Kinetic and mechanism of alkene polymerization

Article abstract of DOI:10.1134/S0023158406020029

Triphenylmethyl salts of the very weakly-coordinating borate anions [CN{B(C₆F₅)₃}₂]^- (1), [H₂N{(C₆F₅)₃}₂] ^- and [M{CNB(C₆F₅)₃} ₄]^2- (M = Ni, Pd) have been prepared in simple one-pot reactions. Mixtures of (SBI)ZrMe₂/1/AlBu ₃ ⁱ (SBI = rac-Me₂Si(Ind)₂) are 30-40 times more active in ethylene polymerizations at 60-100°C than (SBI)ZrCl₂/MAO. The quantification of anion effects on propene polymerization activity at 20°C gives the order [CN{B(C₆F₅)₃}₂] ^- > [H₂N{(C₆F₅)₃} ₂]^- ≈ B(C₆F₅) ₄ ^- ? [MeB(C₆F₅)₃]^-. The highest productivities were of the order of ca. 3.0 × 10⁸ g PP (mol Zr)^-1 h^-1 [C₃H₆]^-1, about 1.3-1.5 times higher than with B(C₆F₅) ₄ ^- as the counter anion. The titanium system CGCTiMe ₂/1/AlBu ₃ ⁱ gave activities that were very similar to the zirconocene catalyst. The concentration of active species [C*] as determined by quenched-flow kinetic techniques indicates typical values of around 10%, independent of the counter anion, for both the borate and MAO systems. Pulsed field-gradient spin echo and nuclear Overhauser effect NMR experiments on systems designed to be more realistic models for active species with longer polymeryl chains, (SBI)M(CH₂SiMe₃)(μ-Me) B(C₆F₅)₃ and [(SBI)MCH₂SiMe ₃ ⁺ ...B(C₆F₅) ₄ ^- ] (M = Zr, Hf), demonstrated the influence of bulky alkyl chains on the ion pair solution structures: while the MeB(C₆F ₅)₃ compound exists as a simple inner-sphere ion-pair, the B(C₆F₅) ₄ ^- compound is an outer-sphere ion pair (OSIP), a consequence of the relegation of the anion into the second coordination sphere by the γ-agostic interaction with the alkyl ligand. The OSIP aggregates to ion hextuples (10 mM) or quadruples (2 mM). Implications for the polymerization mechanism are discussed; the process follows an associative interchange (I _a) pathway. MAIK Nauka/Interperiodica , 2006.

Full text of DOI:10.1134/S0023158406020029

ISSN 0023-1584, Kinetics and Catalysis, 2006, Vol. 47, No. 2, pp. 160–169. © MAIK “Nauka /Interperiodica” (Russia), 2006.
Kinetic and Mechanism of Alkene Polymerization1
M. Bochmann, R. D. Cannon, and F. Song
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy,
University of East Anglia, Norwich NR4 7TJ, UK
e-mail: m.bochmann@ula.ac.uk
Received July 11, 2005
Abstract—Triphenylmethyl salts of the very weakly-coordinating borate anions [CN{B(C6F5)3}2]– (1),
[H2N{B(C6F5)3}2]– and [M{CNB(C6F5)3}4]2– (M = Ni, Pd) have been prepared in simple one-pot reactions. Mix-
tures of (SBI)ZrMe2/1/AlBui (SBI = rac-Me2Si(Ind)2) are 30–40 times more active in ethylene polymerizations
3
at 60–100°C than (SBI)ZrCl2/MAO. The quantification of anion effects on propene polymerization activity at
20°C gives the order [CN{B(C6F5)3}2]– > [H2N{B(C6F5)3}2]– ≈ B(C6F5)–4 ꢀ [MeB(C6F5)3]–. The highest pro-
ductivities were of the order of ca. 3.0 × 108 g PP (mol Zr)–1 h–1 [C3H6]–1, about 1.3–1.5 times higher than with
B(C6F5)– as the counter anion. The titanium system CGCTiMe2/1/AlBui gave activities that were very similar
4
3
to the zirconocene catalyst. The concentration of active species [C*] as determined by quenched-flow kinetic
techniques indicates typical values of around 10%, independent of the counter anion, for both the borate and
MAO systems. Pulsed field-gradient spin echo and nuclear Overhauser effect NMR experiments on systems
designed to be more realistic models for active species with longer polymeryl chains, (SBI)M(CH2SiMe3)(µ-
Me)B(C6F5)3 and [(SBI)MCH2SiMe+ ···B(C6F5)– ] (M = Zr, Hf), demonstrated the influence of bulky alkyl
3
4
chains on the ion pair solution structures: while the MeB(C6F5)3 compound exists as a simple inner-sphere ion-
pair, the B(C6F5)– compound is an outer-sphere ion pair (OSIP), a consequence of the relegation of the anion
4
into the second coordination sphere by the γ-agostic interaction with the alkyl ligand. The OSIP aggregates to
ion hextuples (10 mM) or quadruples (2 mM). Implications for the polymerization mechanism are discussed;
the process follows an associative interchange (Ia) pathway.
