Novel cross-linking monomer based on benzocyclobutene for an application in microelectronics

Article abstract of DOI:10.1007/s11172-020-3049-3

A new cross-linking monomer, allyl-bis(benzocyclobuten-4-yl)methylsilane (ABCBMS), was synthesized and its physical properties were investigated. Copolymers of ABCBMS with triethylene glycol dimethacrylate (TGM-3) and bis(methacryloylethylene glycol) carbonate (BMCC-2) were obtained by photo- and thermal polymerization. The final cross-linking of the copolymers took place at 200 °C. Homo- and copolymers of ABCBMS, ABCBMS-TGM-3 (50: 50) and ABCBMS—BMCC-2 (50: 50) exhibit high thermal stability (T_d5% = 460, 359, and 352 °C, respectively) and good dielectric properties (ε = 2.7–2.8 and tg δ = (3.7–6.7)?10^?4 at 10 GHz).

Full text of DOI:10.1007/s11172-020-3049-3

Russian Chemical Bulletin, International Edition, Vol. 69, No. 12, pp. 2396—2400, December, 2020
2396
Brief Communications
Novel cross-linking monomer based on benzocyclobutene
for an application in microelectronics
K. S. Levchenko,a G. E. Adamov,b K. A. Chudov,b P. S. Shmelin,b A. Yu. Kalashnikov,c and E. P. Grebennikovb
аN. S. Enikolopov Institute of Synthetic Polymer Materials of Russian Academy of Sciences,
70 ul. Profsoyuznaya, 117393 Moscow, Russian Federation
bCentral Technological Research Institute "Technomash",
4 ul. Ivana Franko, 121108 Moscow, Russian Federation
cState Space Corporation ROSCOSMOS,
53 ul. Aviamotornaya, 111250 Moscow, Russian Federation.
E-mail: k.s.levchenko@gmail.com
A new cross-linking monomer, allyl-bis(benzocyclobuten-4-yl)methylsilane (ABCBMS), was
synthesized and its physical properties were investigated. Copolymers of ABCBMS with tri-
ethylene glycol dimethacrylate (TGM-3) and bis(methacryloylethylene glycol) carbonate
(BMCC-2) were obtained by photo- and thermal polymerization. The final cross-linking of the
copolymers took place at 200 C. Homo- and copolymers of ABCBMS, ABCBMS—TGM-3
(50 : 50) and ABCBMS— BMCC-2 (50 : 50) exhibit high thermal stability (Td5% = 460, 359,
and 352 C, respectively) and good dielectric properties ( = 2.7—2.8 and tg  = (3.7—6.7)•10–4
at 10 GHz).
Key words: benzocyclobutene, polymers, oligomers, monomers, dielectric materials, in-
sulating materials, thermal polymerization, photopolymers, silanes, acrylates.
Polymers based on benzocyclobutene (BCB, 1) exhibit
exceptional properties and can be used in microelectronics
as dielectric materials.1 Fragments of BCB can be ther-
mally isomerized to form o-xylylene derivatives (2). The
latter, in turn, can react with active double bonds or
dimerize in the absence of such with the formation of
structures 3, 4, or 5, respectively (Scheme 1).2 This affords
BCB-based polymers unique properties, namely, ther-
mal stability (up to 550 C),3,4 low dielectric constant
( = 2.3—2.9 at 1 MHz), high chemical resistance, and
low moisture absorption.
The properties and options for the practical use of
materials based on BCB are described in a number of
publications.2,5—10 The BCB-based polymers are used in
electronics and photonics5—7 as dielectric layers in high-
density electronic modules.
The method of synthesis and properties of allyl-
bis(benzocyclobuten-4-yl)methylsilane (ABCBMS), a new
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2396—2400, December, 2020.
1066-5285/20/6912-2396 © 2020 Springer Science+Business Media LLC

