Study of the supramolecular Architecture-Directed synthesis of a Well-Defined Triple-Chain ladder polyphenylsiloxane

Article abstract of DOI:10.1021/ma100145j

High molecular weight (M_w) triple-chain ladder polyphenylsiloxane (TCLP) was synthesized by a supramolecular architecture-directed approach. First,. a bis(phenyldihydroxysiloxy) dimethoxysilane ladder monomer was self-assembled via hydrogen bonding interactions in acetonitrile/toluene (1:1, v/v) solution to form a ladder superstructure (LS). Then the LS was used as a template to direct the whole polymerization process. Lyophilization and surface-enhanced synchronous growth polycondensation process of the LS gave a ladder dimethoxysiloxy-bridged polyphenylsiloxane (DCLP) with gaseous triethylamine as condensation catalyst. Then DCLP was hydrolyzed to form a triple-chain ladder superstructure (TCLS), which was further converted into the target TCLP via subsequent in situ dehydration condensation. The three ladder entities formed during the polymerization, that is, LS, DCLP, and TCLP, have been well characterized. X-ray diffraction shows two Bragg reflections representing the ladder width and ladder thickness, respectively. ²⁹Si NMR analysis illustrates narrow peaks with the peak width at half-height of 0.5-2.5 ppm for the repeat units of the entities, indicating fine ladder regularity. In addition, an investigation of the dependence of the intrinsic viscosity [η] on molecular weight (M _w) in Mark-Houwink-Sakurada equation gave the exponent factor α = 1.19, suggesting the target TCLP had a semirigid ladder structure. Meanwhile, high-resolution transmission electron microscopy observations showed a regular morphological structure for TCLP with a molecular width of ca. 1.4 nm. This value is quite close to the X-ray diffraction data. Dynamic mechanical analysis experiments also indicated TCLP has high storage modulus and high thermal stability.

Full text of DOI:10.1021/ma100145j

2130 Macromolecules 2010, 43, 2130–2136
DOI: 10.1021/ma100145j
Study of the Supramolecular Architecture-Directed
Synthesis of a Well-Defined Triple-Chain Ladder Polyphenylsiloxane
Zhongjie Ren,† Ping Xie,‡ Shidong Jiang,† Shouke Yan,*,† and Rongben Zhang*,‡
†State Key laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing
‡
100029, China, and Laboratory of Polymer Science and Material, Beijing National Laboratory for Molecular
Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received December 5, 2009; Revised Manuscript Received February 4, 2010
ABSTRACT: High molecular weight (Mw) triple-chain ladder polyphenylsiloxane (TCLP) was synthesized
by a supramolecular architecture-directed approach. First, a bis(phenyldihydroxysiloxy)dimethoxysilane
ladder monomer was self-assembled via hydrogen bonding interactions in acetonitrile/toluene (1:1, v/v)
solution to form a ladder superstructure (LS). Then the LS was used as a template to direct the whole
polymerization process. Lyophilization and surface-enhanced synchronous growth polycondensation pro-
cess of the LS gave a ladder dimethoxysiloxy-bridged polyphenylsiloxane (DCLP) with gaseous triethylamine
as condensation catalyst. Then DCLP was hydrolyzed to form a triple-chain ladder superstructure (TCLS),
which was further converted into the target TCLP via subsequent in situ dehydration condensation. The three
ladder entities formed during the polymerization, that is, LS, DCLP, and TCLP, have been well character-
ized. X-ray diffraction shows two Bragg reflections representing the ladder width and ladder thickness,
respectively. 29Si NMR analysis illustrates narrow peaks with the peak width at half-height of 0.5-2.5 ppm
for the repeat units of the entities, indicating fine ladder regularity. In addition, an investigation of the
dependence of the intrinsic viscosity [η] on molecular weight (Mw) in Mark-Houwink-Sakurada equation
gave the exponent factor R = 1.19, suggesting the target TCLP had a semirigid ladder structure. Meanwhile,
high-resolution transmission electron microscopy observations showed a regular morphological structure for
TCLP with a molecular width of ca. 1.4 nm. This value is quite close to the X-ray diffraction data. Dynamic
mechanical analysis experiments also indicated TCLP has high storage modulus and high thermal stability.
