High temperature thermosets derived from benzocyclobutene-containing main-chain oligomeric carbosilanes

Article abstract of DOI:10.1016/j.polymer.2014.09.030

A series of benzocyclobutene-carbosilane thermosets derived from benzocyclobutene-containing oligomeric silylenephenylene (PBSEPs) were synthesized. DSC analysis results demonstrated that the reaction of PBSEPs presumably took place within the temperature

Full text of DOI:10.1016/j.polymer.2014.09.030

Polymer 55 (2014) 5680e5688
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Polymer
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High temperature thermosets derived from benzocyclobutene-
containing main-chain oligomeric carbosilanes
Ke Cao, Lu Yang, Yawen Huang*, Guanjun Chang, Junxiao Yang*
State Key Laboratory Cultivation Base for Nonmetal Composite and Functional Materials, Southwest University of Science and Technology, Mianyang
621010, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of benzocyclobutene-carbosilane thermosets derived from benzocyclobutene-containing oligo-
meric silylenephenylene (PBSEPs) were synthesized. DSC analysis results demonstrated that the reaction
of PBSEPs presumably took place within the temperature range of 200e350 ꢀC. FT-IR, 1H NMR and 13C
NMR results demonstrated that this exothermic reaction was attributed to a [4 þ 2] cycloaddition. DSC
results further revealed that benzocylcobutene linked with silylene exhibited lower exothermic tem-
perature compared with that linked with Si(CH3)2. Chemical simulation results attributed the lower
temperature to lower steric hindrance and greater activity. DSC and TGA analysis of the thermosets
showed high Tg and thermostability, and good pyrolysis yield.
Received 12 June 2014
Received in revised form
9 September 2014
Accepted 14 September 2014
Available online 20 September 2014
Keywords:
Benzocyclobutene
Carbosilanes
Thermosets
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
thermosets. Moore et al. have prepared main-chain BCB-contain-
ing polyether, polyester and polyamide via condensation poly-
BCB-based thermosets have emerged as a new generation of
high-performance materials due to the good thermal stability,
chemical resistance, and low dielectric constants and dissipation
factors [1,2]. In addition, the reaction of benzocyclobutene (BCB)
has several interesting features, such as free of catalyst, initiator
and gaseous by products, and nonpolar chemical character [3,4].
BCB resins were generally prepared from BCB-bicapped monomer
and polymer via intramolecular DielseAlder reaction of o-quino-
dimethanes which are formed upon thermolysis of cyclobutene [5].
A typical BCB resin is the CYCLOTENE (trademark of the DOW
Chemical Company), which is derived from bis (benzocyclobutenyl)
divinyltetramethylsiloxane (DVSBCB) monomer. Recently, poly-
mers bearing benzocyclobutene pendant groups have aroused
much interest [6e8]. Typically, poly (4-vinylBCB), developed by
Baskaran and co-workers [6], have exhibited good processing
property, thermal stability and low dielectric constant [6,7]. How-
ever, most of current BCB-based thermosets hardly exhibit
acceptable thermal stability over 450 ꢀC.
merization of 3,6-disubstituted BCB [9]. However, since then, new
main-chain BCB-containing polymers has never been reported.
The integration of organic-inorganic hybrid backbone with BCB
structure is another effective strategy. Polycarbosilanes are a
typical family of organiceinorganic hybrid polymers with SieC
bonds in the backbone [10]. Compared with the well-established
siloxane-BCB polymers, high temperature polycarbosilanes have
been studied to a less degree in despite of their unique properties
such as good thermal and hydrolytic stability, low Tg, as well as
serving as high-yield precursors to SiC ceramics [11e15]. Con-
ventional routes used to prepare polycarbosilanes include ring-
opening polymerization (ROP), polycondensation, hydrosilylation
reaction, etc [10]. Accordingly, cross-linking of these poly-
carbosilanes generally involved post-functionalization, end-
capping and adding cross-linking agents. These cross-linking
methods generally result in the introduction of impurities or low
degree of cross-linking.
Integrating carbosilane and main-chain BCB structure may give
rise to a series of new thermosets because they effectively combine
several structural elements which were commonly required in high
temperature polymers: 1) organic-inorganic hybrid backbone
affording high bonding energy; 2) main-chain rigid species; 3)
main-chain cross-linkable sites. To the best of our knowledge, this
type of structures was not studied, because synthetically some
BCB-based thermosets derived from main-chain BCB-contain-
ing polymers may provide alternative route to high temperature
* Corresponding authors.
