9648
Ind. Eng. Chem. Res. 2010, 49, 9648–9654
Retarding Effect of Aromatic Solvents on Cobalt(II)-Based Catalyst System
during Synthesis of High cis-1,4-Polybutadiene
Archana Singh,† Siddharth Modi,† N. Subrahmanyam,† Pradip Munshi,‡ Vinod K. Upadhyay,‡
Raksh Vir Jasra,‡ and Madhuchhanda Maiti*,‡
Reliance Technology Group, Vadodara Manufacturing DiVision, Reliance Industries Ltd., Vadodara-391346,
Gujarat, India, and Institute of Technology, Nirma UniVersity, Sarkhej-Gandhinagar Highway,
Ahmedabad-382481, Gujarat, India
The present study discloses the influence of aliphatic (cyclohexane) and aromatic solvents with different
π-electron densities viz. toluene, o-xylene, ethylbenzene (EB), and p-diethylbenzene (PDEB) on the cobaltcatalyzed high cis-polymerization of 1,3-butadiene. Comparative study in cyclohexane and toluene has been
conducted in view to understand the effect of solvent on catalyst reactivity, molecular weight of the polymer,
and polymerization kinetics. At given reaction conditions conversion is highest (68%) in cyclohexane, followed
by toluene (27%) and EB (8%). Respective polymerization rate constants are found to be 0.0170, 0.0020, and
0.0008 min-1. Higher viscosity average molecular weight (M) of polymer observed in toluene (M ) 4.68 ×
105) compared to that in cyclohexane (M ) 2.37 × 105) indicates that the presence of lower number of active
sites leads to higher molecular weight. Cyclohexane shows activity at a lower water:alkylaluminum ratio
than does toluene; as the π-electron of toluene reduces the Lewis acidity of hydrolyzed diethylaluminum
chloride (DEAC) more than cyclohexane, where influence of π-electron is absent. The effect of different
solvents has also been demonstrated by respective shifting of wavelength (λ) of solvent π f π*, cobalt 4A2
f 4T1(P), as well as 4T1 g (F) f 4T1 g (P) and activated butadienyl π(HOMO) f π*(LUMO) transitions.
1. Introduction
Butadiene rubber (BR) is the second largest volume synthetic
rubber produced worldwide, next to styrene-butadiene rubber
(SBR). The major use of high cis-BR is in tires, and more than
70% of the polymer produced goes into tire-treads and sidewalls.
This is due to its excellent abrasion resistance, or less tread
wear, and low rolling resistance, which leads to good fuel
economy.1 The global demand for rubber can be estimated at
about 30 million tonnes by 2020 with synthetic rubber share
going up to nearly 67%.2 In other words, even a small
improvement in the process can be of great benefit because of
the huge demand of high cis-BR. This necessitates continuous
research in this area.
Polymerization of 1,3-butadiene (BD) to high cis-BR is
commonly done by solution polymerization technique using
Ziegler-Natta catalysts. The catalysts are based on titanium,
cobalt, nickel, or neodymium in presence of alkylaluminum as
a cocatalyst.3-8 Among them, a cobalt-based system has
received the earliest recognition at the commercial level and
thus a sizable literature is also available.7-12
With cobalt-based systems, benzene is the most commonly
used solvent.7-12 However, primarily because of environmental
and health concerns of benzene, attempts were made in finding
alternative solvents.13-21 In that aspect, different aliphatic,
aromatic, and olefinic solvents are being tried as alternatives.
But polymerization rate in aliphatic solvents, e.g., cycloalkanes
and reactivity of catalysts in aromatic solvents except benzene
are the critical issues.14-19 In addition, there are only a few
open references available detailing the effect of polymerization
parameters in cycloalkane solvents.13,20,21
As study of solvent is an important aspect of the process,22
we are motivated to understand the effect of different solvents
* Corresponding author. E-mail: [email protected]. Fax:
+91-265-6693934.
†
Nirma University.
‡
Reliance Industries Ltd.
in butadiene polymerization.13,23 There are reports available
citing consequences of different solvents, but the actual reason
has not been disclosed anywhere. Gippin revealed that polarity
of the solvent is inversely related to the rate of polymerization.12
Without elaborating, author also mentioned that solvent polarity
should not have any influence on the microstructure of the
polymer. Porri et al. remarked in their work on transition-metalbased catalyst system, while reasoning the stereoregularity, that
donating solvent may take part in coordinating vacant site of
the metal.24 In a separate report, he also discussed the comparative effect of solvents with increasing basicity.25 Sluggish rate
of reaction in solvents with higher electron density was
interpreted by the coordinating effect of the aromatic solvent.
