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Full Paper
Received: 15 January 2009
Revised: 9 April 2009
Accepted: 9 April 2009
Published online in Wiley Interscience: 18 May 2009
(www.interscience.com) DOI 10.1002/aoc.1511
UV-activated hydrosilylation: a facile approach
for synthesis of hyperbranched
polycarbosilanes
Guo-Bin Zhanga,b, Jie Kongb∗ , Xiao-Dong Fanb , Xin-Gui Lic , Wei Tianb and
Mei-Rong Huangc
A facile approach for synthesis of hyperbranched polycarbosilane from AB2 monomer via UV-activated hydrosilylation
is presented in this communication. The polymerization process was monitored using real-time FTIR spectroscopy and
the resulting hyperbranched polycarbosilanes were characterized using 1 H-NMR, 13 C-NMR, 29 Si-NMR and SEC/MALLS. It is
found that hyperbranched polycarbosilane can be synthesized from methyldiallylsilane via UV-activated hydrosilylation with
bis(acetylacetonato)platinum(II) as catalyst. The polymerization activated by UV irradiation was much faster than that under
thermal conditions. The similar degree of branching, average number of branch units and the exponent of the Mark–Houwink
equation demonstrate that the hyperbranched polycarbosilane synthesized via UV-activated polyhydrosilylation possesses
c 2009 John
almost the same branching structure as that synthesized via thermal-activated polyhydrosilylation. Copyright Wiley & Sons, Ltd.
Keywords: hyperbranched; polycarbosilane; UV; hydrosilylation
Introduction
Appl. Organometal. Chem. 2009 , 23, 277–282
Experimental
Materials
Methyldichlorosilane (98%), bis(acetylacetonato)platinum(II)
[Pt(acac)2 ] and Karstedt’s catalyst (platinum and 1,3-divinyl-1,1,3,3tetramethyldisiloxane complex) were purchased from Alfa Aesar
∗
Correspondence to: Jie Kong, Northwestern Polytechnical University, Department of Applied Chemistry, Youyi west road 127#, Xi’an, Shaanxi, 710072,
People’s Republic of China. E-mail: mfkongjie@hotmail.com
a The Second Artillery Equipment Academy, Beijing 100085, People’s Republic of
China
b Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China
c Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering
Materials, College of Material Science and Engineering, Tongji University,
Shanghai 200092, People’s Republic of China
c 2009 John Wiley & Sons, Ltd.
Copyright 277
Since Mathias reported the synthesis of hyperbranched
poly(siloxysilanes) for the first time,[1] synthesis, modification
and application of these hyperbranched organosilicon polymers
have attracted tremendous interest in both fundamental and
applied research fields due to their unique structures and properties including low viscosity, multifunctionality and facility in
preparation.[2] So far, many hyperbranched organosilicon polymers, such as hyperbranched polycarbosilanes,[3] hyperbranched
polycarbosiloxanes[4] and hyperbranched polycarbosilazane, have
been designed and prepared[5] that show potential application
in extended fields, such as heat-resistant polymers,[3b,d,4e] catalyst carriers,[6] luminescent polymers[7] and magnetic ceramic
materials.[8]
These hyperbranched organosilicon polymers are mainly prepared via thermal-initiated polyhydrosilylation from ABx (x ≥ 2)
monomers which containing Si–H and carbon–carbon unsaturated groups. Karstedt’s catalyst or platinum on activated carbon
(Pt/C) is usually employed as the catalyst. However, thermalinitiated polyhydrosilylation often takes a long time, ranging
from hours to days, especially when the monomers employed
consists of allyl and Si–H groups. It is well known that ultraviolet (UV) irradiation can initiate a rapid hydrosilylation reaction
between Si–H and carbon–carbon unsaturated groups under
appropriate catalyst conditions. Up to now, UV-activated polyhydrosilylation has been applied in the synthesis of linear polycarbosilane from dimethylvinylsilane,[9] surface grafts of polybutadiene or polyphenylenevinylene derivatives with silanes.[10] If
the UV-activated hydrosilylation is employed to synthesize hyperbranched organosilicon polymers from ABx monomers, it will
provide a novel strategy to synthesize hyperbranched organosili-
con polymers ceramic precursors with facility in synthesis and low
energy consumption.
