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BF 3 OEt 2 -coinitiated cationic polymerization of cyclopentadiene in the presence of water at room temperature

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The cationic polymerization of cyclopentadiene (CPD) with 1-(4-methoxyphenyl)ethanol (1)/BF3OEt2 initiating system in CH2Cl2:CH3CN 4:1 (v/v) mixture at room temperature and in the presence of water ([H2O]/[BF3OEt2] up to 8) is reported. The number-average molecular weights of obtained polymers increased in direct proportion to monomer conversion or initial monomer concentration (M n ≤ 4,000 g mol−1) in agreement with calculated values, and were inversely proportional to initiator concentration. Polymer MWDs were relatively narrow (M w/M n = 1.4–1.7) up to 60% of monomer conversion. It was also shown that regioselectivity of CPD polymerization with 1/BF3OEt2 initiating system did not depend significantly on water, monomer, or initiator concentration (1,4-structures content was nearly 60% in all cases).
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ORIGINAL PAPER
BF
3
OEt
2
-coinitiated cationic polymerization
of cyclopentadiene in the presence of water
at room temperature
Alexei V. Radchenko
Sergei V. Kostjuk
Ludmila V. Gaponik
Received: 8 October 2010 / Revised: 17 January 2011 / Accepted: 8 February 2011 /
Published online: 19 February 2011
Ó Springer-Verlag 2011
Abstract The cationic polymerization of cyclopentadiene (CPD) with 1-(4-
methoxyphenyl)ethanol (1)/BF
3
OEt
2
initiating system in CH
2
Cl
2
:CH
3
CN 4:1 (v/v)
mixture at room temperature and in the presence of water ([H
2
O]/[BF
3
OEt
2
]upto8)
is reported. The number-average molecular weights of obtained polymers increased
in direct proportion to monomer conversion or initial monomer concentration
(M
n
B 4,000 g mol
-1
) in agreement with calculated values, and were inversely
proportional to initiator concentration. Polymer MWDs were relatively narrow
(M
w
/M
n
= 1.4–1.7) up to 60% of monomer conversion. It was also shown that
regioselectivity of CPD polymerization with 1/BF
3
OEt
2
initiating system did not
depend significantly on water, monomer, or initiator concentration (1,4-structures
content was nearly 60% in all cases).
Keywords Cationic polymerization Cyclopentadiene Poly(cyclopentadiene)
Water-tolerant coinitiators
Introduction
Cyclopentadiene (CPD) is one of the most active monomers in cationic polymer-
ization leading to polymers with rigid cyclic repeated units and one double bond in
every monomer unit [1]. Therefore, poly(cyclopentadiene)s exhibit attractive
thermal and electronic properties such as low dielectric constants, excellent
transparency, and high glass transition temperatures [1, 2], on the one hand. On the
other hand, poly(cyclopentadiene)s easily crosslink into stable films by heat or air
[2]. The first study on the cationic polymerization of cyclopentadiene with different
A. V. Radchenko S. V. Kostjuk (&) L. V. Gaponik
Research Institute for Physical Chemical Problems of the Belarusian State University,
14 Leningradskaya st., 220030 Minsk, Belarus
e-mail: kostjuks@bsu.by; kostjuks@rambler.ru
123
Polym. Bull. (2011) 67:1413–1424
DOI 10.1007/s00289-011-0455-6
Lewis acids (SnCl
4
, FeCl
3
, BCl
3
, and TiCl
4
) was performed by Staudinger et al. in
the middle of 1920s [1]. In the late 1960s, important kinetic investigations were
carried out by Imanishi et al. using BF
3
OEt
2
alone or CCl
3
COOH in conjunction
with SnCl
4
or TiCl
4
[3]. Detailed investigations on the microstructure of
polycyclopentadienes (polyCPDs) were performed by Aso et al. at the same time
[4]. The very interesting studies were also performed in this period by Sigwalt and
Vairon, who used such weak Lewis acids as TiCl
3
OBu [5, 6] and Ph
3
C
?
