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Chapter 2
Ring Opening Metathesis Polymerization
Alexey Lyapkov, Stanislav Kiselev,
Galina Bozhenkova, Olga Kukurina,
Mekhman Yusubov and Francis Verpoort
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/intechopen.71085
Abstract
In recent years, the olefins metathesis has established itself as a powerful tool for carboncarbon bonds forming and has found numerous applications in polymer chemistry. One
of the important directions of metathesis is the polymerization with cycle opening. A
study of new ruthenium catalysts, resistant to the many functional groups effects, has
showed the possibility of synthesizing functionalized polymers with unique properties.
In this chapter, reactivity and activation parameters of eight different norbornene dicarboxylic acid alkyl esters in the presence of a Hoveyda-Grubbs II catalyst for the ring
opening metathesis polymerization were determined by 1H NMR analysis in-situ. The
molecules of esters differ in the aliphatic radical structure and the location of the substituent groups. Kinetic studies have shown that effective polymerization constants and
activation parameters strongly depend on the monomer structure. It is shown that the
elongation of the aliphatic radical does not significantly affect the reactivity, but significantly changes the activation parameters. The branching of the aliphatic radical significantly affects both the reactivity of the corresponding ester and the activation
parameters of the polymerization. The position of the substituents in the norbornene
ring of the ester also has a significant effect on the activation parameters of metathesis
polymerization.
Keywords: ring opening metathesis polymerization, nuclear magnetic resonance,
dicyclopentadiene, alkyl esters of norbornene dicarboxylic acid, Hoveyda-Grubbs
catalysts, observed rate constant, activation energy
1. Introduction
Ring opening metathesis polymerization (ROMP) is a process of one or more cyclic olefins
transformation to polymer catalyzed by metal carbene compounds. Indeed, the number of
© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
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Recent Research in Polymerization
double bonds both in polymer and in monomer is equal [1–4]. As well as in any other type of
polymerization, ROMP can be divided into several stages (Scheme 1).
The initiation begins with the coordination of the cycloolefins’ double bond with the metalcarbene complex. The next step is a formation of a metal-cyclobutane intermediate. This
process occurs in each act of addition of the monomer during the chain growth. The intermediate decomposes to form a new metal-carbene complex, with the growing chain being a
ligand attached to the metal via a double bond.
The polymerization continues until the monomer is completely reacted or an equilibrium state
is reached or the reaction is terminated by the addition of a special reagent that blocks the
catalyst.
Living polymerization with ring opening metathesis is terminated by removing the transition
metal from the end of the growing chain and its further deactivation. Deactivation in this case
involves the formation of a complex unable to initiate polymerization [5]. Ethyl vinyl ether
is an effective stopper for most ruthenium catalysts. It forms a very stable complex of the
[Ru]=CHOEt type and ensures the functionalization of the polymer end-group. Acrylate derivatives of 2-butene-1,4-diol, succinic anhydride or butyl acrylate can also stop the growth of the
macromolecule [6].
ROMP, as well as most metathesis reactions, is reversible, so the described transformations can
proceed both in the forward and backward directions. The direction of the reaction can be
predicted using the Gibbs energy:
Scheme 1. Stages of ROMP.
Ring Opening Metathesis Polymerization
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ΔG ¼
RT ln Kравн ¼ ΔH
T ΔS
(1)
The enthalpy factor makes an essential contribution to ROMP’s Gibbs energy. This fact is
explained by a high strain energy of cyclic unsaturated compounds participating in the polymerization [7]. The cycle strain energy of most cycloalkenes is more than 20 kJ/mole. For
example, norbornene has an energy of about 100 kJ/mole [8]. The strain energy of the cycle
releasing during the decomposition of the metal-cyclobutane complex and maintaining the
forward direction of the reaction. However, if the deformation energy of cycle is low, the
contribution of the entropy factor into the Gibbs energy becomes more significant in comparison with a smaller enthalpy factor. In this case, the entropy factor must be properly reduced in
order to implement the direct process by increasing the monomer concentration or decreasing
the temperature of the process [9].
Secondary metathesis reactions can occur in addition to the main described reactions during
the polymerization process. Intermolecular and intramolecular chain transfers are two main
side reactions of ROMP (Scheme 2).
During the intermolecular transferring, the active metal-carbene complex located at the end of
one macromolecule interacts with the double bond of the adjoined macromolecule, which
leads to fragment exchange process. The reaction proceeds simultaneously in two directions.
One of them provides two polymer chains with the active ruthenium on both. The second
leads to one inactive chain and one chain with two active centers.
At the intramolecular chain transferring, the active metal-carbene complex reacts with the
double bond of the same macromolecule led to a cyclization of the polymer chain. The listed
side reactions, eventually, lead to a broadening of the molecular mass distribution (MMD) and
a decrease in a molecular weight of the polymer [10].
