Transcript Modeling of the active site in TiCl4/MgCl2 based Ziegler

Modeling of the active site
in TiCl4/MgCl2 based
Ziegler-Natta
heterogeneous catalysts
L. Petitjean and T. Ziegler
University of Calgary
2500 University Drive, NW
Calgary, Alberta
T2N 1N4 CANADA
1
-2-
1. Introduction
The catalytic polymerization of -olefin, known as Ziegler-Natta catalysis, is of high
interest for industry.[1] This reaction is indeed able to produce polymer from olefin under mild conditions with high activity and is the only way to get regular
polymers, such as isotactic polypropylene (iso-PP), which have a wide range of
applications. Since the structure of the active species involved in this catalysis has
still not been yet clearly identified experimentally, molecular modeling seems to be
a good tool to understand how this catalytic system works. Some theoretical works
[2-13] have already attempted to give some insight on this topic, but the complexity
of the active site has been for a long time a significant problem for the calculations.
Fortunately, recent advances in theoretical methods (i.e. QM/MM methods) give us
now access to larger molecular models, by treating only a small part of the system
(reaction site) by quantum mechanical method and the other part by molecular
mechanics potential (environment). The objective of this work is to apply such
hybrid QM/MM method to investigate the structure of the active site in order to
bring a new insight to this problem.
-3The catalytic systems used in industrial
process are basically composed by
molecular TiCl4 supported on MgCl2
crystal to which is added an AlEt3 as
co-catalysts.
In order to improve the activity and
stereo-selectivity of the catalyst two
Lewis bases have been added to initial
catalytic system. (called ILB and ELB).
These two compounds have a dramatic
impact on the catalyst performance.
Their mode of action is not clearly
determined at this time but they are
supposed to be a part of, or at least to
interfere with, the actual active center.
It is consequently very interesting for
both theoretical and experimental point
of view to imagine and calculate a
model, which includes the presence of
a Lewis Base.
iBuO
O
OiBu
O
OEt
Ethyl benzoate
O
Diiso-butyl phtalate
Typical ILB
R
R
MeO
MeO
R'
Si
R'
Alkoxy-Silanes
MeO
MeO
Alkoxy-Propane
Typical ELB
-4-
2. Choice of the model
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Mg
Cl
An important part of our work was to
define a potential model of the active
site. We have chosen to describe it as
the product of the adsorption of a single
TiCl4 molecule on the [100] surface of
the MgCl2 support. In this structure the
octahedral sphere of Ti is not filled (5fold species). The main reasons that
brought us to this choice are developed
hereafter.
We have considered the formal
oxidation state of the catalysts to be
Ti (IV). Even if other oxydation states of
Ti may be present on the active
catalysts, we believe, like other authors
on the basis of experimental evidences,
[16] that the dominant catalytic species
adopts this oxydation state.
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg Mg
Mg
Cl
Cl
Mg
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Ti
Cl
Mg
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Ti
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
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Cl
Mg
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Mg
Cl
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Cl
Mg
Cl
Mg
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Mg
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
M g Cl
Mg
Mg
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
TiCl4 on [100] MgCl2 surface
-5The MgCl2 crystal gives rise principally to hexagonal shaped crystallites. This
means that there is mainly two host surfaces for the adsorption of TiCl4 molecules :
[100] and [110].
On the [110] surface, it has been shown that the adsorption of TiCl4 is more likely
to form 6-fold species (octahedral sphere of Ti filled).[4] The reason being that a
6-fold structure is able to form 4 bonds with the surface while the 5-fold only forms
3. This stabilizes the 6-fold structure by 8,1 kcal/mole with respect to the other,
even if the deformation necessary to form this 6-fold species is higher than for the
5-fold one.
