Olefin Polymerizations Catalyzed by Late Transition Metal Complexes

Olefin Polymerizations Catalyzed by Late Transition Metal Complexes Maurice Brookhart University of North Carolina

Polyolefins Total : 100 billions / year 16 lbs / person on Earth / year ! • Inexpensive monomers • Little waste in production • Attractive physical properties, long term stabilities

Polymer Microstructure — Key to Properties Polypropylene Tm = 160°C Stereoregular Tm = 165°C Completely amorphous Polyethylene High Density PE (HDPE) Tm= 136°C Linear Low Density PE (LLDPE) Tm = 115~130°C Low Density PE (LDPE) Tm= 105~115°C

Polyolefins Primarily Produced via Metal-Catalyzed Processes Catalyst Structures Control: — polymer microstructures — polymer molecular weights, molecular weight distributions — comonomer incorporation Early Metal Catalysts (Ti, Zr, Cr) Late Metal Catalysts (Pd, Ni, Co)

General Mechanism for Polymer Formation

Olefin Polymerizations Using Late Metal Catalysts (Ni, Pd) Why Late Metals ? 1. Potentially different enchainment mechanisms => new microstructures 2. Less oxophilic — functional group compatible But… 1. Normally lower insertion barriers 2. Chain transfer competitive with propagation => dimers, short chain oligomers

α–Diimine Based Catalysts ■ High molecular weight polymers with unique microstructures from: ● ethylene ● α – olefins ● cyclopentene ● trans-1, 2 -disubstituted olefins ■ Copolymers of ethylene with certain polar vinyl monomers

Catalysts Modeled on α–Diimine Systems

Polyethylene

Poly (α–Olefins)

1, 2–Disubstituted Olefins

Mechanistic Studies Generation of Cationic Alkyl Complexes

1 H, 13 C NMR Studies – Pd(II)

Insertion Kinetics – Ni(II)

Activation Barriers to Insertion (ethylene)

Mechanistic Model

Blocking of Axial Coordination Sites

Chain Transfer Mechanisms

Mechanistic Model

Formation of Agostic Ethyl Complex

Dynamics of Agostic Ethyl Complex

Cationic Metal Alkyl Intermediates – Ethylene Trapping Experiments

Cationic Metal Alkyl Intermediates – Ethylene Trapping Experiments

Mechanistic Model

Commercial Copolymers of Ethylene and Polar Vinyl Monomers ● Radical Initiation ● High temperatures, very high ethylene pressure

Examination of Pd and Ni Diimine Catalysts for Copolymerizations of Ethylene and:

Problems Connected with Copolymerization 1. Monomer Binding through the Functional Group 2. β-Elimination of G

3. Weak Competitive Binding of 4. Strong Chelate Formation Following Insertion

5. High Barrier to Insertion of Open Chelate

Examples: G = -CN ; -Br, -Cl

Ethylene / Acrylate Copolymerization - Pd

Mechanism of Copolymerization

Examination of Pd and Ni Diimine Catalysts for Copolymerizations of Ethylene and:

Ethylene / Alkoxy Vinyl Silane Copolymers Versipol Group - Du. Pont

Vinyl Alkoxy Silane Insertion Chemistry -

Evidence for Reversible C 2 H 4 Coordination

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.