Macromolecular Science and Engineering

Tuesday June 1 PM MA3 Regatta Advances In Olefin Polymerization Organizer - H. ZahalkaChair - H. Zahalka

13:30-14:00 0080 Olefin Polymerization Catalysts Beyond Group-IV Metallocenes Ziegler T., Deng L., Schmid R., Margl P.

Polyethylene• … is one of the commercially most

important plastics

• … is conveniently produced by the Ziegler-Natta catalytic process

• … can be tailor-made to suit specificpurposes of the customer

n

n

Successful Catalysts• Commercial homogeneous catalysts are usually

of the group 4 “metallocene” type.

• Metallocene catalysts have severalundesirable drawbacks and are heavilyshielded by patents.

TiSiN

R1R2

R3

++CR1

R2Zr

Potential Alternatives

For ligands: everything organic chemistry can offer!

Fe

N

NFe

iPr

iPrPri

Pri

MeMe

N

McConville

Getting the idea

Cr

R

X

ZrSi

R'

R

NR"

do

Bercaw/Marks

ZrSi

R'

R'

Rdo

Kaminsky/Brinzinger

do

d3

Brookhart/Gibson

M=Fe,Cod6-d7 N

NM

R'R

'R

R"

R"

Brookhart

M=Ni,Pd

d8

NTi

NR'

R'

R"R

"R

NM

N RR'

R'

"R

"RTheopold

d1 - d3 M=Ti,V,Cr

M= ?

L= ?d?

Focus: d0 and d0fn Catalysts

• Most viable catalysts are systems with(formally) no d electrons

• We shall first develop a picture of how d0

systems work ...

• … and later generalize to systems withhigher d occupation

+ Lanthanides, Actinides

The Sample

NH

MNH

Et

NM

N

Et

BH2

BH2

OM

O

Et OMO

Et

O

PrM

NH

NH

PrM

O

O

EtM

EtM

NHSiH2

EtM

CH3

CH3

90oEt

MOH

OH90o

EtM

NH2

NH2

90o

M = Sc(III),Y(III), La(III), Lu(III), Nb(III), Ti(III), Ti(IV), Zr(IV), Hf(IV), Ce(IV), Th(IV), V(V)

LM

Η2CαCβ

P

H

HL

MH

H2Cα

HCβ

P

L

(c) Termination

LL

MH

L

-CH2=CHP

(d) Ejection

≠

LM

CαΗ2

CβP

H

HL

M

Ca

Cb P

H H

HHL

L

Front-Side

(a) UPTAKE

Principles for d0 Polymerization Catalysts

LM

CαΗ2H2Cb

P

LM

L

LH

P

≠

(b) Propagation

Margl et al. Organometallics 1998, 1998,17, 933

Margel et al. J ACS 1999,121,154

Margl et al JACS, 1998,120,5517

Margel et al, Top.in Catl. 1999,7,187

A Test for Activity:Ethylene π- Complexation Energetics• Ethyleneπ-complexation precedes insertion

as well as the dominant chain termination step• If ethylene does not stick , to the catalyst there

will be no insertion• If ethylene sticks too well, the insertion

barrier will be high

L2

M

H

P

+

L2

M

H

P

Insertion

Termination

-200

-150

-100

-50

0

50

Ethylene Complexation Energy (kJ/mol)

0 2 0 4 0 6 0 8 0 1 0 0Acces s ib le Surface (bohr2 )

Sc-4

Sc-1,2,7,8,9Y-1,7,8,9

La-1,7,8,9Ti-4

V-6

Zr-4

Hf-4

Ti-5

Ti-1,2,7,8,9

Sc-3

Zr-1,7,8,9Hf-1,7,8,9

Th-1Ce-1

Ti-3 Zr-3

Ethylene Uptake Energies

Sampling the C2H4 Uptake

Uncharged

Cations (+1)

Cations (+2)

0 20 40 60 80 100

-160

-140

-120

-100

-80

-60

-40

-20

0

Olefin

Complexation

Energy (kJ/

mol

)

Accessible Surface of Metal Ion (bohr2)

• Residual spread of complexation energies– due to varying d orbital energies of the metal and

deformability of the metal-ligand framework

Dominant Factors forEthylene Complexation

• Accessible surface area– must be large enough to allow interaction

• Total charge on the complex– positive charge lowers d orbitals and enhances

electrostatic interactions

The Essential Step:The Olefin Insertion Barrier

• The olefin insertion barrier is the key factorfor catalyst performance

• A high insertion barrier will retardpolymerization

• If the insertion barrier is higher than thetermination barrier, no polymer will beproduced at all

L2

M

H

P

ML2

H

P

ML2

H

P

QuickTime™ and aGraphics decompressor

are needed to see this picture.