The chain transfer as well as the spontaneous termination of the growing chain is highly improbable, so ROMP is a living polymerization. ROMP polymers are characterized by high molecular
weights and a narrow MMD, as well as for products of other living polymerizations [11].
Scheme 2. Intermolecular and intramolecular chain transfers in ROMP.
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2. Well-defined ruthenium metathesis catalysts
Up until the mid-1970s, the information about the structure of the active center of olefin
metathesis catalysts was not known. Catalytic systems were mixtures of various compounds
containing a transition metal. For example, in the late 1960s, Calderon and Goodyear employees published a number of papers about usage of a catalyst consisting of WCl6, AlEtCl2, and
ethanol [12, 13]. Cyclic olefins form polymers (copolymers) of a high tacticity in a presence of
this catalyst [14]. Catalytic systems prepared from compounds based on transition metals such
as vanadium VCl4/Al(Hex)3; V(Ac)3/AlC1(C2H5)2; titanium TiCl4/Al(C2H5)3 and TiCl4/AlCl3/
Al(C2H5)3 [15]; molybdenum MoCl5/Al(C2H5)3 [16] as well as catalysts based on osmium,
ruthenium, and iridium chlorides [17] were used in ROMP.
In 1976, through the example of the Fisher catalyst WCPhR(CO)5 (where R]Ph or dOCH3),
described by Casey and Fisher [18, 19], Katz was the first who showed the ability of a metalcarbene complex to catalyze the process of metathesis polymerization independently without
additional compounds [20]. In the history of metathesis, catalytic complexes with a wellstudied structure were called “well-defined” catalysts. The discovery of “well-defined” catalysts had significantly increased the ability of ROMP to obtain polymers that have unique
properties.
The first “well-defined” ruthenium catalyst was synthesized by Grubbs in 1992. The alkylidene
source was 3,3-diphenylcyclopropene [21] (Scheme 3).
Unfortunately, this catalyst had a low activity in comparison with already available metathesis
catalysts. Replacing triphenylphosphine ligands with tricyclohexylphosphine significantly
improved the activity of the catalyst (Figure 1—1 and 2).
Later, in 1995, catalytic complexes known as first-generation Grubbs catalysts (Figure 1—3
and 4) were prepared using phenyl diazomethane. These catalysts not only had equal activity
to molybdenum catalysts but also were indifferent to the polar groups in the monomer [22, 23].
In 1999, Grubbs reported the synthesis of second-generation catalysts (Figure 1—5), showing
better activity and more stability at air. This catalyst was obtained by replacing tricyclohexylphosphine with an N-heterocyclic carbene ligand [24]. A year later, Hoveyda’s group reported
on a new type of catalytic system based on the catalysts of Grubbs of the first and second
generations (Figure 1—6 and 7). These complexes include a chelating ester ligand [25].
Recently, a new type of ruthenium catalyst has appeared where the N-heterocyclic carbene
Scheme 3. Scheme for the Grubbs I catalyst synthesis.
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Figure 1. Main types of ruthenium catalysts.
ligand chelates the metal through the Ru-carbon bond (Figure 1—8). Such complexes possess
high cis-selectivity in ROMP [26].
3. Reactivity of esters of 5-norbornene-2,3-dicarboxylic acid in ROMP
Currently, despite the fact that ethers of 2,3-norbornene dicarboxylic acid appear to be potential material for synthesizing polymers via ROMP, the interrelation between the molecule
structure and their reactivity for metathesis polymerization with full ring opening has not
been stated. A few polymerization mechanisms with different catalysts (including ruthenium)
are known; however, there is no detailed description of how ethers of 5-norbornene-2,3-dicarboxylic acid behave. The study of reaction activity of 5-norbornene-2,3-dicarboxylic acid ethers
with different structure using an appropriate catalyst (carbene complex of ruthenium (1,3-bis(2,4,6-trimethylphenyl)-2-imidoazolidevynilidene)dichloro(ortho-N,N-dimethylaminomethylphenylmethylene)-ruthenium—1 (Figure 2) [27] has filled this gap.
In this research, we used alkyl diesters of bicyclo[2.2.1]hept-5-en-2,3 dicarboxylic acid, obtained
according to the technique given in the paper (Figure 3) [28].
Polymerization was carried out in NMR tubes, concurrently measuring the proton spectrum
after a certain period using AU-program zgser.
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Recent Research in Polymerization
Figure 2. Catalyst complex of ruthenium used as an initiating agent for polymerization.
Figure 3. Alkyl diesters of bicyclo [2.2.1]hept-5-en-2,3 dicarboxylic acid, used as monomers.