DOI: 10.1134/S0023158406020029
1
Highly electrophilic metallocene cations paired with large a volume as possible. This can be achieved, for
non-nucleophilic, very weakly coordinating anions are
known to be highly active catalysts for the Ziegler-type
polymerization of olefins (Scheme 1) [1–5]. For the
effective binding of substrates as weakly basic as alk-
enes, a strongly Lewis acidic metal centre is required.
This Lewis acidity is controlled by the nature of the
counter anion, and it is therefore not surprising that sig-
nificant efforts have been invested in the way of anion
engineering [6, 7]. A few pertinent examples are shown
example, by the facile adduct formation between com-
mercially available B(C6F5)3 and strongly coordinating
anions such as CN–. For example, stirring a suspension
of KCN with two equivalents of B(C6F5)3 · Et2O in
diethyl ether at room temperature followed by treat-
ment with Ph3CCl gives [CPh3][CN{B(C6F5)3}2] (1) as
orange crystals. The linear CN-bridged geometry of 1
in Scheme 2. Of these, B(C6F5)–4 and the methylborate
MeB(C6F5)–3 derived from B(C6F5)3 are the most
Functionality
L
R
widely used, while other examples are synthetically
less accessible [8–38].
M
RESULTS AND DISCUSSION
X
Lewis
acid
Anion synthesis. We are interested in methods of
generating new bulky anions in a more straightforward
manner. The overriding principle for this anion design
is the desire to delocalize the negative charge over as
L
Weakly coordinating
anion
Protecting ligands
1
Scheme 1.
The text was submitted by the authors in English.
160

KINETIC AND MECHANISM OF ALKENE POLYMERIZATION
161
F
F
F
F
F
F
F
F
F
F
F
F
F
F
FAl
F
F
F
B
F
B
F
B
Si
F
F
F
F
F
4
3
4
F
3
F
F
F
B(C6F5)2
B(C6F5)2
F
F
B(C6F5)2
H
Et
X
H
F
B(C6F5)2
H
H
Cl
Cl
H
H
F
F
F
F
F
F
F
F
X= OMe, Cl
Cl
Cl
F
F
F
F
Cl
F
Cl
F
(C6F5)3B
B(C6F5)3
F
Scheme 2. Representative examples of weakly coordinating anions.
B(C6F5)3
N
C
[CPh3]+[(C6F5)3B–CN–B(C6F5)3]–
K2 (C6F5)3B NC M CN B(C6F5)3
1
K2M(CN)4
C
N
1) KCN
2) Ph3CCl
B(C6F5)3
B(C6F5)3
1) NaNH2
2) Ph3CCl
M = Ni, Pd
1) K2Pd(CN)4
2) Ph3CCl
Ph3CCl
[CPh3]+[H2N{B(C6F5)3}2]–
+
[CPh3]2[M{CNB(C6F5)3}4]2–
3
2
Scheme 3.
was confirmed by X-ray crystallography, the νCN
Similarly, the reaction of NaNH2 with two equiva-
lents of B(C6F5)3 gives [Na(OEt2)4][H2N{B(C6F5)3}2],
which is readily converted into the trityl salt
[CPh3][H2N{B(C6F5)3}2] (3); the whole process can con-
veniently be conducted as a one-pot reaction [42, 43].
