Structure and dynamics in solution of bis(phenoxy-amine)zirconium catalysts for olefin polymerization

Article abstract of DOI:10.1021/om2001926

The activity of two bis(phenoxy-amine)ZrR₂ precatalysts (bis(phenoxy-amine) = N,N'-(3-^tBu-5-OMe-2-C₆H ₂OCH₂)₂-N,N'-Me₂-(NCH ₂CH₂N); R = Me (1), Bn (2, benzyl)) toward propene polymerization has been evaluated using different activators and cocatalysts: MAO, MAO/TBP, B(C₆F₅)₃/TIBA, and [CPh ₃][B(C₆F₅)₄]/TIBA (MAO = methylalumoxane, TBP = 2,6-di-tert-butylphenol, TIBA = triisobutylaluminum). It was found that the nature of the activator affects the activity only to a small extent. NMR studies in solution and DFT calculations on the 3a-c and 4a-c(a, MeB(C₆F₅)₃^-; b, BnB(C ₆F₅)₃^-; c, B(C₆F ₅)₄^-) ion pairs deriving from the activation processes of 1 and 2, respectively, showed that three isomers can form. All of them have the anion in the second coordination sphere, whereas the binding modality of the ligand leads to the mer-mer most stable isomer, fac-mer isomer of intermediate stability, and fac-fac least stable isomer. Notably, the energy of the fac-fac isomer, which is supposed to be the active species in the polymerization process, depends more on the R group and not much on X-, in agreement with the small influence of the activators on the polymerization activity.

Full text of DOI:10.1021/om2001926

ARTICLE
pubs.acs.org/Organometallics
Structure and Dynamics in Solution of Bis(phenoxy-amine)Zirconium
Catalysts for Olefin Polymerization
Gianluca Ciancaleoni,†,‡ Natascia Fraldi,‡,§ Peter H. M. Budzelaar,*,|| Vincenzo Busico,§ and
Alceo Macchioni*,†
†Dipartimento di Chimica, Universitꢀa di Perugia, via Elce di Sotto, 8, 06123 Perugia, Italy
‡Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
§Dipartimento di Chimica, Universitꢀa di Napoli Federico II, Via Cintia, 80126 Napoli, Italy
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
S Supporting Information
b
ABSTRACT: The activity of two bis(phenoxy-amine)ZrR2
precatalysts (bis(phenoxy-amine)
=
N,N0-(3-tBu-5-OMe-
2-C6H2OCH2)2-N,N0-Me2-(NCH2CH2N); R = Me (1), Bn
(2, benzyl)) toward propene polymerization has been evaluated
using different activators and cocatalysts: MAO, MAO/TBP,
B(C6F5)3/TIBA, and [CPh3][B(C6F5)4]/TIBA (MAO =
methylalumoxane, TBP = 2,6-di-tert-butylphenol, TIBA =
triisobutylaluminum). It was found that the nature of the
activator affects the activity only to a small extent. NMR studies in solution and DFT calculations on the 3aꢀc and 4aꢀc
(a, MeB(C6F5)3ꢀ; b, BnB(C6F5)3ꢀ; c, B(C6F5)4ꢀ) ion pairs deriving from the activation processes of 1 and 2, respectively, showed
that three isomers can form. All of them have the anion in the second coordination sphere, whereas the binding modality of the
ligand leads to the mer-mer most stable isomer, fac-mer isomer of intermediate stability, and fac-fac least stable isomer. Notably, the
energy of the fac-fac isomer, which is supposed to be the active species in the polymerization process, depends more on the R group
and not much on Xꢀ, in agreement with the small influence of the activators on the polymerization activity.
’ INTRODUCTION
metals, the ligand wraps the octahedral metal center in a fac-fac
mode (Scheme 1), leading to the C2-symmetrical neutral pre-
cursors (ONNO)MR2 (R = Bn,2a,7 Me2CHO8a,10).14 This has
In recent years, a significant effort has been devoted to the
discovery of new postmetallocene R-olefin polymerization cata-
lysts and rationalization of their performance on the basis of their
structures.1 Tetradentate bis(phenolate) ligands of the type
[OYYO] (Y = heteroatom donor N,2 P,3 S4, or O5) have attracted
interest because they might mimic the steric environment of the
surface Ti atoms in classical heterogeneous ZieglerꢀNatta sys-
tems used for industrial production of isotactic polypropylene.6
Furthermore, the steric and electronic properties of the [OYYO]
ligands can be easily modulated, also taking advantage of the high-
throughput parallel screening technologies.7 From a catalytic
point of view, the [ONNO] bis(phenoxy-amine)2a,8 class of
complexes generally shows low activity and high isotacticity for
1-hexene and propene polymerization,2a,9 although higher activ-
ities can be obtained by ligand modification10a or with different
monomers, such as rac-lactide.10b
The structures of complexes bearing the bis(phenoxy-amine)
ligand are rather diverse, since the inherent flexibility of the ligand
backbone allows several coordinative geometries to be adopted.