The monomer concentrations were determined based on decrease and growth of integrated
intensities of resonances of olefinic protons of monomer—SM and polymer—SP (Figure 4)
CM ¼ CM0
SM
SM þ SP
(2)
CK ¼ CK0
SK0 SS
SS0 SK
(3)
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Figure 4. Combining the fragments of NMR spectra 2 in the beginning of a reaction and after 20 min.
where CK0 and SS0 —squares of integrated intensities of a catalyst and solvent, measured in the
beginning of the reaction; SK and SS—current squares of integrated intensities of a catalyst and
solvent during the reaction.
The AU-program multintegr was used to gauge the integrated intensities and time of the
experiment. The using of low-viscosity solvents allowed obtaining high-resolution proton
NMR spectra. Thus, kinetic studies should be carried out in the solution. And the set of
monomer concentrations was defined to get kinetic correlations based on the spectral data.
The solvent should be used as a diluent. Figure 5(a) demonstrates the curves describing the
changes of concentration 2 in the course of time. According to the literature data, chloroform-d
was taken as a solvent.
The molecules of chloroform-d do not react with active ruthenium and play a role of a polar
medium, which stabilize 14-electron state of the active ruthenium [29]. Initially, toluene-d8 was
suggested as a possible solvent, but the catalyst and monomers dissolve better in chloroform-d,
which is also a widely used and more available solvent for NMR studies than toluene-d8.
21
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Figure 5. The decrease of monomer 2 (a) and its semi-logarithmic anamorphoses (b) during polymerization with catalyst
1 with varying initial concentration of the monomer (CK0 = 0.0087 mole l 1, 50 C).
Also, it was shown that reactivity of dimethyl ether of exo,exo-norbornene dicarboxylic acid is
higher in chloroform-d [30]. Since chloroform-d boils at 60.9 C in ambient conditions, the
operational temperature range was limited to 50 C to prevent any changes in the reactant
concentration which could be caused by evaporation.
Studies [31, 32] considered the ring opening metathesis polymerization as pseudo first-order
reaction as regards to the monomer concentration, which is valid for polymerization of abovementioned monomers.
Figure 5(b) demonstrates that there can be seen three regions in the semi-logarithmic anamorphoses. The first region has non-linear segment of curve corresponding to the initiation stage.
The second one is the straight-line segment prolongs to the extent of 70% monomer conversion
(till one on the logarithmic scale, Figure 5(b)). The third region is a noticeable non-linear
segment of curves, which is observed after 70% conversion. The appearance of such nonlinear segments is due to the viscosity of the reaction mixture increasing, owing to the polymer
molecular weight growth. This results to the fact that the polymerization rate is limited by the
diffusion of monomer molecules to the active ruthenium.
Figure 6 shows the straight-line ranges of semi-logarithmic anamorphoses of polymerization
2, catalyzed by 1. The slope of the right lines corresponds to the observed constant of polymerization ko.
Based on correlation coefficients given in Table 1, we can conclude that semi-logarithmic
anamorphoses are linear in the noticed interval. Figure 6 shows the correlation of the constant
ko and initial monomer concentration.
Figure 7 shows that ko linearly depends on the monomer concentration within the following
range from 0.2 to 1.0 mole l 1, which allows to vary the monomer concentration in this range
to implement kinetic experiments.
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Figure 6. Linear segments of semi-logarithmic anamorphoses of monomer 2 polymerization over catalyst 1 (CK0 = 0.0087
mole l 1, 50 С, dependences are marked in accordance with Figure 5).
CM 0 , mole l
1
a
b
r
103ko
1.01
0.00183
0.3417
0.999
1.83
0.69
0.00131
0.2416
0.999
1.31
0.49
0.00094
0.1992
0.999
0.94
0.39
0.00089
0.1614
0.999
0.89
0.33
0.00070
0.1116
0.998
0.70
0.19
0.00042
0.0751
0.999
0.42
Table 1. Values of ko which calculated out of linear dependences on Figure 6.
Ruthenium complex should be activated to initiate polymerization. This is carried out by the first
addition of monomer, which is initiation stage as well. There exist several possible mechanisms
of activation; however, based on the literature data, it is assumed that bulky olefins, including
the research monomers, interact with active ruthenium on a dissociative mechanism [33]
k1
þM k2 ∗
K∗ !
K !
P
k
1
The initiation rate equals the rate of active centers formation P*. The active centers formation
occurs in two stages. As it can be seen from Figure 8, the concentration of ruthenium complex
slightly changes.
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Recent Research in Polymerization
Figure 7. Correlation of the observed constant ko of monomer 2 polymerization, catalyzed by 1 with the initial monomer
concentration (CK0 = 0.0087 mole l 1, 50 С).
Its decrease is 1–2% from the initial catalyst concentration. Synthesized polymers at these
conditions possess high molecular weight (Table 2).