E-mail addresses: huangyawen@swust.edu.cn (Y. Huang), yangjunxiao@swust.
edu.cn (J. Yang).
http://dx.doi.org/10.1016/j.polymer.2014.09.030
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

K. Cao et al. / Polymer 55 (2014) 5680e5688
5681
challenges exist, no matter in constructing main-chain curable
polycarbosilanes or in introducing BCB moieties into main-chain of
polymers.
2.3. Monomer and precursors synthesis and curing
2.3.1. Synthesis of 3,6-dibromobenzocyclobutene
As part of continuous interest in developing high performance
benzoyclclobutene derived thermosets [16], we attempted to syn-
thesize a new family of BCB/carbosilane-based thermosets derived
from BCB/silylene-containing main-chain oligomeric silylenephe-
nylene (PBSEPs). The synthesis and the curing behaviors of these
precursors and the thermal stability of the final resins have been
investigated systematically. Moreover, it should be noted that
silylene was introduced into main-chains of precursors. We expect
that it can act as a dienophile to undergo [4 þ 2] cycloaddition
reaction [17] with benzocyclobutene to lower the reaction tem-
perature and enhance the cross-linking. Hence, the chemistry of
main-chain BCB in the presence of the dienophiles has also been
investigated in depth.
This compound was synthesized according to reported method
[18]. IR (KBr),
y
(cmꢁ1): 3063 (AreH), 1458 (in-plane aromatic ring
stretching), 1088 (CeBr). 1H NMR (400 MHz, CDCl3, ppm):
(s, 2H, ArH), 3.09 (s, 4H, eCH2CH2).
d
¼ 7.16
2.3.2. Synthesis of 3,6-bis(vinyldimethylsilyl)benzocyclobutene
Magnesium 1.28 g (52.5 mmol) was added to a dried flask. Then
a solution of 6.59 g (25 mmol) 3,6-(dibromo)benzocyclobutene in
20 mL freshly distilled THF was added dropwise over a period of
2 h. The reaction mixture was heated to 40 ꢀC and stirred contin-
uously for 2 h. After cooled to room temperature, a solution of
6.65 g (55 mmol) of (vinyl-dimethyl-chloro)silane in 15 mL freshly
distilled THF was added into the reaction mixture over a period of
1 h at ambient temperature. The reaction mixture was then heated
to 40 ꢀC, and stirred for 2 h. Then the mixture was allowed to stand
overnight and was poured into distilled water. The aqueous phase
was extracted with petroleum ether (30 mL ꢂ 3). The organic layers
were combined and dried over anhydrous sodium sulfate. After
removal of the solvents under reduced pressure, the residue was
purified by chromatography (petroleum ether) to give 2 of 5.57 g
2. Experimental
2.1. Materials
Chlorodimethylvinylsilane (97%) was purchased from Alfa
Aesar Company. Chlorodimethylsilane (95%) was purchased from
TCI Chemicals. Tri-o-tolylphosphine and Palladium (II) acetate
were purchased from the Acros Chemicals. Tetrahydrofuran (THF)
and toluene were distilled over sodium under nitrogen before
used. N, N-Dimethylacetamide (DMAc) and tributylamine were
distilled over CaH2 under reduced pressure. All other chemicals
were purchased from Chengdu Kelong Reagent Corp (China), and
used as received unless otherwise stated.
(82% yield). IR (KBr),
y
(cmꢁ1): 3008 (C]CeH), 1593 (C]C), 1457
(in-plane aromatic ring stretching), 1248 (SieCH3 bending), 1128
(Si-Ph stretching). EI-MS (m/z, %): 272 (Mþ, 15), 257 (Mþ-15, 100).
1H NMR (600 MHz, CDCl3, ppm):
J ¼ 20.2, 14.6 Hz, eCH]CH2), 6.10 (dd, 2H, J ¼ 14.6, 3.8 Hz, eCH]
CH2), 5.81 (dd, 2H, J ¼ 20.2, 3.8 Hz, eCH]CH2), 3.26 (s, 4H,
eCH2CH2), 0.34 (s, 12H, eCH3). 13C NMR (600 MHz, CDCl3, ppm):
d
¼ 7.36 (s, 2H, ArH), 6.35 (dd, 2H,
d
¼ 150.98, 137.90, 133.47, 132.50, 131.77, 31.19, e2.91.