For example, the ligand disposition is intermediate between
square pyramidal and trigonal bipyramidal in five-coordinated
aluminum complexes;11 a fac-mer geometry (Scheme 1) is pre-
ferred with octahedral chromium(III),12 while a tetrahedral
arrangement is adopted with manganese(II).13 With group 4
1
been confirmed by both X-ray crystallography and H NMR
spectroscopy.2a Much less is known about the ligand conforma-
tion after activation, since the resulting ion pair is often not
stable2a or exhibits significant dynamic behavior, precluding an
accurate NMR study.15
In one case the structure of a bis(phenoxy-amine)zirconium ion
pair was fully elucidated.16 Particularly, NMR and DFT (density
functional theory) studies were carried out for the [(N,N0-(3-
CMe2Ph-5-Me-2-C6H2OCH2)2-N,N0-Me2-(NCH2CH2N))ZrMe]-
[MeB(C6F5)3] (5a) ion pair, which is a competent catalyst for
olefin polymerization.7 It was found that the most stable isomer
in solution is the outer-sphere ion pair (OSIP)17 cis(N,N)-cis-
(O,O) (C2x,18 or mer-mer, Scheme 1), with a distorted-square-
pyramidal geometry, having the ZrꢀMe bond in an apical
position and the anion on the opposite side. If 5a were exclusively
present as the C2x isomer, it would be inactive in polymerization,
since the entering olefin would be trans to the alkyl chain and the
insertion could not take place. This led us to hypothesize that
the C2x isomer transforms into the cis(N,N)-trans(O,O) (C2, or
Received: March 1, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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Scheme 1. Geometrical Isomers of [(ONNO)ZrX2]
Complexes
Figure 1. Dependence of catalyst productivity on the choice of
activator.
productivity is quite low,9 less than 250 kgpp/(h molZr [C3H6]),
indicative of an inherent slowness of the catalyst toward propene.18
The three boron-based anions give essentially the same very low
productivity (Figure 1), around 10ꢀ20 kgpp/(h molZr [C3H6]),
independent of the precise choice of the anion.
Scheme 2. Syntheses and Numbering of Ion Pairs
These results are anomalous if compared with those for the
archetypal metallocene catalysts, where there is a large influence of
the activator on the productivity:26 the use of [CPh3][B(C6F5)4]
instead of B(C6F5)3 usually leads to a 200-fold increase in propene
polymerization productivity, even in the presence of TIBA.23 This
difference has been attributed to the different coordinating abilities
of B(C6F5)4ꢀ and MeB(C6F5)3ꢀ, implying that the energy cost of
displacement of the counterion contributes to the rate-limitingstep.
In the present case, the increase is small and is believed to be within
experimental error, giving us an important piece of information
about the polymerization mechanism: displacement of the anion is
not part of the rate-determining step and may even not be involved
at all in the mechanism (see below). The use of MAO alone results
in only a modest increase in activity (by a factor of about 2); a larger
increase in productivity (factor of 10) can be achieved by combin-
ing MAO with TBP (Figure 1).22 This still corresponds to a rather
modest change in activation energy.