Distinctly, Mello and co-workers evaluated effect of the solvents
with neodymium catalyst system.26 However, the study remained
focused on only two solvents, namely, hexane and cyclohexane.
This study also could not add information beyond Porri’s
clarification. Therefore, the cause of influence of solvents in
butadiene polymerization is still elusive. Ricci et al. discovered
a new cobalt phosphine complex CoCl2(PiPrPh2)2 as an efficient
catalyst for the preparation of 1,2 syndiotactic polybutadiene,
one of the diene polymers of industrial interest.27 The metal
complex associated with methylaluminoxane was found to be
the active catalyst for the reaction. The authors also observed
the effect of solvents that resulted in high productivity in heptane
compared to toluene, for the same reason as mentioned earlier
by Porri et al. However, to the best of our knowledge, solvent
coordination to the metal center has not been directly shown
so far.
In the present work, we have tried to address this concern by
screening various solvents and studying spectroscopically to
understand the interaction of solvent with respective reaction
components. The absorbance maxima (λmax) in UV-visible
spectroscopy of different solvents in reaction mixture helped
us to discuss the possible interactions with solvent molecules
affecting rate of the reaction. UV-visible spectroscopic study
10.1021/ie101324d  2010 American Chemical Society
Published on Web 09/16/2010
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
of solvents in butadiene polymerization has not been looked
into before and thus leaves great opportunity for us to explore.
Additionally, we have studied the effect of catalyst concentration, water:alkylaluminum ratio (W/Al), and chain transfer agent
on polymerization of BD in cyclohexane and toluene. The
selected studies in toluene reported by Honig et al. are shown
to be in agreement with our values.28,29 Noticeably, important
variables like water:alkylaluminum ratio, molecular weight,
polydispersity index (PDI), and rate constants are studied as
valuable information for butadiene polymerization.
2. Experimental Section
2.1. Materials and Methods. 1,3-Butadiene (BD), ethylbenzene (EB), p-diethylbenzene (PDEB) were obtained from
Reliance Industries Limited, India. Cobalt(II) octanoate,1, was
procured from Maldeep Catalysts Pvt. Ltd., India. Diethylaluminum chloride (DEAC) solution and triethylaluminum (Et3Al)
solution were obtained from Akzo Nobel, India. Ditertiary-butylp-cresol (DTBPC) was procured from Quality Industries, India.
The mixture of reactants consists of proportional amount of
BD and solvent will be herein after called “feed”. BD was
purified by passing through two successive columns containing
DEAC solution and molecular sieves 3A. This was then
solubilized in respective solvents to prepare feed. Different
solvents viz., cyclohexane, toluene, tetrahydrofuran (HPLC
grade) were procured from Labort Fine Chem. Pvt. Ltd., India
and o-xylene was obtained from Merck Specialties Pvt. Ltd.,
India. Methanol was procured from Fisher Scientific, India.
Polymerization was carried out in a jacketed 1 L laboratory
glass reactor (Buchi SFS, Switzerland). All the glasswares were
oven-dried prior to use and used under strict exclusion of
moisture. Only dried and purified solvents were used throughout
the study.30 Utmost care was taken while handling pyrophoric
alkylaluminums. External solutions and reaction feed were
prepared using vacuum manifold.
Microstructure of BR was determined using a Fourier
Transformation Infrared (FTIR) spectrophotometer (PerkinElmer 1600 Series) in the range of 600-1100 cm-1 by
dissolving the rubber sample in carbon disulfide (75 mg in 10
mL of carbon disulfide), according to the literature reported
Morero method.31
Perkin-Elmer UV/vis/NIR (Lambda 19) spectrometer was
used for obtaining the UV spectra. PDI was determined using
gel permeation chromatography (GPC, Perkin Elmer series 200,
USA).
2.2. Polymerization of 1,3-Butadiene in Toluene. In a
typical polymerization process, 350 mL of feed (21% (w/w) of
BD in solvent) was taken in a Buchi reactor. Water as a
promoter (W/Al ) 0.3) was added into the feed under an inert
atmosphere. The cocatalyst, DEAC, 15 mmol per 100 g of
monomer (mmphgm), was then added gently into the above
mixture under stirring. The addition of reactant was completed
by introducing the catalyst, 1, (0.04 mmphgm), whereupon the
reaction was considered to be started. The temperature was
maintained at 26 ( 1 °C. Stirring was done at 200 rpm.
