Synthesis of poly(hydrosilane)s, s(-) [R1 (H)Si]n s(-) (R1 = (CH2)3 SiRR2′ ; R, R′ = Me, Et or Ph) and their reactivity studies towards allyl/vinylsilanes

Article abstract of DOI:10.1016/j.jorganchem.2006.04.007

New poly(hydrosilane)s, s(-) [RR₂^′ Si(CH₂)₃ SiH]_n s(-) (R = R^′ = Et (1 a) ; R = Ph, R^′ = Me (2 a) ; R = Me, R^′ = Ph (3 a)) have been synthesized by catalytic dehydroco

Full text of DOI:10.1016/j.jorganchem.2006.04.007

Journal of Organometallic Chemistry 691 (2006) 3310–3318
www.elsevier.com/locate/jorganchem
Synthesis of poly(hydrosilane)s, –½R1ðHÞSiꢀ – ðR1 ¼ ðCH2Þ SiRR20 ;
n
3
R; R0 ¼ Me; Et or PhÞ and their reactivity studies towards
allyl/vinylsilanes
*
Ravi Shankar , Arti Joshi
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
Received 3 March 2006; received in revised form 11 April 2006; accepted 11 April 2006
Available online 19 April 2006
Abstract
New poly(hydrosilane)s, –½RR02SiðCH2Þ3SiHꢀn– ðR ¼ R0 ¼ Et ð1aÞ; R ¼ Ph; R0 ¼ Me ð2aÞ; R ¼ Me; R0 ¼ Ph ð3aÞÞ have been syn-
thesized by catalytic dehydrocoupling of the corresponding carbosilane monomers 1–3 using Cp2TiCl2/2.2BuLi as the catalyst. Hydro-
silylation reaction of 1a or 2a with allyltrimethyl/triethoxyvinyl/trichlorovinylsilane in the presence of AIBN as free radical catalyst
affords the polysilanes 1b–d and 2b–d, respectively, bearing Si–Me/Si–OEt/Si–H functional groups appended on the carbosilyl side chain.
UV absorption spectra of 1a–3a reveal a shoulder like absorption without any peak at 250–260 nm while the polysilanes 1b–d and 2b–d
exhibit a distinct absorption with kmax 274–299 nm and e: 671–887 (Si repeat unit)ꢁ1 dm3 cmꢁ1. A discernable bathochromic shift in the
kmax value is invariably observed in the polysilanes bearing appended SiMe3 or Si(OEt)3 group in comparison to that observed for the
polysilanes bearing sterically less bulky SiH3 group incorporated on carbosilyl side chain. The polysilanes, 4b–d derived from chemical
modification of poly(phenylsilane) also exhibit similar spectral behavior (kmax 314–328 nm; e = 1500–2700 (Si repeat unit)ꢁ1 dm3 cmꢁ1).
These changes indicate that electronic properties of these polysilanes are sensitive to nature of pendant substituents on silicon backbone.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Carbosilanes; Dehydrocoupling; Poly(hydrosilane)s; Chemical modification
1. Introduction
sor, reaction conditions etc. in order to improve the yield
and molecular weight of these linear polymers [3].
Following the pioneering work of Harrod et al. [1], the
reaction involving chemical transformation of primary
organosilanes, RSiH3 (alkyl or aryl) to linear polysilanes,
–[RSiH]n– using Group 4 metallocene catalysts has been
actively pursued in a number of research groups [2]. This
synthetic approach is unique in providing a direct access
to low molecular weight polysilanes bearing SiH groups
which can not be synthesized by classical Wurtz coupling
method due to harsh reaction conditions. Considerable
attention has been devoted to understand several factors
such as nature of the metal catalyst, organosilane precur-
Chemical transformation of Si–H groups in poly(phe-
nylsilane), –[PhSiH]n– using a variety of reagents is well
documented [4]. The polysilane is known to undergo hydro-
silylation reaction with aldehydes, ketones or functional
olefins in presence of AIBN as free radical catalyst to afford
new polysilanes with pendant alkoxy groups or side chain
alkyl substituents appended with hydroxyl, amino or car-
boxylic acid [4a]. Dehydrohalogenation of Si–H groups in
poly(phenylsilane) has been reported under mild conditions
to generate reactive Si–X (X = Cl or Br) functionalities
which can be subsequently transformed using nucleophilic
reagents such as alcohols or mercaptans to afford
hetero-atom substituted polysilanes [5]. Another example
of chemical modification of poly(hydrosilane)s includes
Rh-catalyzed carbenoid insertion into the Si–H bond [6].
*
Corresponding author. Tel.: +91 11 26591513.
E-mail address: shankar@chemistry.iitd.ac.in (R. Shankar).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.04.007

R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
3311
R'
Si
R'
Si
R'
Si
R
R
R'
R
R'
R'
(CH2)3
(CH2)3
(CH2)3
(ii), (iii)
(i)
Si
Si
RR'2Si(CH2)3SiH3
Si
n
H
1-m
m
n
(1-3)
H
(CH2)y
(1a-3a)
SiX3
1b-d 2b-d
); (
(
)
Ph
Ph
Si
Ph
(ii), (iii)
Si
H
Si
n
H
1-m
m
n
(CH2)y
4a
(4b-d)
SiX3
Polysilane
Polysilane
X(y)
Me (3); OEt (2); H (2)
R, R'
Et
1a
2a
1b-d
2b-d
Ph, Me
Me (3); OEt (2); H (2)
-
3a
-
Me, Ph
-
Me (3); OEt (2); H (2)
-
4b-d
Scheme 1. Reagents: (i) Cp2TiCl2 Æ 2.2BuLi; (ii) CH2 = CHCH2SiMe3/CH2 = CHSi(OEt)3/CH2 = CHSiCl3; (iii) AIBN.