SbCl
6
-
[7]
to synthesize poly(cyclopentadiene)s with high molecular weights (up to
M
n
* 200,000 g mol
-1
). However, no sufficient control over cationic polymeri-
zation of CPD was attained so far due to the low stability of growing cations and the
high reactivity of polymer double bonds [1]. Poor controllability was also seen in
recent reports on the polymerization of CPD catalyzed by transition metal
complexes [8, 9] or methylaluminoxane [2].
The possibility to control the molecular weight of poly(cyclopentadiene)s was
first reported by Sigwalt and Vairon at the beginning of 1980s [10, 11]. Recently,
Sawamoto et al. reported the possibility to control the molecular weight (up to
M
n
B 12,000 g mol
-1
) and molecular weight distribution (M
w
/M
n
= 1.2–1.4) of
poly(cyclopentadiene)s obtained by cationic polymerization of CPD using adduct
of CPD with hydrogen chloride as an initiator and SnCl
4
as a coinitiator [12].
The same authors showed that weak Lewis acids such as ZnBr
2
or ZnI
2
allowed to
control the regioselectivity in cationic polymerization of CPD: 70 and 76% of
1,4-structures in a polymer chain were obtained, respectively [13]. However, in all
these systems, strict anhydrous conditions and very low temperatures (e.g. –78 °C)
were required [1013].
During the last decade, the cationic polymerization in aqueous media has attracted
considerable attention [14]. This promising process allowed to overcome the two
main problems of the cationic polymerization, i.e. the necessity to conduct
polymerizations (i) in anhydrous conditions and (ii) at low temperatures. Particu-
larly, we developed an efficient initiating system, 1-(4-methoxyphenyl)ethanol (1)/
B(C
6
F
5
)
3
, for the cationic polymerization of p-methoxystyrene [15], p-hydroxysty-
rene [16], styrene [17], and also cyclopentadiene [16, 18] in aqueous media.
However, to date only low molecular weight polymers (M
n
B 3,000 g mol
-1
) could
be obtained using this technique [14]. An alternative to the polymerization in
aqueous media is the cationic polymerization in undried organic solvents, i.e. in the
presence of excess water toward Lewis acid (LA). Two water-tolerant Lewis acids,
i.e. BF
3
OEt
2
[1923] and B(C
6
F
5
)
3
[15, 17, 24], were effective in quasi-living
cationic polymerization of styrene and its derivatives in CH
2
Cl
2
or its mixtures with
CH
3
CN at [H
2
O]/[LA] = 1 to 100, and between 0 and 20 °C. Polymers with
M
n
B 15,000 g mol
-1
and relatively narrow molecular weight distribution (M
w
/
M
n
B 1.8) could be obtained under these conditions. The main advantage of this
technique is the simplification of the polymerization procedure, working with
solvents that do not require any purification or drying.
We have recently shown that quasi-living cationic polymerization of cyclo-
pentadiene could be performed using 1/B(C
6
F
5
)
3
initiating system in the presence of
excess water toward Lewis acid ([H
2
O]/[B(C
6
F
5
)
3
] = 0–5.5) at 20 °CinaCH
3
CN/
CH
2
Cl
2
mixture [18]. Since BF
3
OEt
2
is a cheaper and more widely used Lewis acid
1414 Polym. Bull. (2011) 67:1413–1424
123
than B(C
6
F
5
)
3
, we investigate in this work the cationic polymerization of
cyclopentadiene with BF
3
OEt
2
as a coinitiator. Particularly, the effect of water,
initiator, and monomer concentrations on the cationic polymerization of the
cyclopentadiene is discussed.
Experimental
Materials
Cyclopentadiene (CPD) was obtained by the retro Diels–Alder reaction of
dicyclopentadiene at 180 °C over calcium hydride and then distilled in argon
atmosphere over CaH
2
and kept at -30 °C. 1-(4-Methoxyphenyl)ethanol (Alfa
Aesar, 95%) was distilled over CaH
2
under reduced pressure. CH
2
Cl
2
was treated
with sulfuric acid until the acid layer remained colorless, then washed with aqueous
NaHCO
3
, dried over CaCl
2
, refluxed for 3 h with CaH
2
, and distilled twice from
CaH
2
under an inert atmosphere. Acetonitrile (Carlo Erba, 99.5%) was refluxed with
P
2
O
5
for 4 h, distilled over P
2
O
5
and finally distilled in argon atmosphere over
CaH
2
.BF
3
OEt
2
(Fluka) was distilled twice under reduced pressure before use.
