Mechanistic Aspects of Alkene Polymerization

Clark R. Landis

Dow Chemical CompanyMarch, 2002

Doug SillarsKim RosaaenCurtis WhiteDr. Zhixian Liu

FundingDow Cooperative Research

Department of Energy

Plastics Industry: Prediction vs. Reality

20 Year Prediction made in 1975(anonymous top 5 chemical company)

20%Polyolefins

(PE, PP, LLDPE, EPDM, ...)

20%Nylons, ABS, PS, SAN, ...

HighPerformanceEngineering

Thermoplastics(PEEK, Sulphones, PPS, ...)

60%

Nylons,ABS, PS, SAN, ...

HighPerformance

19%

80%

1%

1995 Reality

Polyolefins(PE, PP, LLDPE,

EPDM, ...)

What Makes a Catalyst Impressive?

“ The use of chiral catalysts to obtain high optical yields … representsone of the most impressive achievements to date in catalytic selectivity,rivaling the corresponding stereoselectivity of enzymic catalysts.”

These catalyst systems are impressive … also for their very highactivities.

… in respect of both selectivity and rate, the behavior of these syntheticrivals, to an unprecedented degree, that of enzymic catalysts.”

Halpern, J. Science 1982, 217, 401-407.

Metallocene Single Site CatalystsIndustrially Significant Enzyme-Like Behavior

Exquisite Ligand-Based Control of Selectivity

Me2SiNt-Bu

TiMeMe

B(C6F5)3

Bu

US Annual Production > 1,000 Metric Tons

RR R R RR

R

R

Rates ≈ 104 insertions/sec.Stereospecificity > 99%Regiospecificity > 99.5%

Linear-Low Density PE

ZrCH2R

R

H

HZr C HH

R

ZrC

CH3

H3C

CH3

isotactic

Kaminsky, W.; Külper, K.; Brintzinger, H. H.;Wild, F. R. W. P. Angew. Chem. Int. Ed. Engl. 1985, 24, 507.

C2-Symmetric Catalysts - Isotactic Polymer

ZrC

R

Zr

H HCH3

ZrH3C

ZrCH3

syndiotactic

Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc. 1988, 110, 6255.

Cs-Symmetric Catalysts - Syndiotactic Polymer

Cs-symmetric ligand

Our Research ActivitiesGoal: To develop a fundamental understanding of the mechanistic details of alkene polymerization through detailed kinetics

Ion-Pair Dynamics via NMR and high sensitivity conductivity studies

Creation of new active site counting methods

Fabrication of novel time-resolved calorimeters and quenched-flow reactors

Determination of rate laws for initiation, propagation, and termination.

Counter-ion influences on reaction mechanisms

Heavy-Atom Kinetic Isotope Effects: Exp. And Ab Initio Computations

Systems Under Investigation

Catalyst Precursors

Catalyst Activators

Alkenes

(EBI)Zr(CH3)2 (Me4Cp)Zr(CH3)2 CGC-1

B(C6F5)3, R3NH+ B(C6F5)4-, Ph3C+ B(C6F5)4

-, MAO

Ethene, Propene, 1-Hexene

ZrMe

Me

ZrMe

MeZr

NMe

Me

Si

Active Site Counting MethodsQuenching with 14CONon-stoichiometric, very sensitive, radioactive, does not indicate type of alkyl

Labeling with CS2

incomplete labeling

Labeling with CH3OT stoichiometric, very sensitive, radioactive, Kinetic Isotope Effect, does not indicate type of alkyl

Marques, M. M. et alia, J. Polym. Sci.: Part A, Polym. Chem 1998, 36, 573-585.

Zr1 4CO

ZrO

active

dead

*

Zr CH3OTTH2C

active + Zr(OCH3) dead

Zr CS2

S

SZr

activedead (analyze by IR, ICP)

Berger Ball Mixers

Catalyst

monomer

Quench agent

product

Quench Flow Reactor

Information: The fraction of Zr centers that are attached to polymers at the time of quench.