Although there has been numerous reports on the tran-
sition-metal mediated dehydrocoupling of primary aryl sil-
anes, related studies on primary alkyl silanes are quite
meager [1a–c,2l,n,7]. This may partly be attributed to low
reactivity of alkyl silanes towards dehydrocoupling and/
or their tendency to undergo autoxidation under aerobic
conditions [8]. In addition, relatively high volatility of
lower alkyl silanes limits their use as precursors. Within
the framework of our research interest on functional poly-
silanes, we have recently reported that carbosilanes of the
general formula, R2R0SiXSiH3 (X = CH2CH2; CH(CH3);
R = R0 = Me, Et, Ph or H) are useful candidates which
undergo efficient Ti-catalyzed dehydrocoupling at elevated
temperature to afford polysilanes –[R2R0SiXSiH]n– bearing
carbosilyl moieties as the pendant groups [7]. Our interest
in this class of polysilanes stems from a desire to expand
the scope of reaction chemistry on the appended silyl moi-
eties in addition to the reactivity of the Si–H groups. In a
related study, we also reported high molecular weight poly-
silanes –[R3Si(CH2)xSiR1]n– (R = Me, Et or Ph; R1 = Me,
Ph; x = 2 or 3) by Wurtz coupling method [9]. It has been
observed that electronic properties of these polysilanes are
influenced by the pendant sila-alkyl chain length as well as
by steric effect of substituents on the appended silyl groups.
In continuation of this work, we report herein the synthesis
of new poly(hydrosilane)s, –[R2R0Si(CH2)3SiH]n– (1a–3a,
Scheme 1) which have been synthesized by Ti-catalyzed
dehydrocoupling of appropriate carbosilane precursors.
Subsequent chemical modification of these polysilanes by
free radical hydrosilylation with allyl/vinyl silanes has
allowed the formation of functional polysilanes bearing
asymmetrically substituted side chain carbosilyl moieties
(Scheme 1). A detailed analysis of UV spectral data of these
polysilanes and its comparison with those of the precursor
poly(hydrosilane)s (1a–3a) has allowed to delineate the ste-
ric influence of carbosilyl substituents on the electronic
properties of these hitherto unknown polysilanes. The
details are reported herein.
2. Results and discussion
2.1. Synthesis and characterization of carbosilane monomers,
RR02SiðCH2Þ SiH3
3
The carbosilanes, RR02SiðCH2Þ3SiH3 ðR ¼ R0 ¼ Et ð1Þ;
R ¼ Ph; R0 ¼ Me ð2Þ; R ¼ Me; R0 ¼ Ph ð3ÞÞ have been
synthesized in two steps. In the initial step, hydrosilylation
[10] reaction between equimolar quantities of allyltrichlo-
rosilane and an appropriate hydrosilane RR02SiH in pres-
ence of Karstedt’s [11] catalyst proceeds regioselectively
to afford the corresponding b-addition products, RR02Si
ðCH2Þ3SiCl3. Subsequent treatment of these compounds
with LiAlH4 results in the isolation of the desired carbosil-
anes 1–3 as colorless, distillable liquids. The presence of
only one isomer in these carbosilanes is elucidated by
1
HPLC and identified by IR, H, 13C{1H}, 29Si NMR and
EI-Mass spectroscopy. The primary SiH3 group in these
compounds gives rise to characteristic IR absorptions at
2149–2148 (tSiH) and 924–923 (d SiH2) cmꢁ1 and a
distinct 29Si NMR resonance at d ꢁ56.46 to ꢁ63.97 (q,
1
SiH3, JSi–H = 191.5–192.3 Hz). In addition, each carbosi-
lane exhibits a sharp 29Si resonance in the characteristic

3312
R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
chemical shift region (d6.40 to ꢁ6.92) of the pendant
250–320 °C. However, subsequent weight loss is observed
in a continuous manner up to 500 °C leaving a residual
yield of 22–28%. GPC data shows a monomodal molecular
weight distribution in each case with Mw = 2168–2309 and
PDI = 1.16–1.28. A representative GPC profile of polysi-
lane 2a is shown in Fig. 1. These polysilanes have been
characterized by IR, and multinuclei (1H, 13C{1H},
29Si{1H}) NMR studies and the pertinent data are given
in Table 1. By analogy with literature precedence [2l], IR
absorption (Fig. 2a) at 2095–2105 cmꢁ1 is attributed to
Si–H groups of the silicon backbone. The absence of IR
bands in the region ꢂ2070–2090 cmꢁ1 suggests that the
polysilanes are devoid of cyclic species. It is noteworthy
to mention that IR spectral characteristics remain identical
upon ageing the polysilanes under inert conditions. How-
ever, polysilane 1a undergoes autooxidation upon exposure
leading to some degree of siloxane formation. This is evi-
dent from the appearance of new bands at 1080 and
2156 cmꢁ1 characteristic of siloxane containing Si–H
groups [8]. The polysilanes 2a and 3a bearing appended
PhMe2Si/Ph2MeSi groups in the carbosilyl chain are rela-
tively stable under these conditions, presumably due to ste-
1
RR02Si moiety. H and 13C{1H} NMR spectra are quite
straightforward and corroborate well with the composition
of these compounds. The mass spectra (EI) reveal [MꢁH]+
ion in each case, while structurally important fragment ions
are discernable from Si–C bond cleavage. The relevant
spectral data are summarized in Section 3.
2.2. Polymerization of 1–3
Following the procedure known in literature [2n,2o],
dehydrocondensation reaction of the carbosilane mono-
mers 1–3 in presence of catalytic amount of Cp2TiCl2/
2.2BuLi (50 °C, 48 h) results in a viscous mass in each case.