Herein, a facile approach for synthesis of hyperbranched
polycarbosilane via UV-activated hydrosilylation is presented.
The hyperbranched polycarbosilane was synthesized from AB2
monomer of methyldiallylsilane under UV irradiation and platinum
catalyst. The real-time FTIR spectroscopy was used to monitor the
polymerization process, and 1 H-NMR, 13 C-NMR, 29 Si-NMR and
size exclusion chromatography/multi angle laser light scattering
(SEC/MALLS) were employed to describe the branching structure
and molecular configuration of the resulting hyperbranched
polycarbosilanes.
G.-B. Zhang et al.
Scheme 1. The schematic synthesis route of hyperbranched polycarbosilane from AB2 monomer via UV-activated or thermal-activated polymerization.
Co. of China. Dimethylallylsilane (98%) was received from ABCR Co.
of Germany. Allyl chloride (AR) was purchased from Guoyao Chemical Regents Centre of China. Tetrahydrofuran (AR), magnesium
powders (AR), pyridine (AR), toluene (AR), hexane (AR), THF (AR),
magresium sulfate anhydrous, diethylether and acetonitrile were
all received from Tianjin Bodi Chemical Reagents Ltd of China. THF
used as the eluent for SEC/MALLS (HPLC grade) was purchased
from Dikma Tech (USA).
Synthesis of AB2 -type Monomer
The synthetic route of the AB2 monomer (methyldiallylsilane) was
similar to the literature.[12a] It was distilled at 44–46 ◦ C under
−0.095 MPa and obtained as colorless liquid in a 45% yield
(boiling point 121–122 ◦ C at ambient atmosphere[12a] ) (AB2 ). IR
(KBr): 2118 (Si–H), 3083, 1630 (CH CH2 ), 1255 cm−1 (Si–CH3 ). 1 HNMR (CDCl3 ): δ = −0.05–0.05 (d, 3H, 3 JHSiCH = 3.5 Hz, Si–CH3 ),
1.63–1.66 (m, 4H, 3 JHSiCH = 3.5 Hz, Si–CH2 ), 3.78–3.82 (oct, 1H,
3
JHSiCH = 3.5 Hz, Si–H), δ 4.80–4.85 (2H, CH CH2,trans ), 4.85–4.91
(2H, CH CH2,cis ), 5.73–5.86 (2H, CH CH2 ,). 13 C-NMR (CDCl3 ):
δ = −5.05 (SiCH3 ), 20.5 (Si–CH2 –CH CH2 ), 113.2 (–CH CH2 ),
134.8 (CH CH2 ). 29 Si–NMR (CDCl3 ): δ = −12.20. (C7 H14 Si)n (127)n :
Calcd. C 66.67, H 11.11, Found C 66.62, H 11.14.
Synthesis of Hyperbranched Polycarbosilane by UV-activated
Route
278
Novacure 2100 type High Pressure Mercury Vapor Short Arc
apparatus (100 W, EXFO Photonic Solutions Inc., USA) was
employed to generate UV light. UV light was firstly filtered by
a broadband filter (320–500 nm) and a pair of neutral density
filters (1 or 10%) and then was transmitted to the sample via dual
light guide. The distance between the liquid sample surface and
the end of the light guide was about 10 mm and the light intensity
was recorded.