Ethanol (Sigma-Aldrich, C96%) was used as received.
Polymerization
The polymerization reactions were carried out under argon atmosphere at 20 °Cin
glass tubes. As an example of a typical procedure, polymerization was initiated by
adding a solution of BF
3
OEt
2
(0.061 g, 2.8 9 10
-4
mol) in 0.5 mL of CH
3
CN to a
mixture of a total volume of 10.3 mL consisting of cyclopentadiene (0.66 g,
1 9 10
-2
mol), 1-(4-methoxyphenyl)ethanol (0.031 g, 2 9 10
-4
mol), CH
2
Cl
2
(8 mL), acetonitrile (1.5 mL), and water (0.023 g, 1.3 9 10
-3
mol). After a
predetermined time, 1.0–1.5 mL aliquots were withdrawn and poured into an excess
of ethanol containing a small amount of ammonia. The precipitated polymer was
separated from the solution by centrifugation and dried in vacuum. Monomer
conversions were determined gravimetrically. Polymer samples were stabilized by
N-phenyl-2-naphthylamine and kept under argon atmosphere at -30 °C[12, 13].
Polymer characterization
Size-exclusion chromatography (SEC) was performed on a Agilent 1200 apparatus
with Nucleogel GPC LM-5, 300/7,7 column thermostated at 30 °C. The detection
was achieved by a differential refractometer and tetrahydrofuran (THF) was eluted
at a flow rate of 1.0 mL/min. The calculation of molar mass and polydispersity was
based on polystyrene standards (Polymer Labs, Germany). The molecular weights
measured by SEC were corrected using the following equation: M
n
= (M
n
(SEC)
- 684)/1.28 [18].
1
H NMR (400 MHz) spectra were recorded in CDCl
3
at 25 °Con
a Bruker AC-400 spectrometer calibrated relative to the solvent peak in reference to
tetramethylsilane standard (
1
H NMR).
Polym. Bull. (2011) 67:1413–1424 1415
123
Results
In our previous article on the B(C
6
F
5
)
3
-coinitiated cationic polymerization of
cyclopentadiene, we have shown that control over molecular weight (M
n
) and
molecular weight distribution (MWD) could be achieved only if a mixture of
CH
2
Cl
2
with CH
3
CN was used as a polymerization solvent [18]. Moreover, the
optimal ratio (in terms of reaction rate as well as the molecular weight control) of
CH
2
Cl
2
to CH
3
CN was found to be 4:1 v/v [18]. Therefore, the cationic
polymerization of cyclopentadiene coinitiated by BF
3
OEt
2
was investigated in the
same solvent mixture.
Influence of water
In this series of experiments, the influence of added water concentration ([H
2
O] = 0
to 0.31 M) on the BF
3
OEt
2
-coinitiated cationic polymerization of cyclopentadiene
was investigated in order to find an optimal [H
2
O]/[BF
3
OEt
2
] ratio.
The cationic polymerization of CPD was extremely fast (Fig. 1a) and exothermic
(DT = 20–30 °C, up to boiling) in the absence of additional water ([H
2
O] = 0 M).
The addition of water ([H
2
O] = 0.12 M; [H
2
O]/[BF
3
OEt
2
] = 3:1) allowed to
depress the polymerization rate and control the reaction exotherm (DT B 8 °C).