ZrMe

MeB(C6F5)3-

RZr

MeB(C6F5)3-

RR MeOD

n

RRn

D

“Count” D-terminatedChains by 2H NMR asa function of time

Timed ReactionInterval (t) Quench

CH2D

BuBun

CDCl3

Time (s)

FractionActiveSites

Average of 5 runs

Initiation Kinetics, Active Site Counts by CH3OD Quench

Conditions

0C, Toluene Solution

1M 1-hexene

8 x 10-4M (EBI)ZrMe2

8 x 10-4M B(C6F5)3

ZrCD3

CD3B(C6F5)3-

RZr

CD3B(C6F5)3-

CD3

RR

CD3

RRH

MeOH

n

n

Information:The fraction of Zr centersthat produced polymer atsome time before quench.

Comparison of Two Labeling Methods

FractionActiveSites

Time (s)

Active Site Counting with CD3

40s reaction time1.45 M propene4x10-4 M (EBI)ZrMe2

4x10-4 M B(C6F5)3

20°C, Toluene

15% active sites

Active Sites and Polypropene

Int. Std.

LabeledPolymer

Solvent

2H NMR of MeOD quenched product

Label found only at terminal methyl groups

Active Site Growth Kinetics

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.00 20.00 40.00 60.00 80.00 100.00

Reaction Time (s)

• Kinetics at 0°C in Toluene• Each observed k is average of three runs• Initiation rate is unaffected by excess borane

Rate = ki [Zr][1-hexene]

ki = 2.1 x 10-2 M-1s-1 at 0°C

= 0.25 M-1s-1 at 24°C

H‡= 11.2(1.5) kcal/mol

S‡= -24(5) cal/mol-K

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.3 0.6 0.9 1.2 1.5

[1-hexene]

2O°C

1O°C

O°C

-1O°C

kIobs (s-1)

Kinetic Data: Initiation

Catalytic Kinetics:[(EBI)Zr(Me)](MeB(C6F5)3]-Catalyzed

Polymerization of 1-Hexene

General Conditions

• [Zr]: 2x10-4 - 2x10-3 M

• [1-Hexene]: 0.15 M - 3.0 M

• Temperatures: -40 - 60°C

• Activator: 1-5 equiv.

• Solvent : Toluene

General Observations

• Clean, Reproducible Kinetics• Exotherm < 1°C• Polymer Molecular weights: 1,000 - 30,000 depending on quench time

Convolution of Initiation and Propagation Kinetics

Polymer mass(t) = 84.16kp [Zr]tot[1-hexene](t+(e-ki[1-hexene]t)) + C

• kp : propagation rate constant• ki : initiation rate constant• [Zr]tot = concentration of all Zr species• C= constant of integration = -84.16 kp[Zr]tot/ki

ZrMe

AR

Zr

A

R

Chain Initiation Chain PropagationR

Zr

A

R RInactive Catalyst

Initiated Catalyst

ki

Propagating Catalyst

kp

n

Weight of Polymer vs. Reaction Time

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 50

Reaction time(s)

Observed CalculatedConditions

0˚C, Toluene Solution

1M 1-hexene

8 x 10-4M (EBI)ZrMe2

8 x 10-4M B(C6F5)3

kp = 2.1 M-1s-1

Complicated Kinetics Are Good

“There is no such thing as a free lunch”Milton Friedman

“There is no such thing as free information”Jack Halpern, Kinetics Course, Spring 1980

Propagation Kinetics-High Conversion50°C, Polymer Mass vs. Time 0°C, Hexene Disappearance (IR)

Propagation Rate = kp[Zr][1-hexene]kp = 8.1 M-1s-1 at 25°CH‡= 6.4(1.5) kcal/molS‡=-33(5) cal/mol-K

kp = 2.2 M-1s-1

At 0°C, propagation is70-times faster thaninitiation!

Excess B(C6F5)3 or PhNMe3+ BMe(C6F5)3

-: No Effect on Propagation Rate

ZrMe

MeB(C6F5)3-

ZrMe

MeB(C6F5)3-

+

ZrMe

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 20 40 60 80

Wei

gh

t o

f p

oly

mer

(g)

Reaction time(s)

B/Zr=2

B/Zr=4

B/Zr=1• Reactions with excess B(C6F5)3 indicateno “double activation effect”

• Reactions with added BMe(C6F5)3- are ambiguous:

the lack of an inhibitory effect contradicts the schemeshown above only if all the ions are free ions. In low dielectric mediaone anticipates tight ion-pairing and no common ion effect.