New benzocyclobutene-based cross-linking agent
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
2397
Scheme 1
monomer containing two benzocyclobutene and one allyl
fragment capable of polymerization, are described in
this work.
chloromethylsilane (8) with water in the presence of pyr-
idine.12 The formation of product 10 may indicate an incom-
plete reaction between intermediate 8 and Grignard´s
reagent. Siloxane 10 is formed during purification of the re-
action product, including aqueous extraction. An increase in
the reaction time and temperature (up to 65C) upon addi-
tion of an excess of compound 6 and Mg led to the dis-
appearance of the siloxane in the reaction mixture. How-
ever, the yield of the target product did not increase, which
is obviously due to the accumulation of the by-products.
Synthesis of polymers and copolymers based on ABCBMS
with triethylene glycol dimethacrylate (TGM-3) and bis-
(methacryloylethylene glycol) carbonate (BMCC-2).
ABCBMS homopolymers were obtained by heating the
monomer at 200 C. Copolymers with TGM-3 and
BMCC-2 were synthesized in two steps using photo- and
thermal polymerization and compositions with different
monomer ratios (25 : 75, 50 : 50).
Results and Discussion
Synthesis of ABCBMS. Monomer ABCBMS was
synthesized from BCB in two steps (Scheme 2). First, BCB
(1) was brominated using molecular bromine in a two-
phase aqueous system according to a known method11 in
70—72% yield. Then, the target product 9 was obtained
in the form of a colorless liquid as a result of the reaction
of 4-bromobenzocyclobutene (6) with Mg in THF pro-
ceeding at 30—40 C in the presence of allyldichloromethyl-
silane 7. Two fractions were isolated after purification by
column chromatography. The main product is allyl-bis-
(benzocyclobuten-4-yl)methylsilane (9) (48% yield). In addi-
tion, siloxane 10 was present in an insignificant amount
(5—10%). The compound 10 was synthesized by us earlier by
the reaction of allylbicyclo-[4.2.0]octa-1,3,5-trien-3-yl-
In the first step, photopolymerizable compositions
were prepared by mixing appropriate amounts of mono-
Scheme 2
Reagents and conditions: i. Br2, H2O, –10—25 С, 24 h; ii—iv. Mg, THF, 30—40 С, 12 h.

Levchenko et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
Sample 2
Sample 3
Sample 4
ABCBMS/TGM-3
(50 : 50)
ABCBMS/TGM-3
(25 : 75)
Sample 1
TGM-3
Sample 5
Sample 6
Sample 7
ABCBMS
ABCBMS/BMCC-2
(50 : 50)
ABCBMS/BMCC-2
(25 : 75)
BMCC-2
Fig. 1. Photos of the obtained polymer samples.
mers with 3 wt.% of Darocur™ 4265 photoinitiator. The
compositions were then placed using a microdispenser
between two glass substrates pretreated with a dimethyl-
dichlorosilane/trimethylchlorosilane release agent (1 : 1).
The distance between the substrates was about 480 μm.
The polymerization process was carried out under irra-
diation with a UV lamp (395 nm) for 1 h. At the second
step, the sample was heated at 200 C for 2—3 h. As
a result, yellow or pale-yellow polymer films were ob-
tained (Fig. 1).
the [4+2]-cycloaddition products, was not clearly mani-
fested in the spectrum of the cross-linked ABCBMS
polymer in contrast to compound 10 with a similar struc-
ture described earlier.12 This is due to the fact that in the
ABCBMS molecule, there are two BCB fragments per
allyl substituent. As a result, transverse [4+2]-poly-
merization competes with the mechanism of [4+4]-cyclo-
addition and the mechanisms leading to the appearance
of structures 3 and 4 (see Scheme 1).
Thermal stability of ABCBMS and its copolymers with
TGM-3 and BMCC-2. The thermal behavior of polymers
The completeness of polymerization was monitored
by FTIR spectroscopy. Figure 2 shows the data obtained
for the ABCBMS homopolymer at various temperature
and various time conditions of the polymerization.
According to the data,13 the bands at 1462 cm–1 and
1491 cm–1 belong to the vibrations of the CH2 groups of
the cyclobutene ring in the initial state and after thermal
ring opening in the cross-linked polymer, respectively.
This is also confirmed by our results, namely, in the course
of polymerization, the band at 1462 cm–1 gradually de-
creases, while the intensity of the band at 1491 cm–1 in-
creases. We also observed an increase in the intensity of
the band at 1431—1433 cm–1, which characterizes the
vibrations of the C—H bond on the hybridized carbon
atom sp3.13 These vibrations are nonspecific and they are
also present in the spectrum of the starting monomer
(allyl substituent, methyl group, and cyclobutene ring).
In the course of polymerization, the number of sp3-hy-
bridized carbon atoms increases. Therefore, the intensity
of this band increases. The band at 1418 cm–1 character-
izes the vibrations of the C—H bond on the hybridized
carbon atom of the sp2-allyl fragment.14 Its monotonic
decrease in the course of polymerization indicates that
this atom is involved in the formation of a cross-linking
polymer chain. The band at 1452 cm–1, characteristic of
log (1/R)
1395
1433
1431
1418
1462
1491
3
2
1
1480 1460 1440 1420 1400 1380 T/C
Fig. 2. Fourier transform infrared spectroscopy data for the
ABCBMS homopolymer at various temperatures and duration
of thermal polymerization: the initial monomer (1), after
thermal polymerization at 200 C for 30 min (oligomer) (2)
and for 2.5 h (polymer) (3).