A 1.26 g aliquot of methyldiallylsilane, 4 mg of Pt(acac)2 and
2.5 g of toluene were charged into a pre-dried 50 ml two neck
flask packed with aluminum foil and equipped with a magnetic
stirrer. The role of aluminum foils was to avoid UV irradiation
leakage from the flask. A water-bath cooling system was employed. The monomer and solvent mixture were irradiated under
www.interscience.wiley.com/journal/aoc
UV light with intensity of 110.9 mW/cm2 . The polymerization was
conducted for 40 min then toluene was eliminated in a rotary evaporator. The synthesized crude product was precipitated in ether
and acetonitrile several times and finally hyperbranched polycarbosilane was obtained as sepia viscous liquid in an 89% yield
(P1). IR (KBr): 3076, 1630 (CH CH2 ), 1250 cm−1 (Si–CH3 ). 1 H-NMR
(CDCl3 ): δ = −0.26–0.23 (Si–CH3 ), 0.33–0.77 (Si–CH2 CH2 CH2 –Si),
0.84–1.10 [Si–CH(CH3 )CH2 –Si], 1.19–1.47 (Si–CH2 CH2 CH2 –Si),
1.47–1.66 (CH2 –CH CH2 ), 1.76–1.93 [Si–CH(CH3 )CH2 –Si],
4.76–4.95 (CH CH2 ), 5.71–5.88 (CH CH2 ). 13 C-NMR (CDCl3 ):
δ = −6.0 to −4.5 (Si–CH3 ), 17.1–19.6 (Si–CH2 –CH2 –CH2 –Si),
21.2–23.2 (CH2 –CH CH2 ), 112.3–114.4 (CH CH2 ), 133.5–135.7
(CH CH2 ). 29 Si–NMR (CDCl3 ): δ = 0.18 (terminal unit), 0.65 (linear
unit), 0.96 (dendritic unit). Tg = −85.1 ◦ C.
Synthesis of Hyperbranched Polycarbosilane by the Thermalactivated Route
A 1.26 g aliquot of methyldiallylsilane, 8 mg of Karstedt’s catalyst and 2.5 g of toluene were charged into a pre-dried
50 ml two-neck flask packed with aluminum foil and equipped
with a magnetic stirrer. A water-bath cooling system was employed. The polymerization was conducted for 5–6 h under
50–60 ◦ C then toluene was eliminated in a rotary evaporator.
The synthesized crude product was precipitated in ether and
acetonitrile several times and finally hyperbranched polycarbosilane was obtained as sepia viscous liquid in a 92% yield (P2).
IR (KBr): 3080, 1630 (CH CH2 ), 1255 cm−1 (Si–CH3 ). 1 H-NMR
(CDCl3 ): δ = −0.17–0.16 (Si–CH3 ), 0.36–0.74 (Si–CH2 CH2 CH2 –Si),
0.94–1.03 [Si–CH(CH3 )CH2 –Si], 1.25–1.42 (Si–CH2 CH2 CH2 –Si),
1.49–1.61 (CH2 –CH CH2 ), 1.76–1.90 [Si–CH(CH3 )CH2 –Si],
4.78–4.94 (CH CH2 ), 5.73–5.87 (CH CH2 ). 13 C-NMR (CDCl3 ):
δ = −6.1 to −4.1 (Si–CH3 ), 17.3–20.1 (Si–CH2 –CH2 –CH2 –Si),
21.2–22.8 (CH2 –CH CH2 ), 112.2–113.7 (CH CH2 ), 134.5–135.6
(CH CH2 ). 29 Si-NMR (CDCl3 ): δ = 0.18 (terminal unit), δ 0.67 (linear
unit), δ 0.98 (dendritic unit). Tg = −91.5 ◦ C.
Synthesis of the Linear Analog Polycarbosilane
A 2.0 g aliquot of dimethylallylsilane (AB type monomer), 10 mg
of Karstedt’s catalyst and 2.5 g of toluene were charged into
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 277–282
UV-activated hydrosilylation
(a)
(a)
(b)
(b)
(c)
Figure 1. Real-time FTIR analysis results: (a) real-time FTIR spectra of the
mixture of monomer during UV irradiation; (b) plots of conversion of Si–H
and allyl groups vs UV irradiation time.
a pre-dried 50 ml flask. The polymerization was conducted for
16 h under 60–70 ◦ C then toluene was eliminated in a rotary evaporator. The linear analog polymer was obtained as
a sepia vicious liquid in a 58% yield (P3). IR (KBr): 1250 cm−1
(Si–CH3 ). 1 H-NMR (CDCl3 ): δ = −0.10–0.15 (Si–CH3 ), 0.53–0.62
(Si–CH2 CH2 CH2 –Si), 0.92–0.98 [Si–CH(CH3 )CH2 –Si, weak],
1.23–1.43 (Si–CH2 CH2 CH2 –Si), 1.79–1.84 [Si–CH(CH3 )CH2 –Si,
weak]. Mw = 9400 g/mol, Mw /Mn = 4.3. Tg = −95.3 ◦ C.