Based on Figure 8 and Table 2, we can conclude that the formation of active centers is slower
than the growth of polymer chain. The research [34] also confirmed this, stating that for
polymerization of exo-exo-5,6-bi(methoxycarbonyl)-7-oxabicyclo[2.2.1]hept-2-ene over Grubbs
catalyst of the first generation the correlation of constants is ki/kg = 0.23. Moreover, the study
[29] suggests that the correlation ki/kg is even lesser and equals 0.03 for catalyst with Nchelating ligand. In addition, based on the data presented, we can assume that disassociating
of nitrogen defined by constant k1 is limiting in the initiation reaction. Notably that the monomer molecule does not interact during initiation, that is why the formation rate of the active
ruthenium complex K* only depends on the temperature and initial concentration of ruthenium complex. Thus, the structure of the monomer molecule can affect the second stage of
initiation defined by constant k2 and the stage of polymer chain growth defined by constant kg
(it is suggested that constants of different stages of polymerization are equal k1g ¼ k2g ¼ … ¼ kg )
k1g
P∗ þ M!P∗ M
k2g
P∗ M þ M!P∗ M2
…
kg
P∗ Mn þ M!P∗ Mnþ1
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Figure 8. Ruthenium complex decrease during polymerization of monomer 2 with different initial concentration of
catalyst 1 (CM0 = 0.35 mole l 1, 50 С).
CK0 , mole l
1
CM0 =CK0
10 5Mn, g mole
0.021
17
8.5
0.012
30
10.1
0.006
57
7.6
1
Table 2. Average molecular weight of the obtained polymers depending on the number of initial reagents.
Kinetics of monomer consumption is complicated (Figures 5(a) and 9).
In polymerization, a monomer is used during initiation and growth of the polymer chain
dCM
¼ k2 CK∗ CM þ kg CP∗ CM
dt
(4)
The concentration of active ruthenium complex CK* and concentration of active chains CP* are
low, with CP* due to the absence of reactions of termination [5, 6] and transfer [22] of the chain
constantly increases during the reaction. Since k1 is much lesser than constants k 1 and k2, it is
possible to apply the principle of quasistationary for concentration of the active form CK*:
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Recent Research in Polymerization
Figure 9. Monomer 2 consumption in the polymerization reaction over varying initial catalyst concentration CM0 = 0.35
mole l1, 50 С).
dCK∗
¼ k1 CK k1 CK∗ k2 CK∗ CM ¼ 0
dt
CK∗ ¼
k1 CK
k1 k2 CM
(5)
(6)
The second stage of the initiation reaction can be viewed as pseudo first-order one proceeding
with effective constant k2e ¼ k2 CM0 . This assumption is fair as CK0 ≪ CM0 and CK0 ≫ CK∗ .
Taking into consideration that the catalyst concentration slightly changes during the reaction,
it could be considered that CK ffi CK0 . Then, changes in the concentration of active chains over
time are defined by the following equation:
dCP∗
k1 CK0
¼
k2 CM0
k1 k2 CM0
dt
(7)
After integrating we get:
CP∗ ¼
k1
k1
k2 CK0 CM0
1 þ kk12 CM0
t
(8)
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The amount of active chains is equal to the decreasing of the monomer, which is forming these
chains. Knowing the active chains’ concentration from Eq. (8), the change in monomer concentration in time can be described by Eq. (9):
k
k
dCM k 11 k2 CK0 CM0 k 11 k2 CK0 CM0
þ
kg CM t
¼
dt
1 þ kk2 CM0
1 þ kk2 CM0
1
(9)
1
To simplify the equation and implement semi-logarithmic coordinates for defining the rate
constant, we can ignore the first component of the right side of the equation, since it contributes less if compared with the second component. This assumption is fair for the later stages of
polymerization. The formula
k1
k 1 k2 CK0 CM0
k
1þk 2 CM0
1
t can be expressed as the following product CK0 f ,
k1
k2 CM0 t
1
k
1þk 2 CM0
1
where f is the effectiveness of initiation equal to CCKP∗ ¼ k
0
. Then, we can put it down the
following way:
dCM
¼ kg CK0 f dt
CM
(10)
C M0
¼ kg CK0 f t
CM
(11)
After integrating, we would acquire:
ln
Taking into consideration that f for each monomer differs only by the value of k2 constant,
which depends on the structure of monomer, it is possible to compare reaction capacity and
values of activation parameters using product fkg.
The chain growth rate constant of polymer kg times the effectiveness of initiation f corresponds
the tangent of the slope in the straight-line segment of semi-logarithmic correlation, which
equals the product of the observed constant ko times the initial catalyst concentration CK0
(Figure 10).
Correlations in Figures 7 and 11 demonstrate that the observed constant of polymerization ko
linearly depends on both the initial concentration of monomer and the initial catalyst concentration.
