Transcript Mechanistic Aspects of Alkene Polymerization

Mechanistic Aspects of Alkene
Polymerization
Clark R. Landis
Dow Chemical Company
March, 2002
Doug Sillars
Kim Rosaaen
Curtis White
Dr. Zhixian Liu
Funding
Dow Cooperative Research
Department of Energy
Plastics Industry: Prediction vs. Reality
High
Performance
High
Performance
Engineering
Thermoplastics
60%
Nylons,
ABS, PS,
SAN, ...
1%
19%
(PEEK, Sulphones, PPS, ...)
Nylons, ABS, PS, SAN, ...
Polyolefins
(PE, PP, LLDPE, EPDM, ...)
20 Year Prediction made in 1975
(anonymous top 5 chemical company)
20%
Polyolefins
(PE, PP, LLDPE,
EPDM, ...)
20%
1995 Reality
80%
What Makes a Catalyst
Impressive?
“ The use of chiral catalysts to obtain high optical yields … represents
one 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 high
activities.
… in respect of both selectivity and rate, the behavior of these synthetic
rivals, to an unprecedented degree, that of enzymic catalysts.”
Halpern, J. Science 1982, 217, 401-407.
Metallocene Single Site Catalysts
Enzyme-Like Behavior
Industrially Significant
R
Ti Me
Me
Me 2Si
R
R
R
R
R
R
B(C 6F5) 3
N
t-Bu
R
Bu
Rates ≈ 104 insertions/sec.
Stereospecificity > 99%
Regiospecificity > 99.5%
Linear-Low Density PE
US Annual Production > 1,000 Metric Tons
Exquisite Ligand-Based Control of Selectivity
C2 -Symmetric Catalysts - Isotactic Polymer
R
Zr
CH2 R
CH3
Zr
H3C
C
H
H
Kaminsky, W.; Külper, K.; Brintzinger, H. H.;
Wild, F. R. W. P. Angew. Chem. Int. Ed. Engl.
1985, 24, 507.
H
H
C
Zr
Cs -Symmetric Catalysts - Syndiotactic Polymer
H H
C CH3
CH3
Zr
Cs -symmetric ligand
R
R
CH3
Zr
Zr
H3 C
Zr
isotactic
syndiotactic
Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am.
Chem. Soc. 1988, 110, 6255.
Our Research Activities
Goal: 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
Zr
Me
(EBI)Zr(CH3)2
Me
Me
Me
Zr
Zr
Me
(Me4Cp)Zr(CH3)2
Si
Me
N
CGC-1
Catalyst Activators
B(C6F5)3, R3NH+ B(C6F5)4-, Ph3C+ B(C6F5)4-, MAO
Alkenes
Ethene, Propene, 1-Hexene
Active Site Counting Methods
Quenching with 14CO
Non-stoichiometric, very
sensitive, radioactive, does not
indicate type of alkyl
Zr
Zr
stoichiometric, very sensitive,
radioactive, Kinetic Isotope Effect, active
does not indicate type of alkyl
incomplete labeling
CO
active
dead
*
CH 3OT
TH 2C
+ Zr(OCH 3) dead
CS 2
Zr
O
Zr
active
Labeling with CH3OT
Labeling with CS2
14
Zr
S
S
dead (analyze by IR, ICP)
Marques, M. M. et alia, J. Polym. Sci.: Part A, Polym. Chem 1998, 36, 573-585.
Quench Flow Reactor
product
Quench
agent
Berger Ball
Mixers
Catalyst
monomer
Initiation Kinetics, Active Site
Counts by CH3OD Quench
Timed Reaction
Interval (t)
Quench
MeB(C 6F5)3Zr
Me
MeB(C 6F5)3Zr
R
R
R
CDCl3
D
R
n
MeOD
0.8
Average of 5 runs
CH2D
Bun Bu
R
n
“Count” D-terminated
Chains by 2H NMR as
a function of time
0.6
Fraction
Active
Sites 0.4
0.2
0
0
20
40
60
Time (s)
Information: The fraction of Zr centers that are attached to polymers at
the time of quench.
Active Site Counting with CD3
CD3B(C6F5)3Zr
CD3
CD3B(C6F5)3Zr
R
Conditions
8 x 10-4M (EBI)ZrMe2
8 x 10-4M B(C6F5)3
Information:
The fraction of Zr centers
that produced polymer at
some time before quench.