Figure 2. NMR spectra of hyperbranched polycarbosilanes [P1 (UVactivated), P2 (thermal-activated)]: (a) 1 H-NMR, (b) 13 C-NMR, (c) 29 Si-NMR.
Characterization
Elements analysis
IR analysis
Appl. Organometal. Chem. 2009, 23, 277–282
The composition of the monomer was determined using Vario ELIII elements analysis instrument (Vario, Germany) via C/N/H mode.
Molecular weight and macromolecular structure parameters were
measured by size exclusion chromatography/multi-angle laser
light scattering (SEC/MALLS) equipment coupled with viscometer
(Wyatt Technology, St. Barbara, USA). The THF elution velocity
was set as 1.0 ml/min. The chromatography system consisted of
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
279
The polymerization process was monitored by real-time FT-IR
spectroscopy (WQF-31 model, RuiLi Co., Beijing, China). NMR
measurement: 1 H-NMR, 13 C-NMR and 29 Si-NMR spectra were
recorded on an Avance 500 spectrometer (Bruker BioSpin,
Switzerland) at room temperature using CDCl3 as solvent.
Tetramethylsilane (TMS) in CDCl3 was used as internal standard.
G.-B. Zhang et al.
Table 1. Molecular structure parameters of hyperbranched polycarbosilanes and their linear analog [P1 (UV-activated), P2 (thermal-activated) and
P3 (linear analog)]
Integral in 29 SiNMR spectra
Sample
index
P1
P2
Theoretical value[14]
P3
dn/dc
Mw
(g/mol)
0.098
0.095
–
0.064
12 220
15 880
–
9 400
Mw /Mn
ηw
(ml/g)
K
(ml/g)
α
D
L
T
DB
ANB
2.7
2.6
–
4.3
5.3
5.8
–
5.5
0.275
0.107
–
0.059
0.34
0.41
–
0.58
1.0
1.0
1
–
1.33
1.45
2
–
1.02
0.63
1
–
0.60
0.58
0.5
–
0.43
0.41
0.33
–
Figure 3. SEC curves of hyperbranched polycarbosilanes [P1 (UVactivated), P2 (thermal-activated)].
a pump (Waters 515), an autosampler, a differential refractometer
(Optilab rEX) and two columns of MZ 103 Å, 300 × 6.8 mm and
MZ 105 Å, 300 × 6.8 mm. The refractive index increment (dn/dC)
value of polymer in THF solution was determined by Optilab rEX
detector at 25 ◦ C. Glass transition temperature (Tg ) of polymer was
determined on a DSC instrument (MDSC 2910, Waters-TA, USA) at
a heat scanning rate of 10 K/min.
Results and Discussion
as internal reference. Here, the IR absorption intensity of allyl
groups seems to increase at the last stage of irradiation [Fig. 1(a)]
due to the volatilization of toluene solvent. In contrast, the AB2
monomer was also polymerized by the thermal-activated route
with Karstedt’s catalyst. The IR absorption peak of Si–H groups
disappeared completely even after about 5–6 h under 50–60 ◦ C.
Therefore the result clearly indicates that UV irradiation can
efficiently activate hydrosilylation of Si–H and allyl groups in
the presence of Pt(acac)2 . From the viewpoint of reactive time,
UV-activated polyhydrosilylation of AB2 monomer is much faster
than thermal-activated hydrosilylation.
Using Pt(acac)2 as the catalyst, UV-activated hydrosilylation
is conducted through the coordination mechanism.[11,9a] In this
process, Pt(acac)2 is first activated by UV irradiation and is thought
to produce a super active catalyst by loss of one acac ligand.