2.2. Measurements
2.3.3. Synthesis of 1,4-bis(vinyldimethyl)silylbenzene
This compound was prepared by an identical procedure as 3,6-
Fourier transform infrared (FTIR) measurements at
400e4000 cmꢁ1 were conducted on a Nicolet FTIR 5700 spec-
trophotometer at room temperature and the sample films were
prepared by casting solution on KBr plates. 1H NMR and 13C NMR
spectra were recorded on the Bruker 400 MHz and 600 MHz
instruments using deuterated chloroform as the solvent and
tetramethylsilane as the internal reference. The molecular
weight and polydispersity index were measured by size exclu-
sion chromatography (SEC) equipped with a refractive index
detector (Optilab rEX) and a Dawn Heleos light scattering de-
tector. Gel permeation chromatography (GPC) was performed in
bis((vinyl-dimethyl)silyl)benzocyclobutene. Yield: 84%. IR (KBr),
y
(cmꢁ1): 3048 (C]CeH stretching), 2958, 2899 (CeH stretching),
1593 (C]C stretching), 1249 (in plane aromatic stretching), 1133 (Si-
Ph stretching). EI-MS (m/z, %): 246 (Mþ, 2), 231 (Mþ-15,100).1H NMR
(400 MHz, CDCl3, ppm),
d
¼ 7.55 (s, 4H, AreH), 6.31 (dd, 2H, J ¼ 20.2,
14.6 Hz, eCH]CH2), 6.08 (dd, 2H, J ¼ 14.6, 3.8 Hz, eCH]CH2), 5.81
(dd, 2H, J ¼ 20.2, 3.8 Hz, eCH]CH2), 0.37 (s, 12H, eCH3). 13C NMR
(400 MHz, CDCl3, ppm):
d
¼ 139.21, 137.87, 133.13, 132.83, ꢁ3.0.
2.3.4. Synthesis of precursors
A typical experimental procedure to synthesize precursor, (P1,
shown in Scheme 2) was as following: to a two-necked round-
bottom flask were sequentially added 1,4-di(bromo)benzene
(0.2361 g, 1 mmol), compound 1 (0.2727 g, 1 mmol), palladium(I1)
THF using gel columns (5
mm, 105 nominal pore diameter and
5
m
m, 104 nominal pore diameter, Mz-Gel SDplus) and a flow
rate of 1 mL/min. Differential scanning calorimetry (DSC) was
performed on powdered samples using a TA Q200 differential
scanning calorimeter at a heating rate of 10 ꢀC/min under a ni-
trogen atmosphere. Thermal analysis was conducted in nitrogen
atmosphere from 10 ꢀC to the desired temperature using TA/TGA
Q500 thermogravimetric (TG) analyzer. UVevis spectra were
recorded in a 1-cm-length path quartz cell with a Shimadzu
Spectronic UV3150 (Shimadzu, Japan) apparatus at a resolution
of 0.5 nm. Dielectric constants were measured using a Novo-
control dielectric analyzer on thin films which were spin-coated
on a silicon wafer from the polymer solution in Mesitylene. Gold
electrodes were sputtered on the surface and back of the film.
Dielectric constants of polymer were obtained in the frequency
range (1e10 MHz), in which the sample films were prepared
acetate (0.0112 g,
5 mol %), tris(o-methylphenyl)-phosphine
(0.0914 g, 30 mol %), and DMAC (1 mL) under a nitrogen atmo-
sphere. After a homogeneous solution was obtained, tributylamine
(0.7 mL, 3 mmol) was added to the solution. The reaction mixture
was stirred at 110 ꢀC for 48 h and then poured into an excess
amount of ethanol (100 mL). The oligomer was collected by filtra-
tion, purified by chromatography using chloroform as eluent and
precipitated from methanol. The collected orange solid was dried in
a vacuum oven at 40 ꢀC to a constant weight. Yield: 48%.
Mn ¼ 2000 Da, Mw/Mn ¼ 1.5 (GPC, polystyrene calibration). IR (KBr),
y
(cmꢁ1): 3033, 2956, 2919, 1602, 1459 (in-plane aromatic ring
stretching),1251 (SieCH3 bending),1129 (Si-Ph stretching). 1H NMR
(600 MHz, CDCl3, ppm):
d
¼ 7.42 (br, ArH), 7.11 (br, ArH), 6.93e6.96
with a thickness of 5
mm and a diameter of 2 cm. Polymer
(br, eCH]CHe), 6.58e6.61 (br, eCH]CHe), 3.23 (br, 4H,
thicknesses were measured on an AMBAOS XT-200 Surface
Profilometer.
eCH2CH2), 0.44 (br, SieCH3). 13C NMR (600 MHz, CDCl3, ppm):

5682
K. Cao et al. / Polymer 55 (2014) 5680e5688
Scheme 1. Synthesis route to 3,6-bisubsituted benzocyclobutene derived monomers.
d
¼ 149.86, 143.27, 130.66, 129.93, 127.07, 125.63, 125.46, 30.13,
stretching). 1H NMR (600 MHz, CDCl3, ppm):
d
¼ 7.58 (br, ArH), 7.11
ꢁ3.72.