MeOH
R
n
R
n
Comparison of Two Labeling Methods
0C, Toluene Solution
1M 1-hexene
R
CD3
R
CD3
H
0.80
0.60
Fraction
Active
Sites 0.40
0.20
0.00
0
20
40
Time (s)
60
80
Active Sites and Polypropene
2H
NMR of MeOD quenched product
Label found only
at terminal methyl
groups
Int. Std.
40s reaction time
1.45 M propene
4x10-4 M (EBI)ZrMe2
4x10-4 M B(C6F5)3
20°C, Toluene
15% active sites
Solvent
Labeled
Polymer
Kinetic Data: Initiation
• Kinetics at 0°C in Toluene
• Each observed k is average
of three runs
• Initiation rate is unaffected by
excess borane
0.18
0.16
0.14
2O°C
Rate = ki [Zr][1-hexene]
ki = 2.1 x 10-2 M-1s-1 at 0°C
= 0.25
D H ‡=
M-1s-1
at 24°C
kIobs (s-1)
0.12
0.1
1O°C
0.08
0.06
O°C
0.04
0.02
11.2(1.5) kcal/mol
DS‡= -24(5) cal/mol-K
-1O°C
0
0
0.3
0.6
0.9
[1-hexene]
1.2
1.5
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
Chain Initiation
Me
R
R
A
Zr
Chain Propagation
ki
A
A
Zr
kp
R
Inactive Catalyst
Zr
R
nR
Initiated Catalyst
Propagating Catalyst
Polymer mass(t) = 84.16kp [Zr]tot[1-hexene](t+(e-ki[1-hexene]t)) + C
Conditions
• kp : propagation rate
0˚C, Toluene Solution
constant
1M 1-hexene
• ki : initiation rate
8 x 10-4M (EBI)ZrMe2
constant
8 x 10-4M B(C6F5)3
• [Zr]tot = concentration
of all Zr species
kp = 2.1 M-1s-1
• C= constant of
integration
= -84.16 kp[Zr]tot/ki
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 Conversion
1
50°C, Polymer Mass vs. Time
1.50 M hexene
0.202
1.00 M hexene
0.8
0°C, Hexene Disappearance (IR)
0.222
995 cm-1
0.182
911 cm-1
0.50 M hexene
0.162
0.15 M hexene
0.142
0.6
kp = 2.2 M-1s-1
0.122
0.102
0.4
0.082
0.062
0.2
0.042
0.022
0
0
20
40
60
80
100
120
Time (s)
Propagation Rate = kp[Zr][1-hexene]
kp = 8.1 M-1s-1 at 25°C
DH‡= 6.4(1.5) kcal/mol
DS‡=-33(5) cal/mol-K
0
500
1000
1500
2000
2500
3000
time(s)
At 0°C, propagation is
70-times faster than
initiation!
Excess B(C6F5)3 or PhNMe3+ BMe(C6F5)3-:
No Effect on Propagation Rate
0.40
• Reactions with excess B(C6F5)3 indicate
no “double activation effect”
Weight of
polymer(g)
0.35
0.30
B/Zr=1
0.25
B/Zr=2
0.20
B/Zr=4
0.15
0.10
MeB(C 6F5)3-
0.05
Zr
Me
0.00
0
20
40
60
Zr
+
MeB(C 6F5)3-
Me
80
Reaction time(s)
BMe(C6F5)3-
Zr
Me
• Reactions with added
are ambiguous:
the lack of an inhibitory effect contradicts the scheme
shown above only if all the ions are free ions. In low dielectric media
one anticipates tight ion-pairing and no common ion effect.
Interception of the Propagating Species
CH 3
1-hexene
Zr
CH 3B(C6F5)3
POL
Zr
CH 3B(C6F5)3
1-Hexene Polymerization Followed by 1H NMR
1 spectrum every 2 minutes
[Zr]0 = 8 mM
[1-hexene]0= 0.6 M
Temp. =-40°C
Characterization of the Propagating Species
Initiation and Propagation Kinetics
[1 hexene]

 k prop [Zr][1  hexene]
t

POL
Zr
CH 3B(C6F5)3
[(EBI)ZrMe(MeB(C6F5 )3 )]
 k prop [Zr][1  hexene]
t
-40°C obs.
Previous (extrapolated)
kinit (M-1s-1)
11.5  10-4
8.78  10-4
kprop(M-1s-1)
0.256
0.299
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.
