Transcript On the Electronic Character of Oxygen

ACS Meeting, Chicago
August 28, 2001
The Challenge of the Copolymerization
of Olefins with Nitrogen-Containing
Polar Monomers
Dirk V. Deubel and Tom Ziegler
Dept. of Chemistry, University of Calgary, Canada
[email protected], [email protected]
New Catalysts for Olefin Copolymerization
with O-Containing Monomers
• Incorporation of polar monomers in the polymer chain of
polyolefins is of industrial interest
• Common random copolymers such as polystyreneacrylonitrile are still produced in radical processes
• Promising Nickel(II) and Palladium(II) catalysts with
diimine ligands (“Brookhart”) and salicylaldiminato
ligands (“Grubbs”) were recently reported:
2
New Catalysts for Olefin Copolymerization
with N-Containing Monomers?
• The Brookhart and Grubbs late transition metal (late TM)
catalysts are less oxophilic than their Ziegler-Natta and
metallocene counterparts and have been used for
copolymerization with oxygen-containing monomers
• Despite the industrial importance, little is known about
whether copolymerization of olefins with nitrogencontaining polar monomers can also be achieved
• Objective: Initial screening of late TM catalysts and polar
monomers toward an incorporation of amines or nitriles in
the polymer chain of polyolefins
• Quantum-chemical methods: Gradient-corrected density
functional theory (DFT) at the BP86 level; Basis sets
VTZP at the metals and VDZP at the other atoms
• Quantum-chemical software: ADF 2000
3
DFT Model Study on Polar Monomer
Binding to Late TM Catalysts
• N-containing polar monomers can bind either with the 
moiety or with the N-containing polar site to the catalyst
• The  coordination leads to polymer growth while the N
coordination is a dead end
• Catalyst-monomer combinations that prefer 
coordination over the N coordination are promising
4
Model Catalysts
• Model catalysts have been used, because steric effects on
monomer coordination energies are comparably small
C
C
C
C
C
N
N
O
N
Ni
Ni
C
C
C
Brookhart
1: Ni 3: Pd
C
Grubbs
2: Ni 4: Pd
5
Model Monomers
• Monomers of the type CH2=CH(CH2)n(PolarGroup) have been
considered
• Conjugated systems (n = 0) have explicitly been investigated
• Non-conjugated systems (n ≥ 1) have been studied efficiently
using CH2=CHCH3 and CH3(PolarGroup) as models
• A large number of catalyst-monomer combinations was
considered at a high computational level
6
Model Complexes
Example: Nitriles and Brookhart Nickel
C
C
• CH2=CHCN
C
C
N
N
N
N
Ni
Ni
C
C
N
N
C
C
C
C
C
C
:
• CH2=CH(CH2)nCN, n ≥ 1
N:
C
C
C
C
C
C
N
N
N
N
Ni
C
Ni
C
C
:
C
C
C
N:
C
N
C
C
7
 versus N Coordination: Calculated
Stabilization Energies for the Ni Catalysts
-10
kcal  E
mol-1
1 
-15
N 2
 2
-20
 1
2 
2 N
-25
1
N 1
N
-30
CH2=CHCN
CH2=CHNH2
CH2=CH(CH2)nCN
-35
CH2=CH(CH2)nN(CH3)2
CH2=CH(CH2)nNH2
8
 versus N Coordination




Large effect of  conjugation in the polar monomer on 
binding with the Brookhart catalyst: electron-rich C=C
bonds increase  complex stability
The polar monomers form very strong N complexes with
the cationic Brookhart catalysts
Vinylamine (CH2=CHNH2) prefers  coordination over N
coordination
Small effect of  conjugation in Grubbs catalysts: both
electron-rich and electron-poor C=C bonds slightly
increase the  complex stability
The polar monomers form N complexes with the Grubbs
catalysts of the same stability as  complexes
Destabilization of amine-N complexes by N-alkyl
substituents (Grubbs ligands have a larger bite angle than
Brookhart ligands)
9
Large Differences in  Complex Stability:
Rationalization by Orbital Interactions
• Donation from the monomer to the catalyst is predominant
in Brookhart complexes
• Considerable amount of backdonation from the catalyst to
the monomer in the Grubbs complexes
10
Calculated  and N Coordination Energies
for the Pd Catalysts
-10
kcal  E
mol-1
-15
-20
-25
1 
3 
N 2
 2
 1
2 
2 N
4
N
 3
4

N 4
 4
3
N
1
N
N 1
N 3
-30
CH2=CHCN
CH2=CHNH2
CH2=CH(CH2)nCN
-35
CH2=CH(CH2)nN(CH3)2
CH2=CH(CH2)nNH2
11
Ni versus Pd Catalysts: Systematic
Trends in  and N Coordination
 The  complexes with the Brookhart Pd catalysts are more
stable than the corresponding Ni complexes  by 3 kcal/mol
 The N complexes with the Brookhart Pd catalysts are as
stable as the corresponding Ni complexes 
 The  complexes with the Grubbs Pd catalysts are more
stable than the corresponding Ni complexes  by 6 kcal/mol
 The N complexes with the Grubbs Pd catalysts are more
stable than the corresponding Ni complexes  by 3 kcal/mol
• Replacing Ni by Pd favors  coordination relative to N
coordination by 3 kcal/mol
• Explanation by larger overlap between Pd d orbitals and C=C
ligand orbitals
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Summary
• The stabilization energies for the  and N binding modes of
unsaturated amines and nitriles to Brookhart and Grubbs
polymerization catalysts have been calculated using DFT
• A reasonable choice of computational models has enabled us
to study a large number of catalyst-monomer combinations at a
high level of theory
• -Conjugated amines prefer  binding mode in its complexes
with all investigated model catalysts, including the cationic
Brookhart catalysts
• The  complexes formed by the polar monomers and the
Grubbs catalysts are as stable as the corresponding N
complexes, indicating a very promising research direction
• The Pd  complexes are more stable than their Ni counterparts
• N-alkyl substituents destabilize the N complexes and therefore
indirectly favor  coordination
13
Acknowledgments
• Dr. Artur Michalak and the other members of the Ziegler
research group
• Multimedia Advanced Computational Infrastructure
(MACI), University of Calgary, Canada
• German Academic Exchange Service (NATO Fellowship)
• Alexander-von-Humboldt Foundation
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