Cl
Cl
Cl
Cl
Ti
Cl
Cl
Ti
Cl
Cl
Mg MgMg
Cl Cl
Cl
Mg
Cl
Mg
Mg
Mg
Mg
Mg
Cl Cl Cl
Cl
Mg
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
6-fold
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
5-fold
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Mg
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Cl
Cl
Cl
Mg
Mg
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Mg
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg Mg Mg
Cl Cl
MgMg
Cl
Cl
Cl
Mg
Cl
Cl Cl
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
ClCl
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Mg
Mg
Mg
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
MgMg
Cl
Mg
Mg
Mg
ClCl Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl Cl
MgMg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl Cl
Cl Cl
Mg
Mg
Cl
Cl
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
-6On the [100] surface, there can be also formation either of 6-fold or 5-fold
structures. The 6-fold structure is formed via adsorption of Ti2Cl8 dimeric molecules
on the surface.[11] However, the number of shared bonds with the surface per
titanium atom is only 5/2 in the case of dimeric Ti2Cl8 while it is 3 for the mononeric
TiCl4. Since, the deformation necessary to adsorb the original TiCl4 molecule is also
larger in the case of the dimeric species, we have concluded that a 5-fold
structure would be favored on the [100] surface. Since, as explained in the
following a 5-fold species is more likely to form a polymerization center, we have
chosen this surface as a model for MgCl2 support.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Ti
Ti
Ti
Cl
Cl
Cl
Cl
Cl Cl
Cl Cl
Cl Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
MgMgMg
Mg Mg Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
6-fold
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
5-fold
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Mg
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Cl
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Mg
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Mg
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Cl
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Mg
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Cl
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Cl
Mg
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Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
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Cl
Cl
Mg
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Mg
Mg
Mg
Mg
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Cl
Cl
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Cl
Cl
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Mg
Mg
Mg
Mg
MgMg
Cl
ClCl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
Cl Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Cl
Cl
Cl
Cl Cl Cl
Cl
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Cl
Cl ClCl
Cl
Mg
Mg
Cl
Cl Cl
Cl
Cl
Cl
Mg
Cl
Mg
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Cl
Cl
Mg
Cl
Cl
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-7-
Cl
After the adsorption on the surface and
prior to the olefin insertion, the active
center must get in some way its first TiC bond. Experimentally, in most of the
case this activation step is done by
reaction with AlEt3.
Cl
+ AlEt3
Cl
Cl
Mg
Mg
Cl
Cl
Cl
DE = -9,3
kcal/mole
In the case of 6-fold models,[11-12] in
which the octahedral sphere of the Ti is
filled, this step involves the abstraction
of a Cl atom before the chlorine/alkyl
exchange with AlEt3 can take place.
This reaction is quite energetically
expensive.
With a 5-fold structure, the activation
involves only a chlorine/alkyl exchange,
which is quite cheap.
+ AlEt2Cl
C
C
Cl
Cl
Ti
C
A 5-fold species is consequently
more likely to form an actual
polymerization center.
Ti
Cl
Cl
Cl
Ti
Cl
Cl
C
Cl
Cl
Mg
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Alkyl exchange with AlEt3
Cl
-8-
It is known experimentally that by adding an ELB before the AlEt3 the molecule
instead of activating the catalyst kills its activity.This effect is very difficult to
understand with a 6-fold site model in which the octahedral sphere of the Ti is
filled. In that case, no easy reaction is possible before the activation.
By contrast, with a 5-fold model we can imagine that a ELB will be able to
complex easily the TiCl4 center. This will fill the octahedral sphere of the Ti and
hinder further activation by AlEt3.
Cl
Cl
C
C
Cl
O
+
Ti
O
Cl
Cl
Cl
Cl
C
C
DE = -16,8 kcal/mole
Cl
Mg
Cl
Ti
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Cl
-9-
3. Building the QM/MM model
High Resolution 13C NMR spectra can be best fitted using a 3-sites type
statistics.[15] This statistical model corresponds to the mixing of an enantiomorphic
model site (ES), a chain-end model site (CE), and a more general model site for
polymerization called C1. It corresponds to the fact that the polymer is composed
by 3 different stereo-blocks : highly-isotactic, syndiotactic and isotactoïd (i.e.
isotactic with low regularity) corresponding respectively to ES, CE and C1 models.