The isolation of low molecular weight linear polysilanes
–½RR02SiðCH2Þ3SiHꢀn– 1a–3a (Scheme 1) from the crude
mixture has been effected by repeated fractional precipita-
tion with n-hexane/methanol mixture. These are obtained
as pale yellow viscous gums and are soluble in common
organic solvents such as dichloromethane, chloroform,
hexane, toluene, THF, etc. The results obtained from ther-
mogravimetric analysis reveal that these are stable up to
1
ric interference of the bulky silyl-substituents. H NMR
spectrum of each polysilane shows broad and featureless
signals associated with side chain methylene protons while
resonances due to silyl-substituted methyl, ethyl or phenyl
groups are well resolved. A similar spectral behavior is
observed in the 13C{1H} NMR spectra as well, the terminal
silyl substituents featuring relatively sharp resonances. The
spectral behavior shows a close analogy with those of pre-
viously reported polysilanes bearing carbosilyl groups [9] as
well as with poly(methyl-n-alkylsilane)s [12] prepared by
1
Wurtz coupling method. Accordingly, the unresolved H
and 13C{1H} resonances of methylene groups in the poly-
mers 1a–3a may be attributed to short spin–spin relaxation
1
time of these nuclei. The assignments of overlapping H
and 13C{1H} signals of methylene and ethyl groups in 1a
are made from DEPT-135 13C{1H} and HSQC (1H–13C)
NMR spectra (Fig. 3) which shows distinct cross peaks at
d 23.26–22.38/1.38 (b-CH2), 15.96–15.52/0.64 (c-CH2)
Fig. 1. GPC profile of polysilane: (a) 2a (solid line); (b) 2c (dotted line).
Table 1
1H, 13C{1H}, 29Si{1H} NMR, and GPC data of polysilanes
Polysilane
d
1H NMR (ppm)
d
13C{1H} NMR (ppm)
d
29Si{1H} NMR (ppm)
Mw
PDI
1.17
1a
3.52 (br, SiH backbone), 1.38 (br,
2H, b-CH2), 0.85 (t, JH–H
7.8 Hz, CH3–Et + a-CH2), 0.64
(br, 2H, c-CH2), 0.56–0.48 (q,
6H, CH2–Et, JH–H = 7.8 Hz)
23.26–22.38 (b-CH2), 15.96–
15.52 (c-CH2), 14.62–13.15
(a-CH2), 7.48 (CH3–Et),
3.41 (CH2–Et)
6.96 (Et3Si) ꢁ57.18 to ꢁ65.43 (br, SiH
2309
3
=
backbone)
3
2a
3a
7.49, 7.33 (br, 5H, SiPh), 3.51 (br,
SiH backbone), 1.44 (br, 2H,
b-CH2), 0.81 (br, 4H, a- and
c-CH2), 0.26 (br, 6H, PhMe2Si)
7.54, 7.28 (br, 10H, SiPh), 3.37
(br, SiH backbone), 1.42 (br, 2H,
b-CH2), 0.81 (br, 2H, a-CH2),
0.64 (br, 2H, c-CH2), 0.48 (br,
3H, Ph2MeSi)
139.39, 133.52, 128.80, 127.73
(Si–Ph), 23.52–22.42 (b-CH2),
20.50–19.65 (c-CH2), 14.81–13.52
(a-CH2), ꢁ2.90 (PhMe2Si)
ꢁ2.63 (PhMe2Si) ꢁ57.30 to ꢁ65.15 (br,
2168
2264
1.28
1.16
SiH backbone)
139.43, 134.12, 129.53, 128.87
(Si–Ph), 23.45–22.03 (b-CH2),
20.26–19.97 (c-CH2), 14.63–13.79
(a-CH2), ꢁ3.84 (Ph2Me2Si)
ꢁ7.02 (Ph2MeSi) ꢁ57.02 to ꢁ65.70 (br,
SiH backbone)

R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
3313
Fig. 3. (1H–13C) HSQC NMR spectrum of 1a.
The polysilanes, thus obtained are pale yellow viscous
gums and are soluble in common organic solvents such
as dichloromethane, chloroform, hexane, toluene, THF,
etc. The results of thermogravimetric analysis reveal that
these are stable up to 300–350 °C. However, a continuous
weight loss is observed up to 500 °C leaving a residual yield
of 23–36%. These polysilanes show a monomodal molecu-
lar weight distribution in each case with Mw = 2289–4780;
PDI = 1.25–1.78. A GPC profile of a representative polysi-
lane 2c along with the parent poly(hydrosilane) 2a is shown
in Fig. 1 for comparison and the relevant data are given in
Table 2. These results apparently suggest that free radical
hydrosilylation of the parent poly(hydrosilane)s occurs
without scission of the silicon backbone. However, a per-
ceptible reduction of molecular weight for 1d and 2d in
comparison to those of the corresponding poly(hydrosi-
lane)s may be attributed to arise during the step involving
reduction of Si–Cl groups with lithium aluminium hydride
[5a]. IR spectra of the polysilanes reveal a very weak
absorption at 2092–2101 cmꢁ1 (except for 1d and 2d) due
to residual Si–H group of the backbone. Characteristic
absorptions for the functional groups associated with car-
bosilyl chain appear at 1250–1246 (Si–Me), 1080–1079,
(Si–OEt) and 2147–2145 (mSiH3); 922–921 (dSiH3) cmꢁ1
(Fig. 2c). These results coupled with 1H NMR spectral data
suggest that the chemical transformation of poly(hydrosi-
lane)s under these conditions occurs with >90% inclusion
Fig. 2. IR spectra of: (a) 1a; (b) hydrosilylated product of 1a with
vinyltrichlorosilane; (c) 1d.
and d 7.48 (CH3–Et), 14.62–13.15 (a-CH2)/0.85. 29Si{1H}
NMR spectra invariably show a singlet at d 6.96 to
ꢁ7.02 and a broad featureless signal at d ꢁ57.30 to
ꢁ65.70 which are attributed to the carbosilyl side groups
and to silicon backbone, respectively.
2.3. Chemical modification
The Si–H groups in polysilanes 1a and 2a have been
subjected to free radical hydrosilylation reaction with allyl-
trimethyl/triethoxyvinyl/trichlorovinylsilane (toluene, 80–
90 °C) in presence of AIBN as the catalyst. The extent of
hydrosilylation was monitored at different time intervals
by IR (Fig. 2b) and 1H NMR spectroscopy. The concentra-
tion of Si–H content in the reaction mixture was found to
decrease with time and finally remains practically
unchanged after 48 h. The isolation of the corresponding
polysilanes 1b–d and 2b–d (Scheme 1) has been achieved
by fractional precipitation of the crude product using
THF/methanol mixture.