Importantly, the reaction proceeded with a reasonable rate even at [H
2
O]/
[BF
3
OEt
2
] = 8:1 ([H
2
O] = 0.31 M), although the monomer conversion in this
particular case did not exceed 50% (see Fig. 1a). In all cases, the polymerization
processes were characterized by an initial period of high reaction rate followed by
considerably slower monomer consumption (Fig. 1a). This observation is consistent
with larger ionization rate for initiator relative to polymer chain ends [2527]. A
similar behavior was observed by us earlier during the investigation of the cationic
polymerization of styrene with 1/BF
3
OEt
2
[23], 1/B(C
6
F
5
)
3
[17], and 2-phenyl-2-
propanol/AlCl
3
OBu
2
[28, 29] initiating systems, respectively. On the other hand, the
irreversible termination through b-H elimination as well as due to slow hydrolysis
of Lewis acid [23], which operated under investigated conditions, is also responsible
for the observed decrease in the reaction rate.
Figure 1b shows the evolution of M
n
and MWD with conversion for the CPD
polymerization with 1/BF
3
OEt
2
initiating system at different water concentrations.
The molecular weights of the obtained poly(cyclopentadiene)s were directly
proportional to the conversion and followed the theoretical line in all experiments,
i.e. the cationic polymerization of cyclopentadiene using 1/BF
3
OEt
2
initiating
system proceeded in a quasi-living (in the sense of Ivan [30]) fashion even in the
presence of excess of water toward Lewis acid. In addition, the water concentration
in the reaction mixture almost did not influence the molecular weight distribu-
tion evolution with conversion (Fig. 1b); MWD remained relatively narrow
(M
w
/M
n
B 1.6) up to 60% of monomer conversion but some broadening of
MWD was observed at higher conversions (Fig. 1b).
SEC traces, normalized by their conversion, of polymers obtained with
1/BF
3
OEt
2
initiating system are shown in Fig. 1c. The peak maximum shifts with
conversion to high molecular weight region but distributions overlap in the low
1416 Polym. Bull. (2011) 67:1413–1424
123
molar mass range. This observation also indicates that irreversible termination
(but at a low extent) leading to the formation of dead chains took place under
investigated conditions.
From the obtained results, we can conclude that optimal concentration of water in
the systems is 0.12 M ([H
2
O]/[BF
3
OEt
2
] = 3:1). At this water concentration, the
polymerization proceeded at a high reaction rate while keeping a good control over
M
n
. Interestingly, almost the same amount of water is contained in wet (not purified)
mixture of CH
2
Cl
2
with CH
3
CN [24]. These results also suggest that the added
water serves as a reversible chain transfer agent providing an efficient intercon-
version of the active species into dormant hydroxyl-terminated species [1923].
Further investigations were conducted at [H
2
O] = 0.12 M.
Effect of initiator concentration
To confirm the quasi-living nature of cyclopentadiene polymerization with 1/
BF
3
OEt
2
initiating system, the influence of initiator concentration on the polymer-
ization process was studied. As shown in Fig. 2a, the rate of reaction increased with
increasing initiator concentration, while the polymerization was very slow in the
absence of an initiator. Besides, at low initiator concentrations, the polymerizations
0306090120
25
50
75
100
[H
2
O]=0 M
[H
2
O]=0.12 M
[H
2
O]=0.31 M
Conversion (%)
Time (min)
(a)
0255075100
1000
2000
3000
[H
2
O]=0 M
[H
2
O]=0.12 M
[H
2
O]=0.31 M
M
n
Conversion (%)
(b)
1.0
1.5
2.0
2.5
M
w
/M
n
6789
Conv.=65%
M
n
=2460 g/mol
M
w
/M
n
=1.9
Elution Volume (mL)
Conv.=55%
M
n
=2150 g/mol
M
w
/M
n
=1.69
Conv.=40%
M
n
=1750 g/mol
M
w
/M
n
=1.66
(c)
Fig. 1 a Conversion vs. time, b M
n
and M
w
/M
n
vs. conversion dependences and c SEC traces for CPD
polymerization with 1/BF
3
OEt
2
initiating system at different water concentrations and at 20 °Cin
CH
2
Cl
2
:CH
3
CN 4:1 (v/v): [BF
3
OEt
2
] = 0.04 M; [CPD] = 0.93 M; [1] = 0.019 M. The straight line in
b corresponds to theoretically calculated M
n
.[H
2
O] for c is 0.12 M
Polym. Bull. (2011) 67:1413–1424 1417
123
terminated at incomplete monomer conversion indicating that irreversible termina-
tion took place (Fig. 2a).