New benzocyclobutene-based cross-linking agent
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
2399
m (%)
served for the copolymer ABCBMS—TGM-3 (50 : 50)
and up to 352 C for the copolymer ABCBMS —BMCC-2
(50 : 50).
90
80
70
60
50
40
Dielectric properties of ABCBMS polymers. Dielectric
characteristics were measured at a frequency of 10 GHz
using the volume resonance method. For this purpose,
a measuring stand consisting of a G 4-83 high-frequency
generator, a DK 565 resonator, a V8-7 voltage ratio meter,
and an S1-76 oscilloscope, was used. We used a cylin-
drical cavity resonator with vibration of the H01p type
(where p is the number of half-waves that fit along
the length of the resonator) for measurements. The inner
diameter of the resonator was 50 mm, and the length
of the resonance cavity was 80 mm. The relative di-
electric constant ε was determined from the difference
between the resonant length of the resonator without
a sample and after placing a sample in it at a fixed reso-
nance frequency.
30
20
10
1
5
4
3
2
The tangent of dielectric losses tg was determined by
measuring the intrinsic Q-factor of the resonator with and
without sample, taking into account the change in the field
distribution and ohmic losses in the resonator walls when
the sample was placed. The obtained dielectric character-
istics for the ABCBMS homopolymer and copolymers
with BMCC-2 and TGM-3 indicate a high perspective of
using the synthesized benzocyclobutene derivative and
copolymers based on it as insulating materials, including
microwave technology.
Thus, we have successfully synthesized a new monomer,
allyl-bis(benzocyclobuten-3-yl)methylsilane (ABCBMS)
and a series of polymers based on it. The obtained polymers
are characterized by high thermal stability (>350 C), low
dielectric constants ( = 2.68 at 10 GHz), and good film-
forming ability. They can find application as dielectrics in
the microelectronic industry.
100
200 300
400
500 600 T/C
Fig. 3. TGA thermograms of ABCBMS (1), TGM-3 (2),
BMCC-2 (3) and their copolymers ABCBMS—TGM-3
(50 : 50) (4), ABCBMS—BMCC-2 (5).
was studied by thermogravimetric analysis (TGA) carried
out in a dynamic mode in the range of 50—700 C using
a TG50 M3 instrument (Mettler Toledo) with an accuracy
of determining the sample weight of up to 1 μg. The heat-
ing rate was 10 C min–1
.
The measurements were carried out under nitrogen
flow (200 mL min–1). Figure 3 shows the TGA data for
homopolymers and copolymers of ABCBMS, TGM-3,
and BMCC-2.
The highest thermal stability was found for the
ABCBMS homopolymer (Тd5% = 460 С); the weight loss
upon heating to 358 С was 0.5%. The lowest values were
determined for homopolymers TGM-3 (Td5% = 276 С)
and BMCC-2 (Td5% = 295 С). It was found that the ad-
dition of ABCBMS to TGM-3 and BMCC-2 significantly
increases the thermal stability of the copolymers. An in-
crease in the degradation temperature to 359 C was ob-
Experimental
NMR spectra of the samples were recorded using a Bruker
Avance 600 spectrometer in CDCl3 (1H, 500 MHz; 13C, 125 MHz).
High resolution mass spectra were obtained using a Bruker-
micrOTOF II spectrometer with electrospray ionization (ESI).
Table 1. Thermal and dielectric characteristics of ABCBMS homopolymer and co-
polymers at a frequency of 10 GHz at 25 C
Sample
 (Т = 25 C)
tg
T5%/C
ABCBMS (homopolymer)
ABCBMS—TGM-3 (50 : 50)
ABCBMS—TGM-3 (25 : 75)
ABCBMS—BMCC-2 (50 : 50)
ABCBMS—BMCC-2 (25 : 75)
BMCC-2
2.68
2.81
2.86
2.66
2.80
3.186
3.324
3.7•10–4
1.3•10–3
.1—
460
359
—
352
—
295
276
5.7•10–4
6.7•10–4
1.54•10–2
3.64•10–2
TGM-3