(br, ArH), 6.90 (br, eCH]CHe), 6.50 (br, eCH]CHe), 3.34 (br, 4H,
Other polymers, P2, P3 and P4, were prepared by similar pro-
eCH2CH2e), 0.43 (br, SieCH3). 13C NMR (600 MHz, CDCl3, ppm):
cedure and their characterization data are given as following.
d
¼ 142.40, 141.96, 132.31, 131.36, 128.73, 125.60, 29.59, ꢁ3.51.
P2: Yield: 56%. Mn ¼ 1300 Da; Mw/Mn ¼ 1.2 (GPC, polystyrene
calibration). IR (KBr),
y
(cmꢁ1): 3025, 2959, 2917, 1601, 1457 (in-
2.3.5. Curing of precursors
plane aromatic ring stretching), 1260 (SieCH3 bending), 1121 (Si-Ph
In a dry and clean reactor, the precursors (100 mg) were dis-
solved in 1 mL of mesitylene and were refluxed for 10 h under the
nitrogen atmosphere at 240e250 ꢀC. Transparent polymer solu-
tions with relatively high viscosity were obtained. Polymer films
were fabricated from pre-crosslinked polymers by spin-coating
method (KW-4A Spin Processor, 2000 rpm), using silicon wafer as
the substrate and mesitylene as solvent. The residue solvent was
allowed to evaporate under vaccum at 140 ꢀC. The film was cured in
muffle furnace at 300 ꢀC for 4 h and 400 ꢀC for 1 h under the ni-
trogen atmosphere to produce highly cross-linked materials.
stretching). 1H NMR (600 MHz, CDCl3, ppm):
d
¼ 7.35e7.62 (br,
ArH), 7.19 (br, ArH), 7.01 (br, eCH]CHe), 6.53 (br, eCH]CHe),
3.23 (br, 4H, eCH2CH2), 0.44 (br, SieCH3). 13C NMR (600 MHz,
CDCl3, ppm):
d
¼ 150.11, 143.44, 136.33, 130.92, 129.93, 127.23,
126.12, 125.97, 30.33, ꢁ3.72.
P3: Yield 50%. Mn ¼ 2300 Da; Mw/Mn ¼ 1.4 (GPC, polystyrene
calibration). IR (KBr),
y
(cmꢁ1): 3023, 2955, 2917, 1598, 1459 (in-
plane aromatic ring stretching), 1256 (SieCH3 bending), 1129 (Si-Ph
stretching). 1H NMR (600 MHz, CDCl3, ppm):
d
¼ 7.35 (br, ArH),
7.10e7.18 (br, ArH), 6.93 (br, eCH]CHe), 6.50 (br, eCH]CHe),
3.12e3.43 (br, 8H, eCH2CH2), 0.41 (br, SieCH3). 13C NMR (600 MHz,
3. Results and discussion
CDCl3, ppm):
d
¼ 150.07, 142.28, 141.79, 131.29, 130.87, 128.76,
127.26, 125.67, 30.37, 29.60, ꢁ3.48.
3.1. Monomer and precursor synthesis
P4: Yield: 46%. Mn ¼ 1100 Da; Mw/Mn ¼ 1.2 (GPC, polystyrene
calibration). IR (KBr),
y
(cmꢁ1): 3045, 2955, 2918, 1597, 1480 (in-
The synthesis routes to 3,6-disubsituted benzocyclobutene
derived monomers, including 3,6-dibromobenzocyclobutene and
plane aromatic ring stretching), 1249 (SieCH3 bending), 1133(Si-Ph
Scheme 2. Synthesis of benzocyclobutene-containing polysilylenephenylenes, P1eP4.

K. Cao et al. / Polymer 55 (2014) 5680e5688
5683
Table 1
Supporting Information). In contrast to PBSEPs, C]C bonds were
replaced by CeC bonds in MP-1 and benzocyclobutene units were
replaced by benzene units in MP-2. As a result, the reaction of
benzocyclobutene on MP-1 only involves a coupling reaction be-
tween benzocyclobutene itself, while that of MP-2 only involves a
coupling reaction between C]C bond itself.
Synthesis and molecular weight characteristics of PBSEPs.