1
of the carbosilyl moiety. H and 13C{1H} NMR spectra
show broad and overlapping signals due to methylene
and/or ethyl (for 1b–d only) groups associated with the
1
carbosilyl moiety. The overlapping H and 13C{1H} reso-
nances of methylene groups has been assigned from HSQC

3314
R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
Table 2
1H, 13C{1H}, 29Si{1H}/29Si NMR, UV and GPC data of polysilanes
1H NMR (ppm)
d
13C{1H} NMR (ppm)
d
29Si{1H}/29Si NMR
UVa
k
max/e Mw
PDIc
1.78
b
Polysilane
d
(ppm)
1b
1.44 (br, b-/b0-CH2), 0.94 (t,
22.47 (c0-CH2), 21.88 (b-CH2),
6.02 (Et3Si), 0.00
(SiMe3) ꢁ30.03
296/671
4780
3JH–H = 7.6 Hz, CH3–Et + a-/a0- 20.15 (a-CH2), 17.64 (a0-/b0-
CH2), 0.62 (br, c-/c0-CH2), 0.51
(br, CH2–Et), 0.00 (br, SiMe)
CH2), 16.61 (c-CH2), 7.54 (CH3– (br, Si backbone)
Et), 3.46 (CH2–Et), ꢁ1.52 (Si–
Me3)
1c
1d
2b
2c
3.72 (br, CH2–OEt), 1.36 (br, b- 58.86 (CH2–OEt), 21.46 (b-CH2), 5.93 (Et3Si), ꢁ29.89
293/732
3310
2394
2952
3801
1.36
1.41
1.25
1.63
CH2), 1.13 (br, CH3–OEt), 0.85
(br, CH3–Et + a/a0-CH2), 0.54,
0.44 (c-/b0-CH2 + CH2–Et)
20.38 (a-CH2), 19.05 (CH3–OEt), (br, Si backbone),
17.56 (c-CH2), 7.89 (CH3–Et),
6.97, 5.42 (a0-/b0-CH2), 3.12
(CH2–Et)
ꢁ46.25 (Si–(OEt)3)
3.49 (br, SiH3), 1.32 (br, b-CH2), 21.83 (b-CH2), 19.39 (a-CH2),
6.86 (Et3Si), ꢁ30.55 (br, 278/824
Si backbone), ꢁ48.03,
3
0.86 (t, JH–H = 7.6 Hz, CH3–
Et + a-CH2), 0.67 (br, a0-/b0-
CH2), 0.55 (br, c-CH2), 0.44 (br, 1.02 (b0-CH2)
CH2–Et)
17.20 (c-CH2), 9.56 (a0-CH2),
7.53 (CH3–Et), 3.44 (CH2–Et),
ꢁ51.28, ꢁ54.52, ꢁ57.76
1
(q, SiH3, JSi–H
=
193.1 Hz)
7.50, 7.28 (br, Si–Ph), 1.42 (br,
b-/b0-CH2), 0.84 (br, a-/a0/c-
CH2), 0.57 (c0-CH2), 0.28 (br,
PhMe2Si), 0.00 (br, SiMe3)
139.42, 133.44, 128.76, 127.73
1.26 (SiMe3)-3.10
299/822
(Si–Ph), 22.35 (c0-CH2), 21.83 (b- (PhMe2 Si)-30.78
CH2), 20.17 (c-/a-CH2), 17.68
(a0-/b0-CH2), ꢁ1.41 (Si–Me3),
ꢁ2.81 (PhMe2Si)
(br, Si backbone)
7.44, 7.25 (br, Si–Ph), 3.73 (br,
CH2–OEt), 1.42 (br, b-CH2), 1.15 (Si–Ph), 58.21 (CH2–OEt), 21.38 (br, Si backbone),
139.39, 133.41, 128.72, 127.68
ꢁ3.18 (PhMe2Si) ꢁ27.23 296/887
(br, CH3–OEt), 0.78 (br, a-/c-/
a0-/b0-CH2), 0.21 (br, PhMe2Si)
(b-CH2), 19.64 (c-CH2), 18.40
(CH3–OEt), 17.62 (a-CH2), 6.65,
4.50 (a0-/b0-CH2), ꢁ2.94
(PhMe2Si)
ꢁ45.35 (Si–(OEt)3)
2d
7.50, 7.35 (Si–Ph), 3.54 (br,
139.24, 133.48, 128.80, 127.74
ꢁ3.15 (PhMe2Si) ꢁ28.67 274/873
2289
1.54
SiH3), 1.42 (br, b-CH2), 0.82 (br, (Si–Ph), 20.30 (b-/c-CH2), 16.90 (br, Si backbone),
a-/c-/a0-/b0-CH2), 0.27
(PhMe2Si)
(a-CH2), 8.76 (a0-CH2), 2.54 (b0- ꢁ48.79, ꢁ52.02, ꢁ55.25,
CH2), ꢁ2.86 (PhMe2Si)
ꢁ58.48 (q, SiH3,
1JSi–H = 192.5 Hz)
0.78 (SiMe3) ꢁ32.46
(br, Si backbone)
4b
4c
4d
7.18 (br, SiPh), 1.10 (br, a0-/b0-/ 136.68, 134.23, 127.46 (Si–Ph),
c0-CH2), 0.00 (br, SiMe)
328/2400
323/2700
314/1500
4002 (2628)d 1.52 (1.32)d
7122 (3505)d 2.05 (1.38)d
2348 (3175)d 1.33 (1.46)d
21.83 (a0-/b0-/c0-CH2), ꢁ1.64
(SiMe3)
7.00 (br, SiPh), 3.46 (br,
CH2–OEt), 1.01 (br, a0-/b0-
CH2 + CH3–OEt)
7.18 (br, SiPh), 3.31 (br, SiH3),
0.81 (br, a0-/b0-CH2)
136.55, 134.16, 127.53 (Si–Ph),
58.06 (CH2–OEt), 18.22 (CH3–
OEt), 5.42 (a0-/b0-CH2)
136.39, 133.77, 127.89 (Si–Ph),
8.69 (a0-CH2), 1.84 (b0-CH2)
ꢁ30.17 (br, Si
backbone), ꢁ46.19
(Si–(OEt)3)
ꢁ32.01 (br, Si
backbone), ꢁ48.51,
ꢁ51.75, ꢁ54.99, ꢁ58.24
1
(q, SiH3, JSi–H
=
193.1 Hz)
a
e units: (Si repeat unit)ꢁ1 dm3 cmꢁ1; kmax units: nm.
