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Chain Propagation for Polyethylene and Polypropylene
Polymerization with Late Metal Homogeneous Catalysts
Dean M. Philipp, Richard P. Muller, and William A. Goddard, III.
I. Why Late Metal Homogeneous Catalysts?
II. What Makes a Good Catalyst?
III. General Mechanism for Polymerization by Late Metal Homogeneous
Catalysts
IV. What Is Involved in Chain Propagation Calculations?
V. Results for Chain Propagation Calculations Using Various Metals, Ligands,
and Monomer Units
VI. Conclusions
I. Why Late Metal Homogeneous Catalysts?
• Alternative to other methods of polymerization - could be more active.
• Like Ziegler-Natta catalysts, they offer control of branching and
stereoselectivity.
Linear Polyethylene
Isotactic Polypropylene
Highly Branched Polyethylene
Syndiotactic Polypropylene
• Homogeneous catalysts are relatively easy to model.
N
N
+
Pd
H
R
Can use Mixed QM/MM methods
N
N
+
Pd
H
Can use full QM
II. What Makes a Good Catalyst?
• Low barrier to insertion.
• Strong enough affinity for the incoming monomer - but not too strong
• Large barriers for termination pathways.
• Ability to control branching
• Ability to control tacticity
• Other factors that are more difficult to address theoretically such as:
–
–
–
–
Stability under reaction conditions
Ease of synthesis
Cost
Ability to for from precursor + co-catalyst
III. General Mechanism for Polymerization by Late Metal
Homogeneous Catalysts
precursor +
co-catalyst
Initialization:
N
+
N
N
+
R'
R
N
N
N
+
N
+
N
+5.0
N
+
N
N
-17.1
N
-25.2
Pd
+
N
+
N
N
Pd
Pd
R
R
+2.6
+5.1
0.0
H
H
+5.8
+5.6
N
Pd
Pd
-33.9
-41.5
Chain Branching:
+5.2
+0.4
N
H
N
R
Chain Propagation:
0.0
+
Pd
=
N
+
Pd
N
N
N
Pd
Pd
R'
-17.6
0.0
N
N
Pd
H
H
+
-1.1
N
R
H
N
H
H
-14.9
N
R
R
+
Pd
Pd
R
R
+
N
N
+
N
R
Pd
R
-25.8
+3.8
Chain Termination:
0.0
N
+
N
+
N
Pd
-4.3
N
Pd
N
+
+R
Pd
H
H
R
R
H
N
*Data given are from Morokuma
IV. What Is Involved in Chain Propagation Calculations?
• Need to find complexation energy of incoming monomer unit, Ecomp
• Need to find energy for insertion of monomer into growing polymer chain, Ein
• Need to find barrier to monomer insertion, E†in
L
+
L
M
L
+
L
Ecomp
Ein
M
H
R
L
+
L
M
R
L
+
L
Ein
H
M
R
• Computational details:
R
– B3LYP density functional theory
– Jaguar program
– 6-31G** basis set used, except for LACVP** on metal and 6-31G on coordinating
ligand atoms not directly connected to metal
Calculated Results for Varying the Metal from Group 10
+
N
N
=
N
N
M
M
N
0.0
0.0
A
N
+0.2
-0.4
B
N
M
N
M
Ni
Pd
Pt
C
-11.4
H
N
-18.1
N
M
H
-26.5
N
N
M
-27.6
B
-26.6
-27.9
C
Calculated Results for Varying the Metal from Group 9
+
N
N
=
N
N
M
N
M
N
+5.1
0.0
N
+3.9
A
M
M
-3. 8
N
N
N
B
N
Co
Rh
Ir
C
N
-12.4
H
N
N
-20.5
N
M
H
-25.9
-26.1
N
C
N
-29.0
B
M
-26.3
N
Calculated Results for Varying the Metal from Group 8
+
N
N
=
N
N
M
N
M
N
N
0.0
+0.1
-4.8
A
N
N
M
N
-12.4
N
-8.6
B
M
N
Fe
Ru
Os
C
H
-24.8
-25.4
-25.2
-25.8
N
N
M
N
N
M
H
N
-40.1
B
N
Summary of Chain Propagation Calculations Using Various
Metals from Groups 8-10
Ni
Pd
Pt
Co
Rh
Ir
Fe
Ru
Os
Ecomplexation=EB-EA
-11.4
-18.1
-27.6
-12.4
-20.5
-29.0
-12.4
-24.8
-40.1
Einsertion=EC-EB
-15.1
-8.5
-0.3
-13.7
-5.5
2.7
-12.8
-0.6
14.3
E†insertion=EBC-EB
10.9
18.3
27.6
8.6
24.3
34.1
3.8
24.8
35.3
CalculatedResults
Results for
Varying
the the
Ligand
on Pdon Pd
Calculated
for
Varying
Ligand
+5.9
0.0
+
0.0
A
L
L
-5.2
L
Pd
L
L
=
Pd
Pd
-11. 2
H
N
C
L
Pd
B
E complexation=E B-E A
-11.2
-18.1
-25.5
=
L2
=
L3
Pd
+
O
Pd
L
L
L1
N
O
-25.5
=
+
-18.1
L1
L2
L3
P
Pd
L
B
P
-26. 1
-26.6
-27. 8
L
Pd
H
C
E inser tion=E C-E B
E †inser tion=E BC-E B
-2.3
17.2
-8.5
18.3
-14.9
20.3
Calculated
Results
Propyleneasas
Monomer
Calculated
Resultsfor
forPropylene
Monomer
UnitUnit
+3.1
+1.4
0.0
-2.2
A
N
N
N
Pd
N
N
Pd
B
Path (1)
(1)
C
(2)
N
Path (2)
Pd
H
-18.7
(1)
N
N
B
Pd
H
(2)
-21.0
-24.3
-21.7
N
Pd
C
N
N
H
N
N
N
Pd
Pd
H
N
N
Pd
(1)
(2)
(1)
(2)
Ecomplexation=EB-EA
(1)
-18.7
(2)
-19.4
Einsertion=EC-EB E†insertion=EBC-EB
-2.3
21.8
-2.7
23.0
VI. Conclusions
• Ethylene complexation energy increases as metal is changed to one further
down or further to the left in the periodic table
• Insertion energy barrier increases as metal is changed to one further down. It
increases as one moves to the left for the second and third transition series, but
decreases towards the left for the first series.
• The weaker the trans influence of the coordinating ligand, the larger the
observed complexation energies and insertion energy barriers.
• Using propylene instead of ethylene yields slightly larger complexation
energies and insertion barriers.
• The two propagation pathways explored for polypropylene were energetically
similar, with the second pathway slightly lower in energy for all points, but
with a slightly larger insertion barrier.
Acknowledgements:
• The Dow Chemical Company
References:
• Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117,
6414-6415.
• Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120,
4049-4050.
• Musaev, D. G.; Svensson, M.; Morokuma, K. Organometallics 1997, 16,
1933-1945.
• Musaev, D. G.; Froese, R. D. J.; Morokuma, K. Organometallics 1997, 17,
1850-1860.
• Deng, L.; Woo, T. K.; Carallo, L.; Margl, P.M.; Ziegler, T. J. Am. Chem. Soc.
1997, 119, 6177-6186.