CH3B(C6F5)3

ZrCH3

ZrPOL

CH3B(C6F5)3

1 spectrum every 2 minutes

1-Hexene Polymerization Followed by 1H NMR

1-hexene

Interception of the Propagating Species

[Zr]0 = 8 mM[1-hexene]0= 0.6 MTemp. =-40°C

kinit (M-1s-1) 11.5 10-4 8.78 10-

4

kprop(M-1s-1) 0.256 0.299

-40°C obs. Previous (extrapolated)

Other evidence…

•Resonances disappear in 0 to -1 region with (EBI)Zr(CD3)2.

•19F NMR exhibits new ortho peak upon initiation.

•1H and 19F NMR shifts suggest coordinated -CH3B(C6F5)3.

•1H{11B} NMR demonstrates CH3-B topology of peaks at

-0.62 and -0.85 ppm.

Initiation and Propagation KineticsZr

POL

CH3B(C6F5)3

€

−∂[1−hexene]

∂t=kprop[Zr][1−hexene]

−∂[(EBI)ZrMe(MeB(C6F5)3)]

∂t=kprop[Zr][1−hexene]

Characterization of the Propagating Species

ZrPOL

CH3B(C6F5)3

Using 1D-Pulse Field Gradient Spin EchoNOESY, irradiate one of the indenyl peaks

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0 0.5 1 1.5

mix time

peak intensity

Intensity at 5.8ppmIntensity at 5.6ppmCalc. Int. at 5.8ppmCalc. Int. at 5.6ppm

Ion-Pair Dynamics of Propagating Species

ksym (

s-1)

CH3B(C6F5)3

ZrCH3

ZrPOL

CH3B(C6F5)3

8.4 mM (EBI)Zr(CH3)2

-36 °C

8.2 mM (EBI)Zr(CH3)2

-40 °C

0

1

2

3

4

0 10 20 30 40 50 60 70

Concentration of free B(C6F5)3 (mM)

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20

Effect of Excess B(C6F5)3 on Exchange Rates

Measurements demonstrate:• Similar symmetrization rates in the limit of no free borane.• Free borane does not promote symmetrization of the propagating species.

Termination KineticsTwo types of vinyl end groups are found via proton NMR:Vinylidene4.7, 4.78 ppm (singlets)

Internal Alkene 5.4 ppm (broad multiplet)

Zr

CH3B(C6F5)3

PH

ZrH

CH3B(C6F5)3

P

Zr

CH3B(C6F5)3

ZrH

CH3B(C6F5)3

P

H P

RegioerrorNormal Insertion

Vinylene:vinylidene ratio depends on [1-hexene]

0.15M 1-hexene

1.5 M 1-hexene

5 . 5 5 . 0 4 . 5 P P M

Termination Rate Measurements

[Vinyl]t =[Zr]0ktobs(t+1

kiobs

e−kiobst −1

kiobs

)

All runs conducted with < 10% 1-Hexene conversion

Termination Kinetics, 0°C, 0.5M 1-Hexene

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

0 20 40 60 80 100 120 140time (sec)

[unsat. end groups] M

Internal Alkene

Vinylidene

kobs = 8.9 x 10 -4 s-1

kobs= 7.1 x 10 -4 s-1

Vinylidene and Internal Alkene Formation Have Different Rate Laws

Vinylidene

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.00 0.50 1.00 1.50

0°C

10°C

20°C

50°C

[1-hexene]

Log

(kvi

nylid

en

e )

Rate = kvinylidene[Zr]kvinylidene=1.3x10-3s-1(25°C)

H‡=16(3)kcal,mol, S‡=-13(6)cal/mol-K

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

2.5E-02

3.0E-02

3.5E-02

4.0E-02

4.5E-02

0 0.3 0.6 0.9 1.2 1.5

[1-hexene] (M)

0°C

10°C

20°C

50°C

kviny

lene

obs (

s-1)

Internal Alkene (vinylene)

Rate=kvinylene[Zr][1-hexene]kvinylene=9.7x10-3M-1s-1

H‡=9.7(12)kcal,mol, S‡=-35(4)cal/mol-K

Are Internal Alkenes Formed by Chain Transfer to Monomer?

Conventional Wisdom* •Vinylidene = Mononuclear -Hydride Elimination

Why must secondary alkyls wait for a monomer whereas primaryalkyls do not?