As the crystal structure of this anion shows, it is signif-
icantly stabilized by NH···F hydrogen bonds to five
ortho-F atoms (Fig. 1). The resistance of the diborate
anion towards protolysis is illustrated by the conversion
stretching frequency at 2305 cm–1, and the observation
of two 11B NMR signals at δ –11.94 and –21 [39, 40].
A similar reaction between B(C6F5)3 · Et2O and the very
stable nickel and palladium tetracyanometallates
[M(CN)4]2– leads to the dianions [M{CNB(C6F5)3}4]2–
(2, M = Ni, Pd), which were isolated as NHMe2Ph+ and
CPh+3 salts (Scheme 3) [41].
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

162
BOCHMANN et al.
F(45)
F(23)
F(24)
C(230)
C(450)
F(46)
F(44)
F(64)
C(240)
F(22)
C(250)
F(25)
C(440)
C(220)
F(12)
H(2*)
F(62)
H(3*)
C(410)
C(130)
C(140)
C(160)
F(14)
N(3)
C(640)
F(36)
F(56)
B(1)
C(310)
C(320)
B(2)
C(510)
C(150)
F(65)
F(16)
F(15)
F(66)
F(42)
F(52)
C(520)
C(350)
F(32)
C(340)
C(330)
C(530)
F(53)
F(33)
F(34)
C(540)
F(54)
–
Fig. 1. Structure of the anion [H N{B(C F ) } ] in the solid state, illustrating the H···F hydrogen bonding to five of the six o-F
2
6 5 3 2
atoms.
of
the
sodium
salt
into
crystalline not least kindled by our observation that the ethylene
[H(OEt2)2][H2N{B(C6F5)3}2]. The formation of this
strong Brønsted acid is favored over protolytic anion
decomposition, e.g., to give H3N · B(C6F5)3 + Et2O ·
B(C6F5)3.
polymerization activity of a standard catalyst like
(SBI)ZrMe2 (SBI = rac-Me2Si(Ind)2) was increased by
a factor of 30–40 if a mixture of AlBui3 (TIBA) and 1
was used as the activator instead of methylaluminoxane
(MAO) [39, 40]. In effect, the activity (productivity) of
a catalyst is one of the most difficult parameters to
quantify objectively. This is perhaps best illustrated by
Möhring and Coville’s compilation of literature data for
the activity of the “standard” ethene polymerization
catalyst Cp2ZrCl2/MAO, which cover a range of four
orders of magnitude [44]. In order to compare anion
effects with metallocene catalysts, we have developed a
protocol that eliminates concentration and mass trans-
port effects. To reduce the problem of monomer deple-
tion in reactions under 1 bar monomer pressure, pro-
pene rather than ethylene polymerization was investi-
gated. The results show that at 20°C catalyst
productivity decreases in the order [CN{B(C6F5)3}2]– >
Quantification of anion effects. Our interest in new
catalyst activators based on complex borate anions was
Activity, 106 g PP (mol Zr)–1 h–1 bar–1
300
250
[CN{B(C6F5)3}2]–
200
[H2N{B(C6F5)3}2]–
150
[H2N{B(C6F5)3}2]– ≈ B(C6F5)4– ꢀ [MeB(C6F5)3]–. With
(SBI)ZrMe2/1/AlBu3i and [Zr] = 5 × 10–6 mol/L, pro-
100
ductivities were of the order of ca. (2.8–3.0) × 108 g PP
(mol Zr)–1 h–1 [C3H6]–1, about 1.3–1.5 times higher than
50
with B(C6F5)–4 as the counter anion. The titanium sys-
tem CGCTiMe2/1/AlBui3 gave activities that were very
[MeB(C6F5)3]–
0
1
2
3
4
5
similar to the zirconocene catalyst. The almost linear
dependence of catalyst activity on [Zr] makes it possi-
ble to extrapolate to zero catalyst concentration in order
Zr, µmol
Fig. 2. Dependence of propene polymerization activity on [Zr].
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

KINETIC AND MECHANISM OF ALKENE POLYMERIZATION
163
to determine the “intrinsic” activity of the system
(Fig. 2) [41].