Introduction
Supramolecular template-directed polymerization, which is of
particular interest in the field of polymer synthesis, provides an
High molecular weight (Mw) ladder polymers bring about the
persistent interests to scientists for their unique structures as well
as their superior mechanical, thermoresistant, and electrolumi-
nescent properties compared with their single chain counter-
parts.1 Among the ladder polymers, ladder polysiloxanes have
been especially emphasized due to their hybrid composite and
excellent resistances to thermal and irradiation degradations
caused by the stronger bond energy of Si-O linkages than that
of the common C-C and C-O ones.2-4 Brown et al.5 has first
reported a high Mw ladder polyphenylsilsesquioxane (Ph-LPSQ)
in the early 1960s. The reported structure was, however, refuted
later by Frye et al.,6 who indicated that the so-called Ph-LPSQ
was actually “partially opened polycyclic cages”, and Brook,7
who claimed that the reported high Mw ladder polysilsesquiox-
anes are generally random networks. The difficulty stems from
the fact that the preparation of ladder polysiloxane needs multi-
functional monomers and therefore requires precise configura-
tion control during polymerization process. Otherwise, cycliza-
tion and gelation reactions will take place, resulting in the
formation of very complicated products with cyclic, ladder,
cage and cross-linked structures.8 As proposed by Bailey,9 the
most desirable type of reaction for preparing perfect ladder
polymers is the one in which both sides of the ladder grow
synchronously. This is, however, quite difficult to control during
polymerization.
effective method to synthesize polymers with designed struc-
tures.10 As examples, while Stupp et al.11 has prepared the first
two-dimension planar polymer, Harada et al.12 has successfully
prepared the cyclodextrins-based tube polymers. Moreover, a
series of polymers with peculiar structures, such as nanosphere
and nanotube, have also been prepared by combining lyophiliza-
tion with surface confined synthesis.13-15 Taking this into ac-
count, we have put forward a supramolecular template strategy
named “supramolecular architecture-directed stepwise coupling
and polymerization” to realize the synchronous growth polym-
erization. As a result, a series of ordered double-chain ladder
polymers, such as, organo-bridged polysiloxanes and polysilses-
quioxanes, have been successfully prepared.16-21
It should be pointed out that triple-chain ladder polymers are
expected to have even superior chemical and physical properties
comparing to the double-chain ladder polymers. For example,
the well-known fiberlike collagen protein, which exhibitsa unique
barlike three-ply spiral structure formed through hydrogen
bonding between the polypeptides chains, shows excellent phy-
sical and mechanical properties (e.g., high-tensile strength etc.) in
addition to its important biochemical functions.22 Taking the
excellent properties into account, the triple-chain ladder poly-
mers may find many applications.23-25 Therefore, the synthesis
of triple-chain ladder polymers is of great significance both from
scientific and practical view points. Recently, Swarger et al.26
have achieved the synthesis of three-strand conducting ladder
polymers by two-step electropolymerization of metallorotaxanes.
Dehydration condensation-based synthesis of the triple-chain
*To whom correspondence should be addressed. E-mail: (S.Y.) skyan@
mail.buct.edu.cn; (Z.R.) zhangrb@iccas.ac.cn. Tel. þ86-10-6445 5928.
Fax þ86-10-6445 5928.
pubs.acs.org/Macromolecules
Published on Web 02/12/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 5, 2010 2131
ladder polysiloxane (TCLP) is, however, even more difficult than
the synthesis of the double-chain analogues considering hexa-
functional monomers should be used. From the viewpoint of
polymer chemistry, the preparation of triple-chain ladder poly-
mers requires precisely confined synthesis of multifunctional
monomer, which has not been touched so far. Herein, we report
the details about the supramolecular architecture-directed syn-
chronous growth polymerization for preparing the soluble high
Mw well-defined TCLP.
mode in a temperature range of -60 to 350 °C at a frequency of
1 Hz and 3 °C /min. Storage modulus and tg δ were recorded as a
function of temperature.
Synthesis. Preparation of Phenylsilanetriol.28 A mixture
of phenylamine (14.02 g, 150 mmol), deionized water (2.7 g,
50 mmol) and 350 mL of anhydrous ether was placed into a
500 mL round bottom flask in ice bath. Then phenyltrichloros-
ilane (10.58 g, 50 mmol) and 50 mL anhydrous ether were
added stepwise over 5 h. The reaction was continued for a
further 2 h at room temperature after addition of phenyltri-
chlorosilane. After removing the sediment through filtration,
the solution was then condensed and precipitated with n-hexane
Experimental Section
1
to give the white product (6.1 g, 78%). H NMR (400 MHz,
Materials. Toluene, 1,4-dioxane, hexane, and tetrahydrofur-
an (THF) were distilled over sodium and benzophenone;
triethylamine was distilled over sodium before use. All other
reagents were purchased without further purification.
CD3COCD3) δ (ppm): 7.44 (dd, 2H), 7.36 (q, 3H), 5.55 (s, 3H).