Sample Monomer a Monomer b Temp. Time Yield (%) Mn
Mw/Mn
(ꢀC)
(h)
(g/mol)
P1
P2
P3
P4
2
2
2
4
4
3
105
100
105
105
50
48
36
48
48
12
48
56
50
46
67
2000
1300
2300
1100
4700
1.5
1.2
1.4
1.2
1.7
DBBPa
1
1
1
FITR spectra of P1eP4 and MP-1 were shown in Fig. 1. All the
spectra, except that of MP-1, showed the stretching of C]C on
benzene ring at 1600 cmꢁ1 and the bending of CeH on ethylene at
985 cmꢁ1. The band at 985 cmꢁ1 was absent in the spectrum of MP-
1. The in-plane aromatic ring stretching showed considerable
change when different groups were substituted on 3,6-sites of
benzocyclobutene [23]. When 3,6-sites of benzocyclobutene were
linked with ethylene (P4), this stretching vibration occurred at
1480 cmꢁ1. When linked with Si(CH3)2 (P1 and P2), it shifted to
lower wavenumber at 1457 cmꢁ1. Both of two bands were observed
in the spectrum of P3. The lower wavenumber indicated that the
in-plane aromatic ring stretching vibration was inhibited to some
degree. The inhibition of vibration implied the release of the ring
stress, which stabilized the cyclobutene, in other words, reduced
the ring-opening activity of cyclobutene.
MP-1
Polymerization conditions: monomers
¼
1
mmol, DMAC
¼
1.0 mL,
Pd(OAc)2 ¼ 0.05 mmol, P(o-Tol)3 ¼ 0.3 mmol, NBu3 ¼ 3 mmol. Molecular weight
was measured by GPC/LS.
a
DBBP: 4,40-dibromobiphenyl.
3,6-bis(vinyldimethylsilyl) benzocyclobutene, are listed in Scheme
1. 3,6-bis[(vinyl-dimethyl)silyl]benzocyclobutene was synthesized
by Grignard reaction of 3,6-dibromobenzocyclobutene with vinyl-
dimethylchlorolsilane. The silylethylene group on this monomer
enables the Heck reaction with 3,6-dibromobenzocyclobutene or
dibromobenzene to produce oligomeric carbosilanes (Scheme 2).
As is well known, Heck reaction presented advantages in producing
alternating conjugated copolymers and introducing functional/
reactive groups in main-chain of polymer [19e21]. 3,6-
dibromobenzocyclobutene was synthesized according to the re-
ported method [18]. Pd(OAc)2/P(o-Tol)3 was used as catalytic sys-
tem as generally reported [21,22]. Prior to the synthesis of
benzocyclobutene-containing polymers, a model monomer, 1,4-
bis(dimethylvinyl)silyl benzene, was used for Heck coupling reac-
tion to screen reaction conditions. From the data shown in Table S1
(see Supporting Information), it is apparent that the use of equiv-
alent molar ratio of two monomers is required to give acceptable
yield. Good yield of 47% was finally obtained when using feed ratio
of 1:1, Pd(OAc)2/P(o-Tol)3 ratio of 5.6:1 and DMAC as solvent. Under
this screened conditions, benzocyclobutene-containing polymers
were prepared. Table 1 shows that good yields in the range of
45e70 % were attained. It appears that the replacement of benzene
by benzocyclobutene affected the polymerization to low degree.
As-prepared polymers, except P2, are soluble in CH2Cl2, CHCl3,
DMF, THF and DMSO.
Fig. 2 showed 1H NMR spectra of P1eP4. The peaks at 6.35e6.65
and 6.75e7.00 were easily assigned to the hydrogen on ethylene
neighbored to silicon and benzene ring, respectively. The assign-
ment of hydrogen signal on benzene ring was complicate as indi-
cated in Fig. 2b. It is worthy noted that the protons of
benzocyclobutene aromatic ring showed two signals in the range of
7.1e7.2 and 7.3e7.3 ppm for P1eP3. However, only one signal was
observed for the protons on benzene ring. One possible explanation
for this result is that the presence of cyclobutene resulted in the
structure asymmetry of aromatic ring. Moreover, these signals were
also absent for P4 and MP-1. By comparing the structures of these
polymers, one can conclude that the splitting appeared under two
conditions: 1) the main-chain should have double bonds. The
double bonds possibly induced cis-trans isomerism. 2) 2- and 5-
sites of benzocyclobutene were linked with Si(CH3)2. The steric
hindrance of Si(CH3)2 groups hindered the rotation of benzocy-
clobutene. These two structure features diversified chemical envi-
ronment around the protons on aromatic ring.