After 60 min, the reaction was terminated by adding methanolic solution of 0.5% (w/v) DTBPC, while the rubber was
found to be precipitated. The filtered rubber was washed with
fresh methanol and kept for air drying overnight. Then the
sample was cut into small pieces and oven-dried at 40 °C for
6 h under a vacuum.
The efficiency of the reaction was evaluated as the weight %
of rubber produced with respect to the monomer fed (eq 1).
Reactions in different solvents were carried out in similar
9649
manner as mentioned above using desired catalyst:cocatalyst
ratios.
% conversion )
produced
× 100
( rubber
monomer fed )
(1)
2.3. Viscosity Average Molecular Weight. The viscosity
average molecular weight (M) of the sample was determined
from the intrinsic viscosity ([η]) using the Mark-Houwink
equation, eq 2.
[η] ) kMR
(2)
Where k and R are Mark-Houwink constants; k ) 0.000305,
and R ) 0.725 for the solvent toluene and polybutadiene rubber
at 30 °C.32
2.4. UV-Visible Spectroscopy. The solutions for spectroscopic study were prepared in a 100 mL three necked round
bottomed flask attached with vacuum manifold maintaining the
ratio of 1:BD ten times higher than what was used in polymerization reaction, to observe the measurable absorbance.
Desired solvent was chosen as per the requirement of the study.
A stock solution of 1 used in polymerization was prepared in
the concentration of 0.54 g in 100 mL of solvent. BD was
dissolved in chilled solvent in the concentration of 17 g in 100
mL. DEAC was diluted to a concentration of 12.2 g in 100 mL
of solvent. The desired proportion of the above solutions was
mixed in a round-bottomed flask and transferred to a UV cell
in a nitrogen atmosphere for recording the spectrum.
2.5. Gel Permeation Chromatography (GPC). Polydispersity index of the polymers was determined using GPC with
refractive index (RI) detector at 30 °C. Samples were dissolved
in HPLC grade tetrahydrofuran at concentration 60 mg/5 mL.
For proper dissolution of the sample, reasonable sonication was
performed. Response of the desired molecular weight was
achieved in Mixed-Bed PLgel, 5 µm, 300 mm ×7.5 mm at flow
rate of 1.0 mL/min using polystyrene as standard (molecular
weight range was 582 to 6 × 105). The GPC procedure
established was able enough to distinctly show the desired peak
to be well-separated from solvent peak.
3. Results and Discussion
3.1. Comparative Study of BD Polymerization in Cyclohexane and Toluene. While searching for an alternative, successful
solvent other than benzene for polymerization of BD, toluene
and cyclohexane come as the second best candidates as far as
industrial applications are concerned. In that aspect, at first, we
have studied polymerization in cyclohexane and toluene. Polymerization in toluene has already been reported but when
compared with cyclohexane, we found substantial difference
between the solvents in terms of reactivity, molecular weight,
and other important parameters.
The results show that the polymer microstructure is not
influenced markedly by changing solvents, as disclosed by
Gippin.12 In both the cases, high cis (g95%) polymer was
obtained. The trend of activity in both the solvents was almost
similar. However, a significant decrease (3-fold) in the catalyst
activity has occurred by changing the solvent from cyclohexane
(aliphatic) to toluene (aromatic) as seen in Figure 1a. The
minimum catalyst concentration of 0.03 mmphgm was used in
toluene; below this concentration, the yield was observed to be
very low. Additionally, in cyclohexane, the reaction was
observed to be spontaneous above 0.05 mmphgm of 1. Hence,
the run was taken up to this catalyst concentration. Both the
solvents show a similar downward trend in a molecular weight
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Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
Figure 1. (a) Comparative catalyst reactivity in cyclohexane and toluene solvents and (b) effect of cyclohexane and toluene solvents on molecular weight
of polymer. Feed, 21% (w/w); water/DEAC, 0.3; DEAC/1, 375; 26 ( 1 °C; 1 h.