FT-IR (cm-1): 3000-3600 (Si-OH), 1430 (-Ph) and 907
(Si-OH). 29Si NMR (59.6 MHz, 1,4-dioxane), δ = -52.7 ppm.
Elem. Anal. Calcd for C6H8O3Si: C, 46.13; H, 5.16. Found: C,
46.21; H, 5.10.
Preparation of Dimethoxydichlorosilane.29 A mixture of tet-
ramethyl orthosilicate (51.7 mL, 348 mmol), silicon tetrachlo-
ride (40 mL, 348 mmol) was added into an autoclave and heated
to 150 °C for 30 h. After that, the mixture was then separated by
fractional distillation. The boiling point of the product is
100-102 °C (62.6 g, 61%). 1H NMR (400 MHz, CD3COCD3)
δ (ppm): 3.68 (s, 6H). 29Si NMR (59.6 MHz, 1,4-dioxane),
δ=-55.2 ppm. Elem. Anal. Calcd for C2H6O2Cl2Si: C, 14.91;
H, 3.75; Cl, 44.02. Found: C, 14.88; H, 3.82; Cl, 44.09.
Preparation of Monomer (M), Bis(phenyldihydroxysiloxy)-
Characterizations. Fourier transform infrared (FTIR) spectra
were recorded using a NICOLET spectrophotometer in the
range of 400-4000 cm-1
.
1H and 29Si NMR spectra were obtained at room temperature
by using Bruker DX400 and Bruker DX300 spectrometers at
400 MHz and 59.6 MHz, respectively, with tetramethylsilane as
the reference.
X-ray diffraction (XRD) was performed on a Rigaku D/max
2400 diffractometer with Cu KR radiation.
Elemental analysis was conducted by Heraeus CHN-RAPID,
DATEL System, Inc. (Germany).
Vapor pressure osmometry (VPO) analysis was measured in
dry toluene at 40 °C on a Knauner VPO instrument at 20%
relative humidity.
The DSC investigation was carried out on a Mettler Toledo
Star 822 differential scanning calorimeter at a heating rate of
10 °C/min under nitrogen atmosphere.
Thermogravimetric analysis (TGA) of TCLP was performed
on Perkin-Elmer TGA7 thermal analyzer under a 50 mL/min
nitrogen flow. The samples were heated from 25 to 800 °C at a
rate of 10 °C/min.
Laser light scattering (LLS) measurements were performed
on an ALV/DLS/SLS-5022F spectrometer equipped with a
multi-τ digital time correlator (ALV5000) and a cylindrical
22 mW UNIPHASE He-Ne Laser (λ0 = 632.8 nm) as the light
source. The spectrometer has a high coherence factor of β ∼ 0.95
because of a novel single-mode fiber optical coupled with an
efficient avalanche photodiode. The LLS cell was held in a
thermostat index matching vat filled with purified and dust-free
THF with the temperature controlled to within (0.02 °C. The
details of the LLS instrumentation and theory can be found
elsewhere.27
dimethoxysilane.
A mixture of phenylsilanetriol (1.35 g,
8.6 mmol) and 40 mL anhydrous 1.4-dioxane was added into
a three-neck flask protected by Argon atmosphere. The flask
was equipped with two constant pressure funnels, which con-
tain dimethoxydichlorosilane (0.644 g, 4 mmol), 15 mL of
1,4-dioxane, and triethylamine(0.808, 8 mmol), 15 mL of
1,4-dioxane, respectively. First, 1 mL of dimethoxydichlorosi-
lane solution was added dropwise at room temperature, and
then dimethoxydichlorosilane and triethylamine were added
dropwise at the same speed over 6 h. The reaction was continued
for another 2 h at the room temperature after addition. The
sediment was removed by filtration and then the solvent was
removed by reduced pressure at room temperature. After that,
the mixture was dissolved in ether and washed three times by
deionized water. The solution was dried over anhydrous sodium
1
sulfate to get the product (1.0 g, 81%). H NMR (400 MHz,
CD3COCD3) δ (ppm): 7.7 (dd, 4H), 7.36 (q, 6H), 5.53 (s, 6H),
3.53 (s, 6H). 29Si NMR (59.6 MHz, 1,4-dioxane), δ = -62.4,
-71.7 ppm. Elem. Anal. Calcd for C14H20O6Si3: C, 45.62; H,
5.47. Found: C, 45.75; H, 5.39.
Gel permeation chromatography (GPC) analysis was also
performed by a set of a Hitachi/Merck L-7100 pump, a Waters
2414 refractive index detector, and a Waters 486 ultraviolet
detector, the combination of Hersteller MZ-Gel SDplus 5 μm,
porosity 100 A, 10 A, 10 A, and 10 A. THF with 1 g/L LiBr
was used as eluent at flow rate of 1 mL/min at 35 °C.