The protons of cyclobutene of P3 showed obvious two peaks,
while this phenomenon was absent for other polymers. Summari-
zing the chemical shifts of the proton signal, it was apparent that
In order to investigate the structures and curing behavior of
benzocyclobutene-containing oligomeric (silylenephenylene)s
(PBSEPs), model oligomers, MP-1 and MP-2 were synthesized (see
Fig. 1. FT-IR spectra of P1eP4 and MP-1.

5684
K. Cao et al. / Polymer 55 (2014) 5680e5688
Fig. 2. 1H NMR spectra of P1eP4 and MP-1 (in CDCl3).
the hydrogen signal of the benzocyclobutene linked with Si(CH3)2
is in the range of 3.1e3.3 ppm, while that of the benzocyclobutene
linked with ethylene is in the range of 3.3e3.5 ppm. We suspected
that the methyl groups on Si(CH3)2, which were close to cyclo-
butene groups, may induce shielding effect on cyclobutene to some
degree, consequently leading to a shift to high magnetic field. This
shielding effect implied a bulky steric encumbrance.
possible explanation of this result is that the bis-phenyl structure
improves the stacking of polymer main-chains.
The most unusual feature of exothermic behavior seen in Fig. 3 is
the high temperatures for MP-1, at which the reaction exotherm
was observed. From data in Table 2, the temperatures of maximum
heat release of MP-1 were observed to be around 425 ꢀC, which was
apparently higher than that of P1eP4. This result reveals that the
coupling reaction between benzocyclobutene to generate cyclo-
octyl takes place at high temperature. The DHrxn of MP-1 was also
3.2. Curing of precursors
greatly lower than other polymers. When the polymers contained a
dienophile (polymers P1eP4), an exotherm also takes place around
400 ꢀC. Apparently, this should be attributed to the intramolecular
DielseAlder reaction between o-quinodimethanes derived from
benzocyclobutene, consequently producing mainly cyclooctyl. Be-
sides, a reaction exotherm, which has a greatly lowered maxima
within the range of 270e280 ꢀC, appeared.
The obvious drop of reaction temperature in the presence of
dienophiles may be related to the occurrence of [4 þ 2] cycload-
dition reaction. It should be noted that the contribution of the re-
action of the dienophiles by themselves was excluded because DSC
DSC scans of P1eP4 provided the information related to glass
transition temperature (Tg) and exothermic reaction (Fig. 3). In
general, the Tg points were reversible provided that the sample
remained sufficiently below the reaction exotherm. The exothermic
transition was irreversible as shown by subsequent cooling and
heating cycles. The Tg data from DSC curves was summarized in
Table 2. The highest Tg of MP-1 (117 ꢀC) was theoretically explained
by the absence of double bonds, which resulted in a reduction of
the mobility of polymer chains. The higher Tg of P2 compared with
P1 could be attributed to the presence of rigid bis-phenyl in P2. A
melting transition was only observed in DSC curve of P2. One
curve of MP-2 showed
a slight exotherm (See Supporting
Information Fig. S1). To verify the [4 þ 2] cycloaddition reaction,
the structure of cured resins was monitored by FT-IR and 1H NMR
analysis. Fig. S2 showed FT-IR spectra of P3 heated at 220 ꢀC with
time (see Supporting Information). As can be seen, the in-plane
aromatic ring stretching around 1480 cmꢁ1 almost disappeared.
This result indicated that most of the benzocyclobutene linked with
ethylene reacted. The disappearance of the bending of CeH on
ethylene at 980 cmꢁ1 pointed to the possible occurrence of [4 þ 2]
cycloaddition reaction. Fig. S3 showed the typical 1H NMR spectra
of original PBSEP and slightly cross-linked one which was obtained
under heating at 220 ꢀC for 0.5 h. Obviously, a new hydrogen signal
appeared around 2.30 ppm. This chemical shift was close to that of
cyclooctyl (2.6e2.8) as reported [20]. 13C NMR spectra of P3 and the
one cured at 200 ꢀC were shown in Fig. 4. One can see that a series
of new signals at 13.1, 20.3, 21.7 and 42.7 ppm appeared after
curing. This result was different from the singlet signal generally
observed for cyclooctyl [23]. In addition, as reported earlier the
signals at 13.1 and 42.7 ppm are assignable to the carbons on
cyclohexyl which were linked with Si(CH3)2 and arene, respec-
tively. All of above results support the formation of cyclohexyl
through [4 þ 2] cycloaddition reaction.
The occurrence of [4 þ 2] cycloaddition reaction was also sup-
ported by photoluminescence. Fig. 5 showed photoluminescence
spectra of PBSEPs, typically P3, with curing process. One can see
Fig. 3. DSC curves of polymers P1eP4 and MP.