Then it is coordinated with Si–H and olefin. Once this super
active catalyst is formed, it can initiate AB2 monomer to conduct
polyhydrosilylation at a high rate, as presented in Fig. 1(b).
Because of the rearrangement or exchange reaction that leads
to the formation of oligomers containing more than one Si–H
groups, gelation is often difficult to avoid during the thermalactivated polyhydrosilylation.[12] It should be pointed out that
the gelation can also occur in UV-activated polyhydrosilylation
bulk polymerization. However, in toluene solution, UV-activated
polymerization can be conducted successfully without gelation in
a broad range of monomer concentration. It may be attributed to
the enlarged distance between reactive molecules in toluene
solution that can restrain the rearrangement and exchange
reactions.[12a]
280
UV-activated Polymerization Process Analysis
Molecular Structure Characterization of Hyperbranched Polycarbosilanes
The schematic synthesis route of hyperbranched polycarbosilane
from AB2 monomer via UV-activated or thermal-activated hydrosilylation is presented in Scheme 1. As shown in Fig. 1, the
polymerization process of AB2 monomer via UV-activated hydrosilylation was monitored using real-time FTIR spectroscopy.
When the mixture of monomer, Pt(acac)2 catalyst and solvent
was stored in a darkroom, the IR absorption intensity of either Si–H
or allyl groups showed no variation during 48 h. This indicates
that the mixture is stable in the ambient environment without
visible light or UV irradiation. When UV irradiation was conducted
on the mixture, the IR absorption intensity of Si–H and allyl
groups decreased rapidly [Fig. 1 (a)]. After about 40 min of UV
irradiation, the IR absorption peak of Si–H groups disappeared
completely. At the same time, the IR absorption intensity of allyl
groups decreased to ca 51% of that in the original monomer
if the IR absorption intensity of Si–CH3 groups was employed
The suggested hyperbranched polycarbosilane structure is shown
in Scheme 1. 1 H-NMR and 13 C-NMR spectra of hyperbranched
polycarbosilane synthesized via UV-activated polyhydrosilylation
(P1) and hyperbranched polycarbosilane synthesized via thermalactivated polyhydrosilylation (P2) are presented in Fig. 2.
Hydrosilylation reaction can be classified into two categories: α
addition to form –CH3 groups as the side chain and β addition
to form the linear chain.[5a,13] Compared with the monomer, there
are four new chemical shifts in 1 H-NMR spectrum of P1. The strong
chemical shift near δ 0.6 and δ 1.3 is assigned to β addition product
and the weak chemical shift near δ 1.0 and δ 1.9 can be ascribed
to α addition product.[5a] Furthermore, it can be calculated from
quantitative analysis that β addition product accounts for 96%
and 98% for P1 and P2, respectively. The results show that both
α and β addition can occur during the UV-activated and thermalactivated hydrosilylation and β addition plays a dominative role
www.interscience.wiley.com/journal/aoc
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 277–282
UV-activated hydrosilylation
(a)
(b)
(c)
Figure 4. Double logarithmic plots and linear fit curves of intrinsic viscosity versus molar mass for hyperbranched polycarbosilanes: (a) P1 (UV-activated);
(b) P2 (thermal-activated); (c) P3 (linear analog).
compared with α addition. Because the α addition product is
so small, its chemical shift is difficult to detect in the 13 C-NMR
spectrum [Fig. 2(b)].