In a paper by Kol and co-workers about similar catalyst precursors,
having chlorine atoms on the phenol ring, it was shown that also a
change of solvent did not substantially alter the 1-hexene polymer-
ization activity.8a The same group reported that, for 1-hexene
polymerization using catalyst precursors based on a C1-symmetric
(ONNO0)Zr framework,10a the isotacticity and polymer molecular
weight were unaffected by the activator used to generate the active
species. The independence of the catalytic parameters on the anion
and activation conditions appears to be a general issue for the
bis(phenoxy-amine)-based catalysts. For this reason, a more detailed
comprehension of the structure and dynamics of the resulting active
species in solution is desirable.
fac-fac) isomer having all the correct features to be the active
species for the polymerization of olefins.7 Consistent with this
hypothesis, we recently showed that only the inclusion of the
C2xꢀC2 isomerization process in the reaction pathway of
propene polymerization, mediated by a bis(phenoxy-amine)-
ZrBn2 complex, allows the experimental kinetic parameters to
be well reproduced by DFT calculations.19
In this work, we explore the scope of such an unusual reaction
pathway by investigating the influence of the activator on the ion
pair structure and catalytic performance. For this purpose, ion
pairs 3a,c and 4b,c were synthesized from precursors 1 and 2
(Scheme 2), fully characterized both experimentally (19F,
1H-HOESY, 1H-EXSY, and 1H-PGSE NMR studies) and com-
putationally (DFT calculations) and tested in the polymerization
of propene.
Syntheses and Characterization of Ion Pairs in Solution.
Thereactions of precursors 1 and 2 with theLewisacids B(C6F5)3
and [CPh3][B(C6F5)4] afforded the desired ion pairs 3a,c and 4b,
c (Scheme 2). 3c was not soluble enough in benzene to allow
detailed characterization and NMR studies. The solubility of 3a
’ RESULTS AND DISCUSSION
Polymerization Tests. Catalyst precursors 1 and 2 were tested
in the polymerization of propene in toluene, using different
activators and cocatalysts: MAO (MAO = methylalumoxane),20ꢀ22
MAO/TBP (TBP = 2,6-di-tert-butylphenol),23 B(C6F5)3/TIBA
1
was sufficient to obtain H exchange spectroscopy (EXSY)27,28
(TIBA
=
triisobutylaluminum),24 and [CPh3][B(C6F5)4]/
NMR spectra but was too low for 19F,1H heteronuclear Over-
hauser effect spectroscopy (HOESY)27,29 NMR experiments.
TIBA.23,25 The results are shown in Figure 1. In all cases the
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Scheme 3. Geometry of Ion Pair 5a
BnB(C6F5)3 ,
ꢀ 33 it can be safely assumed that it too remains in
the second coordination sphere.
The fact that even a rather “sticky” anion such as MeB-
(C6F5)3ꢀ does not interact with the metal is an indication that
the coordination vacancy is probably protected, either sterically
or electronically.34 In a recent communication16 we reported the
first solution-phase characterization of a related complex (5a,
Scheme 3): also in that case the methyl borate was in the second
coordination sphere. Furthermore, the 1H-NOESY experiments
showed that the halves of the ligand were in slow chemical
exchange and the anion showed the strongest contacts with the
NMe group of one ligand half and the NCH2Ar group of the other
ligand half. This is incompatible with the fac-fac geometry and is
in agreement with the mer-mer geometry. DFT calculations
confirmed that the latter is the most stable geometry.16
Figure 2. 19F,1H-HOESY NMR spectra for 4b (left) and 6b (right)
(C6D6, 296 K).
Table 1. Relevant 1H and 19F NMR Data (ppm) for Ion Pairs
ZrꢀCH
BꢀCH
|Δδ(m,p-F)|
3a
3c
4b
6b
4c
0.61
0.62
2.68
2.68
2.66
1.47
2.74
4.03
2.99
2.87
4.01
For ion pair 4b, in contrast to 5a, exchange between the halves
of the ligand is fast on the relaxation time scale, though slow on
the chemical shift NMR time scale. As a consequence, the left and
right halves of the ligand give rise to separate NMR signals, but in
the 19F,1H-HOESY NMR spectrum the NOE contacts between
the anion and the ligand halves have the same intensities.
However, the pattern of NOE cross-peaks for 4b is similar to
that reported for 5a,16 with strong contacts between the m-F of
the anion and protons of the N-methyl, the methoxy group, and
one of the aromatic protons (1, 2, and 6, Figure 2). This fact and
DFT calculations (see Computational Studies) confirm that also
4b adopts the mer-mer geometry in solution. The absence of NOE
contacts between the benzyl group of the anion and any proton of
the cation indicates that the anion orients the fluorꢀin3a1tae,3d5,r3i6ngs
3.59
3.52
Since olefin complexes of cations 3þ and 4þ are obviously
unstable, we decided to use the THF complex of 4b as a model
for an olefin complex. Compound 4b was reacted with an excess
of dry tetrahydrofuran to generate the ion pair 6b (Figure 2,
right). The coordination of one molecule of THF is clear from
the ꢀOCH2ꢀ 13C chemical shift value (δC ∼76.5 ppm), which
is about 9 ppm higher than that of free THF in benzene,30 and
from integration of the respective 1H NMR resonances.