Ion-Pair Dynamics of Propagating Species
peak intensity
90.0
80.0
70.0
Intensity at 5.8ppm
Intensity at 5.6ppm
Calc. Int. at 5.8ppm
Calc. Int. at 5.6ppm
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
0.5
1
1.5
mix time
POL
Zr
CH 3B(C6F5)3
Using 1D-Pulse Field Gradient Spin Echo
NOESY, irradiate one of the indenyl peaks
Effect of Excess B(C6F5)3 on Exchange Rates
1.2
1
0.8
4
0.6
0.4
ksym (s-1)
3
0.2
0
2
0
10
20
8.2 mM (EBI)Zr(CH3)2
-40 °C
1
0
0
10
20
30
40
50
60
Concentration of free B(C6F5)3 (mM)
70
CH 3
Zr
CH 3B(C6F5)3
Measurements demonstrate:
8.4 mM (EBI)Zr(CH3)2
• Similar symmetrization rates in the
-36 °C
limit of no free borane.
• Free borane does not promote symmetrization
of the propagating species.
Termination Kinetics
Two types of vinyl end groups are found via proton NMR:
Vinylidene
Internal Alkene
4.7, 4.78 ppm
1.5 M 1-hexene
5.4 ppm
(singlets)
(broad multiplet)
Normal Insertion
Regioerror
CH3B(C6F5)3
Zr
CH3B(C6F5)3
Zr
H
H
P
P
CH3B(C6F5)3
0.15M 1-hexene
CH3B(C6F5)3
Zr
Zr
H
H
P
P
5.5
Vinylene:vinylidene ratio depends on [1-hexene]
5.0
4.5
Termination Rate Measurements
All runs conducted with < 10% 1-Hexene conversion
[Vinyl ]t  [Zr ]0 k tobs (t 
1 k
e
k iobs
i obst

1
)
k iobs
Vinylidene and Internal Alkene
Formation Have Different Rate Laws
Internal Alkene (vinylene)
Vinylidene
Rate = kvinylidene[Zr]
kvinylidene=1.3x10-3s-1(25°C)
DH‡=16(3)kcal,mol, DS‡=-13(6)cal/mol-K
Rate=kvinylene[Zr][1-hexene]
kvinylene=9.7x10-3M-1s-1
DH‡=9.7(12)kcal,mol, DS‡=-35(4)cal/mol-K
4.5E-02
-0.5
Log(kvinylidene)
-1.0
kvinyleneobs (s-1)
50°C
-1.5
20°C
-2.0
10°C
-2.5
-3.0
4.0E-02
0°C
3.5E-02
10°C
20°C
3.0E-02
50°C
2.5E-02
2.0E-02
1.5E-02
1.0E-02
0°C
5.0E-03
-3.5
0.00
0.50
1.00
[1-hexene]
1.50
0.0E+00
0
0.3
0.6
0.9
[1-hexene] (M)
1.2
1.5
Are Internal Alkenes Formed by
Chain Transfer to Monomer?
Conventional Wisdom*
•Vinylidene = Mononuclear -Hydride Elimination
Bu Bu
Zr
POL
Zr
MeB(C6F5)3 -
H
Bu Bu
+
POL
MeB(C6F5)3-
• Internal Alkene = Bimolecular Chain Transfer to Monomer?
Bu
Zr
POL
Bu Bu
MeB(C6F5 )3-
Bu
Zr
POL
H Bu Bu
MeB(C6 F5)3Zr
MeB(C6F5)3 - Bu
Bu
POL
Bu
Why must secondary alkyls wait for a monomer whereas primary
alkyls do not?
*Resconi et al. Chem. Rev., 2000, 100, 1253-1345.
Alternate Model: Every 2,1-Insertion
Leads to Termination
Bu
Zr
POL
Bu Bu
MeB(C6 F5 )3
X
Zr
H
MeB(C6 F 5 )3
-
Bu
POL
Bu
No Chain Extension
The steady-state concentration of the secondary alkyl (shown
above) 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
k1,2 p = 2.0 M-1 s-1
1,2-2,1
2,1
k2,1 p = 0.0016 M-1 s-1 k
p[1-hexene]<< k
t
k1,2 t = 0.0006 s-1
Zr-1,2-1,2-Pol
Zr-H + vinylidene
k1,2 t
k1,2 p[1-hexene]
[Zr]TOT - [Zr-1,2-Pol]
Zr - 1,2-Pol
k2,1 p[1-hexene]
k1,2-2,1p[1-hexene]
Zr-1,2-2,1-1,2-Pol
Zr-2,1-1,2-Pol
Zr-H + internal vinyl
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 k2,1p[1-hexene][Zr]TOT/k2,1t
= k2,1p[1-hexene][Zr]TOT
= Rate of 2,1 insertion
Polymer Microstructure via 13C NMR
Strategy: Use 13C label in 1-position of 1-hexene to look for
enchained regioerrors and to examine microstructure
Bu
Bu
Normal (1,2)
Insertion
...