Linear correlation of ko from CK0 is observed because Eq. (1) takes the initial concentration of
catalyst into consideration. In turn, ko linearly depends on CM0 since Eq. (11) includes parameter f, which depends on the initial concentration of monomer. Based on the data presented, we
o
to compare reaction
can conclude that it is possible to use the effective constant ke ¼ CK kC
M
0
0
capacity of the ethers under study. The dimensionality of constant ke correspond the dimensionality of second-order constant since the concentration of monomer is included in numerator
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Recent Research in Polymerization
Figure 10. Semi-logarithmic correlations of polymerization 2 over catalyst 1 with varying catalyst concentration (CM0 =
0.35 molel 1, 50 С, dependences are marked in accordance with Figure 9).
Figure 11. Correlation of the observed constant ko of polymerization of monomer 2, catalyzed by 1 with the initial
concentration of catalyst (CM0 = 0.35 mole l 1, 50 С).
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and denominator of the equation of the initiation effectiveness. Since constant ko depends on
the initial concentration of catalyst linearly, we can use the noticed range of concentration to
estimate reactivity of esters.
4. Reactivity-structure relationship of esters of 2,3-norbornene
dicarboxylic acid
Based on the values of effective constants, we compared reactivity and activation parameters
of polymerization of diesters exo,exo-2,3-norbornene dicarboxylic acid, which are differ by
length and branched chain of ester substituent. To define the activation parameters, we used
Arrhenius equation (12) and calculation results are shown in Figure 12
ln ke ¼ ln A
Ea
RT
(12)
This correlation between ln ke and 1/T for each researched ester has linear character. This
proves that the interaction mechanism of ruthenium complex and corresponding ester at the
different temperatures is unchanged. Table 3 presents data on effective constants and activation parameters of polymerization of diesters exo,exo-2,3-norbornene dicarboxylic acid.
Figure 12. Arrhenius correlations of polymerization of diesters exo,exo-5-norbornene-2,3-dicarboxylic acid.
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Substituent
ke, l mole1 s1 (30 С)
Ea, kJ mole1
A, l mole1 s1
Methyl
0.11
82
2 1012
Propyl
0.10
89
2 1013
Butyl
0.08
92
7 1013
Iso-butyl
0.01
72
6 109
Pentyl
0.21
105
2 1016
Octyl
0.17
121
2 1018
Table 3. Effective constants and activation parameters of polymerization of diesters exo,exo-2,3-norbornene dicarboxylic
acid.
It was expected that aliphatic radical elongation from the first to the eighth atoms of carbon
would lead to gradual decrease in reactivity of esters row. However, according to Table 5,
aliphatic radical elongation insignificantly affects the reactivity of esters.
On the contrary, branched substituent chain affects reactivity greatly. Constant ke of an ester
with iso-butyl radical is six times lesser than constant ke of a similar ester with linear butyl
radical. The steric hindrances significantly decrease the reactivity of diesters with branching
aliphatic radical under interaction with active form of ruthenium complex. In study [35], the
researchers attempted to make a quantitative estimation of the initiation and growth constants.
As shown in Table 5, the increase of aliphatic radical length leads to gradual increase of
activation parameters. To explain changes in activation parameters, we should define the rate
constant, which is dependent from monomer structure in more degree. Effective constant of
polymerization includes four true constants. Constants k1 and k1 are determined by the
structure of ruthenium complex and do not depend on the monomer structure. Nevertheless,
the influence of the ester structure could be indirect. When bond Ru-N is disassociated, a
14-electron state is formed. This state is more polar than the initial 16-electron state (Scheme 4).
Scheme 4. Dissociation the Ru-N bond of catalyst.
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Polar media stabilize 14-electron state and make disassociation easier. Solutions of esters,
which are different in structure, may possess different dielectric permittivity and, thus, could
affect constant k1 and k 1. However, in polymerization, solutions of esters have low concentration and the contribution of ester into the polarity of medium remains insignificant.
Esters structure would affect more constants k2 and kg. First, we should understand the way
monomer structure can affect constant k2. This constant defines the reaction rate, which identifies the process of monomer addition to the active form of ruthenium complex. In this
process, the double bond of ester molecule occupies the vacant position in the coordination
sphere of ruthenium complex (Scheme 5).
While the activation energy Ea defines excess of energy, which molecules in the reaction should
possess to form transition state. Pre-exponential factor A can correlate with steric factor. Both
parameters define the process of reaching the top of a potential barrier and are calculated from
the initial state of the system. It is unlikely that the length of aliphatic radical affected the rate and
activation parameters of this reaction. It is more probable that constant k2 and activation parameters are nearly equal for molecules with varying length of aliphatic radical. It is also unlikely
that branching substituent can affect both the rate and activation parameters of this process.
It is necessary to mention that the influence of the previous monomer unit may affect rate and
activation parameters of monomer addition reaction to one of the active forms of ruthenium.
However, this factor is absent on this stage of the reaction.
Having analyzed the experimental data, we concluded that the structure of monomer is more
likely to affect the growth reaction of polymer chain with constant kg.