Sampling the Insertion Barriers

-20

0

20

40

60

80

100

120

140

160

180

200

220

240

Insertion Barrier (kJ/

mol

)

Metal-Ligand Combination

Sc(III)

Y(III)

La(III)

Lu(III)

Ti(III)

Nb(III)

Ti(IV) Zr(IV) Hf(IV)

Ce(IV)Th(IV)

V(V)

Important Trends forInsertion Barriers

• d0 systems in general have small barriers

• insertion barriers increase down a triad

• Especially for group 4

• Group 3 potentially better than group

Barrier Height isRegulated by Deformability!

• Insertion process deforms the catalyst framework

strong deformation

MLL

C

H

P

π Complex pyramidal

MLL

C

H

P

- / -Front Side Back Side Insertion Transition State

planar

Means to Controlthe Insertion Barrier

• The insertion barrier is determined by thedeformability of the metal-ligandframework

• Insertion step gets faster ...– ... from group 4 to group 3 and...

– ... up a triad and...– ... from bad π donor ligands to good ones

Make or Break:The Chain Termination Barrier

• Even if the insertion barrier is low, an evenlower chain termination barrier will renderthe catalyst ineffective

L2

M

H

P

L2

M H

P

L2

M

P

H

Activation Barriers

Reaction (ADF) (CP/PAW)

1 Front-Side Insertion 20 n/a

2 Back-Side Insertion 22 n/a3 β- Hydrogen Transfer 40 43 +/- 84 β - Hydrogen Elimination 54 57 +/- 35 - Polymer C H Activation 72 70 +/- 36 - Ethylene C H Activation 92 87 +/- 5

• Future investigations can focus on 1-3 reactions !

ΔF‡ (300 )K

QuickTime™ and aGraphics decompressor

are needed to see this picture.

0

20

40

60

80

100

Termination Barrier (kJ/mol)

Metal-Ligand Combination

Sampling the Termination Barrier

Sc(III)

Y(III)

La(III)

Lu(III)

Ti(III)

Ti(IV)Zr(IV)

Hf(IV)

Th(IV)

V(V)

Dominant Factors forTermination Barriers

• Termination barriers rise down the triad

• Termination barriers are higher for group 3metals than for group 4 metals

• Termination barriers are very high forsterically encumbered systems– since there is no space to form the termination

transition state

10203040506070

La[III]Y[III]Sc[III]

Barrier (kJ/mol)

Species

BHT FSP

020406080

100

Ti[III][7]-Ti[III]

283236404448

Th[IV]Lu[III]

020406080

100

V[V]Hf[IV]Zr[IV]Ti[IV]

Barrier (kJ/mol)

Barriers of Insertion and Termination

Z-N

The Final Step:Combining Insertion and Termination

Sc(III)Y(III)

La(III)

Lu(III)Ti(III)

Ti(IV)Zr(IV)

Hf(IV)

Ce(IV)Th(IV)

V(V)

Nb(III)

-20

0

20

40

60

80

100

120

140

160

180

200

220

240

Insertion Barrier

Termination Barrier

Barrier Heigh

t (kJ/

mol

)

Metal-Ligand Combination

A Set of Rules (I)• To achieve better sticking probabilities:

– use a group 4 cation instead of a group 3 neutral– or increase the surface area of the metal– do not increase the d electron count

• To achieve a faster insertion process:– use a 3d or 4d metal instead of a 5d one or an f

element– use a bulky ligand or a ligand with good π-

(donor abilitiesamido)– 3, 4 5 never use a group or transition metal with

- a non zero d electron count

A Set of Rules (II)

• To better balance insertion vs. termination– use a 3d or 4d metal

– using Hf is dangerous

– termination can be selectively curtailed byincreasing the steric bulk of the auxiliaryligands

• These rules are currently in use in our andour industrial collaborator’s laboratory

MeTi

NR

NR

R'

Me+ B(C6F5)3

23oC R'

x

Ti

NR

NRMe

x

Me - B(C6F5)3-

+

McConville Systems: Living Polymerization of α-Olefins

(a) R = 2,6-iPr2C6H3

(b) R = 2,6-Me2C6H3

R' = n-Bu, n-Pr, n-Hex

the chelating diamide complexes of titanium proved to be efficient catalystsfor living polymerization of α-olefinsthe zirconium analogue displayed little on no activity for the polymerizationof olefins

McConville et al., J ACS, 1996, 118, 10008; Organometallics, 1997, 16, 1810;Macromolecules, 1996, 29, 5241

Observations:

precursors

cocatalyst

catalysts

Ax

Eq Eq

Ax

R

RR

R

RR

R

R R

R

R R

Active Site in McConville Catalyst

MN N

C

NC

C

C

M

C

C

C

CN

NC

C

C

C

M

C

C

C

C

N

Insertion

Ti: 5.6Zr: 6.0Hf:9.6 (8.6)