According to Fig. 2b, the molecular weights of synthesized poly(cyclopentadi-
ene)s increased with monomer conversion irrespective of the initiator concentration in
contrast to cyclopentadiene polymerization with H
2
O/BF
3
OEt
2
initiating system
(without an addition of initiator) where M
n
s decreased with increasing monomer
conversion. Moreover, for cyclopentadiene polymerization with 1/BF
3
OEt
2
initiating
system the M
n
s were inversely proportional to initiator concentration, while
experimental values of M
n
correlated well with theoretical ones. In contrast, for
polymerization done at low initiator concentration, the deviation of M
n
from
calculated value was observed due to competitive initiation by water (Fig. 2b). The
observed deviation of experimental values of M
n
from the theoretical line at the earlier
stages of the polymerization could be attributed to a fast addition of ca. 20 molecules
of CPD to ionized 1 during the first ionization-propagation-ion pair collapse cycle
leading to higher DP
n
than predicted by [M]/[I] ratio [28, 29]. In addition, the
molecular weight distribution almost did not depend on the initiator concentration and
leveled off at M
w
/M
n
B 1.95 for higher initiator concentrations ([1] = 3.8 9 10
-2
and 1.9 9 10
-2
M, respectively), while broader MWD (M
w
/M
n
* 2.2) was obtained
at low initiator concentration (Fig. 2b). These data indicated that the cationic
polymerization of cyclopentadiene with 1/BF
3
OEt
2
initiating system proceeded in a
quasi-living fashion, at least for [1] C 1.9 9 10
-2
M.
Influence of monomer concentration
Most quasi-living cationic polymerizations are successfully carried out at low
monomer concentrations to avoid a high exothermicity of the reaction. However,
from a practical point of view, a high monomer concentration would be desired.
Therefore, the influence of monomer concentration on the cationic polymerization
of cyclopentadiene with 1/BF
3
OEt
2
initiating system was investigated.
According to Fig. 3a, the reaction rate slightly decreased with increasing
monomer concentration, most probably, due to the decrease of solvent polarity
024650100
25
50
75
100
[I]= 3.8·10
-2
M
[I]= 1.9·10
-2
M
[I]= 9.1·10
-3
M
[I]= 0 M
Conversion (%)
Time (min)
(a)
0255075100
500
1000
1500
2000
2500
3000
3500
4000
[I]= 3.8·10
-2
M
[I]= 1.9·10
-2
M
[I]= 9.1·10
-3
M
[I]=0 M
M
n
Conversion (%)
1.0
1.5
2.0
2.5
M
w
/M
n
(b)
Fig. 2 a Conversion vs. time, b M
n
and M
w
/M
n
vs. conversion dependences for CPD polymerization
with 1/BF
3
OEt
2
initiating system at different initiator concentrations and at 20 °CinCH
2
Cl
2
:CH
3
CN 4:1
(v/v): [BF
3
OEt
2
] = 0.04 M; [CPD] = 0.93 M; [H
2
O] = 0.12 M. The straight lines in b correspond to
theoretically calculated M
n
1418 Polym. Bull. (2011) 67:1413–1424
123
through replacing part of solvent by less polar monomer. The number-average
molecular weights and molecular weight distributions versus conversion plots are
shown in Fig. 3b. The molecular weights of poly(cyclopentadiene)s were directly
proportional to the conversion and followed the theoretical lines, with the exception
of polymerizations performed at high monomer concentration ([M] = 3.72 M),
where slight deviation of M
n
from calculated value was observed. Note, in order
to control the polymerization reaction at highest monomer concentration ([M] =
3.72 M), the higher concentration of water ([H
2
O = 0.31 M]) was used (polymer-
ization is uncontrolled and highly exothermic at [H
2
O] = 0.12 M). Whatever the
monomer concentration, the molecular weight distribution of obtained polymers
almost did not change with conversion and typically lied below 1.95 (Fig. 3b).
These results indicate that polymerization proceeds in a quasi-living fashion even at
high monomer concentration ([M] = 3.72 M).