It is known from literature data that esters of 5-norbornene-2,3-dicarboxylic acid can chelate
the active forms of ruthenium complex with carbonyl oxygen of ester group, thus, forming
hexatomic intramolecular complex [36]. Therefore, two active forms of ruthenium complex can
take part in the polymer chain-growth reaction (Figure 13).
RudO bond strength depends on donor properties of carbonyl oxygen. In esters row, the
donor properties of oxygen will enhance as there will increase inductive effect of growing
radical. At the same time, RudO bond strength will increase. Reinforcement of RudO bond
decreases mobility of ester fragment and makes its intramolecular complex more rigid.
Scheme 5. Coordination of the monomer’s molecule with ruthenium complex.
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Recent Research in Polymerization
Figure 13. Non-chelated (а) and chelated (b) active forms of ruthenium complex.
When transition state is formed, monomer molecules occupy the position of oxygen in coordination sphere of ruthenium, what is accompanied by destruction of intramolecular complex
(Scheme 6).
To degrade RudO bond, it is necessary to spend some energy. Lengthening of aliphatic
radical, which promotes improvement of donor properties of carbonyl oxygen and intensification of RudO bond, increases the amount of energy needed to degrade RudO bond. That is
why activation energy rises as the length of aliphatic radical increases. If the activation energy
corresponds to the excessive energy that reacting molecules should possess to pass the potential barrier, then pre-exponential multiplier defines peculiarities of interaction of these molecules. Pre-exponential multiplier can correlate with the change of activation entropy, which
depends on changes in the number of freedom degrees of the reacting molecules. Ruthenium
and the previous monomer unit can form a ring with lesser number of freedom degrees than
the complex they form of non-ring structure. Besides the rigidness of intramolecular complex
depends on RudO bond strength (the more strength RudO bond, the more stable is intramolecular complex). Therefore, the increase of pre-exponential multiplier defined by the growth
of aliphatic radical is explained by the increase in the number of freedom degrees, which
appear when intramolecular complex degrades during the formation of transition state.
To form RudO bond, carbonyl oxygen and ruthenium should be positioned in a certain way.
When RudO if formed, the molecule geometry is changed. Steric factor is one of the
Scheme 6. The destruction of the intramolecular complex with the addition of a new monomer’s molecule.
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hindrances making the formation of intramolecular complex harder. In the case of ester with
branched substituent, bulky iso-butyl radicals cannot set near each other properly for carbonyl
oxygen to form strength bond with ruthenium due to steric hindrances. This reduces the
activation energy and pre-exponential multiplier. In addition, iso-butyl fragments of the previous monomer unit hinder the monomer placement in the coordination sphere of ruthenium,
which cuts reactivity of this ester.
Figure 14 demonstrates Arrhenius correlations of constant ke in three 3-dimensional isomers of
dimethyl ester of 5-norbornene-2,3-dicarboxyl acid. The correlations are linear in the range of
temperatures, which proves that the interaction mechanism of ruthenium complex and the
corresponding ester is permanent.
Based on the correlations in Figure 14, we calculated effective constants and activation parameters of polymerization. The results are in Table 4.
Table 4 shows that the orientation of ester substituents to the norbornene ring affects both
reactivity and activation parameters of polymerization.
The presence of substituent in endo-position reduces reaction capacity of ester. This corresponds
with the data shown in other studies [34, 37–40], which estimated reaction capacity of endo- and
exo-isomers of dicyclopentadiene and 2,3-dicarbomethoxy-5-norbornene. In the research of
Delaude at al. [40] measured the initiation constants for monomers 2, 3, and 4 over [RuCl2(pcymene)]2 complex activated with trimethylsilyldiazomethane; their values at 25 С were 0.040,
Figure 14. Arrhenius correlations of polymerization of three-dimensional isomers of dimethyl ester 5-norbornene-2,3dicarboxylic acid.
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Position
ke, l mole1 s1 (30 С)
Ea, kJ mole1
A, l mole1 s1
Exo,exo-
0.47
82
9 1013
Exo,endo-
0.20
105
2 1017
Endo,endo-
0.02
72
7 1010
Table 4. Effective constants and activation parameters of polymerization of three-dimensional isomers of dimethyl ester
2,3-norbornene dicarboxylic acid.
0.025, and 0.05 l mole1 s1 for 2, 3, and 4, respectively. At the same time, the constant of chain
growth remains the same for all monomers and is within the range of 0.003–0.006 l mole1 s1.
The initiation stage of monomer 2 catalyzed by 1 is much slower than the chain growth stage. If
we compare the constant of initiation and growth of monomer 2 catalyzed by 1 and [RuCl2(pcymene)]2, then we would notice that initiation catalyzed by 1 is slower than chain growth in
distinction from [RuCl2(p-cymene)]2, which affects initiation in a way that it is 10-fold faster
than the growth of polymer chain. Comparison of constants defining polymerization initiated
by these complexes is not adequate since these complexes have different structure and may
have different activation mechanisms. However, in both the cases, ester groups in endo-position
are located not far enough from the double bond of norbornene ring and sterically hinder the
monomer attack by double bond of ruthenium. This would affect the polymerization rate of
these esters both in the case of initiation by complex 1 and in the case of initiation by [RuCl2(pcymene)]2. In the first case, the steric factor would affect both constants k2 and kg.