Levchenko et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
Analysis of the reaction mixtures to confirm the purity of all
products was carried out using thin layer chromatography on
Merck Silicagel 60 F254 UV-254 plates.
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4-Bromocyclobutene (6). Benzocyclobutene (23.7 g, 22.8 mmol)
was emulsified in water (240 mL) and cooled in an ice bath.
Bromine (11.7 mL) was added to the resulting mixture in small
portions at a temperature of 0—5 C. The reaction mixture was
allowed to warm to 20 C and then stirred for 24 h. The reaction
was monitored by thin layer chromatography. Then n-hexane
(50 mL) and sodium sulfite were added to the mixture. The
mixture was stirred at 20 C for 30 min until complete discolor-
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reduced pressure, 30 g of a colorless liquid were obtained.
1H NMR (300 MHz, CDCl3) : 7.40 (d, 1 H, J = 7.8 Hz); 7.26
(s, 1 H, J = 7.0), 6.99 (d, 1 H, J = 7.8 Hz); 3.31—3.10 (m, 4 H).
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benzocyclobutene (6.1 g, 33.3 mmol) was added dropwise to
a mixture of magnesium (900 mg, 37.5 mmol) and allylmethyl-
dichlorosilane (2.62 g, 16.9 mmol) in THF (30 mL) at a tem-
perature of 30—40 C. Thereafter, the mixture was kept for 12 h,
and then water (10 mL) was added thereto and the product was
extracted with ethyl acetate. The product was purified by column
chromatography. Yield 2.4 g. 1H NMR (500 MHz, CDCl3) :
7.40 (d, 2 H, J = 7.2 Hz); 7.24 (s, 2 H); 7.09 (d, 2 H, J = 7.2 Hz);
5.91—5.71 (m, 1 H); 4.95 (d, 1 H, J = 17.0 Hz); 4.90 (d, 1 H,
J = 11.0 Hz); 3.22 (s, 8 H); 2.09 (d, 2 H, J = 8.0 Hz); 0.55
(s, 3 H). 13C NMR (75 MHz, CDCl3) : 147.33, 145.58, 134.99,
134.51, 132.90, 128.63, 122.25, 113.97, 29.96, 29.83, 22.61,
–4.32. Found: m/z 313.1373 [M + Na]. Calculated: 313.1383
[M + Na].
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The work was financially supported by the Ministry of
Research and Education of the Russian Federation (Agree-
ment on the provision of grants from the federal budget
in the form of subsidies dated November 22, 2019
No. 075-15-2019-1694; internal number of the Agreement
05.604.21.0227, unique identificator RFMEFI60419X0227).
Received August 28, 2020;
accepted September 30, 2020