Figure 2. Effect of W/Al ratio on (a) conversion and (b) molecular weight of polymer. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; 26 ( 1
°C; 1 h.
vs catalyst concentration plot. To achieve polymer conversion
similar to cyclohexane, we require a higher catalyst dose in the
case of toluene. On the other hand, molecular weight shows a
higher value in toluene at a particular catalyst concentration
(Figure 1b). This result corroborates the hypothesis that at
definite monomer concentration, having fewer available active
sites provokes chains to grow to have a higher molecular
weight.33
Apart from catalyst concentration, another important factor
in the polymerization system of 1 and DEAC is the water
concentration. Water, acting as a promoter, is known to be
required to produce different Lewis acidity12 or aloxane34 type
of structure generated after reacting with DEAC. Moreover,
addition of water initiates selective hydrolysis of DEAC by
sequential removal of alkyl group and thereby increasing the
Lewis acidity, as observed by Gippin.12 Figure 2a shows the
conversion as a function of water concentration in toluene as
well as in cyclohexane. In both the solvents, in terms of the
commonly used water:alkylaluminum (W/Al) ratio, conversion
and molecular weight pass through maxima (Figure 2a,b),
indicating a certain degree of modification by water on DEAC
is required to achieve the best efficiency. The W/A1 ratio was
varied by changing the water concentration at fixed alkylaluminum concentration. In the case of cyclohexane, the maximum
is achieved at lower W/Al ratio with higher conversion.
Distinctly, in the case of toluene, the maximum is achieved at
higher W/Al ratio but with lower conversion. This clearly
indicates that the Lewis acidity of hydrolyzed DEAC (at certain
W/Al) has been affected greatly by solvent. In other ways,
toluene, having π-electrons, as solvent reduces the Lewis acidity
of hydrolyzed DEAC more than cyclohexane, where the
influence of π-electrons is absent. That is perhaps the reason
why cyclohexane shows activity at lower W/Al ratio than does
toluene. Difference in solubility of water in cyclohexane (0.01
g/100 mL, 20 °C) and toluene (0.033 g/100 mL, 25 °C) should
not be the reason in this case because the water used in the
reaction condition is within the solubility limit. Thus, the
maximum conversion can be observed in case of cyclohexane.
The decrease in molecular weight with increasing water can be
observed in both the cases, which is in line with the work by
Saltman and Kuzma.35 Water concentration beyond the critical
value allows complete hydrolysis of aluminoxane framework,
reducing the Lewis acidity greatly.
In the commercial process of synthesis of cis-1,4-polybutadiene using this catalyst system, an additional chain transfer
agent is usually employed to regulate the molecular weight in
the desirable range of 0.2-0.4 millions for applications in
automobile tires. A number of organic compounds, e.g.,
hydrogen, ethylene, propylene, alpha-olefins and allene, nonconjugated dienes, etc., are known to be used for this purpose.14
The effect of 1,5-cyclooctadiene (COD), as polymer chain
regulator, is studied in this paper (Figure 3a,b). Up to a certain
concentration of COD, the conversion is not affected significantly but molecular weight is observed to drop continuously.
After this critical concentration of COD, both conversion and
molecular weight decrease. Effect of chain regulator is more
pronounced in toluene than in cyclohexane. With the addition
of 0.2% COD with respect to monomer, the decrease in
molecular weight is more significant in the case of toluene (48%)
than in cyclohexane (3%). This behavior can be attributed to
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
9651
Figure 3. Effect of chain transfer agent on (a) conversion and (b) molecular weight of polymer. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm;
W/Al, 0.3; 26 ( 1 °C; 1 h.
Figure 4. Conversion of BD in different solvents at a given condition. Feed,
21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1
°C; 1 h.
Figure 5. Conversion of BD as a function of time in different solvents.
Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26
( 1 °C; 1 h.
the fact of the chain transfer reaction being more favored than
propagation in presence of aromatic solvent.36 This is supported
by broadening of the molecular weight distribution (PDI value
changes from 2.1 in cyclohexane to 3.8 in toluene at same COD
dosing) in changing the polymerization solvent. The above
difference between cyclohexane and toluene can be explained
by intervention of π-electrons in toluene. From the above
studies, the concentrations of catalyst components and chain
transfer agent were optimized. A noticeable point is that, as
explained earlier, the number of active sites is the same but the
accessibility of the active centers is lower in case of toluene.
At a given monomer concentration, aromatic solvent comes in
competition for coordination to the active center. Because of
this competition of the solvent molecules with monomer at
similar concentration, the reaction rate decreases and favors
chain transfer.
3.2. Kinetic Behavior in Different Solvents. Progress of
reaction in solvents having different electron-donating groups
(+I effect), e.g., toluene, o-xylene, EB, and PDEB, under similar
reaction conditions was also studied. The polymerization was
conducted for 1 h. The final conversion of BD to polybutadiene
is shown in Figure 4. Highest conversion is achieved with
aliphatic solvent, cyclohexane, having no π-electrons. Conversion is observed to decrease with the use of aromatic solvent,
toluene, and further with EB, matching with their increasing
trend in π-electron density.37 Reaction is extremely slow for
the solvents having disubstituted benzene, e.g., o-xylene and
PDEB, and at studied conditions, no polymer formation occurs.