K. Cao et al. / Polymer 55 (2014) 5680e5688
5685
Table 2
Summary of DSC data of P1eP4 and MP-1.
a
Sample
Tg
32
50
73
39
171
Trxn
D
Hrxn (KJ/mol)b
P1
P2
P3
P4
282
278
ꢁ85
ꢁ74
264, 389c
279
ꢁ79, ꢁ51c
ꢁ99
MP-1
427
ꢁ18
a
Temperature of maximum heat release.
b
Exotherm per mole which was obtained by curing exotherm per weight divides
molecular weight of repeating unit.
c
Two exothermic peaks were observed for P3, thus corresponding to two Trxn and
DHrxn.
that original P3 displayed light emission centered at 532 nm. This
emission peak gradually shifted to low wavelength with curing at
200 ꢀC. This phenomenon was possibly explained by the shortening
of conjugation length resulted from the destruction of silylene
through the [4 þ 2] cycloaddition reaction.
Although the benzocyclobutene linked with Si(CH3)2 groups
presumably took [4 þ 2] cycloaddition reaction, almost identical
exothermic reaction temperature around 280 ꢀC was observed. In
contrast, the effect of substituents may mainly reflect on the change
Fig. 5. Fluorescence spectra of polymer P1 after curing at different time.
In order to further explore the effect of substituents on 3,6-sites
of benzocyclobutene, the quantum chemical calculations of relative
energies, vibrational frequencies, and transition state structures
have been carried out at the B3-LYP/TZVP level of theory using the
TURBOMOLE program package. The transforming process of two
model molecules, 3,6-vinylbenzocyclobutene (A) and 3,6-
bis(trimethylsilyl) benzocyclobutene (B), are shown in Fig. 6. It is
of
in Table 2, maximum
butene linked with ethylene. When benzocyclobutene was linked
with Si(CH3)2 (P1 and P2), Hrxn was lowered to different degrees.
By comparing the Hrxn of P1 and P2, one can find an inverse
correlation between Hrxn and Tg. This result indicated that the
benzocyclobutene reaction was inhibited when chain mobility
D
Hrxn, which was indicative of the degree of reaction. From data
D
Hrxn was found for P4 whose benzocyclo-
D
D
D
apparent that DH of the reaction for these two molecules is almost
decreased. In a sense, the lowest DHrxn of MP-1 was also related to
its highest Tg.
the same. These results revealed that the lower reaction tempera-
ture for vinyl linked benzocyclobutene is mostly attributed to ki-
netic factors, not thermodynamic factors. In contrast, the active
energy of A, ~46.2 kcal/mol, is greatly lower than that of B,
~72.5 kcal/mol. According to the simulated transformation pro-
cesses of A and B, the higher active energy is because the Si-
aromatic bonds and CeC bonds in cyclobutene experience great
transformation of configuration and conformation.
Another notable observation indicated in Fig. 2 is the different
chemistry of 3,6-disubstituted benzocyclobutene linked with
Si(CH3)2 and ethylene. In detail, the intensity of hydrogen signal
assigned to Si(CH3)2-linked benzocyclobutene at 7.3e7.4 decreased
by ~30%, while that assigned to ethylene-linked benzocyclobutene
at 7.1e7.2 decreased by ~50%. This result revealed that the [4 þ 2]
cycloaddition reaction took place presumably on ethylene-linked
benzocyclobutene.
The electronic factors were considered as a possible explanation
for the different kinetic of reaction. Kirchhoff et al. have shown that
13
Fig. 4. C NMR spectra of P3 and the one cured at 200 ꢀC for 0.5 h (CDCl3 as solvent).

5686
K. Cao et al. / Polymer 55 (2014) 5680e5688
Fig. 6. The energy, bonding angels in aromatic ring and configuration transformation with reaction process for compounds A (left) and B (right).
Table 3
It is well known that steric hindrance can significantly affect the
Quantum chemical calculations of 3,6-vinylbenzocyclobutene (A) and 3,6-
bis(trimethylsilyl)benzocyclobutene (B).
rates of [4 þ 2] cycloaddition reactions. Reaction rates are
decreased by bulky substituents that either impede the approach of
diene/dienophile or reduce the stability of the required s-cis
conformation of the diene. This explains well that when benzocy-
clobutene linked with Si(CH3)2, the reaction was inhibited. Chem-
ical simulation results as indicated in Fig. 7 revealed that as
compared with that linking with ethylene, the rotation activation
energy of benzocyclobutene linked with Si(CH3)2 was elevated by
as much as 75 kcal/mol. This result indicated that benzocyclobu-
tene linked with Si(CH3)2 was more difficult to reach a proper
conformation for the reaction. Based on the above analysis, steric
factors was also possibly responsible for the inhibition of the
cycloaddition reaction of benzocyclobutene linking with Si(CH3)2.