 ES site
Isotactic Polymer
 CE site
Syndiotactic Polymer
 C1 site
Isotactoïd (Low Regular) Polymer
- 10 We have tried to interpret the NMR features in terms of molecular structure and
imagined a molecular model, which is able to have 3 possible reaction centers. The
idea is that the presence of ELB or ILB molecules around the Ti active site would
create an asymmetry, which would be at the origin of 3 reactive centers (A, B,
C). The purpose of the QM/MM calculations has been to check if those 3 sites
could be related to the stereo-blocks described by NMR.
Cl
Ti
Bulk
Cl
Cl
1
Cl
1
2
Cl
Reaction Center A
Cl
Ti
2
Cl
Cl
Reaction Center B
Cl
Cl
Ti
Cl
2
Cl
Reaction Center C
Bulk
Ti
Cl
Bulk
Cl
Bulk
Cl
1
- 11 Our hypothesis can be associated to many molecular structures. Indeed, it is
generally accepted that the active center lies in many different environment leading
to a distribution of properties of the product. Unfortunately, for the calculations
we had to choose one. We decided to pick up the one in which reaction centers
B and C would be most differentiating.
C
C
C
C
C
C
C
C
C
C
N
C
C
C
C
C
Cl
N
C
Cl
Mg
Cl
Cl
ClCl
Mg
Mg
Mg
Mg
Ti
Cl
Cl
Cl
Cl
Cl
C
Mg
C
Cl
Mg
Cl
Ti
C
Cl
Mg
Cl
C
Cl
Cl
C
Cl
C
C
C
C
Cl
C
C
Cl
Cl
C
Cl
C
Cl
Mg
C
C
Cl
Mg
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Mg
Mg
Cl
Cl
C
C
Cl
C
ClClCl
Cl
C
C
QM system
C
C
C
C
N
QM system
Cl
Cl
Cl
Cl
Cl
Mg
Mg
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
C
C
C
Cl
Mg
Cl
C
C
N
C
Cl
Mg
Cl
Cl
- 12 -
4. Computational details
We have used a QM/MM code implemented at the University of Calgary[18] using
the ADF density functional package developed by Baerends et al.[19] The
molecular mechanics part has been treated using the Sybyl force field [20] for C, N,
O, H and Cl atoms. Van der Waals parameters, which are not present in Sybyl,
have been added from Dreiding [21] for Ti and Mg atom type.
For the quantum part, the electronic configuration of the molecular systems were
described by a triple- basis set on Ti atom for 3s, 3p, 3d and 4s, while double-
STO basis set with polarization functions were applied for Mg, Cl, C and H atoms.
[22]The 1s electron of C atoms and the 1s-2p electrons for Mg, Cl and Ti atoms
were treated as frozen core. The auxiliary s, p, d, f and g STO functions, [23]
centered on all nuclei, were used to fit the electron density and the Coulomb and
exchange potentials in each SCF cycle. The B-LYP exchange-correlation functional
[24]was used in all the calculations.
In all the calculations, the atoms of the surface model have been fixed using the Xray parameters published in the literature [17] for the crystal bulk. For the QM part,
the surface model is Mg2Cl4, which may be seen as modest. However, the use of a
larger and much more realistic cluster model (Mg9Cl4H6) for the MgCl2 layer didn’t
affect dramatically the results.
- 13 -
5. Results
Here is a summary of the QM/MM calculations carried out on the active site
model with 2,2,6,6 tetra-methyl piperidine (TMP) as ELB (p.11). We want to
be able to compare qualitatively the results of the calculations with the NMR data.
This means that we have to determine the type polymer produced by each
reaction center A,B,C.
A first step to evaluate the type polymer produced by each center is to determine
the selectivity of the insertion at each of the potential position of coordination of
the olefin (A1,A2,B1,B2,C1,C2). For each position, this involves the calculations
of two paths corresponding to the two face of the olefin (re or si). By comparing
the energy of the transition states we will know which path is favorable.