R
X
R
Polymerization kinetics. The classical picture of
the mode of action of metallocene-based catalysts
assumed dissociation into an ion pair, with the cationic
metallocenium alkyl species being responsible for
chain propagation, while the anion was not thought to
be involved (Scheme 4) [3, 4]. However, the fact that
the anion does contribute to the activation barrier of
polymerization must mean that it is involved in the tran-
sition state. The question therefore was whether the
anion-dependent differences in activity were due to dif-
ferences in the concentration of active species [C*] or
to the lowering of the transition state.
K
M
M
X
Scheme 4.
result in values that approach 100%. On the other hand,
the [C*] values determined by quenched-flow kinetics
were much smaller, 8%. Surprisingly, this value was the
same for both the borate and the MAO-activated sys-
tem. It follows that there must be a distribution between
active and dormant species that is the same for both cat-
alyst systems, as well as that the MAO-catalyst takes
significantly longer to complete one insertion cycle.
Other systems, such as the CGCTi catalysts, show sim-
ilar [C*] (table) [45, 49].
Further information about the possible nature of the
dormant sites comes from the polymer microstructure
and end group analysis [53, 54]. Both vinylidene and
cis-butenyl (vinylene) end groups are found, the latter
being dominant in this system. There is also a contribu-
tion from internal C=C bonds, which are a follow-on
product of vinylidene end groups. Taking this into
account gives a vinylidene : butenyl ratio of 34 : 66.
There is a small number of enchained regioerrors due to
2,1-insertions followed by 1,2-chain growth, on aver-
age one per 500 1,2-insertions (Scheme 5). Since most
of the end groups are generated following a 2,1-misin-
sertion, and since termination reactions in polymeriza-
tion systems are orders of magnitude slower than prop-
agation reactions, it is clear that a substantial amount of
total [Zr] must be present as Zr-sec-alkyl species; these,
therefore, make an important contribution to the total
resting states in this system.
This problem was probed using quenched-flow
kinetic techniques [45–49]. Two catalyst systems were
used: (a) (SBI)ZrMe2/TIBA/CPh3[CN{B(C6F5)3}2] and
(b) (SBI)ZrCl2/MAO, both at 1 bar propene in toluene.
This system allowed us to investigate the onset of poly-
merization under homogeneous, as opposed to mass-
transport limited conditions, from tR = 0.2 to 5 s
[50−52].
The results provide two independent types of data.
The time dependence of polymer mass assumes that
100% of the injected pre-catalyst was active and leads
to the “apparent” propagation rate constant, kappp . The
second source of data is the time dependence of the
number-average polymer molecular weight, which
makes no assumptions about the concentration of active
species. From this a second rate constant kp was
obtained for the rate of growth of the polymer chains.
The ratio kappp /kp is therefore a measure of the propor-
tion of total zirconocene that is active at any one time.
Of course (neglecting irreversible deactivation), due to
ligand redistribution most of the injected zirconocene
will be involved in the chain growth process over time
and will carry a polymeryl chain. Measurements of
The mechanistic scheme we used to rationalize our
[C*] by isotopic labelling techniques will therefore findings and the relevant rate constants for propagation
Proportion of active species in propene polymerizations with various metallocene catalysts
kappp (yield vs. time),
Fraction
of active species, kappp /kp
kp (Mn vs time),
Catalyst
l mol–1 s–1
l mol–1 s–1
(SBI)ZrMe2/TIBA/CPh3[CN{B(C6F5)3}2]
(SBI)ZrCl2/MAO
1320 20
17200 1400
600 230
8800
0.08
0.08
0.13
0.22
0.15
0.15
48
3
(SBI)ZrCl2/TIBA/CPh3[B(C6F5)4] (1 : 100 : 1)
(SBI)ZrCl2/TIBA/CPh3[B(C6F5)4] (1 : 100 : 3)
(CGC)TiMe2/TIBA/CPh3[B(C6F5)4]
(CGC)TiCl2/TIBA/CPh3[B(C6F5)4]
Note: Toluene, [C H ] = 0.6 M, 25°C.
1100
2500
1100
2800
11200
7400
19300
3
6
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

164
BOCHMANN et al.
P
Zr
Zr
P
P
P
P
P
Vinylidene
24%
Internal C=C
10%
66%
cis-butenyl
Enchainment
Zr
P
Scheme 5.
P2
P3
C*
C*
kbut
kbut
Zr
P2
Zr
P3
…
'
'
kp
kp
ksec
ksec
ksec
kp
kp
kp
…
C*
Zr
P1
Zr
P2
Zr
P3
Zr
P4
kv
kv
kv
kv
C*
C*
C*
C*
P1
P2
Scheme 6.
