPMS shown in Figure 6. Referring to Brown,1 Andrianov,3
and Shi,32 the first peak (d1 ) 2.40 nm) representing the
intramolecular chain-to-chain distance (i.e., the width of lad-
derlike main chain) of the polymer A-LPMS is narrow and
sharp, implying that the polymer has a relatively regular
ladderlike skeleton. The value of d1 (2.40 nm) is approximately
(30) Williams, E. A.; Caryioli, J. D. In Annual Reports on NMR Spectroscopy;
Webb, G. A. Ed.; Academic Press: New York, 1979.
(31) Judith, C. B.; Puscy, P. N. J. Chem. Phys. 1975, 62, 1136.
(32) Shi, L. H.; Zhang, X. S.; Shi, Y. D.; Ye, M. L.; Li, D. C. Chin. J. Polym.
Sci. 1987, 5, 539.
molecular weight (Mw) by Judith’s method31 (here, Mw
)
6.5 × 104 measured by static light scattering), indicating that
the polymer A-LPMS is a mainly linear rather than branched
9
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A R T I C L E S
Tang et al.
Figure 6. XRD pattern of the polymer A-LPMS.
Figure 7. DSC curve of the polymer A-LPMS.
Conclusion
equal to the calculated intramolecular chain-to-chain distance
based on a molecular dynamic simulation (2.27 nm, simulated
by Alchemy 2000) of polymer A-LPMS. The second diffusing
peak (d2 ) 0.48 nm) possibly stands for the thickness of the
macromolecular chain or the intermolecular spacing between
the polymer molecules.
A novel soluble, high molecular weight organo-bridged
ladderlike polymethylsiloxane, A-LPMS, has been synthesized
for the first time through the use of an aryl amido H-bonding-
based self-assembly template. The polymer was characterized
by a combination of FTIR, H NMR, 29Si NMR, XRD, DSC,
1
To study the flexibility of the macromolecular chain of
A-LPMS, one can compare the glass transition temperature, Tg,
between the single-chained and ladderlike polysiloxanes. It is
well-known that the common single-chain poly(dimethylsilox-
ane) is very flexible, with Tg < -50 °C. However, the Tg of
A-LPMS is as high as 125.20 °C (Figure 7). That reveals that
the polymer A-LPMS has a relative stiff backbone. A possible
explanation for this discrepancy is that polymer A-LPMS has a
unique double-chain structure and a rigid ladder rung of strong
H-bonding interactions which remarkably restricts the internal
rotation of the Si-O-Si bond and the segment mobility of
polymer.
and, especially, light scattering and viscometry methods. The
results show that the polymer is a semirigid, highly ordered,
ladderlike macromolecule. Such an H-bonding-assisted synthetic
method can be utilized to prepare a variety of highly ordered
organo-bridged ladderlike polysiloxanes.
Acknowledgment. The authors thank The National Natural
Science Foundation of China (No. 29874034 & 20174047) and
Dow Corning Corp. for the financial support, and we greatly
appreciate Prof. Youhuai Wang for his helpful discussions and
suggestions.
JA025650C
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