Elemental analysis was conducted by Heraeus CHN-RAPID,
DATEL System, Inc. (Germany).
The viscosity was determined with Ubbelohde viscometers
using toluene as solvent at 23 °C. The flow time of pure solvent
was maintained at least 100 s and thus the kinetic correction
could be negligible.
For transmission electron microscopy (TEM) observation,
1% solution of TCLP in toluene was dropped onto the carbon-
coated mica surface. After evaporation of the solvent, the
carbon film together with the sample was then floated onto
the surface of distilled water and transferred onto 400 mesh
copper grids for TEM observation. A JEOL JEM-2100F TEM
operated at 200 kV was used in this study.
Preparation of Dimethoxysiloxy-Bridged Double-Chain Lad-
der Polyphenylsiloxane (DCLP) Precursor via Self-Assembly and
Surface-Enhanced Synchronous Growth Polycondensation. A
saturated solution of bis(dihydroxyphenylsiloxy)dimethoxy-
silane in toluene/acetonitrile (1/1) (0.25 mol/L) was slowly
stirred at 0 °C for about 15 min for self-assembling the bis-
(dihydroxyphenylsiloxy)dimethoxysilane into ladder super-
structure. Then, 0.5 mL of this solution was added into a
250 mL flask. The flask was rotated rapidly to spread the sample
solution on its inner surface. Following that, the flask was
frozen instantly. After removal of the solvent under a vacuum,
ladder superstructures were coated on the inner surface of the
flask as a thin solid layer. The solid phase polycondensation of
the ladder superstructure was then initiated when TEA vapor
(catalyst) was drawn into the flask at room temperature. The in
situ surface enhanced confined polycondensation was allowed
to proceed for 48 h. Subsequently, the in situ polymerized
product on the inner surface of reaction flask was dissolved in
toluene/acetonitrile (1/1) and the dehydration polycondensa-
tion was continued for another 20 h at 45 °C in the presence of
several drops of TEA. The byproduct water was removed by
azeotropic distillation under reduced pressure to increase the
3
4
6
˚
˚
˚
˚
Dynamic mechanical analysis (DMA) was performed on the
sample of 5 ꢀ 5 ꢀ 1 mm in size using a dynamic mechanical
analyzer from Mettler Toledo (DMA/SDTA861e) under shear

2132 Macromolecules, Vol. 43, No. 5, 2010
Ren et al.
Scheme 1. Synthetic Route to TCLP
molecular weight of the final polymer. The final mixture was
precipitated by methanol and filtered to give white solid DCLP
(47 mg, 94%). 1H NMR (400 MHz, CD3COCD3) δ (ppm):
7.7-7.4 (b, 4H), 7.4-7.25 (b, 6H), 3.56 (s, 6H). 29Si NMR (59.6
MHz, toluene), δ = -71.7, -78.5 ppm.
Preparation of the Target TCLP. A mixture of 1.6 g DCLP, 40
mL of THF, 1 mL of water, and 5d 0.1 mol/L hydrochloric acid
was stirred 24 h at room temperature, until the methyl group
absorbance peak disappeared in FTIR. The solvent then was
removed by reduced pressure. The product was dissolved
in toluene/acetonitrile (1/1) again. The solution was added into
1 mL of TEA and stirred for 24 h at room temperature, and
the polycondensation was then continued for another 24 h at
45 °C. Finally, the solution of trimethylchlorosilane (0.1 mL,
0.1 mol/L) in toluene was added, and the end-capping reaction
was allowed to proceed for 6 h. The final mixture was precipi-
tated by methanol and filtered to give white solid (1.47 g,
96%).1H NMR (400 MHz, CD3COCD3) δ (ppm): 7.7-7.4
(b, 4H), 7.4-7.25(b, 6H). 29Si NMR (59.6 MHz, toluene),
δ = -76.8, -109.4 ppm.
Figure 1. 29Si NMR of (a) ladder superstructure LS; (b) intermediate
dimethoxysiloxy-bridged double-chain ladder polysiloxane DCLP.
phenylsilanetriol. In this process, the feed ratio, the addition
order and speed of materials and the reaction temperature
are the important parameters for getting the desired mono-
mer. For this reaction, the phenylsilanetriol must be exces-
sive. Therefore, we set the feed ratio of phenylsilanetriol and
dimethoxydichlorosilane as 2.15:1. The excessive phenylsi-
lanetriol can be removed by washing with water. Moreover,
the dimethoxydichlorosilane and TEA solutions should be
added dropwise into the phenylsilanetriol solution simulta-
neously with the drop speed of TEA slightly slower than
that of dimethoxydichlorosilane to avoid the TEA induced
condensation of phenylsilanetriols or/and the produced
monomers M. In addition, the reaction under room tem-
perature with slow material addition speed is preferred.
