Mechanistic Studies of (a-Diimine)Ni(II)-Catalyzed Olefin Polymerization

1
Olefin Polymerizations Catalyzed by Late
Transition Metal Complexes
Maurice Brookhart University of North Carolina
2
Polyolefins
Total 100 billions / year 16lbs / person on
Earth / year !

• Inexpensive monomers
• Little waste in production
• Attractive physical properties, long term
  stabilities

3
Polymer Microstructure Key to Properties
Polypropylene
Tm 160C
Stereoregular
Tm 165C
Completely amorphous
Polyethylene
High Density PE (HDPE) Tm 136C Linear Low
Density PE (LLDPE) Tm 115130C Low Density PE
(LDPE) Tm 105115C
4
Polyolefins Primarily Produced via
Metal-Catalyzed Processes

• Catalyst Structures Control
• polymer microstructures
• polymer molecular weights, molecular weight
  distributions
• comonomer incorporation

Late Metal Catalysts (Pd, Ni, Co)
Early Metal Catalysts (Ti, Zr, Cr)
5
General Mechanism for Polymer Formation
6
Olefin Polymerizations Using Late Metal Catalysts
(Ni, Pd)

• Why Late Metals ?
• Potentially different enchainment mechanisms gt
• new microstructures
• Less oxophilic functional group compatible
• But
• Normally lower insertion barriers
• Chain transfer competitive with propagation gt
• dimers, short chain oligomers

7
aDiimine Based Catalysts
High molecular weight polymers with unique
microstructures from ? ethylene ? a
olefins ? cyclopentene ? trans-1,2-disubstituted
olefins Copolymers of ethylene with certain
polar vinyl monomers
8
Catalysts Modeled on aDiimine Systems
9
Polyethylene
10
Poly (aOlefins)
11
1,2Disubstituted Olefins
12
Mechanistic StudiesGeneration of Cationic Alkyl
Complexes
13
1H, 13C NMR Studies Pd(II)
14
Insertion Kinetics Ni(II)
15
Activation Barriers to Insertion (ethylene)
16
Mechanistic Model
17
Blocking of Axial Coordination Sites
18
Chain Transfer Mechanisms
19
Mechanistic Model
20
Formation of Agostic Ethyl Complex
21
Dynamics of Agostic Ethyl Complex
22
Cationic Metal Alkyl Intermediates Ethylene
Trapping Experiments
23
Cationic Metal Alkyl Intermediates Ethylene
Trapping Experiments
24
Mechanistic Model
25
Commercial Copolymers of Ethylene and Polar Vinyl
Monomers
? Radical Initiation ? High temperatures, very
high ethylene pressure
26
Examination of Pd and Ni Diimine Catalysts for
Copolymerizations of Ethylene and
27
Problems Connected with
Copolymerization

• 1. Monomer Binding through the Functional Group

2. ß-Elimination of G
28

• 3. Weak Competitive Binding of

4. Strong Chelate Formation Following Insertion
29

• 5. High Barrier to Insertion of Open Chelate

30

• Examples G -CN -Br, -Cl

31
Ethylene / Acrylate Copolymerization - Pd
32
Mechanism of Copolymerization
33
Examination of Pd and Ni Diimine Catalysts for
Copolymerizations of Ethylene and
34
Ethylene / Alkoxy Vinyl Silane CopolymersVersipol
Group - DuPont
35
Vinyl Alkoxy Silane Insertion Chemistry -
36
Evidence for Reversible C2H4 Coordination
37
Advantages of Vinyl Alkoxy Silane Comonomers

1. Insertion barriers of vinyl alkoxy silanes into
   Pd-R and Ni-R bonds are similar to ethylene
   insertion barriers.
2. Chelates resulting from vinyl alkoxy silane
   insertions are readily opened with ethylene.
3. Open chelates readily insert ethylene.
4. Relative binding affinities favor ethylene, but
   not to a prohibitive extent.