Each monomer is determined by its own set of activation parameters different from others. To
explain the way the activation parameters change, we compiled a set of monomers in ascending order to form RudO bond and intramolecular complex. Exo,endo-isomer is more prone to
form RudO bond since its ester substituents are located on different sides in relation to the
norbornene ring and do not hinder each other during the formation of intramolecular complex.
RudO bond is more strength, and intramolecular complex is more rigid in comparison with
other isomers. That is why high activation energy and pre-exponential multiplier are typical
for exo,endo-isomer. Exo,exo-isomer is the second on the capability to form RudO bond. This
ester is inferior to exo,endo-isomer, since its ester substituents are located on one side in
relation to norbornene ring. This sterically hinders their mutual distribution necessary for the
formation of RudO bond. RudO bond has less strength, and intramolecular complex is more
flexible. That is why if compared with exo,endo-isomer, exo,exo-isomer is defined by lower
activation energy and pre-exponential multiplier.
Endo,endo-isomer is the third on the ability to form RudO bond. Because of the way ester
substituents are located inside norbornene ring, this ester cannot form strong RudO bond.
Ester group in endo-position cannot properly distribute in the coordination sphere of ruthenium to form intramolecular complex. That is why this molecule possesses low activation
energy and pre-exponential multiplier.
In the paper [41], the authors estimated reactivity of these esters using the observed polymerization constant ko as the criterion for comparing reaction capacity of monomers.
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5. Structure of polymers
The products of chemical reaction are no less valuable as a source of information for kinetic
parameters. Their structure helps us to learn how reaction components interact with each
other. Using NMR method to study the kinetics of metathesis polymerization of norbornene
acid, ester is more beneficial since it allows estimating the structure of the obtained polymers
immediately [25, 39, 42–44].
To analyze structure of polymers 2, 5–8, we used data from the study [39], which demonstrated that cis-units have resonances of olefinic protons in a stronger field in relation to transunits (Figure 15(a)).
Overlap of resonances corresponding to cis- and trans-structures occurs in polymer obtained
from monomer 4 (Figure 15(c)).
The spectrum of polymer 3 is more complex if compared with spectra of polymers 2 and 4,
since molecule 3 possesses chiral properties. To correlate the shifts, we applied the approach
suggested in the following study [43]; it was used to analyze the structure of polymers
obtained from chiral products of norbornene using NMR-spectra COSY.
Implementation of this approach alongside with assumption that resonances of olefinic protons in cis-fragments are shifted to a higher field in relation to trans-fragments [44] allowed
referring resonances of olefin region to four possible structures (Figures 15(b) and 16).
Figure 15. The region of olefinic protons 1H NMR-spectra of polymers obtained with polymerization of 2–4 catalyzed by 1.
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Figure 16. COSY-spectrum of polymer exo,endo-2,3-dicarbomethoxy-5-norbornene.
Monomer
2
3
4
5
6
7
8
Number of cis-units in polymer, %
57
54
43
56
55
56
55
Number of trans-units in polymer, %
43
44
57
44
45
44
45
Table 5. The number of cis- and trans-units in polymers obtained during polymerization of 2–8 over 1.
Table 5 demonstrates what of cis- and trans-units of polymers obtained from diesters of 5norbornene-2,3-dicarboxylic acid contain.
Data given in Table 5 only offer estimative characteristic of polymers structure but allow
comparing in series monomers under study. Given these data, we can highlight that polymers
obtained from exo,exo-2,3-dicarbomethoxy-5-norbornenes have a similar structure. Neither
elongation of radical of ester substituent nor its branching affects the ratio of cis- and transfragments. The change of substituents orientation in positions 2 and 3 in relation to norbornene
ring causes the change in the number of cis- and trans-structures in the case of monomer 4.
Transfer of one ester substituent from exo- into endo-position would not bring about the
increase of trans-units. The situation observed can be explained if we take into consideration
that there are two ways monomer molecules are attached to active ruthenium with the formation of trans- and cis-structures (Figure 17).
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Figure 17. Two possible orientations 4 when attached to active form of ruthenium complex.
In the case when it is attached with the formation of trans-structure, a methylene bridge of
norbornene ring and bulky H2IMes-ligand hinder the monomer placement near the double
bond of ruthenium. For exo,exo-derivatives, it is a more substantial hindrance if compared
with ester groups which deter the attachment with the formation of cis-structures. On the
contrary, for monomer 4, two atoms of oxygen in esters are more of an obstacle for the
attachment to active ruthenium than a methylene bridge of norbornene ring.