The 29Si NMR spectrum of the obtained monomer M is
presented in Figure 1a. The appearance of two peaks at
δ = -62.4 and -71.7 ppm, respectively (as shown Table 1),
indicates the existence of two kinds of silicon atoms corre-
sponding to the two repeat structures of Ph(OH)2Si-O-
and -O(OCH3)Si(OCH3)O-, respectively.
Results and Discussion
As sketched in Scheme 1, the whole polymerization process
was realized through four steps including the following: (1) the
preparation of monomer bis (phenyldihydroxysiloxy) dimethoxy-
silane (M); (2) ladder monomer M was self-assembled via
hydrogen bonding interactions in acetonitrile/toluene (1:1, v/v)
solution to form a ladder superstructure (LS); (3) the LS, which
was used as a template to direct the whole polymerization
process, underwent first a lyophilization and surface-enhanced
synchronous growth polycondensation process to get ladder
dimethoxysiloxy-bridged polyphenylsiloxane (DCLP) with gas-
eous triethylamine as condensation catalyst; (4) the DCLP was
then hydrolyzed to form a triple-chain ladder superstructure
(TCLS), which was further converted into the target TCLP via
subsequent in situ dehydration condensation. In the following
paragraphs, we will describe each step in details.
Preparation of the Bis(phenyldihydroxysiloxy) Dimethoxy-
silane Monomer. The monomer bis(phenyldihydroxysiloxy)-
dimethoxysilane (M) was synthesized by dehydrochlorizan-
tion coupling reaction of dimethoxydichlorosilane with
Formation and Characterization of Self-Assembled Ladder
Superstructure (LS) in Solution. Kakudo and Watase et al.30,31
have reported a series of crystal structures of organosilanols,
which provided clear information regarding the H-bonding

Article
Macromolecules, Vol. 43, No. 5, 2010 2133
Table 1. Characterization Data of Three Ladder Entities, LS, DCLP, and TCLP
Wa/Tb by XRD (nm)
sample
DP/M
polydisperse index (PDI)
found
calcd.
δd/w1/2e (ppm) by 29Si NMR
LS
DCLP
TCLP
5/1,970f
-/66,028g
-/63,600g
1.33/0.41
1.34/0.45
1.36/0.43
-c
-62.4/0.5; -71.7/0.5
-71.7/0.5; -78.5/2.5
-76.4/2.5; -109.4/2.5
1.37h
1.42h
1.34/0.39
1.36/0.39
aW, the ladder width. b T, the ladder thickness. c -, absence. d δ, chemical shift. w1/2, peak width at half-height. f Mn determined by VPO. g Mw
e
determined by SLLS. h PDI determined by GPC.
Scheme 2. Schematic Illustration of Ladder Superstructure LS Align-
ment (a) in Solution and (b) on the Glass Surface
Figure 2. XRD spectra of (A) DCLP and (B) LS in different solvents
by lyophilization (a) toluene/acetonitrile, (b) acetonitrile, (c) dioxane/
acetonitrile, (d) dioxane, and (e) toluene/acetonitrile at 80 °C. The
inserted chart illustrates the ladder width (W) and ladder thickness (T)
of the LS.
or donor-electron capability of solvents.35,36 In this work, we
between Si-OH groups. They found that these crystal
focused on the influence of solvents on the regularity of
ladder superstructure. To achieve this, we chose four kinds of
solvents, that is, acetonitrile, 1,4-dioxane, acetonitrile/to-
luene (1:1, v/v), and acetonitrile/1,4-dioxane (1:1,v/v), which
may promote the formation of maximum hydrogen-bonding
according to the literature.35,37 The XRD patterns of the
samples lyophilized from different solutions with monomer
concentration of 0.25 mol/L and initially at 0 °C are shown in
Figures 2B(a-d). One can see that the ladder superstructure
self-assembled in acetonitrile/toluene (1:1, v/v) at 0 °C
showed a sharper and more intense peak in the small-angle
region than that self-assembled in 1,4-dioxane, acetonitrile,
and acetonitrile/1,4-dioxane solutions. This implies that LS
formed in acetonitrile/toluene (1:1, v/v) solution at 0 °C
possesses a higher regularity than those formed in other
solution. A possible reason is that acetonitrile/toluene sol-
vent has low donor-electron capacity and does not interrupt
the hydrogen-bonds of silanols. To demonstrate the fore-
going explanation, the acetonitrile/toluene solution of the
monomer was first heated to 80 °C and then lyophilized
quickly. As illustrated in Figure 2B(e), this treatment ap-
proximately leads to the disappearance of Bragg diffraction
corresponding to the ladder width. This indicates that hy-
drogen-bonded ladder superstructures have been greatly
destroyed upon heating the solution to 80 °C. According to
the above experimental results, we have chosen the acetoni-
trile/toluene (1:1, v/v) as solvent in the follow-up polymer-
ization process.