Table 1 shows some relevant 1H and 19F NMR data for the ion
pairs, indicating that in all cases the direct product of the acti-
vation is an OSIP, even in presence of the coordinating MeB-
(C6F5)3ꢀ anion. In fact, the 1H NMR resonance of the methyl
bound to the boron falls at a rather high frequency value (δH 1.47
ppm) for 3a, suggesting that it does not interact with the
zirconium.31,32 As a confirmation, the difference in the 19F
chemical shift values between m-F and p-F resonances of the
anion (|Δδ(m,p-F)|) is 2.74 ppm, which is smaller than the
literature value for having the anion in the first coordination
sphere (usually 4ꢀ.0ꢀ4.5 ppm).30,31 Complexes 4b and 6b, with
the BnB(C6F5)3 anion, which is generally considered less
coordinating than MeB(C6F5)3ꢀ, show essentially the same
|Δδ(m,p-F)| value (Table 1). In this case, it can be also noted
that the interionic NOEs in the 19F,1H-HOESY NMR spectrum
are higher for m-F/H than for o-F/H (Figure 2), strengthening
the hypothesis that the anion is in the second coordination
sphere. Since the B(C6F5)4ꢀ anion is even less coordinating than
toward the cation, as was observed for MeB(C6F5)3
.
For 6b the situation is different. The left and right halves of the
ligand are inequivalent and do not interconvert even on the relax-
ation time scale. The anion interacts not only with the protons
mentioned above but also with the protons of the coordinated
THF andthetert-butyl andbenzylgroups (a, b, 3, and 9, Figure 2).
This is only possible if the anion moves around, having different
contact orientations of very similar stability. It seems that the
anion has just a slight preference for staying close to the ethylenic
bridge of the ligand (the normalized cross-peak37 with the tert-
butyl group is roughly half of that with the N-Me group).
Unfortunately, many resonances overlap, making the exact quan-
tification of the signal difficult. The analysis of the intramolecular
NOE contacts does not yield any additional information about
the ligand arrangement around the metal. The fact that there is no
clearly preferred relative anionꢀcation orientation is probably
due to the absence of any coordination vacancy in 6b. Because of
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Figure 4. Eyring plots for the exchange processes of 3a (black squares,
9), 4b (red circles, b), and 4c (blue triangles, 2). Solid lines represent
the best linear fits.
1
Figure 3. C2xꢀC2x0 exchange process (top) and section of the H-
EXSY spectrum of 4b (bottom) (benzene-d6, T = 293 K, mixing time
100 ms).
Table 2. Rate Constants of the Exchange Process Measured
through 1H-EXSY NMR Data (Benzene-d6, ε20 °C = 2.28)
entry
T (K)
ki (sꢀ1
)
Compound 3a
295.9
1
0.51
1.3
2
306.0
3
313.6
3.9
4
320.6
7.7
5a
6b
295.9
0.47
0.53
295.9
Compound 4b
293.9
Figure 5. Plot of ln(I/I0) versus G2 for 3a in benzene-d6 (c = 1 mM).
7
6.8
24.4
30.3
66.8
94.8
8
307.2
this, it is not easy to unambiguously determine the conformation
of the complex as was done for 5a.16
9
313.6
1H-EXSY Studies. All resonances in the 1H NMR spectra of 3a
and 4b,c (but not 6b) are broadened due to a process that
exchanges the analogous protons of the halves of the ligand
(Figure 3).16 A series of 1H-EXSY NMR spectra were recorded at
different temperatures in order to determine the rate constant of
the process (ki) as a function of temperature and to extract the
activation parameters for the C2xꢀC2x0 exchange process of the
different ion pairs. The results are collected in Tables 2 and 3. The
rate of the exchange process is 0.51 sꢀ1 for 3a at 296 K (entry 1,
Table 2), about half of that found previously for 5a (1.2 sꢀ1).16
Both the addition of an excess of B(C6F5)3 (entry 5, Table 2) and
the use of a more polar solvent (C6D5Cl, entry 6, Table 2) have
very little effect on ki.