A representative plot of conversion vs time in different solvents
Table 1. Effect of Solvents on Reaction Ratea
solvent
rate constant, k (min-1)
cyclohexane
toluene
ethyl benzene
0.0170
0.0020
0.0008
a
Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al,
0.3; 26 ( 1 °C; 1 h.
is shown in Figure 5. The overall rate constant of the reaction
in different solvents are listed in Table 1. Rate constants are
determined considering the reaction to be first-order according
to the information revealed earlier.11 It can be seen that the
polymerization rate follows the order cyclohexane > toluene >
EB, in reverse order with their π-electron density. Reaction is
considered to be pseudo-first-order in toluene according to Honig
et al.28,29 This also signifies that the parameter related to
π-donation ability of the solvent is the determinant factor for
controlling the reaction progress.
Molecular weight of polymer as a function of time and
conversion are described in Figure 6a,b respectively. Molecular
weight and conversion increase rapidly at the initial stage of
polymerization and then become plateau. At any time, conversion of BD is higher in cyclohexane than toluene.
3.3. UV Spectroscopy. To understand the effect of solvent
on BD polymerization, we have attempted to study the interaction of catalyst, cocatalyst and different solvents using UVspectroscopy, Table 2. Interaction of π-electrons reflects in their
absorption maxima, the shift of which is being monitored
through UV spectroscopy. Electronic spectra of blue solution
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Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
Figure 6. (a) Molecular weight of polymer as a function of time and (b) molecular weight vs conversion plot. Feed, 21% (w/w); DEAC, 15 mmphgm; 1,
0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C.
Table 2. UV-Visible Absorption of Different Solvent Combinationsa
entry
solvent
1
2
3
4
5
6
7
8
9
toluene
cyclohexane
EB
PDEB
o-xylene
toluene
cyclohexane
EB
PDEB
a
catalyst co-catalyst monomer absorption (λ, nm)
1
1
1
1
1
1
1
1
1
DEAC
DEAC
DEAC
DEAC
BD
BD
BD
BD
Scheme 1. Proposed Reaction Path for High cis-1,4Polybutadiene Using 1, DEAC System
619, 575, 542, 285
612, 575, 550, 269
625
630
621
512, 288
525
500, 287
492, 304
Temperature 25 °C, λ scanned 250-800 nm.
Figure 7. UV-visible spectra of 1 in toluene, cyclohexane, PDEB, and
1-DEAC-BD system in toluene, cyclohexane, PDEB.
of 1 in toluene shows high energy peak at 619 nm, entry 1,
Table 2 (herein after it will be designated according to E1-T2).
A few representative spectra are given in Figure 7. This
transition corresponds to tetrahedral configuration of respective
transitions 4A2 f 4T1(P).38,39 In cyclohexane, the same band
appears at 612 nm (E2-T2) (Figure 7). The red shift of the
respective band from 612 to 619 nm occurs while the solvent
was changed from cyclohexane to toluene because of stabilization of 4T1(P) in comparison to 4A2, resulting in lower energy
of transition.40,41 The red shift of the aforesaid band is seen for
other solvents in accordance to their electron density, e.g., EB,
PDEB, and o-xylene (E3-T2 to E5-T2). This can be attributed
to π-electron donation of the solvent molecule to 4T1(P), which
becomes prominent at higher electron density.24
Upon addition of DEAC and BD in reaction, the color
changes from blue to pale red. The lowest energy band for the
toluene solution appears at 512 nm (E6-T2). DEAC (Et2AlCl)
is reported to react with butadiene, generating butadienyl ion,
which forms a complex with cobalt, replacing one octanoate.12
The species formed binds with another monomer of butadiene,
Scheme 1, attaining the coordination number 6.12,34,42,43 This
indicates that cobalt most likely gains octahedral structure, as
cubic structure (with 6 coordination number) is not feasible.