structures consist of molecular chains with square-planar
(parallelogram-type) hydrogen bonding between the two
nearest intermolecular Si-OH neighbors as linkages. It is
also confirmed that this type of Si-OH association exists
not only in crystalline phase but also in solution. Recently,
Gunji et al.32 have reported the crystal structure of 1,3-
diphenyl-tetrahydroxy-disiloxane, in which four Si-OH
groups face each other to form a ladder-type network caused
by the intermolecular H-bonding. On the basis of these
studies, it was proposed that ladder superstructure LS could
be formed from the self-assembly of the monomer M via
square-planar H-bonding interactions of silanols in solution
(Scheme 2).
To confirm the presence of LS in solution, we conducted a
series of controlled experiments. As a weak supramolecular
dynamic assembly based-on the hydrogen-bonding of
Si-OH groups, LS displayed an apparent molecular weight
of 1,970 measured by vapor pressure osmometry (VPO) at
40 °C, corresponding to ca. 5 repeat units of the monomer (as
shown Table 1). This suggests the existence of H-bonding
induced supramolecular aggregates. To further confirm the
formation of ladder superstructures, that is, LS shown in
Scheme 2, different solutions of the obtained monomer were
lyophilized by quick freezing in liquid nitrogen. In this way,
thin films on glass slides can be obtained by removing the
solvent under reduced pressure. X-ray diffraction (XRD),
which has been widely used in the structure analysis of ladder
polymers,33,34 was performed. As shown in Figure 2, the
XRD of the above lyophilized samples shows two distinct
diffraction peaks with 2θ at about 6.6 and 20.2°, representing
the ladder width (W) and thickness (T) of the ladder super-
structure, respectively.5 This result is very similar to that of
the usual ladder polymers16 and indicates the presence of LS
in solution.
Preparation of the Dimethoxysiloxy-Bridged Ladder Poly-
phenylsiloxane (DCLP) via Self-Assembly and Surface-En-
hanced Synchronous Growth Polycondensation. In situ
surface-confined polycondensation was an important step
for preparing well-defined ladder DCLP and consequent the
final target triple-chain polymer TCLP. At first, a thin layer
of the self-assembled ladder superstructure was coated on the
inner surface of the reaction flask via rotating the flask and
It is well-known that the H-bonding is very susceptible to
the ambient environment such as temperature and polarity

2134 Macromolecules, Vol. 43, No. 5, 2010
Ren et al.
lyophilization, and then polycondensation was initiated
when TEA vapor was drawn into the flask. The inner surface
condition of the reaction flask, for example, the hydrophilic
or hydrophobic property, was an important factor for the
polycondensation, influencing the adsorption of ladder
superstructure LS onto the inner surface. It was found in
our previous work,37 as far as the regularity of the resultant
DCLP was concerned, that the octadecyl-terminated inner
surface was poorly suited for the in situ surface-confined
polycondensation, whereas both the ordinary and the hydro-
xyl-terminated inner surfaces were similarly effective for the
polycondensation process. Thus, the ordinary inner surface
was chosen. Moreover, the rotating speed of the reaction
flask also affects the surface-confined polycondensation.
The continuous rotation of the flask led to the alignment
of the ladder superstructure along the rotation direction. A
faster rotating speed resulted in a better spreading of the
liquid phase and consequently a thinner layer of the ladder
superstructure. In principle, a thinner layer would be more
favorable for the fabrication of a polymer with high regu-
larity. It was found that a combination of a 500 mL reaction
flask, 0.5 mL of saturated solution of monomer M in
acetonitrile/toluene (1:1, v/v) (0.25 mol/L), rotating speed
of ca. 270 rpm, and a vacuum rate of 0.05 mbar during the
lyophilization process were optimal for this stage of poly-
condensation.
Compared with the usual solution polycondensation, the
in situ surface-confined polycondensation could efficiently
restrain the cyclization and random gelation side reactions.