*Resconi et al. Chem. Rev., 2000, 100, 1253-1345.

• Internal Alkene = Bimolecular Chain Transfer to Monomer?

ZrMeB(C6F5)3

-

POL

Bu Bu

ZrMeB(C6F5)3

-

HPOL

Bu Bu+

Zr

MeB(C6F5)3-

BuPOL

Bu Bu Zr

BuPOL

Bu Bu

Bu

H

MeB(C6F5)3-

ZrMeB(C6F5)3

-

BuPOL

Bu

Alternate Model: Every 2,1-Insertion Leads to Termination

Zr

MeB(C6F5)3-

BuPOL

Bu Bu

X

ZrMeB(C6F5)3

-

HBu

POL

Bu

No Chain Extension

The steady-state concentration of the secondary alkyl (shownabove) resulting from a 2,1-insertion is proportional to the[1-hexene] because it is formed by occasional misinsertion of 1-hexene from the catalyst resting state (a primary alkyl).

The rate of termination is really the rate of 2,1-propagation

Steady-State Analysis: Vinylenes

Zr - 1,2-Pol

k1,2p = 2.0 M-1s-1

k1,2p[1-hexene]

k2,1p[1-hexene]

k2,1p = 0.0016 M-1s-1

Zr-1,2-1,2-Pol

Zr-2,1-1,2-Pol

k1,2t

Zr-H + vinylidene

k1,2-2,1p[1-hexene]

Zr-1,2-2,1-1,2-Pol

k2,1tZr-H + internal vinyl

k1,2-2,1p[1-hexene]<< k2,1

t

Steady-state concentration [Zr-2,1-1,2 ]= k2,1p[1-hexene][Zr]TOT/ (k2,1

t + k1,2-21p[1-hexene])

= k2,1p[1-hexene][Zr]TOT/k2,1

t

Rate of internal vinyl formation = k2,1t [Zr-2,1-1,2]

= k2,1t k

2,1p[1-hexene][Zr]TOT/k2,1

t

= k2,1p[1-hexene][Zr]TOT

k1,2t = 0.0006 s-1

[Zr]TOT ≈ [Zr-1,2-Pol]

= Rate of 2,1 insertion

Polymer Microstructure via 13C NMRStrategy: Use 13C label in 1-position of 1-hexene to look for enchained regioerrors and to examine microstructure

Analyze by 1D 13C NMR, INADEQUATE, HMBC, DEPT, 1HNMR

Normal (1,2)Insertion

Me

Bu Bu Bu Bu

Enchained 2,1 Insertion

*

Me

Bu

Bu

***

*

*

**

...

...BuBu

13C NMR Spectrum of Labeled Polymer: 106-134 ppm

POL

Bu

Trans hexenyl

POL

Bu Bu

POL

Cis hexenyl

vinylidene

Termination after2,1 insertion

Termination after1,2 insertion

13C NMR Spectrum of Labeled Polymer: 10-50 ppm

C1C2

C3

C4

C5

C6

**

C6C5C4C3C1

C2

POL*D

ZrPOL

MeOD

POL

Bupentenyl

POL

Bu

ZrH

1-hexene

Pen

BuPOL

Cishexenyl

Transhexenyl

Analysis of Polymer Microstructure Reveals• No enchainment of 2,1 regioerrors: every misinsertion leads to termination of polymer growth

• Several end groups can be identified • cis and trans hexenyl (after 2,1 insertion) • cis and trans pentenyl (after 2,1 insertion) • vinylidene (after 1,2 insertion) • hexyl endgroup (from first insertion into Zr-H) • D-labeled methyl (from MeOD quench)

• After a misinsertion: • Elimination to form hexenyl end group 4-times more frequent than pentenyl end group formation • cis-hexenyl end group 2.4-times more frequent than trans

• >99% isotacticity (mmmm pentads)

How does anion coordination affinity affect active site counts, propagation, and termination kinetics?