10
11
320.6
328.3
Compound 4c
282.9
12
13
14
2.1
5.6
296.0
309.8
23.4
a In the presence of an excess of B(C6F5)3. b In C6D5Cl (ε20 °C = 5.6).
Table 3. Activation Parameters (from Eyring Plots) for the
C2xꢀC2x0 Exchange Processa
ΔHiq
ΔSiq
r2
The activation parameters of the exchange process (Table 3)
were determined from Eyring plots (Figure 4). ΔHiq is considerably
higher for 3a (21 kcal/mol) than for 4b,c (14 and 15 kcal/mol,
respectively), while ΔSiq is close to zero for all ion pairs.
3a
4b
4c
21 ( 1
14 ( 1
15 ( 1
ꢀ5 ( 2
ꢀ2 ( 2
ꢀ1 ( 3
0.986
0.958
0.941
a ΔHiq is given in kcal/mol and ΔSiq in cal/(mol K); r2 is the correlation
coefficient.
PGSE NMR Studies. Pulsed-field gradient spin echo (PGSE)
NMR experiments27,38 (T = 295.6 K, concentration around
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Table 4. Free Energy Differences (ΔG, kcal/mol) of Differ-
ent Isomers
C2x
C1
C2
C2_ISIP
Gas Phase (273 K, 1 bar)
8þ
3a
4a
7a
0.0
0.0
0.0
0.0
ꢀ5.1
1.4
ꢀ9.38
18.1
13.4
12.9
6.2
4.0
Figure 6. Optimized structure of 3a_C2x (left), 3a_C1 (center), and
3a_C2 (right). All hydrogens are omitted for clarity.
0.44
Values Determined with COSMO (ε = 2.37)
or η3 coordination during the exchange process, provided
it is cis to a coordination vacancy.42 This implies that the
transition state should be closer to a fac-mer or fac-fac
geometry than to the mer-mer arrangement.
3a
4a
7a
0.0
0.0
0.0
2.0
5.2
1.1
18.0
13.9
12.8
7.1
1 mM, Figure 5) were performed for 3a in order to evaluate the
average level of aggregation in solution. Since in all ion pairs the
anion resides in the second coordination sphere, measurements
were performed only for 3a (MeB(C6F5)3ꢀ counterion). OSIPs
Computational Studies. DFT calculations were carried out
for cations 3þ and 4þ in combination with the MeB(C6F5)3
ꢀ
counterion. This counterion was explicitly included for two
reasons. (a) The difference in productivity between MAO/TBP
and borate catalysts shows that the counterionaffects the behavior
of the catalyst somewhat. Presumably, having some kind of
counterion is better than none at all. (b) All NMR studies were
done with borate counterions; therefore, the NOE results can be
directly correlated with the calculated structures. Since the
ꢀ
with various RB(C6F5)3 counterions (R = alkyl, aryl) are
known to have very similar tendencies to self-aggregate.35b
A
self-diffusion coefficient of 4.55 ꢁ 10ꢀ10 m2 sꢀ1 (calculated
using the solvent residue as an internal reference)39 was obtained
from PGSE measurements.40 Introducing this value in a modified
version of the StokesꢀEinstein equation38 leads to a hydrody-
namic volume of 1134 ( 100 Å3 that is equal, within experi-
mental error, to the volume of the ion pair in benzene, as
evaluated from the DFT optimized geometry (1126 Å3).
Mechanism of the Exchange Process. Some conclusions can
be drawn from the NMR studies about the mechanism of the
exchange process:
ꢀ
counterions MeB(C6F5)3ꢀ, BnB(C6F5)3ꢀ, and B(C6F5)4 be-
have so similarly both in NMR and in catalysis, we chose MeB-
(C6F5)3ꢀ throughout for simplicity and computational efficiency.
Species 7þ, having an isobutyl group at Zr, was chosen to simulate
the catalyst during polymerization.43
The energies of all the isomers depicted in Scheme 1 were
calculated in order to evaluate their relative stability in solution and
possibly locate the transition state of the C2xꢀC2x0 exchange
process. In the presence of a coordinated dimethyl ether (8þ), C2
is the most stable isomer, but if a coordination vacancy is present, C2x
is the preferred species (Table 4),16 even relative to the C2-ISIP
(ISIP = inner sphere ion pair), in good agreement with the
experimental findings. Also, the dynamic behavior of these complexes
is supported, the other isomers being not much higher in energy.