The band at 512 nm also supports the literature value for 4T1g(F)
f 4T1g(P) transition in the octahedral environment of Co(II).38
In this case, the effect of solvent is manifested even more
prominently (Figure 7). Contrary to earlier observation, blue
shift is seen in presence of Et2AlCl and butadiene while solvents
were changed to higher polarity (E8-T2, E9-T2). This may
be ascribed to the fact that because of the nonbonding nature
of the interaction with the polar solvent, 4T1g(F) is more
stabilized than 4T1g(P), leading to high energy transition. Thus
a shift in λ for cyclohexane, toluene, EB, and PDEB with
increasing electron density, is observed at 525, 512, 500, and
492 nm, respectively. In addition, a red shift of octanoate ligand,
269 to 285 nm, while the solvent is changed from cyclohexane
to toluene has been observed. This is likely to be the π f π*
transition and in accordance with the literature, that coordinating
solvent has high influence in shifting the absorption maxima.44,45
In case n f π* transition had occurred, blue shift would have
been observed on changing the solvent polarity from cyclohexane to toluene. The n f π* is not observed in our case, most
probably because of its low intensity, which could not be
captured by spectrophotometer. The red shift could be reasoned
for attractive polarization between solvent and absorber that
lowers the energy levels of π* resulting into low energy
transition (E1, E2-T2).46 Analogous observations can be made
in the case of other solvents as well (E3, E4, E5-T2).
Besides, when possible interactions between the solvents and
cocatalyst were investigated, we found remarkable differences
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
Scheme 2. Possible Solvent Interaction with Catalyst That
Hinders Monomer Binding to the Active Center
9653
Acknowledgment
The authors thank Dr. K.V.V.S.R. Murthy, Mr. Dinesh S.
Pandya, Mr. Chirag S. Shah, Mr. Pankaj C. Chawla, Mr. Suraj
K. Kusum, and Mrs. Swati G. Trivedi for their support. The
authors are grateful to Reliance Industries Ltd. for providing
financial support to this work.
Literature Cited
in the UV absorption of the solutions containing Et2AlCl and
triethylaluminum (Et3Al). A band was observed at 265 nm when
BD and Et2AlCl are added in toluene; but in the case of Et3Al,
the band appears at 262 nm, probably corresponding to π f
π* transition of the butadienyl unit. Furthermore, a light yellow
coloration occurred when Et3Al was added to BD, unlike
Et2AlCl, where no color change was observed. Probable reason
may be because of π donation from solvent molecule to LUMO
of butadienyl anion unit (π*), which effectively lowers the π
(HOMO) f π* (LUMO) transition energy.44 Because of weaker
Lewis acidity nature of Et3Al than Et2AlCl, creation of butadienyl anion as well as attractive polarization of solvent
molecule are rather energy intensive.12 This becomes prominent
in case of relatively electron-dense molecule like EB, PDEB,
and o-xylene, where the values are 287, 300, and 304 nm,
respectively, in the presence of Et2AlCl and BD. This emphasizes our hypothesis of solvent π-donation. Though we have
used Et3Al as purchased, the occurrence of color may not be
ruled out from the impurity present even at trace level.
Nevertheless, several reactions with different stock of Et3Al
showed similar observation.
Thus it can be seen in Figure 7 that the extent of band shifting
is a function of the electron density of the solvents. Therefore,
it is evident from above observations that π electrons of aromatic
solvents are in the mode of donation to Co, the metal center.34
In effect, such interaction lessens the partial positive charge on
metal, which discourages butadiene monomer to propagate the
chain length, Scheme 2. Thus the rate of reaction in aromatic
solvent is severely deterred in comparison to the solvents like
cyclohexane. This becomes so prominent that in the presence
of PDEB and even o-xylene, no polymeric product was
observed. The problem for highly electron-dense solvents is that
the allowance of monomer onto active site of catalyst gets
hindered. In view of the above, it is summarized that the solvents
having higher electron density have genuine influence on the
active site of the catalyst, retarding the efficiency greatly.
4. Conclusions
In conclusion, solvents having π-electrons have marked effect
on the activity of catalyst compared to aliphatic solvents
(cyclohexane) as the medium of polymerization of BD by
DEAC-cobalt catalyst system. The π-donation indicated by
W/Al ratio studies has been confirmed by UV-vis spectroscopy
and rate constant measurements. The rate of polymerization is
inversely related to the π-electron density of the solvent. Higher
molecular weight with broad molecular weight distribution is,
however, possible to attain by using aromatic solvent. Therefore,
considering the above consequences, judicious selection of
solvents need to be exercised for purpose of solvent modification
to the existing manufacturing process.
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ReceiVed for reView June 21, 2010
ReVised manuscript receiVed August 6, 2010
Accepted August 28, 2010
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