Barriers kcal/mol

Termination

Generic Transition States

Ti: 7.8Zr: 6.8Hf: 8.1 (7.6)

Barriers kcal/mol

McConville Ti(IV) Insertion and Termination Transition States

Term. Barrier kcal/mol

9.2 (QM)17.8 (QM/MM)

Ins. Barrier kcal/mol

5.6 (QM)9.4 (QM/MM)

MwuptakeΔE≠insertionΔE Δ

≠Δ ( E )

MwuptakeΔE insertion≠ΔE Δ(Δ ≠E )

MH

H

H

H

R

iPr iPr

iPr iPr

13 R' = Me, R" = Me14 R' = Me, R" = iPr

ZrR"

R'

R"

R'

R

iPr iPr

iPr iPr

1A M=Ti1B M=Zr

5.6 x 108 (living)

dimmer (2 - 7)

9.411.8

9.22.9

17.022.4

22.313.8

3.3 x 10 3

1.8 x 104

9.9-0.1

2.83.8

LIGAND MODIFICATIONS

MwuptakeΔE≠insertionΔE Δ

≠Δ ( E )

5.6 x 108 (living)

dimmer (2 - 7)

9.411.8

9.9-0.1

MwuptakeE insertion≠ΔE Δ(Δ ≠E )

9.22.9

22.313.8

3.3 x 10 3

1.8 x 1042.83.8

LIGAND MODIFICATIONS

17.022.4

Zr

H

R

R' R'

R' R'

H

HH

Zr

H

R

R'

H

HH

iPr iPr

15 R' = fluoro- iPr

16 R' = 1-Me- cyclo-Pr

17 2 2 4 2R' = -CH (CH ) CH -

18 R' = -CMeH(CH 2)4CMeH-

Brookhart Polymerization Catalyst

C&EN Feb. 5, 1996:“Polymer Catalyst System:Dupont Eyes New Polyolefin Business”

Brookhartcatalyst

highly linear to moderately branched

Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343.

• high MWs

• good activities

N

Ni+N

RRiPr

iPriPr

iPr

R

• temperature: Temp branching

• monomer pressure: [Et] branching

• bulk of substituents: bulk branching MW

Ax

NiEq Eq

Ax

R

RR

R

RR

R

R R

R

R R

Active Site in Brookhart Catalyst

R R

Pure QM Transition States for Brookharts Ni(II)-catalyst

Insertion TS

Termination TS

10.8 kcal/mol

17.5 kcal/mol

Ni

Ni

NN

Termination18.6 kcal/mol

11.9 kcal/molInsertion

Real Barriers

Gibson Brookhart

- Robust - Low-cost - Simple to make

- High activities- High selectivity

Experimental Observations

1 R = R’ = i-Pr2 R = R’ = Me3 R = t-Bu, R’=H4 R = i-Pr, R’ = H5 R = Et, R’ = H6 R = Me, R’ = H

Fe/Co catalysthighly linear high density

Gibson, V. C. et al. Chem. Commun., 1998, 849.Small, B.L.; Brookhart, M.; Bennett, A.M.A. J. Am. Chem. Soc. 1998, 120, 4049.Small, B.L.; Brookhart, M.; J. Am. Chem. Soc. 1998, 120, 7143.

N

NFe

MeMe

NR

R'R'

R

• monomer pressure: [Et] activities: Fe Co no change

• bulk of substituents: bulk MW no branching

• Metals : activities of Fe complexes = activities of analogous Co ones

no mechanistic details!

C

NC C

C

C

C

N

C

Fe

C C

C

C

NC

CN

C C

C

C

C

N Fe

C

C

C

C

C

NC

Barrier ofinterconversion

ΔEincv = 23 kcal/mol

Generic Iron(II)-Bisimino Pyridine Catalyst

Deng,L.;Margl, P.; Ziegler, T., J.Am. Chem. Soc.,1999, in pressa

b

a. Olefin complex resting state:

axial Cα-conformation preferred electronically

ΔEcomplex = -29.7 kcal/mol

ΔEins = 23 kcal/mol ΔEBHT = 4 kcal/mol

b. Olefin complex: the insertion precursor

equatorial Cα-conformation disfavored electronically

ΔEcomplex = -23.8 kcal/mol

ΔEins = 7.4 kcal/mol

CC

C

C

C

CCN

C

C

C

C

C

N N

C

Fe

C

C

C

C

C

C

C

CC

C

C

C

C

C

C

C

C

CC

C

C

C

C2H4

E = -4.5 kcal/mol

ΔEterm = 9.0 kcal/mol

Formation of olefin-complex as precursorfor termination surpressed

Real Iron(II)-Bisimino Pyridine Catalyst

Olefin-complex

CC

C

C

C

CCN

C

C

C

C

C

N N

C

Fe

C

C

C

C

C

C

C

CC

C

C

C

C

C

C

C

C

CC

C

C

C

Real Iron(II)-Bisimino Pyridine Catalyst

Formation of olefin complexas precursor for insertionnow competative

E = -3.6 kcal/mol

ΔEinsert ~ 0-3 /kcal mol

Neutral Ni(II)-Based Catalyst for Ethylene Polymerization

CH

HC C

H

H

H

H

H

C

C

C

H C C

H

C

C

C

C C

H

C

H

H

C

H

H

C

H

C

C

C

N

H

H

C

O

C

C

C

C

H

C

H

C

C

C

H

H

H

C

Ni

C

C

C

H

H

H

H

H

H

C

H

H

C

H

H

C

H

CH

H

H

Wang and Grubbs et al. Organometallics, 1998, 17, 3149-3151

Experiment:Highly active for Polymerization ofethylene

Calculation:Without bulky substituents, the insertion and termination barrierAbout the same both as high as 26 kcal/mol

M

L

'L

R

M = Ti, V, Cr, Mn

L = NH3, NH2

-

R = Me, Et

Possible Polymerization CatalystsPossible Polymerization Catalysts

First row transition metals Cationic high-spin complexes Two nitrogen ligands Me or Et as model for the growing

polymer chain

Olefin Binding EnergyOlefin Binding Energy

0

10

20

30

40

TiVCrMn

d1 d2 d3 d4

Olefin binding energy for R = Me

Olefin binding energy correlates with the number of d-electrons.

d3 and d4 systems have lowest binding energy because of destabilized the acceptor orbital for the -d-interaction.

M

M R

M

M

R

R

R

M R

M R

M RM R

d-levels

a.b.

b.

b.

sp3

OC IN

π

π

π

Orbital Orbital Interactions Interactions during the during the Olefin Olefin Insertion Insertion

for example:a d1 system

SOMO becomes significantly destabilizedduring the insertion.

b. = bonding; a.b. = antibonding

Termination ReactionsTermination Reactions

BHE reaction is in most cases less facile than the BHT reaction.

BHT reaction coordinate involves a shift of the olefin in the BHT plane similar to the insertion reaction.

The major contribution for BHT barrier stems from the breaking of the C-H bond.

M

CH2

CH 2'L

L

OC BHT

H M

H 2CCH 2

'L

LH

CH 2

H2C

Ligand Design: Ligand Design: Real size non-chelating ligandsReal size non-chelating ligands

Cr

N

N

R

H3Si

H3Si

H3SiH3Si

N(SiH3)2

Cr

Ligand Design: Ligand Design: Promising ResultsPromising Results

UPT INS BHT

NH2 -18.3 6.2 11.4 3

HN-(CH2)3-NH -16.8 13.2 14.8 7

NMe2-14.711.9 18.6 3

N(SiH3)2 -10.49.6 20.2 3(Energies in kcal/mol)

19

42

C7

N2

C 4

5

6

M

C 111

C 2

9

12

C3

8

10

N1

C9

[(Ph)2nacnac]MR+ (M = Ti,V, Cr) Catalysts

Kim and Theopold et al. Organometallics, 1998, 17, 4541-4543

N

N

R

R

R'

R'

Nacnac ligand:

- monoanionic- symmetric- bidentate nitrogen coordination- 0/180 arrangement

R = Ph, R’ = Me

d-electron count:Ti: d1; V: d2; Cr: d3

Polymerization ability (Wn):V >> Ti > Cr

Performance of the model system: electronic behaviers of the nacnac ligand

N

N

R

R

R'

R'

R = Me R’ = H

21

19

22

C8

C7

23

C6

18

N2

C

4

24

N1

5

M

6

25

C1

C5

27

26

11

9

12

C3

8

10

2

C

M

TiVCr

ΔEcomplex

-24.7 < -25.2 -15.8

ΔEins

11.3 > 13.110.4

ΔEBHT

11.6 ~ 9.78.3

ΔEBHE

11.3 13.913.1

ΔEEHT - ΔEins

0.3< -3.4-2.1

- Insertion βαrrier usuαlly higher thαn d0 metαl system- Terminαtion βαrrier is not greαter thαn the insertion βαrrier due to the BΗT trαnsition Stαte lies on the lower sπin stαte surfαce

NSERC

A.BeckeE.J.BaerendsFree UniversityAmsterdam

Queens UniversityKingston,Canada

PRF NovacorDr. Rochus Schmid

Dr. Tom K. Woo

Dr. Peter Margl

Dr. L.Deng

Computerson benchesall linkedtogether

Cobalt

Nobel-Price 1998 in ChemistryNobel-Price 1998 in Chemistryfor “The Theory”for “The Theory”

W. Kohn (DFT) and J. Pople (ab initio)

Theory as a valuable tool in chemical research