Considering the experimental results reported above, it can be
assumed that the C2xꢀC2x0 exchange mechanism involves only
movements of the O arms and the alkyl group. The anion, being
already in the second coordination sphere, can easily “follow” the
modification of the cation without playing an active role, as
suggested by EXSY NMR data. 3a_C1 (Figure 6) can be considered
as an intermediate species of the process. In fact, the Gibbs free
energy of 3a_C1 is slightly higher than that of 3a_C2x (Table 4),
but its geometry is very different: it can be seen as a distorted-
trigonal-bipyramidal complex (fac-mer), with an O arm and one of
the nitrogen atoms occupying the apical positions (Figure 6).
3a_C1 can be easily formed from 3a_C2x by just moving at
the same time the methyl and one of the O arms from their initial
positions in 3a_C2x (axial and equatorial, respectively) to those
shown in 3a_C1 (equatorial and axial, respectively), through
“Berry pseudo-rotation” like movements44 (Scheme 4). At the
same time the anion shifts toward the least sterically hindered
side of the complex, near the ZrꢀMe group.
(i) The absence of the said process in 6b indicates that a
coordination vacancy plays a crucial role in the mechanism.
(ii) The insensitivity of ki to an excess of B(C6F5)3 and to
solvent polarity and the absence of any exchange peak
between the methyl groups bound to zirconium and to
boron rule out a methyl group transfer mechanism (eq 1).41
½ðONNOÞZrMeꢂ½MeBðC6F5Þ3ꢂðONNOÞ / ZrMeMe
þ BðC6F5Þ3 / ½ðONNOÞZrMeꢂ½MeBðC6F5Þ ꢂ
3ð1Þ
(iii) Since the same ki values are obtained in benzene and
chlorobenzene, and the same trends of ki versus 1/T are
observed for 4b,c (Figure 4), it can be concluded that the
process does not depend on the anion. This is consistent
with the fact that all the complexes are OSIPs in solution,
as deduced from NMR studies.
(iv) ΔSiq is rather small for all compounds studied, and only
ion pairs are present in solution, as evidenced by the
PGSE measurements, indicating that neither ion pair
association (to ion quadruples) nor ion pair dissociation
is likely to be involved.
(v) Since the anion does not play a significant role, the
difference in ΔHiq values between complexes containing
3þ and 4þ has to be attributed to the different alkyl groups
bound to the zirconium. In particular, it seems that the
transition state for the exchange process is lower in energy in
the case of the benzyl group. The most likely explanation for
such an effect is the ability of the benzyl group to adopt a η2
Successively, the other O arm moves from the equatorial to the
axial position, and the structurebecomes 3a_C2, a fac-fac trigonal-
bipyramidal complex with both O arms in apical positions
(Figure 6 and Scheme 4). From here, the methyl can move either
right or left, creating a coordination vacancy that can trigger the
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Scheme 4. Proposed Mechanism for the C2xꢀC2x0 Exchange
It is important to outline that, although the counterion does
not directly intervene in the pathway of the exchange process, its
inclusion in the calculations seems to be crucial to reproduce the
experimental activation parameters well. Particularly, the calcu-
lated energy difference of 4a_C2 and 4a_C2x considering the
naked cation is 6 kcal/mol, in strident disagreement with the
experimental ΔGiq of 15 kcal/mol. It seems that the counterion
plays a thermodynamic role in stabilizing the species involved in
the C2xꢀC2 isomerization process. This view could offer a
rationale to the increase of activity observed with MAO that
could be due to a destabilization of the C2x isomer (rather than
to an improbable stabilization of the C2 isomer).