Polymer characterization
The microstructure of the obtained polymers was analyzed by
1
H NMR
spectroscopy (Fig. 4). Characteristic signals appear in two regions, one for the
olefinic protons (D; a
1,2
and a
1,4
) in the region 5.4–6.0 ppm and the other one for the
aliphatic protons, between 1.4 and 2.9 ppm. The latter region consists of three well-
resolved parts: 1.4–1.8 ppm (A; c
1,4
), 1.8–2.2 ppm (B; d
1,2
), and 2.2–2.9 ppm
(C; c
1,2
,b
1,2
, and b
1,4
). The 1,4-structures content ([1, 4]) was calculated by the
following equation: [1, 4], % = [2A/(A ? B ? C)] 9 100, where A, B, C, and D
corresponding peak intensities [4, 12, 13, 18]. The olefinic/aliphatic proton ratio
(D/A ? B ? C) can be easily calculated from
1
H NMR spectrum and should equal
0.5 when the polymer chains contain only 1,2- and 1,4-structures. If the olefinic/
aliphatic proton ratio deviates from 0.5, the polymer chain contains other structures
formed due to the side reactions.
The chain-end structure of poly(cyclopentadiene)s obtained with 1/BF
3
OEt
2
initiating system was also examined by
1
H NMR spectroscopy (Fig. 4). The signals
0306090120
25
50
75
100
[M]=0.93 M
[M]=1.86 M
[M]=3.72 M
Conversion (%)
Time (min)
(a)
0255075100
1000
2000
3000
4000
[M]=0.93 M
[M]=1.86 M
[M]=3.72 M
M
n
Conversion (%)
(b)
1.0
1.5
2.0
2.5
M
w
/M
n
Fig. 3 a Conversion vs. time, b M
n
and M
w
/M
n
vs. conversion dependences for CPD polymerization
with 1/BF
3
OEt
2
initiating system at different monomer concentrations and at 20 °CinCH
2
Cl
2
:CH
3
CN
4:1 (v/v): [BF
3
OEt
2
] = 0.04 M; [1] = 0.019 M; [H
2
O] = 0.12 M ([H
2
O] = 0.31 M for experiment with
[M] = 3.72 M). The straight lines in b correspond to theoretically calculated M
n
Polym. Bull. (2011) 67:1413–1424 1419
123
ascribed to the end groups are clearly separated from the large characteristic signals
of main-chain olefinic (D) and aliphatic (A, B, C) protons: CH
3
–(a; 1.2 ppm),
CH
3
O– (b; 3.8 ppm); CH
3
O–C
6
H
4
–(e; 6.8, 7.1 ppm) at the a-end; the signals at
4.6 ppm and 4.8 ppm were attributed to the –CH–OH (x) with 1,2- and 1,4-chain-
end structure of the last monomeric unit, respectively [18].
Discussion
In this section, the scope and limitation of 1/BF
3
OEt
2
initiating system in the cationic
polymerization of cyclopentadiene will be discussed. Table 1 summarizes the main
results from this work in terms of chain-end functionality and microstructure of the
poly(cyclopentadiene)s synthesized at different reaction conditions. As it is evident
from Table 1, at low initiator concentration the functionality at the a-end is
considerably lower than unity (see runs 1 and 2 in Table 1) indicating that under
these conditions competitive initiation by H
2
O takes place. This conclusion is in
agreement with the observed deviation of experimental M
n
s from the theoretical line
at low initiator concentration (see Fig. 2b and discussion therein). Moreover, the
functionality at the x-end is less than 0.1 at medium monomer conversion and
decreases down to 0 at higher conversions (see runs 1 and 2 in Table 1), i.e. an
irreversible b-H elimination (see Scheme 1) is predominant under these conditions.