Z r

M e

M e

Z r M e+ P h 3 C

+

B ( C 6 F 5 ) 4

-

1

B ( C6

F5

)4

-

+

1 *

+ P h3

C M e

[Ph3C]+ [B(C6F5)4]-=5.0x10-4M[(EBI)Zr(CH3)2]=5.0x10-4M[1-Hexene]=1.0M, Toluenet=20oC

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14

Reaction time(sec)

Observed

Average

Approximately 35% active sites

Anion Effects on Polymerization Mechanism: MeB(C6F5)3

- vs. B(C6F5)4-

B(C6F5)4-: Propagation Kinetics

y = 0.1123x + 0.0038

R2 = 0.9955

0

0.05

0.1

0.15

0.2

0.25

0.00 0.50 1.00 1.50 2.00[1-hexene]

Initial Rate(M/s)

Observed

Linear (Observed)

[CPh3]+ [B(C6F5)4]-=5.0x10-4M[(EBI)Zr(CH3)2]=5.0x10-4M T=60oCToluene

€

∂[1−hexene]∂t

=−kp 1*[ ] 1−hexene[ ]

• Initiation period is not observed• Same rate law as forMeB(C6F5)3

-kp= 125 M-1s-1 at 20°C

Termination Products and Rate Laws1H NMR of vinyl region

vinylene

vinylidene

Mono-substituted alkenetri-substituted alkene

20°C, toluene1.25 M 1-Hexene2.8 sec, 9% conv.

0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

6.0E-05

7.0E-05

8.0E-05

9.0E-05

1.0E-04

0 0.5 1 1.5 2

[1-Hexene](M)

Initial Termination Rate(M/s)

Vinylene

Trisubsituted

Monosubstituted

Vinylidene

€

∂[vinylene]∂t

=kvinylene

1*[ ] 1−hexene[ ]

€

∂[vinylidene]∂t

=kvinylidene

1*[ ]

Comparison of Rate Constants: MeB(C6F5)3

- vs. B(C6F5)4-

Property MeB(C6F5)3- B(C6F5)4

-

%Active Sites >90% 35%

Propagation Rate constant (kp)

6.3 M-1s-1 130 M-1s-1

Vinyl End Groups Vinylene, vinylidene Vinylene, vinylidene, others

Termination Rate Constant, kvinylene

7 x 10-3M-1s-1 3 x 10-1M-1s-1

Termination Rate Constant, kvinylidene

1.1 x 10-2 s-1 2 x 10-2 s-1

• Propagation and Termination to yield vinylene endgroups (i.e. 2,1 propagation) involve significant ion-pair separation (20-40 fold increase)• -Hydride Elimination does not require ion-pair separation (same rate).

(EBI)ZrMe2 + Activator, 20°C, Toluene Solvent

Do catalysts resulting from all activators share a common first irreversible step for 1-hexene incorporation?

More weakly coordinating anions appear to be correlated with• higher catalytic activities• more stereoerrors in syndiotactic polymerizations• rates with greater than 1st order dependence on [propene]?

Hypothesis: •Changes in the nature of the alkene insertion step could be revealed by changes in the Kinetic Isotope Effect (KIE).• Interpretation of heavy atom KIE’s do not depend on well-determined active site counts KIE’s provide empirical bridge from MeB(C6F5)3

- to other anions

Heavy Atom Kinetic Isotope Effects in1-Hexene Polymerization

Measurement of 1-Hexene KIE

Activators

B(C6F5)3

Al(C6F5)3

MAO

[PhNMe2H]+

[B(C6F5)4]-

ZrCH3

CH3

n

+ +

3 M2 x 10-4 M

activator

0 °Ctoluene

ca. 95%

ca. 5%

Recover unreacted1-hexene, quantitateconversion, and integrate (carefully) 13C NMR

C-2

C1

C2

C3

C4

C5

C6

R/Ro = (1-F)(1/KIE)-1

Singelton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357.

•R : minor isotopic component in recovered material

•Ro : minor isotopic component in the original material•F : fractional conversion of reactants•KIE : relative rate of major/minor isotopic components

Empirical 1-Hexene KIEs

C1

C2

C3

C4

C5

C6

• KIE(C2)>KIE(C1)• Weaker Ion-Pairs yield smaller KIE’s?