Figure 17 demonstrates that both carbonyl oxygen hinder the distribution of monomer 4 near
the double bond of active ruthenium in such a way that attachment with the formation of
trans-unit is sterically more beneficial. This is also seen in an increased number of trans-units in
polymer obtained with monomer 4. For monomer 3, only one carbonyl oxygen is a hindrance
and that is why the part of trans-units in the obtained polymer remains practically the same if
compared with monomer 2.
Thus, using monomers 2, 5–8, it is stated that the length and branching aliphatic radical of
exo, exo-derivatives do not affect the ratio of cis- and trans-fragments in the obtained polymers.
The orientation of ester substituents in relation to norbornene ring in 2,3-dicarbomethoxy-5norbornenes affect the ratio of cis- and trans-fragments in polymers obtained from monomers
2–4. The transfer of two ester substituents to endo-position increases the share of trans-units,
which is due to more substantial steric hindrances caused by carbonyl oxygen of ester monomer
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Recent Research in Polymerization
group and H2IMes-ligand of catalyst when forming cis-structure if compared with the obstacles
caused by methylene bridge of norbornene ring and H2IMes-ligand of catalyst when forming
trans-structure.
6. Conclusion
To summarize, it is necessary to note that metathesis polymerization with cycle opening has
proved to be a powerful method of synthesizing polymers. Materials obtained with this method
possess good exploitation characteristics and have already proved effective on the market of
polymer goods. Development of carbene complexes based on ruthenium has made it possible to
synthesize polymers from ester of 5-norbornene-2,3-dicarboxylic acid using ROMP. To obtain
polymer materials by described process, a technology of injection molding in which polymerization rate matters most. Largely, polymerization rate is defined by the structure of catalyst and
monomer. The activity of ruthenium catalyst complexes is well studied and apart from the structure of the complex itself it depends on a number of external factors including temperature, solvent
polarity, presence of acceptor or donor compounds, etc. The structure of monomer also affects
polymerization rate. It is known that steric factor during polymerization of dicyclopentadiene and
oxygenated derivatives of norbornene contribute greatly to the reaction rate.
In most studies, the researchers used kinetic correlations to estimate reaction capacity of
different compounds. As a rule, kinetic data are obtained with NMR method, studying polymerization in-situ. The technique of such experiments is well adjusted by many scholars and
has proved to be effective.
Implementing this approach, it is shown how the structure of esters of 5-norbornene-2,3-dicarboxylic acid affects their reaction capacity and activation parameters in metathesis polymerization with cycle opening initiated by ruthenium complex of Hoveyda-Grubbs type II with
N-chelating ligand. Taking the values of activation parameters, it is assumed that there may
exist active ruthenium in chelated form, which is proved in the following studies [28, 36, 45].
It was established that the increase in the length of hydrocarbon radical does not affect greatly
the reactivity, but it influences substantially the activation parameters. Branching aliphatic
radical affects greatly both reactivity and activation parameters of polymerization. Based on
the change in activation parameters, it might be assumed that active form of ruthenium
complex forms intramolecular complex with different stability of this complex.
It was stated that mutual position of ester substituents in relation to norbornene ring affects
both reactivity and activation parameters of 2,3-dicrabomethoxy-5-norbornenes. The presence
of a substituent for the monomer molecule in endo-position reduces reaction capacity of this
monomer [46]. Activation parameters are directly depended on the ability of monomer to form
intramolecular complex with active form of ruthenium.
Based on NMR-spectra, we estimated the structures of the obtained polymers. Based on
the correlation of cis- and trans-fragments, it was established that the length and branching
aliphatic radical of exo,exo-2,3-dicarboxy-5-norbornenes and exo,endo-orientation of ester
Ring Opening Metathesis Polymerization
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substituents of 2,3-dicarbomethoxy-5-norbornene do not affect the structure of the obtained
polymers. Endo,endo-orientation of two ester substituents of 2,3-dicarbomethoxy-5-norbornene
increases the number of trans-units in the polymer, which is attributed to more substantial steric
hindrances caused by carbonyl oxygen of monomer ester group and H2IMes-ligand of catalyst
when forming cis-structure if compared with hindrances caused by methylene bridge of
norbornene ring and H2IMes-ligand of catalyst when forming trans-structure.
Author details
Alexey Lyapkov1*, Stanislav Kiselev2, Galina Bozhenkova2, Olga Kukurina1,
Mekhman Yusubov1 and Francis Verpoort1,3
*Address all correspondence to: alexdes@tpu.ru
1 National Research Tomsk Polytechnic University, Tomsk, Russia
2 Tomsk Oil and Gas Research and Design Institute, Tomsk, Russia
3 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing; and
Department of Applied Chemistry, Faculty of Sciences, Wuhan University of Technology,
Wuhan, China
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