*
*
Bu
Bu
Me
*
*
Bu
Enchained
2,1 Insertion
Bu
Bu
...
*
*
Me
*
Bu
*
Analyze by 1D 13C NMR, INADEQUATE, HMBC, DEPT, 1HNMR
NMR Spectrum of Labeled Polymer: 106-134 ppm
Trans hexenyl
*
*
POL
Bu
*
POL
*
Cis hexenyl
vinylidene
Bu
Bu
*
13C
Termination after
2,1 insertion
POL
Termination after
1,2 insertion
NMR Spectrum of Labeled Polymer: 10-50 ppm
1-hexene
H
Bu
C6
C5
C4
POL
Pen
C3
*
Zr
*
*
C2
*
C1
C3
C4
Cis
hexenyl
C5
C6
*
POL
Bu
C2
Zr
POL
MeOD
*
Trans
hexenyl
pentenyl Bu
*
C1
*
13C
POL
D
*
POL
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)
Anion Effects on Polymerization
Mechanism: MeB(C6F5)3- vs. B(C6F5)4How does anion coordination affinity affect active site
counts, propagation, and termination kinetics?
+
Zr
1
Me
Me
Zr
+ Ph3 C+ B(C6F5 )4-
Me
B(C 6F5)4 -
+ Ph3CMe
1*
Approximately 35% active sites
[Ph3C]+ [B(C6F5)4]-=5.0x10-4M
[(EBI)Zr(CH3)2]=5.0x10-4M
[1-Hexene]=1.0M, Toluene
t=20oC
B(C6F5)4-: Propagation Kinetics
0.25
y = 0.11 23x + 0.00 38
2
R = 0.9 955
[CPh3]+ [B(C6F5)4]-=5.0x10-4M
[(EBI)Zr(CH3)2]=5.0x10-4M
T=60oC
Toluene
0.2
Initial Rate(M/s)
Observed
Linear (Obs erv ed)
0.15
0.1
0.05
0
0.00
0.50
1.00
[1-hexene]
1.50
[1 hexene]
 k 1*1 hexene 
p
t
kp= 125 M-1s-1 at 20°C
2.00
• Initiation period is not
observed
• Same rate law as for
MeB(C6F5)3-
Termination Products and Rate Laws
1H
NMR of vinyl region
20°C, toluene
1.25 M 1-Hexene
2.8 sec, 9% conv.
vinylene
vinylidene
tri-substituted alkene
Mono-substituted alkene
1.0E-04
9.0E-05
Vinylene
Trisubsituted
8.0E-05
Initial Termination Rate(M/s)
Monosubstituted
7.0E-05
Vinylidene
[vinylene]
k
1*1 hexene 
vinylene
t
6.0E-05
5.0E-05
4.0E-05
3.0E-05
[vinylidene ]
k
1*
vinylidene
t
2.0E-05
1.0E-05
0.0E+00
0
0.5
1
[1-Hexene](M)
1.5
2
Comparison of Rate Constants:
MeB(C6F5)3- vs. B(C6F5)4(EBI)ZrMe2 + Activator, 20°C, Toluene Solvent
Property
MeB(C6F5)3%Active Sites
B(C6F5)4-
>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).
Heavy Atom Kinetic Isotope Effects in
1-Hexene Polymerization
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]?
Do catalysts resulting from all activators share a
common first irreversible step for 1-hexene
incorporation?
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 welldetermined active site counts
 KIE’s provide empirical bridge from MeB(C6F5)3- to other
anions
Measurement of 1-Hexene KIE
Zr
0 °C
toluene
CH3
+
+
CH3
activator
3M
2 x 10-4 M
C-2
C2
C1
C4
C3
ca. 5%
C6
C5
Activators
B(C6F5)3
Al(C6F5)3
MAO
[PhNMe2H]+
[B(C6F5)4]-
n
ca. 95%
Recover unreacted
1-hexene, quantitate
conversion, and
integrate (carefully) 13C
NMR
R/Ro = (1-F)(1/KIE)-1
•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
Singelton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357.