In a second step, we have to imagine a succession of insertions. In the case of
center B and C, we have postulated that the polymerization occurs via a chain
migratory insertion, which means that the insertion takes place alternatively in
position 1 or position 2. Indeed, by looking at the -H agostic resting states it can
be seen that the Ti stays in a pseudo octahedral conformation between two
insertions.
- 14 However, this is not true in the case of center A, in which the Ti adopts a
tetrahedral conformation. This involves a movement of the chain in an
intermediate position between the 1 and 2 (which correspond to octahedral
environment). Moreover the two possible conformations found for the
resting-state have the same energy and can be considered to interconvert
easily.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
N
C
N
Cl
Cl
C
C
Cl
Mg
Cl
Mg
C
Cl
Cl
Ti
Cl
Mg
C
Cl
Mg
Cl
Cl
Mg
Cl
Cl C
Mg
C
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Ti
C
Cl
C
C
Cl
Cl
Cl
Cl
C
C
C
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
ClC
Mg
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
C
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
C
C
N
Cl
Mg
Cl
C
C
C
C
N
Mg
Cl
C
C
C
ClC
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Two conformations of canter A resting-state
Cl
C
- 15 Selectivity of Center A :
The presence of TMP molecule induces an asymmetry between position 1 and
position 2. It turns out that the transitions states corresponding to insertion with
olefin in position 2 are lower in energy. It seems then reasonable to think that a
majority of insertion will occur through a path which involves insertion with olefin in
position 2.
Additionally, due to the presence of surface atoms the chain cannot rotate (pointing
down in the figure). When the olefin is in position 2, the re insertion will occur by
trans approach of propylene while si one will happen by cis approach. The insertion
will be favored when occurring through a trans approach (here re).
Consequently, we believe that Center A will produce isotactic polymer.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
N
N
C
C
N
N
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
C
C
C
Cl
C
Ti
C
C
C
C
Cl
C
Ti
Cl
Cl
Cl
C
Ti
C
C
C
Cl
Cl
C
C
C
C
C
C
C
C
Cl
C
Ti
C
C
C
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
C
Cl
Mg
Cl
C
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
C
C
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
re (E=+3.4)
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Mg
Cl
Cl
Cl
Mg
Cl
C
si (E=+5.6 )
Position A1
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Mg
Cl
Cl
C
Cl
Mg
Cl
Mg
Cl
Cl
C
C
C
Cl
Cl
Mg
C
C
C
C
N
Cl
Cl
Cl
C
C
Cl
Mg
Cl
C
C
Mg
Cl
Mg
Cl
N
N
Mg
Mg
C
C
C
Cl
Cl
C
C
C
Mg
Cl
C
C
C
C
Cl
Mg
Cl
Cl
C
C
C
N
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
re (E= +3.0)
si (E= 0)
Position A2
Mg
Cl
Cl
- 16 Selectivity of Center B :
When the olefin is in position 1, due to the presence of surface atoms nearby, the
chain cannot rotate to avoid the pressure of the incoming olefin. The re insertion
will occur by trans approach of propylene while si one will happen by cis
approach. The more favorable corresponds to the trans approach insertion (here
re).
By contrast, when the olefin is in position 2, the chain can easily rotate to avoid
the pressure of the olefin. Both re and si insertions will occurs favorably by the
less sterically demanding trans approach. The insertion is almost not selective.
Consequently, one can think that B would produce a hemi-isotactic type of
polymer.
