Olefin Polymerizations Catalyzed by Late Transition Metal Complexes

Maurice BrookhartUniversity of North Carolina

Polyolefins

Total : 100 billions / year

16lbs / person on Earth / year !

• Inexpensive monomers

• Little waste in production

• Attractive physical properties, long term stabilities

CH CH2

CH3CH CH2

CH2 CH2Polyethylene,n

6 x 1010 lbs/yr

Polypropylene, n 3 x 1010 lbs/yr

1 x 1010

lbs/yr

nPolystyrene,

Polymer Microstructure — Key to Properties

isotactic

syndiotactic

atactic

H3C H3C H3C H3C H3C

H H H H H Tm = 160°C

R R

Polypropylene

Polyethylene

Tm = 165°C

Stereoregular

Completely amorphous

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)

Ph Ph Ph

Ph Ph Ph

isotactic polypropylene

syndiotactic polypropylene

atactic polypropylene

syndiotactic polystyrene

O

O

O

O

alternating CO / copolymer

high % crystallinity

Tm ~ 250oC

General Mechanism for Polymer FormationInitiation

Chain Growth (RP)

Chain transfer (RCT)

RP >> RCT => High Polymer

RP ~ RCT => Short Chains (oligomers)

RP < RCT => Only C4

LnM H LnM H migratory

insertionLnM

LnM LnMmigratory

insertionLnM

LnMetc LnM

LnMP

H H

-H

elim.LnM

P

H

LnM

H

+ P

etc. LnMnew chain starts

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

GG G

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

N N

RR

H3C solv.

R'

R'R'

R' M

R = H, Me, acenaphthyl

R' = -iPr, -Me, aryl, halogen

A

M = Ni, PdA =

CF3

CF3

B4

Catalysts Modeled on α–Diimine Systems

M

NN

R

R

R

R

M

PN

R

R

Ar

Ar

Ph

N

NN

R

R

R

R

M

X X MMAO

Ni

O N

R

R

O NNi

R

R

N

R

R

N

R

R

N N

M

M = Ni, Pd

Daugulis, BrookhartBennettSmall, Brookhart Gibson

M = Fe, Co

G

GrubbsJohnson (DuPont)

Hicks, Jenkins, Brookhart

M = Ni, Pd

Killian (Eastman)

Polyethylene

N N

R' Solv

R

RR

R Pd

+

N N

Br Br

R

RR

R Ni

/ Et2AlCl

30 °C~500 TO/hr

30 °C

~1-3 x 106 TO/hr

Mn > 105

amorphous PE

hyperbranched,

~100 branches / 1000 C's

Mn 105 - 106

Tm: 25° - 135° C

5 - 80 branches / 1000 C's

increasing [C2H4] decreases branching

increasing T increases branching

Early Metal CatalystsTi (IV), Zr (IV), Cr/SiO2

n

linear PEsemicrystallineTm ~ 136 °C

High Density PE, HDPE

R R

1-10% incorporation, LLDPE

Tm = ~ 115 - 130 oCR

Poly (α–Olefins)

N N ArArM

+R R

Early Metal CatalystsR

R RRR

"chain-straightened" (1, 3 enchainment)

chain-straightened, primarily C1, C4 branches(1,6 enchainment)

1,2-insertion

1,2–Disubstituted Olefins

N N

RR

X Y

ArArM

+ A

cis-1,3-enchainment

chainstraightening

-

1,3 insertion

Mechanistic StudiesGeneration of Cationic Alkyl Complexes

N

NM

RR

RR

X

X

N

NM

CH3

CH3

2 R'MgX

N

NM

CH3

CH3

N

NM

CH3

OEt2

R' = -CH3 -CH2CH3 -CH2CH2CH3 -CH2CH(CH3)2

M = Pd stable at 25 °C

M = Ni stable only below ca. -20 °C

H(OEt2)2+ BAr'4

-

Et20

+ BAr'4-

1H, 13C NMR Studies – Pd(II)

N

N

PdMe

Ar

Ar

OEt2

N

N

Pdpoly

Ar

Ar

N

N

Pd

Ar

Ar

N

N

PdMe

Ar

Ar

+

C2H4 (excess)

+

+

kp, -30 °C

+

-30 °Ck1

catalystresting state

CD2Cl2-80 °C

Insertion Kinetics – Ni(II)

N

N

NiMe

Ar

Ar

OEt2

N

N

NiPr

Ar

Ar

CDCl2F

N

N

NiR

Ar

Ar

N

N

NiMe

Ar

Ar

+ 1. 20 eq C2H4 -130 °C2. -110 °C

+

-80 °C

k1st insertion

+

-70 °C

ksub. insert.

+

Activation Barriers to Insertion (ethylene)

N

N

Pd

Me

N

N

Ni

Me

13.6 kcal/mol (-81 oC) 14.0 kcal/mol (-72 oC)

G (1st insertion) G (subseq. insertions)

G‡ (Pd-Ni) ca. 5 kcal/mol

+

18.4 kcal/mol (-20 oC) 18.6 kcal/mol (-20 oC)

+

Mechanistic Model

MN

N

RM

N

N

R

MN

N

R

MN

N

R

H

MN

N

R

insertion

methyl branchresting state

MN

N

R'

turnover-limiting

"chain running"

MN

N

insertion

ethyl branch

R'

Blocking of Axial Coordination Sites

Chain Transfer Mechanisms

NN

HR

R+NN

HM M

(1) Associative Displacement (retarded by blocking axial postions)

(2) Chain Transfer to Monomer (suggested by Ziegler calculations)

N

N

H3C

H3CNi Ni

N

N

H3C

H3CNi

N

NH

Mechanistic Model

MN

N

RM

N

N

R

MN

N

R

MN

N

R

H

MN

N

R

insertion

methyl branchresting state

MN

N

R'

turnover-limiting

"chain running"