P3
P4
and termination processes is depicted in Scheme 6 [45]. than a prim-alkyl species, long enough to accumulate as
Some doubt has been raised in the literature as to the
existence of dormant states, and the accumulation of
sec-alkyl zirconocene species in particular, based on
spectroscopic studies of the (EBI)ZrMe2/B(C6F5)3 sys-
tem (EBI = rac-C2H4(1-Ind)2), which indeed leads
almost exclusively to vinylidene end groups [55, 56].
However, based on a combination of kinetic results and
the polymer microstructure analysis, we estimate the
lifetimes of the primary and secondary alkyls to be
τ(Zr-Prim) = 1/(kp + ksec)[M] ≈ 10–4 s, and τ(Zr-Sec) =
–1
a dormant state.
Inner-sphere versus outer-sphere ion pairs. The
catalyst systems employed so far have been based on
zirconocene dimethyls or dichlorides in combination
with TIBA and an activator. Although it is generally
assumed that these mixtures will lead to transalkylation
and the formation of zirconium isobutyl species, the
precise chemistry of these mixtures under catalytic
conditions is so far unknown. Zirconocene methyl
derivatives are commonly used as precursors, although
the methyl ligand is a poor model for a polymeryl chain
insofar as it lacks steric bulk, is remarkably slow to
'
1/(kp + kbut)[M] ≈ 10 s, respectively; in other words,
the data suggest that under catalytic conditions a sec-
alkyl zirconocene species persists ca. 103 times longer insert alkenes, and acts readily as a bridging ligand
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

KINETIC AND MECHANISM OF ALKENE POLYMERIZATION
165
Me2Si
CH2SiMe3
M
4
Me B(C6F5)3
B(C6F5)3
H
H
Me2Si
C
M
SiMe3
Me
CPh3+B(C6F5)3–
CH2
Me2Si
B(C6F5)4
M
M = Zr, Hf
SiMe2
H3C
5
Scheme 7.
F(75)
C(15)
C(14)
C(17)
C(22)
C(12)
C(13)
C(18)
F(74)
C(74)
C(75)
C(16)
C(11)
C(19)
Si(1)
C(20)
C(73) C(76)
Si(2)
F(76)
C(71)
Hf
C(21)
F(73)
C(72)
C(30)
C(23)
C(1)
C(9)
C(2)
C(24)
C(8)
B
F(72)
C(3)
F(66)
C(4)
C(7)
F(62)
C(64)
F(82)
C(5)
F(63)
C(84)
F(84)
C(6)
F(64)
C(83)
F(83)
Fig. 3. Molecular structure of (SBI)Hf(CH SiMe )(µ-Me)B(C F ) . Ellipsoids are drawn at a 50% probability level.
2
3
6 5 3
whereas higher alkyls do not. We wished therefore to
use a more realistic catalyst model. Since neopentyl
ligands are known to undergo β-methyl abstraction and
eliminate isobutene [57], we chose to prepare zir-
conocene trimethylsilylmethyl derivatives, Scheme 7
[48].
Ion pairs of 14-electron alkyl complexes
[(SBI)MCH2SiMe3+ ···B(C6F5)–4 ] could not be crystal-
lized but turned out to be of remarkable thermal stabil-
ity, and in toluene solution at room temperature they
showed no deterioration over a period of several days.
The β-methyl branched alkyl ligand acts as a model for
ligated polypropylene. The variable-temperature NMR
spectra confirm that here agostic bonding to β-methyl
substituents is preferred over anion coordination,
resulting in an outer-sphere ion pair structure. There
are, therefore, significant differences in the structures
The readily accessible mixed-alkyl metallocene
complexes (SBI)M(Me)CH2SiMe3 (M = Zr, Hf) provide
a
route for zwitterions (SBI)M(CH2SiMe3)(µ-
Me)B(C6F5)3. The crystal structure of this compound
confirmed the familiar methyl-bridged structure
(Fig. 3) [58].
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

166
BOCHMANN et al.