At higher initiator concentrations, the functionality at the a-end is close to unity (runs
3–8, Table 1), indicating that protic initiation is negligible. A better functionality at
the x-end (F
n
(x) = 0.6–0.65) was obtained at low monomer conversions and at
[H
2
O] = 0.12 M (see runs 4, 5 in Table 1). No hydroxyl groups at the x-end were
7654321
OCH
3
CH
CH
3
OH
a
1,2
a
1,4
n
m
c
1,2
b
1,2
d
1,2
b
1,4
b
1,4
c
1,4
α
ω
ε
β
H
{
1,2-structure
1,4-structure
c
1,4
b
1,4
a
1,4
pp
m
ε
a
1,2
a
1,4
ω
β
b
1,4
b
1,2
c
1,2
d
1,2
c
1,4
α
CHCl
3
5.00 4.75 4.50
Fig. 4
1
H NMR spectrum of poly(cyclopentadiene) obtained by cyclopentadiene polymerization with
1/BF
3
OEt
2
initiating system in CH
2
Cl
2
:CH
3
CN 4:1 (v/v) at 20 °C: [CPD] = 0.93 M; [BF
3
OEt
2
] =
0.04 M; [1] = 0.019 M; [H
2
O] = 0.12 M; Conv. = 43%; M
n
= 2710; M
w
/M
n
= 1.82
1420 Polym. Bull. (2011) 67:1413–1424
123
observed when polymerization proceeded without an addition of water (run 3,
Table 1).
With respect to polymer microstructure, almost all polymers contain 55–60% of
1,4-structures in a polymer chain, i.e. the regioselectivity of the polymerization of
cyclopentadiene coinitiated by BF
3
OEt
2
does not depend significantly on water,
initiator, or monomer concentrations. However, in the absence of an additional
water (run 3, Table 1), an equal amount of 1,2- and 1,4-enchainment ([1, 4] = 51%)
was found. In addition, the olefinic/aliphatic proton ratio is close to 0.5 for all
polymers investigated (with the exception of those obtained at low initiator
concentration) indicating that such side reactions as crosslinking, isomerization or
double bond migration are absent under the investigated conditions.
A tentative polymerization mechanism is presented in Scheme 1. The main
feature of this mechanism is the fact that free Lewis acid, which is formed by the
Table 1 Functionality and microstructure of poly(cyclopentadiene)s obtained with 1/BF
3
OEt
2
initiating
system
Run [I] (mM) [H
2
O] (M) Time (min) Conv. (%) F
n
(a)
a
F
n
(x)
b
[1, 4]
c
(%) D/A ? B ? C
d
1 9.1 0.12 2 44 0.65 0.05 57 0.47
2 9.1 0.12 121 64 0.45 0 55 0.48
3 19 0 1 71 0.80 0 51 0.50
4 19 0.12 2 55 1.05 0.65 56 0.49
5 19 0.31 3 6 1.15 0.60 57 0.49
6 19 0.31 32 36 0.90 0.25 57 0.50
7
e
19 0.31 128 43 0.80 0.30 59 0.50
8 38 0.12 1 60 0.90 0.35 60 0.51
Polymerization conditions: CH
2
Cl
2
:CH
3
CN 4:1 (v/v); temperature 20 °C; [CPD] = 0.93 M;
[BF
3
OEt
2
] = 0.04 M.
a
Functionality at the a-end, calculated based on
1
H NMR data as follows: F
n
(a) = [2I
a
9 DP
n
(SEC)/
3D] 9 100%, see Polymer characterization section for details
b
Functionality at the x-end, calculated based on
1
H NMR data as follows: F
n
(x) = [2I
x
9 DP
n
(SEC)/
D] 9 100%.
c
The content of 1,4-structures, calculated from
1
H NMR spectroscopy data.
d
The olefinic/aliphatic proton ratios by
1
H NMR spectroscopy.