Molecular weights determined by GPC relative to polystyrene standards; eluant: THF; 25 °C.
PDI = polydispersity index (Mw/Mn).
b
c
d
Molecular weight of poly (phenysilane) prepared under different runs.
(1H–13C{1H}) NMR spectrum of a representative polysi-
lane 1b (Fig. 4) which shows cross peaks at d 21.88,
17.64/1.44 (b-/b0-CH2), 16.61, 22.47/0.62 (c-/c0-CH2),
20.15, 17.64, 7.54/0.94 (a-/a0-CH2, CH3–Et). A detailed
assignment for other polysilanes has been made accord-
ingly and pertinent spectral data are given in Table 2.
29Si{1H} NMR spectra reveal sharp resonances in the
chemical shift region which are characteristic of appended
silicon moieties. The 29Si{1H} resonances due to silicon
backbone appear at d ꢁ27.23 to ꢁ30.78 showing broad
and featureless profile. For 1d and 2d, the presence of
appended SiH3 group is confirmed by 29Si signal at d
In order to get an insight into the free radical hydrosily-
lation approach, poly(phenylsilane) was also subjected to
chemical modification by AIBN catalyzed reaction with
allyltrimethyl/triethoxyvinyl/trichlorovinylsilane. This has
resulted in the isolation of linear polysilanes 4b–d bearing
phenyl and carbosilyl groups as pendant substituents.
There is no evidence of Si–Si bond cleavage during hydro-
silylation from the GPC data which shows a monomodal
molecular weight distribution with Mw = 2348–7122 and
PDI = 1.33–2.05. IR and 1H NMR spectra of these polysil-
anes suggest the inclusion of 75–85% carbosilyl moiety as
the pendant groups. The presence of residual Si–H groups
in the polymeric framework is evident by a weak IR
1
ꢁ48.03 to ꢁ58.48 (quartet, JSi–H = 192.5–193.1).

R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
3315
Fig. 5. UV spectra (cyclohexane, RT) of polysilanes 1a–d.
ing due to steric effect of long carbosilyl chain which impart
more extended trans conformation to the silicon backbone.
A scrutiny of the spectral profiles of the polysilanes 1b–d
(Fig. 5) also reveal a significant dependence of the kmax val-
ues on the nature of the appended sila-substituents on the
side chain. A discernable bathochromic shift in the kmax
value is observed in the polysilanes 1b–c bearing appended
SiMe3/Si(OEt)3 group in comparison to that observed for
the polysilanes 1d bearing sterically less bulky SiH3 group.
It is interestingly to note that UV spectra of the polysilanes
2b–d also follow a similar trend. To comprehend these
results, the polysilanes 4b–d bearing phenyl and carbosilyl
groups on the silicon backbone have also been studied with
regard to their electronic properties. In accord with litera-
ture precedence [4a], poly(phenylsilane) obtained under dif-
ferent runs shows a weak absorption with kmax at ꢂ300 nm
due to r–r* transition of the silicon backbone while the cor-
responding absorption for 4b–d are discernable at 314–
328 nm with e = 1500–2700 (Si repeat unit)ꢁ1 dm3 cmꢁ1
(Fig. 6). Interestingly, the relative magnitude of the
observed bathochromic shift in these polysilanes follows
the order: 4d(314) < 4b(328) ꢂ 4c(323) which correlates well
with those of polysilanes 1b–d with similar sila-substituents
on the carbosilyl group.
Fig. 4. (1H– 13C) HSQC NMR spectrum of 1b.
absorption at 2094–2103 cmꢁ1, while characteristic absorp-
tions associated with the carbosilyl moieties appear at 1246
(SiMe), 1079 (Si–OEt) or 2143 (mSiH3); 924 (dSiH3) cmꢁ1
.
1H, 13C{1H} and 29Si{1H} NMR spectra are consistent
with the idealized composition of these polysilanes and
the relevant data are summarized in Table 2. For 4d, 29Si
NMR spectrum reveals a quartet at d ꢁ48.51 to ꢁ58.24
(1JSi–H = 193.1 Hz) due to the presence of appended SiH3
group.
2.4. UV spectral studies
UV spectral studies of all the polysilanes have been per-
formed to gain an insight into the electronic properties asso-
ciated with r delocalized silicon backbone. The relevant
data are summarized in Table 2 while the spectral profiles
of the polysilanes 1a–d are shown in Fig. 5. For poly(hyd-
rosilane)s 1a–3a, the spectra invariably show a shoulder like
absorption without any peak at 250–260 nm. A detailed
comparison of these results with those of previously known
poly(hydrosilane)s, –[RSiH]n– (R = alkyl) is precluded due
to lack of adequate information in the literature [1a,5b,13].
However, the observed UV spectral behavior is believed to
arise as a result of predominantly gauche conformation of
the silicon backbone due to size difference between hydro-
gen and the bulky carbosilyl pendant groups [4a]. Contrary
to this, the modified polysilanes 1b–d and 2b–d derived from
poly(hydrosilane)s 1a and 2a, respectively, exhibit a broad
and distinct absorption with kmax 274–299 nm and molar
absorptivity (e: 671–887 (Si repeat unit)ꢁ1 dm3 cmꢁ1). The
observed bathochromic shift in the kmax values as compared
to those of the precursors may be attributed to chain stiffen-
In conclusion, the poly(hydrosilane)s 1a–3a have been
synthesized by titanium catalyzed dehydrocoupling of car-
bosilane monomers with terminal SiH3 groups. These
Fig. 6. UV spectra (THF, RT) of polysilanes 4a–d.