C
C
C
C
C
C
C
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Cl
C
C
C
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
C
Cl
Ti
Cl
Mg
C
C
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
C
Ti
Cl
Cl
Ti
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
N
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
re (E=0)
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
Cl
C
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
C
C
N
Cl
Mg
Cl
C
C
C
N
Mg
ClC
Mg
Cl
C
C
Cl
Mg
si (E=+2.6)
Position B1
Mg
Cl
C
C
N
Cl
Mg
C
C
C
C
ClC
Mg
Cl
C
C
C
C
Mg
Mg
Cl
C
C
Cl
C
C
ClC
Mg
Cl
C
C
Cl
Mg
Cl
C
C
ClC
Mg
Cl
Cl
Mg
Cl
Cl
C
C
C
Cl
Cl
Ti
Cl
C
C
Mg
Cl
Cl
C
C
Cl
Cl
C
N
C
C
C
C
C
N
C
C
C
C
C
C
C
Cl
C
C
C
C
N
C
Mg
Cl
C
C
C
C
C
C
C
C
N
Mg
C
C
C
C
C
C
C
C
C
C
C
C
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
re (E=0)
si (E=+0.5)
Position B2
Cl
C
- 17 Selectivity of Center C :
As in the case of B and due to the presence of atoms from both surface and TMP
molecule nearby, the chain cannot rotate to avoid the pressure of the incoming
olefin when the olefin is in position 1. As already explained, the more favorable
insertion corresponds to the trans approach insertion, in that case for the si face.
When the olefin is in position 2, the presence of TMP molecule hinders the rotation
of the chain. Again it cannot avoid the pressure of the incoming olefin. The
favorable trans approach insertion is here the re one.
Consequently, center C can be considered to produce syndiotactic polymer.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
N
C
C
C
C
C
C
C
N
C
C
C
C
N
C
N
C
C
C
C
C
C
C
C
C
C
Cl
Mg
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Mg
Cl
Cl
C
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
C
Cl
Mg
Cl
Mg
Cl
Cl
C
C
C
Cl Cl
Ti
C
C
Cl
Cl
Ti
Cl
Cl
Cl
Cl
Ti
C
Cl
Cl
Cl
Ti
C
C
C
C
C
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
C
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Mg
Cl
Cl
N
re (E=+4.6)
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Mg
Cl
Cl
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
C
C
N
Cl
Mg
Cl
C
C
C
N
Mg
ClC
Mg
Cl
C
C
C
C
Cl
Mg
si (E=0)
Position C1
Mg
Cl
C
C
C
C
Cl
Mg
C
ClC
C
C
C
C
N
Mg
Mg
Cl
Cl
C
C
C
C
ClC
Mg
Cl
C
C
C
C
C
ClC
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
Cl
Cl
Mg
re (E=0)
si (E=+2.9)
Position C2
Cl
C
- 18 -
6. Conclusions
We have presented here a new approach for the modeling of the active site of
heterogeneous Ziegler-Natta catalysts. This model consists in a TiCl4 molecule
adsorbed on a [100] MgCl2 surface surrounded by two molecules of ELB. These
molecules create an asymmetric environment that makes possible to imagine a
structure with 3 possible reaction centers.
QM/MM calculations have been carried out on this type of model with 2,2,6,6
tetramethyl piperidine (TMP) as ELB. We have been able to identify the type of
polymer produced by 3 reaction centers (A, B and C). A produces higlhy isotactic
polymer, B seems to produce hemi-isotactic polymer, while C could produce
syndiotactic polymer.
Our model seems in fairly good quliative agreement with NMR datas describing the
formation of stereo-blocks in polypropylene. By this way, it gives some insight on
the influence of the ELB on the catalysts selectivity. The presence of ELB
molecules is at the origin of the formation isotactic polymer by reaction center A.
Syndiotactic blocks formed by center C would not exist without the presence of the
ELB.
In our future works, we will focus on describing the evolution of stereospecificity of
the catalyst using a series of ELB molecules. With our model we will try to
understand what are the mechanism that explain the variations of stereospecificy
in the family of R,R'-dimethoxy propane molecules.
- 19 -
7. References
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Acknowledgments
Authors would like to thank Elf-Atochem and Elf Aquitaine for their financial
support. We would like to thankalso Dr. T. K. Woo (University of Calgary) for his
help in providing the QM/MM code for this work. L. Petitjean would like to thank Dr
J. Malinge, Dr T. Saudemont and Dr D. Pattou from the Groupement de
Recherches de Lacq (Elf-Atochem) for their collaboration in this work.