Average of 3 independent runs, 3 spectra/run0°C, (EBI)ZrMe2 + 2 eq. Activator,Toluene

B(C6F5)3

Al(C6F5)3

MAO

PhNMe2H+ B(C6F5)4-

C1 C2 C3 C4 C5

toluene

B(C6F5)3chlorobenzene

1.009(4) 1.019(6) 0.999(1) 1.001(1) 1

1.010(2) 1.017(3) 1.000(0) 1.000(2) 1

1.009(1) 1.017(1) 1.001(2) 1.001(1) 1

1.007(4) 1.018(1) 1.000(1) 1.000(2) 1

1.003(1) 1.013(2) 0.999(1) 1.000(1) 1

Do KIE’s Reveal More?Computational Model

ZrCp

Cp

ClMeZr

Cp

Cp

+

+

+

ZrCp

Cp

ClMe+

ClMe

What is Computed?• Free Energy: Association and Insertion

• Both 1,2- and 2,1-insertion pathways• 3 trajectories for alkene association

• KIE for k1 and k2

• EIE for K1 (=k1/k-1)• B3LYP/LANL2DZ

k1

k-1

k2

Why?• ClMe as an anion substitute

• computationally accessible • ca. thermoneutral association

1 2 3

1 +propene 2

3

ca. 10kcal/mol

G

Computation:Association

Averages C1 C2 C3EIE 1.003(6) 0.995(7) 0.987(16)KIE 1.009(9) 1.001(4) 0.996(16)

Results• Small KIE

ZrCp

Cp

ClMeZr

Cp

Cp

+

+

+

ZrCp

Cp

ClMe+

ClMe

1,2 Pathway 2,1 Pathway

G‡ = 13.1; KIE 1.007 1.002 0.998

ΔG = 0.4; EIE 0.997 0.991 0.977

A-1b A-1c

A-2 A-3a A-3b

A-1a

ΔG‡ = 17.1; KIE 1.001 0.995 0.981

ΔG = 0.2; EIE 0.999 0.993 0.979

ΔG‡ = 12.5; KIE 1.003 1.000 1.000

ΔG = 2.6; EIE 0.999 0.993 0.989

ΔG‡ = 11.1; KIE 1.023 1.001 0.985

ΔG = -3.3; EIE 1.007 0.991 0.980

ΔG‡ = 12.9; KIE 1.006 1.006 1.018

ΔG = 1.1; EIE 1.010 1.005 1.010

ΔG‡ = 11.6; KIE 1.004 0.997 0.995

ΔG = -0.6; EIE 0.996 0.997 0.996

Computation:Insertion

Averages C1 C2 C3KIE 1.020(7) 1.044(6) 1.007(5)

Results• KIE(C2)>KIE(C1)

ZrCp

Cp

ClMeZr

Cp

Cp

+

+

+

ZrCp

Cp

ClMe+

ClMe

1,2 Pathway 2,1 PathwayI-1a I-1b I-2a

I-2b I-2c I-2d

G‡ = 17.3; KIE 1.024 1.050 1.003 ΔG‡ = 10.8; KIE 1.027 1.043 1.001 ΔG‡ = 23.2; KIE 1.029 1.033 1.009

ΔG‡ = 9.6; KIE 1.010 1.035 1.012 ΔG‡ = 15.9; KIE 1.017 1.047 1.010 ΔG‡ = 15.2; KIE 1.022 1.034 1.015

Does Alkene Bind Reversibly?Scenario 1: Irreversible Alkene Association

ZrCp

Cp

ClMe

ClMe

ZrCp

Cp ZrCp

Cp

ClMe+

+

+ +k1 k2

KIE fixed at the alkene association step.

KIE = KIE1 1.009(9) 1.001(4) 0.996(16)C1 C2 C3

Scenario 2: Reversible Alkene Association

KIE fixed at the alkene insertion step.

ZrCp

Cp

ClMe

ClMe

ZrCp

Cp ZrCp

Cp

ClMe+

+

+ +k2

K1

KIE = EIE1xKIE2 1.023(7) 1.039(7) 0.993(16)

Experiment 1.08(7) 1.018(4) 1.000(2)

Data are NOT Compatible with Scenario 1

Working Mechanism

(EBI)ZrA-

MeR

(EBI)Zr

A-R R

(EBI)Zr

A-

H

A-

(EBI)ZrMe

R R

R

R

R

(EBI)Zr

A-

R

R

R

(EBI)ZrA-

R

(EBI)Zr

A-R R

A-

RRR

(EBI)Zr

R

(EBI)Zr

A-

H

R

++

+

+

Initiation

+

+

n

+

n

+

+

n

Termination

slow

fast