Me
C6F5
C6F5
C6F5
Me
B
B(C6F5)3
H
Si
C6F5
Me3Si
H
H
C
Me
H
H
H
H
Si
Si
Me
Me
Me
Me
Fig. 4. Structures of inner-sphere and outer sphere M–CH SiMe ion pairs.
2
3
of MeB(C6F5)–3 and B(C6F5)–4 complexes: the inner- vs.
outer-sphere ion pairs and outwards vs. inwards point-
ing conformations of the alkyl ligand (Fig. 4) [58].
Pulsed field-gradient spin echo (PGSE) and nuclear
Overhauser effect (NOE) NMR experiments showed
the influence of bulky alkyl chains on the solution
structures of (SBI)Zr(CH2SiMe3)(µ-Me)B(C6F5)3 and
[(SBI)ZrCH2SiMe3+ ···B(C6F5)–4 ]. As expected, the
MeB(C6F5)3 compound in toluene-d8/1,2-difluoroben-
zene 9 : 1 exists as a simple inner-sphere ion-pair
These complexes provide useful information about
the ion pair dynamics and solution structures. Site
epimerization is faster than with Zr–CH3 analogues.
Rates decrease with decreasing metallocene concentra-
tion; the observed values therefore represent upper lim-
its for the exchange rates expected for the low metal
concentrations employed under catalytic conditions.
The data also provide an interesting illustration of the
stereochemistry of chain swinging. Thus, while the
bridge–SiMe2 signals show a typical pattern for the
two-site exchange expected for this symmetrization
process, the Zr–CH2 signals remain unaffected and
appear as an AB pattern throughout the observed tem-
perature range; that is, the methylene hydrogens Ha and
Hb do not interchange. This behavior is consistent with
a chain swinging mechanism that involves a 180° rota-
tion of the alkyl ligand (Scheme 8), as is indeed
required by the symmetry in a system with a C2-sym-
metric ligand framework.
1
(ISIP). 19F, H HOESY NMR investigations showed
that the anion binds to the cation in the usual fashion
with the B–Me vector pointing toward the Zr centre. In
agreement with this structure, only the o-F fluorine
nuclei interact with the CH2SiMe3 signal and with the
CH2SiMe3 resonance, while both the m-F and p-F
nuclei give a weak interaction with the CH2SiMe3 sig-
nal. From these observations, we conclude that the
CH2SiMe3 ligand is preferentially oriented in such a
way that the bulky SiMe3 group is pointing away from
the anion, as shown in the crystal structure.
By contrast, [(SBI)ZrCH2SiMe+3 ···B(C6F5)4– ] in tol-
uene-d8/1,2-difluorobenzene 9 : 1 at higher concentra-
tions (10 mM) forms mainly ion hextuples (aggregation
number N = 3.12), indicative of the more ionic charac-
ter of an outer-sphere ion pair (OSIP). This is a conse-
quence of the relegation of the anion into the second
coordination sphere caused by the γ-agostic interaction.
At lower [Zr] and lower temperature, ion quadruples
predominate. It is possible that further dilution may
lead to simple ion pairs, although these were not
observed (Scheme 9).
Ha
Hb
X
Me3Si
Hb
X
SiMe3
Ha
Scheme 8.
Mechanistic models. The mechanism of 1-alkene
polymerization with such ion pair catalysts has recently
been a matter of debate in the literature. Early kinetic
studies by Fink on titanocene-catalyzed ethylene oligo-
merizations have led to a model in which the catalyst
precursor is activated by some chemical process (such
as anion displacement) to give an active species that is
not spectroscopically detectable, and each monomer
insertion can be followed by the (reversible) stabiliza-
tion of the product as a dormant state, in a model
termed “intermittent mechanism” [59–63]. More
recently, Brintzinger et al. studied the reaction of a
10 mM
2 mM
0.001 mM?
Scheme 9.
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

KINETIC AND MECHANISM OF ALKENE POLYMERIZATION
CH3 B(C6F5)3
167
Bu
Bu
P
P
Bu
P
(C6F5)3B
B(C6F5)3
CH3
Ind
CH3
Ind
Bu
kp
Zr
Zr
Zr
Ind
Ind
Ind
Ind
reinitiation, kri
hexene-1
Scheme 10.
X
Pn
Pn + 1
Pn
Me
X
X
Me
Me
etc.