3316
R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
polysilanes serve as models to probe the reactivity of Si–H
groups towards allyl/vinylsilanes under free radical cata-
lyzed conditions. Using this approach, a number of func-
tionally substituted polysilanes 1b–d and 2b–d
incorporating reactive Si–H or Si–OEt appended func-
tional groups on the carbosilyl chain have been synthe-
sized. It is noteworthy that these polysilanes are
inaccessible via Wurtz coupling route. These results assume
significance in view of lack of reported studies on the reac-
tivity of poly(hydrosilane)s, –[RSiH]n– (R = alkyl). These
polysilanes show interesting variations in the electronic
properties associated with r–r* transition. For 1a–3a,
the appearance of shoulder like absorption at 250–
260 nm in the UV spectra is suggestive of predominantly
a gauche conformation of the silicon backbone. For poly-
silanes 1b–d and 2b–d derived from the precursor poly(hyd-
rosilane)s, the observed spectral parameters [kmax 274–
299 nm and e: 671–887 (Si repeat unit)ꢁ1 dm3 cmꢁ1] are
indicative of chain stiffening and greater population of
trans segments in the silicon backbone. In addition, the
observed variation in the kmax values correlates with the
steric demand of appended sila-substituents, SiH3, SiMe3
and Si(OEt)3. This is evident from the bathochromic shift
of kmax for the polysilanes bearing appended SiMe3/
Si(OEt)3 group in comparison to that observed for the
polysilanes bearing sterically less bulky SiH3 group on
the carbosilyl side chain. These results are further corrobo-
rated from the UV spectral data of polysilanes 4b–d.
on a Perkin–Elmer (Lambada Bio 20) spectrophotometer.
Molecular weights of the polysilanes were estimated using
Waters 510 liquid chromatograph equipped with a Varian
400 refractive index detector and Waters styragel HR3 and
HR4 column in series. The chromatograph was calibrated
with polystyrene standards and THF was used as eluent.
HPLC data were obtained on Waters 1525 binary HPLC
fitted with Waters dual wavelength absorbance detector
and a C-18 column. n-Hexane was used as the mobile
1
phase. H, 13C and 29Si NMR spectra were recorded in
CDCl3 on Bruker spectrospin DPX 300 MHz instrument
at frequency 300, 75.5, 59.6 MHz, respectively, as well as
29Si NMR spectra were also recorded on Bruker avance
II 400 NMR spectrometer at frequency 79.49 MHz and
chemical shifts are quoted relative to Me4Si. 13C{1H} spec-
tra in DEPT-135 mode were obtained using standard pulse
sequence with J modulation time 3.7 ms and 2 s delay time.
HSQC (1H–13C) spectra were recorded using standard
pulse sequence with a relaxation delay of 2 s for each of
512t1 experiments. Thermogravimetric analysis of polysil-
anes was carried out in N2 atmosphere between 50 and
800 °C at a heating rate of 10 °C/min on a Perkin–Elmer
Thermal analysis system.
3.2. Synthesis of carbosilane precursors
The carbosilanes 1–3 were prepared by following the
hydrosilylation approach. To a stirred solution of allyltri-
chlorosilane (8.61 g, 7.0 ml, 49.0 mmol) containing catalytic
amount of Karstedt’s catalyst (10ꢁ7 Pt/mol of silane), tri-
ethyl/phenyldimethyl/diphenylmethylsilane (48 mmol) was
separately added dropwise at room temperature. The reac-
tion mixture turned pale yellow after a few ml of hydrosi-
lane was added. No induction period was observed during
the complete addition. The reaction mixture was heated at
100 °C for 12 h in order to complete the reaction and the
product obtained in each case was fractionally distilled.
The resulting chlorocarbosilanes (40 mmol) were separately
added dropwise to a dispersion of LiAlH4 (2.43 g,
64.0 mmol) in the same solvent at 0 °C. The contents were
gently refluxed for 5–6 h and then hydrolyzed with 1 N
HCl. Ether layer was extracted and dried over anhydrous
Na2SO4. Thereafter the contents were distilled under vac-
uum to yield the corresponding carbosilane 1–3 as colorless
liquids. The relevant spectroscopic data are given below.
For assigning the NMR spectral data, methylene group
attached to SiH3 moiety is represented with the notation,
(a-) and subsequent groups are designated with b- and
c-notations respectively.
3. Experimental
3.1. General comments
All operations were carried out using standard schlenk
line techniques under dry nitrogen atmosphere. Solvents
were freshly distilled under inert atmosphere over sodium
benzophenone (ether, tetrahydrofuran, and toluene), phos-
phorus pentoxide (n-hexane), and magnesium (alcohols)
before use. Glasswares were dried in an oven at 100–
120 °C and further flame dried under vacuum prior to
use. Allyltrichlorosilane and vinyltrichlorosilane (Aldrich)
were freshly distilled over magnesium. Triethoxyvinylsilane
and allyltrimethylsilane (Aldrich) were freshly distilled
before use. Karstedt’s catalyst (platinum(0)-1,3-divinyl-
1,1,3,3-tetramethyldisiloxane complex), lithium aluminium
hydride, butyl lithium (1.6 M in hexane), bis(cyclopenta-
dienyl)titaniumdichloride, triethylsilane, phenyldimethylsi-
lane and diphenylmethylsilane (Aldrich) were used as
received. The molarity of hexane solution of n-BuLi was
confirmed by titration method [14]. Poly(phenylsilane)
was prepared by method known in literature [2n,2o]. AIBN
[2,20-azo(bisisobutyronitrile)] (Merck) was recrystallized
from methanol before use.
3.2.1. Et3SiCH2CH2CH2SiH3 (1)
(bp 48–50 °C/5 mm Hg, yield 60%). EI mass: 187
1
[MꢁH]+, 159 [MꢁCH3CH2]+, 115 [Et3Si]+, 87 [Et3]+. H
3
Infrared spectra were obtained as thin films on KBr pel-
NMR: d 3.47 (t, JH–H = 3.7 Hz, 3H, SiH3), 1.51–1.40 (m,
3
´ ´
lets on a Nicolet FT-IR (Protege) spectrophotometer. EI-
2H, b-CH2), 0.92 (t, JH–H = 7.9 Hz, 9H, CH3–Et), 0.86–
(70 eV) Mass spectra were carried out on VG analytical
(Model 70-s) mass spectrometer. UV spectra were recorded
0.78 (m, 2H, a-CH2), 0.63–0.58 (m, 2H, c-CH2), 0.54–0.46
3
(q, JH–H = 7.9 Hz, 6H, CH2–Et). 13C{1H} DEPT-135

R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
3317
NMR: d 21.29 (b-CH2), 15.16 (c-CH2), 10.80 (aCH2), 7.67
anes 1b–c and 2b–c were separated from the corresponding
crude products by repeated precipitation with THF/meth-
anol mixture (yield = 34–41%).