Degree of branching (DB) and average number of branch units
(ANB) are both important parameters to describe the branching
structures of hyperbranched polymers. At present, they are mainly
determined by NMR analysis by distinguishing different branching
units. For hyperbranched polymer prepared from AB2 monomer, it
contains three different units, as shown in Scheme 1, i.e. dendritic
unit (D), linear unit (L) and terminal unit (T). If D, L and T represent
the chemical shifts integral of dendritic, linear and terminal units
in 29 Si-NMR spectra of the hyperbranched polymer, then DB and
ANB can be calculated from equations (1) and (2), respectively.[14]
2D
2D + L
D
ANB =
D+L
DB =
(1)
(2)
Appl. Organometal. Chem. 2009, 23, 277–282
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
281
29 Si-NMR spectra of P1 and P2 are shown in Fig. 2(c). P1 and
P2 both have three different chemical shifts at δ 1.0, δ 0.65 and δ
0.18, which can be assigned to dendritic, linear and terminal unit,
respectively. The result confirms their hyperbranched molecular
structures.[15] It should be pointed out that the peak at δ
0.18 (T) of P1 is higher than that of P2 if the peak at δ 1.0
(D) is employed as a reference. It may be in correspondence
with the lack of linear unit under the similar molecular weight
(Table 1). Furthermore, if the DB and ANB are calculated using
equations (1) and (2), we can see that both DB and ANB of P1 are
only slightly higher than those of P2, as shown in Table 1. This
indicates that the hyperbranched polycarbosilane synthesized
via UV-activated polyhydrosilylation possesses almost the same
branching structures as those synthesized via thermal-activated
polyhydrosilylation.
Molecular weight and distribution of P1, P2 and their linear
analog P3 synthesized from dimethylallylsilane were determined
using SEC/MALLS. The SEC curves of P1 and P2 are plotted in Fig. 3
and the detailed results are also given in Table 1.
From Figure 3, it can be clearly seen that laser response (LS) and
differential refractive indexes response (DRI) are not synchronous,
which can also be found in hyperbranched polysiloxane and
polyester.[16] This unsynchronization phenomenon may result
from the fact that the polymer consists of large amounts of
low molecular weight fractions. The LS response is sensitive to
high molecular weight fractions while the DRI response is more
sensitive to low molecular weight fractions. Therefore the LS
response is intense when high molecular weight fractions elute at
the beginning and then the DRI response becomes intense when
low molecular weight fractions elute at the last stage. As a result,
DRI response drops behind LS response.[16a]
Macromolecular configuration of the polymer in dilute can
be evaluated from the relationship of the polymer’s intrinsic
viscosity ([η]) vs molar mass (M).[17] In the Mark–Houwink equation
[equation (3)], coefficient K is a constant and the exponent α is
relative to the macromolecular configuration. Especially for α,
hard sphere configuration possesses a value of nearly zero, while
the rigid rod configuration possesses a value ranging from 1.0
to 2.0. In addition, linear polymers often exhibiting random coil
structures that possess values ranging from 0.5 to 0.7. In contrast,
G.-B. Zhang et al.
hyperbranched polymers often possess values lower than 0.5.[17b]
[η] = KMα
(3)
Herein, double logarithm plots of intrinsic viscosity vs molar
mass as well as their linear fit curves for P1, P2 and P3 are
presented in Fig. 4. The α-value is equal to the slopes of the linear
fit curve and calculated to be 0.34, 0.41 and 0.58 for P1, P2 and
P3 (Table 1), respectively. It is clear that the α-value of P1 and
P2 is smaller than that of P3. It indicates that both P1 and P2
consist of more compact structures compared with the random
coil structure of their linear analog (P3). The results further confirm
the hyperbranched molecular structures of P1 and P2.[18]
Conclusion
The hyperbranched polycarbosilane can be easily synthesized
from methyldiallylsilane (AB2 monomer) via UV-activated polyhydrosilylation with bis(acetylacetonato)platinum(II) as catalyst.
The real-time FTIR analysis results indicate that this polymerization process is much faster than that under thermal-activated
conditions. The synthesized polycarbosilane possesses a high
branching structure according to the characterization results
of 1 H-NMR, 13 C-NMR, 29 Si-NMR and SEC/MALLS. Moreover, the
similar degree of branching, average number of branch units
and the exponent of the Mark–Houwink equation demonstrate that the hyperbranched polycarbosilane synthesized via
UV-activated polyhydrosilylation possesses almost the same
branching structure as taht synthesized via thermal-activated
polyhydrosilylation.
Acknowledgment
This project was financially supported by the National Natural
Science Foundation of China (no. 20874080) and the Natural
Science Basic research Plan in Shaanxi Province of China (no.
2006B15). Dr Zhang thanks the Foundation of Visiting Ph.D.
Candidates in Tongji University for financial support.
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