MN

N

insertion

ethyl branch

R'

Formation of Agostic Ethyl Complex

N

NPd

N

NPd

H

Pd

CC

Ha

Hc Hc

Hb

Hb

CH3CH3

H(OiPr2)2BAr'4

CDCl2F, -80 oC

BAr'4

-8.9 ppm

t, 2JHH = 16 Hz1JCH = 67 Hz

1H 2.2 ppm13C 38.5 ppm1JCH = 153 Hz

1H 1.4 ppm13C 19.3 ppm1JCH = 155 Hz

(-130 oC)

Dynamics of Agostic Ethyl Complex

Pd

N

N Ha

Hc Hc

Hb

Hb Pd

N

N Ha

Hb Hb

Hc

HcPd

N

N Ha

k = 1450 s-1, -108 oC

G‡ = 7.1 kcal/mol

**

*

Ni

N

N H

H H

H

H Ni

N

N H

H H

H

HNi

N

N H

k = 170 s-1, 16 °C

G‡ = 14.0 kcal/mol

Cationic Metal Alkyl Intermediates –Ethylene Trapping Experiments

Pd

N

N

Pd

N

N

H(OEt2)2+ BAr'4

-

Pd

N

N H H

1 20

Pd

N

N-80 °C

-80 °C

Pd

N

N

-65 °C

-25 °C

insertion

(several 100 1,2 shifts prior to insertion)

(via reversibleloss of C2H4)

1 20

Cationic Metal Alkyl Intermediates –Ethylene Trapping Experiments

Ni

N

N

Ni

N

NX

Ni

N

N

Ni

N

N

-80 °C -80 °C

etc. etc.

no equilibrationprior to insertion

Mechanistic Model

MN

N

R

HM

N

N H

R

MN

N

R

MN

N

R

H

MN

N

R

insertion

methyl branchresting state

MN

N H

R'

turnover-limiting

"chain running"

MN

N

insertion

ethyl branch

R'

Commercial Copolymers of Ethylene and Polar Vinyl Monomers

CO2Me

CO2Bu

CO2H

CO2H

OAc

CN

Si(OMe)3

● Radical Initiation

● High temperatures, very high ethylene pressure

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

O R

O

OR

O

SiOR

OROR

1.

2.

3.

Problems Connected with Copolymerization

1. Monomer Binding through the Functional Group

G

2. β-Elimination of G

ML

L

R

+ G ML

L

R

G

ML

L

R

G

ML

L

R

GM

L

L G

R

3. Weak Competitive Binding of

4. Strong Chelate Formation Following Insertion

G

ML

L

R

G

insertion isomerizationM

L

L G

ML

L

G

K << 1

ML

L

R

G

+ ML

L

R

+G

K >> 1

5. High Barrier to Insertion of Open Chelate

ML

L

insertionR

G

ML

L

R

G

ML

L

insertionR ML

L

R

G1‡

G2‡

G1‡ G2

‡>

slow

fast

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

PdN

N

CH3

PdN

N

CH3

OEt2

CN

X

PdN

N

CH3

PdN

N X

NC

PdN

N X

X

PdN

N

PdN

N X

X = Br, Cl elim

2+Sen, et. al. 2002Jordan, et. al. 2003

Ittel, Johnson, Brookhart, Chem. Rev. 2000

Ethylene / Acrylate Copolymerization - Pd

N

NPd

CH3

NCCH3

N

NPd

CH3 N

NPd

O

OCH3 N

NPd

O OCH3

N

NPd

OOCH3

CO2CH3

OCH3

O

CO2CH3

CH2Cl2, T = 35 oC

P(C2H4) = 2 atm

MA = 25 vol%

TOF = ca. 100 TO/h (slow!)Branched Copolymer6 mol% MA incorporation103 branches/1000 C

Methyl Acrylate Insertion

-80 oC

2,1-insertion

-60 oC

-30 oC

Mechanism of Copolymerization

N

NPd

rearrangementP

OCH3

O2,1-ins.

G‡ = 16 kcal/mol

N

NPd

P

OOCH3

N

NPd

P OCH3

O

N

NPd

P OCH3

Ochain growth

chain running

resting state

+C2H4

-C2H4

K ~ 0.02 M-1

25 °C

insertion

G‡ ~ 18 kcal/mol

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

O R

O

OR

O

SiOR

OROR

1.

2.

3.

Ethylene / Alkoxy Vinyl Silane CopolymersVersipol Group - DuPont

N N

R'

R'R'

R'M

R''R''

R L

Si(OR)xR'y

N NNi

SiMe3Me3Si

Si(OEt)3

/

25 - 120 °C

random copolymer

linear to highly branched

up to 32 mol% comonomer incorporation

600 psi C2H4, 60 °C

5 vol%toluene

5 eq. B(C6F5)35 eq. LiB(C6F5)4

PE copolymer0.42 mol% silane10 Me branches / 1000 CTm = 121 °CMn = 25.5 K, Mw/Mn = 2.9110 kg PE / gm Ni

Vinyl Alkoxy Silane Insertion Chemistry -

N NNi

Me OEt2

Si(OEt)(Me)2

CD2Cl2-60 °C

N NNi

SiO

Et

N NNi

Si

O Et

no 2-alkene

complexes

observed

15%2,1 insertion

85%1,2 insertion

Si(OEt)(Me)2

Evidence for Reversible C2H4 Coordination

N NNi

SiO Et

N NNi

SiEtO

+ C2H4 4.68 4.59 3.96 3.73

CDCl2F

Keq ca. 0.035 M-1, -120 °C

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.