(CH3–Et), 3.53 (CH2–Et). 29Si NMR: d 6.40 (Et3Si),
1
ꢁ63.97, ꢁ61.55, ꢁ59.14, ꢁ56.73 (q, SiH3, JSi–H
=
191.5 Hz). IR (KBr, cmꢁ1) 2149 (mSiH), 924 (dSiH2).
3.3.2. Reactions of polysilanes 1a and 2a with
trichlorovinylsilane
3.2.2. PhMe2SiCH2CH2CH2SiH3 (2)
(bp 68–70 °C/5 mm Hg, yield 83%). EI mass: 207
To a stirred solution of polysilane 1a or 2a (4.30 mmol)
and AIBN (0.35 mmol) in toluene (5 mL) was added tri-
chlorovinylsilane (7.86 mmol) and the solution was
refluxed at 80 °C for 48 h. Excess of solvent was removed
under vacuum. The resulting polysilane, (3.89 mmol) con-
taining SiCl3 terminal groups was dissolved in diethyl
ether and was added to a dispersion of LiAlH4 (6.0 mmol)
in diethyl ether. The contents were gently refluxed for
4–5 h. The reaction mixture was hydrolyzed using 1 N
HCl. Ether layer was separated and dried over anhydrous
sodium sulphate. The linear polysilane 1d and 2d were
separated from the corresponding crude mixture by
repeated precipitation with THF/methanol mixture (yield
39–42%).
[MꢁH]+, 193 [MꢁCH3]+, 135 [PhMe2Si]+, 121 [PhMe-
1
SiH]+, 105 [PhSi]+. H NMR: d 7.54–7.31 (m, 5H, SiPh),
3
3.47 (t, JH–H = 3.8 Hz, 3H, SiH3), 1.56–1.45 (m, 2H, b-
CH2), 0.90–0.78 (m, 4H, a- and c-CH2), 0.27 (s, 6H,
PhMe2Si). 13C{1H} DEPT-135 NMR: d 134.30, 129.12,
128.83 (SiPh), 21.56 (b-CH2), 17.61 (c-CH2), 10.32 (a-
CH2), ꢁ3.67 (PhMe2Si). 29Si NMR: d ꢁ3.42 (PhMe2Si),
1
ꢁ63.71, ꢁ61.30, ꢁ58.88, ꢁ56.46 (q, SiH3, JSi–H
192.3 Hz). IR (KBr, cmꢁ1) 2148 (mSiH), 924 (dSiH2).
=
3.2.3. Ph2MeSiCH2CH2CH2SiH3 (3)
(bp 150–155 °C/5 mm Hg, yield 75%). EI mass: 269
[MꢁH]+, 255 [MꢁCH3]+, 197 [Ph2MeSi]+, 105 [PhSi]+.
3
1H NMR: d7.43–7.11 (m, 10H, SiPh), 3.36 (t, JH–H
=
3.4. Reaction of poly(phenylsilane) with allyltrimethyl/
triethoxyvinylsilane/trichlorovinylsilane
3.6 Hz, 3H, SiH3), 1.51–1.40 (m, 2H, b-CH2), 1.10–1.04
(m, 2H, c-CH2), 0.77–0.74 (m, 2H, a-CH2), 0.45 (s, 3H,
Ph2MeSi). 13C{1H}NMR: d 137.32, 134.62, 129.34, 128.02
(SiPh), 21.17 (b-CH2), 17.84 (c-CH2), 10.49 (a-CH2),
ꢁ4.24 (Ph2MeSi). 29Si NMR: d ꢁ6.92 (Ph2MeSi), ꢁ63.80,
The reaction between poly(phenylsilane) (0.73 g,
6.88 mmol) and allyltrimethyl/triethoxyvinyl/trichlorovi-
nylsilane (10.32 mmol) in presence of AIBN catalyst was
performed under identical conditions as described above
for similar hydrosilylation reactions of 1a and 2a. After
usual workup, the corresponding polysilanes 4b–d were
obtained as pale yellow solids (yield 25–42%).
1
ꢁ61.38, ꢁ58.96, ꢁ56.54 (q, SiH3, JSi–H = 192.3 Hz). IR
(KBr, cmꢁ1) 2148 (mSiH), 923 (dSiH2).
3.3. Preparation of poly(hydrosilane)s 1a–3a
The carbosilane 1 (4.25 g, 22.5 mmol) was charged with
Cp2TiCl2 (0.14 g, 0.56 mmol) and n-BuLi (0.77 mL, 1.6 M
hexane) under dry N2 conditions. The solution immediately
turned blue in color. The reaction mixture was heated at
50 °C for 48 h and the catalyst was subsequently deacti-
vated by adding n-hexane (30 mL) in air until the intense
color disappeared. The solid residue was filtered and the fil-
trate was kept under vacuum for several hours to remove
the volatile materials. The resulting gummy residue was
dissolved in minimum amount of n-hexane and methanol
was added drop wise until the polysilane was obtained in
a separated layer as an oily viscous gum. The dissolution/
separation procedure was repeated three times to obtain
the linear poly(hydrosilane)s 1a (2.38 g, Yield 57%). Fol-
lowing the similar procedure as above, the polysilanes 2a
and 3a were obtained as gummy material using the mono-
mers 2 and 3 respectively. Yield 64–73%.
Acknowledgements
The authors are grateful to C.S.I.R. (India) for financial
support and S.R.F to A.J. We thank RSIC (Chandigarh)
and SIF Bangalore for providing 29Si NMR data.