This greatly reduced the fractions of cyclo-oligomers and
cages and increased the ladder regularity of the DCLP. The
product obtained at this stage of polycondensation was
HO-terminated DCLP oligomer. Under mild conditions,
the oligomers readily self-organized into a new extended
ladder superstructure. Therefore, a follow-up condensation
in toluene/acetonitrile (1/1) solution was carried out under
suitable conditions to further increase the molecular weight
of DCLP, while the ladder regularity of polymer was effec-
tively maintained.
Figure 3. XRD of the target triple-chain ladder TCLP. The inserted
chart is the molecular simulated TCLP by Chemoffice 2006.
of the methoxy groups. In addition, the 0.1 mol/L hydro-
chloric acid cannot break the formed Si-O-Si chains of
DCLP. FTIR measurement was conducted to follow the
kinetics of the hydrolysis process by monitoring the intensity
of characteristic absorption bands relevant to the Si-OMe
(2910, 2850 cm-1) and Si-OH (3300 cm-1) groups. After
hydrolysis for 24 h at 25 °C, the peak associated to the
Si-OH groups increased remarkably, while the peak corre-
sponding to the Si-OMe groups disappeared completely.
1
Also the HNMR signal of Si-OMe at δ = 3.6 ppm dis-
appeared. All these indicate unambiguously the completion
of the hydrolysis reaction. Because the condensation of
Si-OH groups unavoidably happened partly during the
hydrolysis process under acid condition, the triple-chain
ladder superstructure (TCLS) derived from DCLP cannot
be separated.
Follow-up in situ condensation of Si-OH groups was
conducted in acetonitrile/toluene (1:1, v/v) and catalyst
TEA. The mixed solvent acetonitrile/toluene (1:1, v/v)
favors the formation of ladder superstructure TCLS through
hydrogen-bonding of the neighboring Si-OH groups. Since
the condensation of Si-OH groups was carried out in a
confined environment composed by two ladder Si-O-Si
chains, new triple-chain ladder polymer structure could form
effectively. After a 48 h polycondensation, the peak asso-
ciated to the Si-OH groups disappeared almost completely
in FT-IR spectrum. A small peak at ca. 3600 cm-1 is
associated to free terminal Si-OH groups. These terminal
Si-OH groups were then end-capped by adding trimethyl-
chlorosilane to avoid cross-linking of the molecular chains.
Characterization of TCLP. As the target triple-chain
ladder polymer is prepared via hydrogen bonding-aided in
situ dehydration condensation of triple-chain ladder super-
structure (TCLS), the structure of the final TCLP has a close
resemblance of the ladder superstructure LS obtained in
solution. As shown in Figure 3, the XRD profile of the final
TCLP exhibits also two distinct peaks at 2θ around 6.3
and 19.4°. These two peaks correspond to the ladder width
(W = 1.36 nm) and thickness (T = 0.46 nm), respectively, as
predicted by the molecular simulation with Chemoffice 2006
simulation software. If TCLP has a cis-isotactic structure,
the adjacent phenyl groups are arranged on the same side
of the ladder backbone and the width of the ladder is about
1.36 nm (inset of Figure 3), which is in excellent agreement
with the XRD result. However, if TCLP has a cis-syndio-
tactic structure as shown in Figure S10, the width of the
ladder is about 1.51 nm, which is inconsistent with the XRD
result. So it is reasonable that TCLP is of cis-isotactic
configuration.
As shown in Figure 1b, the 29Si NMR spectrum of the
resulting DCLP consists of two peaks at δ = -71.7 with a
narrow peak width at half-height of 0.5 ppm and δ = -78.5
ppm with a narrow peak width at half-height of 2.5 ppm,
suggesting the existence of two kinds of silicon skeletons. The
former peak is assigned to Si-atom of the middle repeat
structure -O(OCH3)Si(OCH3)O-, and the latter is asso-
ciated to the side PhSiO3/2. In addition, the XRD profile of
DCLP (see Figure 1A) also display two distinct peaks at
2θ around 6.4° (ladder width, W = 1.34 nm) and 19.5°
(ladder thickness, T = 0.45 nm), respectively. XRD and 29Si
NMR results indicate the DCLP possesses a regular ladder
structure. Generally, the gel permeation chromatograph
(GPC) using polystyrene as the standard samples is only
suitable to characterize the flexible single-chain polymers.
For this reason, molecular weight of the DCLP was deter-
mined by static light scattering in THF solution as Mw
=
6.60 ꢀ 104. But polydisperse index still was determined by
GPC as shown in table 1.