Model
compounds
Molecular simulation
HOMO LUMO Band
Energy of reaction Energy of barrier
(kcal/mol)
(kcal/mol)
(eV) (eV) gap
A
B
ꢁ5.628 ꢁ2.622 3.006 20.194
ꢁ5.866 ꢁ1.545 4.321 19.612
ꢁ4.733 ꢁ2.991 1.742
46.182
72.445
Transition
state of A
Transition
state of B
ꢁ5.123 ꢁ2.326 2.797
substituting electron acceptors on the arene ring lowers the reac-
tion temperature [10]. Theoretically, substituting the Si(CH3)2
group, which was generally considered as a weak electron donating
group, would induce an inverse effect. In this work, ab initio cal-
culations were used to explore the effect of electronic factors. From
data indicated in Table 3, the band gap of model compound, 3,6-
vinyl-benzocyclobutene, was ~1.31 eV lower than that of another
model compound, 3,6-bis(trimethylsilyl)-benzocyclobutene. In
addition, the transition state of 3,6-vinyl-benzocyclobutene also
showed lower band gap than that of 3,6-bis(trimethylsilyl)-ben-
zocyclobutene. These lowered band gaps, which were possibly
resulted from the conjugated structure of phenylene, may be
related to the high reactivity of 3,6-vinyl-benzocyclobutene.
An alternative explanation for the different chemistry of 3,6-
disubstituted benzocyclobutenes could be based on steric factors.
3.3. Properties of thermosets
The thermal properties of thermosets were evaluated by DSC
and TGA. No reaction exotherm was found in the DSC curves of
cured PBSEPs (Fig. 8a). In addition, no obvious Tg was found, except
that cured resins derived from P1 and MP1 appeared to exhibit
glass transition within the range of 100e150 ꢀC. These results
revealed that the Tg of thermosets was greatly elevated and lay
above the degradation temperature. The TGA curves in nitrogen are
shown in Fig. 8b. One can see that P3 exhibited the highest 5%
weight loss temperature (T5%) around 492 ꢀC and pyrolysis yield at
51.8%. This result was possibly attributed to the presence of more
cross-linking sites. By comparing the TGA data of other polymers, it
Fig. 7. The energy transformation of benzocyclobutene with the rotation of CaromaticeSi and CaromaticeC bonds for the benzocyclobutene linked with Si(CH3)2 (left) and vinyl (right).

K. Cao et al. / Polymer 55 (2014) 5680e5688
5687
Fig. 8. Properties of thermosets derived from P1eP4 and MP-1. (a) DSC curves; (b) TGA curves; (c) dielectric constantefrequency curve of P3.
was apparent that the decomposition temperature was greatly
lowered in the absence of dienophile. Comparing with most current
BCB thermosets, the main-chain BCB/dienophile-containing poly-
carbosilanes exhibited an enhancement of high temperature
performance.
remains a challenge. Further systematic studies are on-going in
our group.
Acknowledgments
The TGA curves indicated that the polymers in air have relatively
low thermostability as compared with that in nitrogen (see
Supporting Information, Fig. S4). This result may be related to the
oxidization of residual silylene. Moreover, we still find some similar
trend which was shown in the TGA curves in nitrogen. P3 exhibited
the best thermostability with the maximum T5% around 450 ꢀC and
pyrolysis ratio above 20%. As compared with P2 and P4, P1
exhibited relatively poor thermostability in the range of
300e600 ꢀC but relatively high pyrolysis ratio. To our surprise, in
contrast to the case in nitrogen, P2 exhibited the lowest pyrolysis
ratio. We speculate that the higher pyrolysis ratio in nitrogen for P2
is attributed to the rigid diphenyl groups while the lower pyrolysis
ratio in air may be related to its lower reaction exotherm in more
degree. Dielectric constant of thermosets derived from P3 was
around 2.5 when the frequency is within the range of 1e10 MHz
(Fig. 8c). Such polymers had potential use as low dielectric mate-
rials, ceramic precursors, etc.
This work was supported through grant from China-NSFC (Grant
No 51107106), the Innovation Team Foundation Incubation Program
of Sichuan Province (13TD0022) and Open Project of State Key
Laboratory Cultivation Base for Nonmetal Composites and Func-
tional Materials of Southwest University of Science and Technology
(11zxfk26 and 13zxfk06).
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.polymer.2014.09.030.
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