References
[1] (a) C.T. Aitken, J.F. Harrod, E. Samuel, J. Organomet. Chem. 279
(1985) C11;
(b) J.F. Harrod, Y. Mu, E. Samuel, Can. J. Chem. 70 (1992) 2980;
(c) C.T. Aitken, J.F. Harrod, U.S. Gill, Can. J. Chem. 65 (1987)
1804;
(d) C.T. Aitken, J.F. Harrod, E. Samuel, J. Am. Chem. Soc. 108
(1986) 4059;
(e) C.T. Aitken, J.F. Harrod, E. Samuel, Can. J. Chem. 64 (1986)
1677.
[2] (a) J.Y. Corey, Adv. Organomet. Chem. 51 (2004) 1, and references
cited there in;
3.3.1. Reactions of polysilanes 1a and 2a with allyltrimethyl/
triethoxyvinylsilane
(b) H.-G. Woo, T.D. Tilley, J. Am. Chem. Soc. 111 (1989) 8043;
(c) J.P. Banovetz, K.M. Stein, R.M. Waymouth, Organometallics 10
(1991) 3430;
(d) R.M. Shaltout, J.Y. Corey, Tetrahedron 51 (1995) 4309;
(e) V.K. Dioumaev, J.F. Harrod, Organometallics 15 (1996) 3859;
(f) N. Choi, S.-Y. Onozawa, T. Sakakura, M. Tanaka, Organomet-
allics 16 (1997) 2765;
The polysilane 1a or 2a (3.38 mmol) in toluene was sub-
jected to free radical hydrosilylation with allyltrimethyl/tri-
ethoxyvinylsilane (5.60 mmol) in the presence of catalytic
amount of AIBN (0.39 mmol). The contents were refluxed
at 90 °C for 48 h. Thereafter, solvent was removed under
vacuum leaving behind a viscous mass. The linear polysil-
(g) B.J. Grimmond, J.Y. Corey, N.P. Rath, Organometallics 18
(1999) 404;

3318
R. Shankar, A. Joshi / Journal of Organometallic Chemistry 691 (2006) 3310–3318
[5] (a) J.P. Banovetz, Y.-L. Hsiao, R.M. Waymouth, J. Am. Chem. Soc.
(h) V.K. Dioumaev, K. Rahimian, F. Gauvin, J.F. Harrod,
Organometallics 18 (1999) 2249;
115 (1993) 2540;
(i) B.J. Grimmond, J.Y. Corey, Organometallics 18 (1999) 2223;
(j) B.J. Grimmond, N.P. Rath, J.Y. Corey, Organometallics 19
(2000) 2975;
(b) U. Herzog, R. West, Macromolecules 32 (1999) 2210.
[6] Y. Hatanaka, S. Okada, S.-Y. Onozawa, Y. Suzuki, M. Tanaka,
Chem. Lett. (2001) 600.
(k) B.J. Grimmond, J.Y. Corey, Organometallics 19 (2000) 3776;
(l) W.H. Campbell, T.K. Hilty, L. Yurga, Organometallics 8 (1989)
2615;
(m) S. Bourg, R.J.P. Corriu, M. Enders, J.J.E. Moreau, Organomet-
allics 14 (1995) 564;
(n) J.Y. Corey, X.-H. Zhu, J. Organomet. Chem. 439 (1992) 1;
(o) J.Y. Corey, X.-H. Zhu, T.C. Bedard, L.D. Lange, Organomet-
allics 10 (1991) 924;
(p) J.Y. Corey, X.-H. Zhu, Organometallics 11 (1992) 672;
(q) H.-G. Woo, R.H. Heyn, T.D. Tilley, J. Am. Chem. Soc. 114
(1992) 5698;
[7] (a) R. Shankar, A. Saxena, A.S. Brar, J. Organomet. Chem. 628
(2001) 262;
(b) R. Shankar, A. Saxena, A.S. Brar, J. Organomet. Chem. 650
(2002) 223.
[8] C. Chatgilialoglu, A. Guerrini, M. Lucarini, G.F. Pedulli, P.
Carozza, G.D. Roit, V. Borzatta, V. Lucchini, Organometallics 17
(1998) 2169.
[9] R. Shankar, A. Joshi, Macromolecules 38 (2005) 4176.
[10] (a) J.L. Spier, Adv. Organomet. Chem. 17 (1979) 407;
(b) I. Ojima, in: S. Patai, P.Z. Rappoport (Eds.), The Chemistry of
Organosilicon Compounds (Part 2), Wiley, New York, 1989, p. 1479.
[11] (a) B.D. Karstedt, US Patent 3,775,452, 1973;
(b) G. Chandra, P.Y. Lo, P.B. Hitchcock, M.F. Lappert, Organo-
metallics 6 (1987) 191;
(c) P.B. Hitchcock, M.F. Lappert, N.J.W. Warhurst, Angew. Chem.
Int. Engl. 30 (1991) 438.
[12] F.C. Schilling, F.A. Bovey, J.M. Zeigler, Macromolecules 19 (1986)
2309.
[13] Y. Kimata, H. Suzuki, S. Satoh, A. Kuriyama, Organometallics 14
(1995) 2506.
[14] M.F. Lipton, C.M. Soreson, A.C. Sadler, R.H. Shapiro, J. Organo-
met. Chem. 186 (1980) 155.
(r) H.-G. Woo, J.F. Walzer, T.D. Tilley, J. Am. Chem. Soc. 114
(1992) 7047.
[3] (a) T.D. Tilley, Acc. Chem. Res. 26 (1993) 22, and references cited
there in;
(b) V.K. Dioumaev, J.F. Harrod, J. Organomet. Chem. 521 (1996) 133;
(c) Y. Obora, M. Tanaka, J. Organomet. Chem. 595 (2000) 1.
[4] (a) Y.-L. Hsiao, R.M. Waymouth, J. Am. Chem. Soc. 116 (1994)
9779;
(b) J.F. Harrod, Polym. Prep. (Am. Chem. Soc., Div. Polym. Chem.)
28 (1987) 403;
(c) H. Qui, Z. Du, J. Polym. Sci., Polym. Chem. Ed. 27 (1989) 2861.