Preparation of the Target Triple-Chain Ladder Polymer
(TCLP). The target TCLP was prepared via hydrogen
bonding-aided in situ dehydration condensation of triple-
chain ladder superstructure (TCLS) derived by hydrolysis of
DCLP. It is realized by hydrolyzing the methoxy groups of
the DCLP into the hydroxyl groups and follow-up conden-
sation of them. In this step, 0.1 mol/L hydrochloric acid and
THF were used to hydrolyze the methoxy groups into
hydroxyl groups. Polar solvent THF favors the hydrolysis

Article
Macromolecules, Vol. 43, No. 5, 2010 2135
Figure 6. TGA curve of TCLP.
Figure 4. 29SiNMR of TCLP.
Figure 5. DSC curve of TCLP.
In addition, the structural regularity of TCLP was further
confirmed by 29Si NMR analysis. It was known that the
higher the regularity of ladder polymer, the narrower the
resonance peak of silicon atoms in polymer skeleton.5,38 As
shown in Figure 4, the 29Si NMR spectrum of TCLP showed
two signals at δ = -76.4 ppm and δ = -109.4 ppm, corres-
ponding to the Ph-SiO3/2 (side silicon atom of triple-chain)
and SiO4/2 unit (middle silicon atom of triple-chain), respec-
tively. The narrow peaks with a half-height width of 2.5 ppm
indicate that the TCLP possess fine triple-ladder regularity.
Moreover, it is known that the glass transition temperatures
(Tg) can be used to reflect the rigidity and regularity of
ladder chain. The higher the Tg, the higher the ladder rigidity
and regularity. The Tg of TCLP measured by differential
scanning calorimetry (DSC) is ca. 158 °C (Figure 5), suggest-
ing great rigidity and fine ladder regularity.
The thermal decomposition temperature of TCLP is as
high as ca. 600 °C and the weight loss at 800 °C is equal to
27 wt % of the initial amount, showing excellent thermal
stability (Figure 6). However, the slight weight loss of
5% before 500 °C may be caused by condensation of
some remaining silanol groups of the defected polysiloxane
chains.
Figure 7. TEM image of TCLP.
molar mass.
½ηꢁ ¼ KMR
ð1Þ
K and R are constants for a given polymer-solvent system
at a given temperature. The exponent R is characteristic for
the polymer topology and ranges from R = 0 (solid spheres)
over R = 0.5 (random coil under θ conditions) to R = 2 (rigid
rod).39,40 The viscosity index (R) of the TCLP estimated by
the log [η]/log Mw plot (not shown here but can be found in
Supporting Information as Figure S11) is 1.19, indicating a
high stiffness of the ladder chain.
The high-resolution transmission electronic microscopy
image, as shown in Figure 7, shows clear features (three
parallel bright-dark alternative lines) of the extended ladder
chains of TCLP. The width of three parallel bright-dark
alternative lines, as marked by the arrows, is measured to be
1.4 nm, which is quite close to the ladder width of TCLP
estimated by the XRD. Therefore, these dark lines may
correspond to the stacked side phenyl groups and the main
ladder Si-O-Si chains, respectively. This result is similar to
that of the real double-chain ladder polytriphenylenesilses-
quioxane.19
The relationship between the intrinsic viscosity [η] and the
absolute molecular weight of the polymers was studied to
investigate the ladder conformational properties of the
TCLP. Usually, the Mark-Houwink-Sakurada equation
is used to express the intrinsic viscosity as a function of the
Dynamic mechanical analysis (DMA) is an efficient meth-
od to determine the relationship of polymer structure and
materials’ performance. Figure 8 shows the temperature

2136 Macromolecules, Vol. 43, No. 5, 2010
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taken at the maximum of tan δ peaks, is 167 °C. As expected,
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side phenyl groups or the Si-O-Si bonds. The value of E0 of
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in 29SiNMR spectrum was synthesized by a supramolecular
architecture-directed approach. First, bis(phenyldihydroxysiloxy)
dimethoxysilane was self-assembled to form ladder superstructure
(LS). Then the LS underwent first a lyophilization and surface-
enhanced synchronous growth polycondensation process to get
intermediate ladder dimethoxysiloxy-bridged polyphenylsiloxane
(DCLP). Finally, the in situ dehydration condensation of hydro-
gen bonded triple-chain ladder superstructure TCLS derived by
hydrolysis of DCLP gave the target TCLP. The confined environ-
ment has prevented other side reactions such as the cyclization and
gelation, which commonly occur in solution polycondensation of
silanol-containing monomers. This strategy could be suitable to
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Supporting Information Available: Characterization data of
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