Empirical 1-Hexene KIEs
C2
C1
C4
C3
C6
C5
Average of 3 independent runs, 3 spectra/run
0°C, (EBI)ZrMe2 + 2 eq. Activator,Toluene
C1
C2
B(C6F5)3
1.009(4)
1.019(6)
0.999(1) 1.001(1)
1
Al(C6F5)3
1.010(2)
1.017(3)
1.000(0) 1.000(2)
1
PhNMe2H+ B(C6F5)4- 1.009(1) 1.017(1)
1.001(2) 1.001(1)
1
toluene
C3
C4
C5
MAO
1.007(4)
1.018(1)
1.000(1) 1.000(2)
1
B(C6F5)3
1.003(1)
1.013(2)
0.999(1) 1.000(1)
1
chlorobenzene
• KIE(C2)>KIE(C1)
• Weaker Ion-Pairs yield smaller KIE’s?
Do KIE’s Reveal More?
Computational Model
Cp
Cp
+
Zr
ClMe
+
ClMe
k1
Cp
Cp
+
Cp
Cp
k2
Zr
+
ClMe
Zr
k-1
1
What is Computed?
• Free Energy: Association and
Insertion
• Both 1,2- and 2,1-insertion
pathways
• 3 trajectories for alkene
association
G
• KIE for k1 and k2
• EIE for K1 (=k1/k-1)
• B3LYP/LANL2DZ
2
Why?
3
• ClMe as an anion substitute
• computationally accessible
• ca. thermoneutral association
1+
propene
2
ca. 10
kcal/mol
3
ClMe
Computation:
Association
Cp
Cp
+
ClMe
Zr
1,2 Pathway
A-1a
A-1c
DG‡ = 17.1; KIE 1.001 0.995 0.981
DG = 0.2; EIE 0.999 0.993 0.979
A-3a
A-2
DG‡ = 12.9; KIE 1.006 1.006 1.018
DG = -3.3; EIE 1.007 0.991 0.980
DG = 1.1; EIE 1.010 1.005 1.010
EIE
KIE
C2
1.003(6) 0.995(7)
1.009(9) 1.001(4)
DG‡ = 12.5; KIE 1.003 1.000 1.000
DG = 2.6; EIE 0.999 0.993 0.989
A-3b
DG‡ = 11.1; KIE 1.023 1.001 0.985
Averages C1
+
+
Zr
2,1 Pathway
A-1b
DG‡ = 13.1; KIE 1.007 1.002 0.998
DG = 0.4; EIE 0.997 0.991 0.977
Cp
Cp
C3
0.987(16)
0.996(16)
DG‡ = 11.6; KIE 1.004 0.997 0.995
DG = -0.6; EIE 0.996 0.997
0.996
Results
• Small KIE
ClMe
Computation:
Insertion
Cp
Cp
+
ClMe
Zr
Cp
Cp
+
1,2 Pathway
I-1a
I-2a
DG‡ = 10.8; KIE 1.027 1.043 1.001
DG‡ = 23.2; KIE 1.029 1.033 1.009
I-2c
DG‡ = 9.6; KIE 1.010 1.035 1.012
Averages C1
KIE
1.020(7) 1.044(6)
ClMe
Zr
I-2d
DG‡ = 15.9; KIE 1.017 1.047 1.010
C2
+
2,1 Pathway
I-1b
DG‡ = 17.3; KIE 1.024 1.050 1.003
I-2b
Cp
Cp
+
Zr
C3
1.007(5)
DG‡ = 15.2; KIE 1.022 1.034 1.015
Results
• KIE(C2)>KIE(C1)
Does Alkene Bind Reversibly?
Scenario 1: Irreversible Alkene Association
KIE fixed at the
alkene association
step.
KIE = KIE1
ClMe
Cp
Cp
ClMe
+
Zr
+
C1
1.009(9)
Cp
k1
+
Zr
Cp
C2
1.001(4)
k2
Cp
Cp
ClMe
+
Zr
C3
0.996(16)
Scenario 2: Reversible Alkene Association
KIE fixed at the
alkene insertion
step.
ClMe
Cp
Cp
+
ClMe
Zr
+
K1
Cp
Cp
+
Zr
k2
Cp
Cp
KIE = EIE1xKIE2
1.023(7)
1.039(7)
0.993(16)
Experiment
1.08(7)
1.018(4)
1.000(2)
+
Zr
Data are NOT Compatible with Scenario 1
ClMe
Working Mechanism
Initiation
A
+
slow(EBI)Zr
+
(EBI)Zr
Me
R
+
(EBI)Zr
+
(EBI)Zr
AH
R
+
(EBI)Zr
-
A R R
-
A
Me
R
R
A- R R
fast
R
-
A
n
R R
A-
+
(EBI)Zr
R
R
R
R
n
Termination
n
R
R
+
(EBI)Zr
R
+
(EBI)Zr
AH
R
P
ro
p
a
g
a
+
(EBI)Zr
A-
-