Zr
Zr
Zr
L
L
L
L
L
L
Scheme 11.
series of zirconocene methylborates with di-n-butyl butions of the counter anion to the activation barrier,
ether (DBE) as an “alkene surrogate.” Here, anion dis- followed by alkyl/anion site exchange, as required
placement was slow, and the authors suggested that within a catalytic cycle, since the structure of the prod-
these findings pointed towards a mechanism in which uct must correspond to the starting point of the insertion
chain growth was initiated by anion displacement and event. In this respect, the “continuous” ISIP mechanism
proceeded by successive rapid monomer enchainments, and the OSIP reaction sequence are similar in principle:
which were interrupted by slow anion re-coordination both follow an associative interchange (Ia) pathway.
[64]. On the other hand, Landis showed that in
hexene-1 polymerizations with (EBI)ZrMe2/B(C6F5)3
at low temperature, where the system is living and Zr-
polymeryl species are directly NMR-observable, each
monomer insertion was followed by anion reassocia-
tion in a concerted anion displacement—monomer
insertion—anion reassociation reaction sequence, a
process termed the “continuous” propagation mode
(kp = kri, Scheme 10) [65].
The site exchange of the (very large) anion is probably
best understood as a rotation of the metallocenium cat-
ion with respect to the anion.
CONCLUSIONS
In summary, we have shown methods for the quan-
tification of catalyst activity in high-activity metal-
locene systems and have demonstrated, in particular,
the contribution the counter anion makes to catalyst
activity, the structure of the active species, ion aggrega-
tion, and the dynamics of well-defined ion pairs. The
synthesis of zirconocene trimethylsilylmethyl com-
plexes in particular has provided insight into the stere-
ochemistry of site epimerization and the mechanism of
chain propagation and has highlighted important differ-
ences in the structure and ligand conformation of ISIP
Clearly the very high activities observed with OSIP-
type B(C6F5)–4 based catalysts and the low contribu-
tions of the anion to the activation barrier are not con-
sistent with a mechanism in which the anion recoordi-
nates and forms a tight ion pair with the metal centre
after each insertion step. The sequence in this case is
probably better depicted in Scheme 11. Here there is no
anion coordination; the anion is, however, kept within
the solvent cage (in low polarity media there is no free
MeB(C6F5)–3 and OSIP B(C6F5)–4 ion pairs. Both agos-
ion diffusion). Each insertion step involves an increase tic interactions of β-methyl branched alkyl ligands and
in the cation–anion distance, which explains the contri- sec-alkyl species are likely to be resting states in these
KINETICS AND CATALYSIS Vol. 47 No. 2 2006

168
BOCHMANN et al.
23. Chen, Y.X., Stern, C.L., and Marks, T.J., J. Am. Chem.
Soc., 1997, vol. 119, p. 2582.
catalysts, at least with the SBI ligand system investi-
gated here. If one accepts the definitions of “active spe-
cies” and “dormant states” employed here, the chain 24. Chen,Y.X., Metz, M.V., Li, L., Stern, C.L., and Marks, T.J.,
J. Am. Chem. Soc., 1998, vol. 120, p. 6287.
propagation sequence is in agreement with an “inter-
mittent” model along the lines of an associative inter-
change mechanism.
25. Li, L. and Marks, T.J., Organometallics, 1998, vol. 17,
p. 3996.
26. Metz, M.V., Schwartz, D.J., Stern, C.L., Nickias, P.N.,
and Marks, T.J., Angew. Chem., Int. Ed. Engl., 2000,
vol. 39, p. 1312.
ACKNOWLEDGMENTS
This work was supported by the British Engineering
and Physical Sciences Research Council. We are grate-
ful to Prof. A. Macchioni (University of Perugia, Italy)
for PGSE and NOE NMR measurements and helpful
discussions.
27. Sun, Y., Metz, M.V., Stern, C.L., and Marks, T.J., Orga-
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28. Metz, M.V., Sun, Y., Stern, C.L., and Marks, T.J., Orga-
nometallics, 2002, vol. 21, p. 3691.
29. Chen, M.C. and Marks, T.J., J. Am. Chem. Soc., 2001,
vol. 123, p. 11803.
30. Abramo, G.P., Li, L., and Marks, T.J., J. Am. Chem. Soc.,
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