e
[CPD] = 1.86 M
O
CH
3
CH
3
OH
BF
3
BF
3
OEt
2
H
2
OBF
3
O
CH
3
CH
3
[HOBF
3
]
nCPD
O
CH
3
CH
3
(n-m)
(m-1)
[HOBF
3
]
O
CH
3
CH
3
OH
(n-m)
m
BF
3
CPD
O
CH
3
CH
3
(n-m)
(m-1)
H
2
OBF
3
β
-
H
a
b
s
t
r
a
c
t
i
o
n
Scheme 1 Proposed mechanism for cyclopentadiene polymerization with 1/BF
3
OEt
2
initiating system
Polym. Bull. (2011) 67:1413–1424 1421
123
dissociation of BF
3
OEt
2
or BF
3
OH
2
complexes, participates in the initiating and
propagating steps (see Scheme 1) similarly to the styrene polymerization coinitiated
by AlCl
3
OBu
2
[28, 29] or TiCl
4
OBu
2
[31, 32]. At appropriate reaction conditions
([I] C 1.9 9 10
-2
M; [H
2
O] C 0.12 M), the polymerization proceeds in a quasi-
living fashion through the reversible activation of C–OH bond of initiator or
dormant polymer chains. An excess of water toward Lewis acid under investigated
conditions serves as a reversible chain transfer agent providing an efficient
interconversion of the active species into dormant hydroxyl terminated species. At
the same time, however, the polymerization is accompanied by irreversible b-H
elimination leading to the loss of desired C–OH functionality at the x-end. This side
reaction becomes predominant under monomer-starved conditions. In addition,
BF
3
OEt
2
is not really water-tolerant Lewis acid and it slowly hydrolyzed in the
presence of excess water [23]. This process also leads to irreversible termination
leading to incomplete monomer conversion.
Conclusions
We have shown here that BF
3
OEt
2
in conjunction with 1-(4-methoxyphenyl)ethanol
(1) could be used for the quasi-living cationic polymerization of cyclopentadiene
under mild experimental conditions, i.e. at room temperature and in the presence of
excess of water toward Lewis acid ([H
2
O]/[BF
3
OEt
2
] = 3–8). Poly(cyclopentadi-
ene)s with molecular weight (M
n
B 4,000 g mol
-1
) and reasonable MWD (M
w
/
M
n
B 1.9) could be prepared under these conditions. It was shown that the
polymerization proceeded through reversible activation of C–OH terminal group,
but the reaction was also accompanied by irreversible b-H elimination, which
became predominant under monomer-starved conditions. The investigation of
cyclopentadiene polymerization at different monomer concentrations revealed that
the reaction proceeded in a quasi-living fashion even at monomer concentration as
high as 3.72 M. We have also shown that the regioselectivity of the polymerization
of cyclopentadiene using 1/BF
3
OEt
2
initiating system did not depend significantly
on water, initiator, or monomer concentration: all obtained polymers contain
55–60% of 1,4-structures in a polymer chain.
Acknowledgments This work was supported by Belarusian Foundation for Fundamental Research. The
authors thank Dr. Francois Ganachaud and Dr. Irina Vasilenko for their useful comments on the article.
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... However, such monomers as conjugated 1,3-dienes are also active in cationic polymerization and interesting for industrial applications because of availability of raw material and elastomeric properties of the resulting products [14,15]. Despite diene cationic polymerization is usually hard to control due to the presence of many side reactions, isoprene [16][17][18][19][20][21][22][23], 1,3-pentadiene [24][25][26][27][28][29], and cyclopentadiene [30][31][32][33][34][35][36][37] polymerizations were highly reported. Special attention was paid to isoprene cationic polymerization as it can mimic the biosynthesis of natural rubber [38][39][40][41]. ...
... For myrcene polymerization, when the monomer was added to 1 solution at 0°C, the polymerization did not take place if the temperature was maintained at 0°C indicating that 1 did not dissociate at this temperature to generate carbocations (Run 8, Table 3). Increasing the temperature to 20°C led to relatively quick [34][35][36][37][38][39][40][41] oligomerization, yielding very low molar mass compounds (Run 9, Table 3). Interestingly, even at an equimolar ratio between the monomer and the initiator, oligomerization took place producing oligomers with slightly lower molar mass and dispersity (Run 10, Table 3). ...
... Finally, ocimene cationic polymerization was investigated with 1 as the initiator. The reaction rate was lower and the degree of [34][35][36][37][38][39][40][41] unsaturation was higher compared to myrcene (Run 9 vs. 16 and 17, Table 3). It can be explained by the lower steric availability of one double bond in ocimene structure, which led to lower activity in polymerization and in side-reactions. ...
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