Process
’ CONCLUSIONS
The structure and dynamics in solution of some [bis(phenoxy-
amine)ZrR]X ion pairs have been studied by means of NMR
techniques and DFT calculations, showing that the most stable
isomer has the ligand occupying all the equatorial coordination
sites (C2x, mer-mer arrangement). However, the ligand confor-
mation is dynamic in solution and it rapidly equilibrates between
C2x and the C2 species, which has the ligand surrounding the
metal in a fac-fac arrangement and is able to coordinate and
polymerize olefins. Notably, it has been demonstrated that the
anion is not involved in the isomerization process and is located
in the second coordination sphere of the metal. This explains why
essentially the same polymer productivity is obtained with all the
tested boron-based anions.47
C2-symmetric metallocene ion pairs undergo an apparently
similar dynamic process (back skip),40 but in that case the anion is
strongly involved in the mechanism, tuning its rate and activation
parameters: the apparent kinetic rate constant is usually around
0.5ꢀ30 sꢀ1 for ISIPs and around 0.0001 sꢀ1 for OSIPs.48 For the
[ONNO] catalysts studied here, even though they are OSIPs, the
rates are similar to those of metallocene ISIPs. This is due to the
fact that the two processes are in reality rather different. For a
metallocene complex, epimerization simply comprises a shift of
the alkyl group from one coordination position to the other one
(preceded by counterion dissociation for an ISIP). For [bis-
(phenoxy-amine)ZrR]þXꢀ ion pairs, on the other hand, the
process involves a drastic change in geometry of the coordinated
[ONNO] ligand.
Although only a few ONNO complexes have been studied so
far, the C2xꢀC2 isomerization process occurs in all of them, and
it seems likely this is a general phenomenon. It might be
important for other [(OYYO)MR][X] catalysts as well. Our
studies indicate that the performance of the catalysts depends
critically on the relative stability of the “active” C2 isomer relative
to the “inactive” C2x isomer. Since the C2xꢀC2 isomerization
process could represent the bottleneck of the polymerization
process, conformational studies such as those reported here
should be very helpful in arriving at an understanding of the
factors controlling the C2xꢀC2 isomerization reaction and,
consequently, the polymerization process.
exchange to 3a_C2x0 or back to 3a_C2x. Alternatively, the
vacancyꢀcan be filled, in principle at least, by the “sticky” MeB-
(C6F5)3 anion (3a_ISIP, Scheme 4). All efforts to locate
transition states for these steps failed: the potential energy surface
appears to be too flat to allow a successful TS search.
It cannot be excluded that 3a_ISIP is involved in the exchange
process pathway, since its energy is lower than that of 3a_C2.
However, since its energy is higher than that of resting state C2x,
its possible involvement in the sequence is kinetically irrelevant.
This agrees with our observation that catalyst productivity does
not depend on the nature of the borate counterion.
Although the transition state of the process has not been
calculated, it can be noted that the energy of 3a_C2 (18 kcal/mol
above 3a_C2x) is just slightly lower than the experimental ΔGiq
(21 kcal/mol). Assuming that DFT results can be directly
compared with the experimental NMR data, this leads us to
conclude that the C2 f C1 barrier is not very high. In other
words, the transition state is probably very similar to C2 in
geometry and close to it in energy.
For the benzyl cation 4þ, the energy difference of 4a_C2 and
4a_C2x was calculated to be “only” 13 kcal/mol due to the
stabilization of 4a_C2 by η3 coordination of the Zr-bound benzyl
group (see the Supporting Information).45 Consistent with this,
also the observed ΔGiq value (from NMR) is reduced relative to
the methyl analogue (to about 15 kcal/mol). Again, the C2 f C1
barrier is probably not very high.
Similarly to the benzyl group, the isobutyl chain is able to
stabilize the C2 structure, in this case by a β-agostic interaction46
(CHꢀZr distance equal to 2.21 Å): the calculated 7a_C2/
7a_C2x free energy difference is 13 kcal/mol. The energy cost
of the isomerization from the “inactive” C2x ion pair to the
“active” C2 pair and the unfavorable polymerization activation
entropy known for this class of catalysts18 seem to offer a
rationale for their low productivity in polymerization.
’ EXPERIMENTAL SECTION
All manipulations were performed in flamed Schlenk-type glassware
interfaced to a high-vacuum line (<10ꢀ5 Torr) or in an argon-filled Braun
LabMaster 130 glovebox (<1 ppm O2). Molecular sieves (MS) were
activated for 24 h at 200ꢀ230 °C under dynamic vacuum. All the solvents
and liquid reagents were freezeꢀpumpꢀthaw degassed on the high-
vacuum line, dried over the appropriate drying agent, vacuum-transferred
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Organometallics
ARTICLE
to a dry storage tube with a PTFE valve, and stored over activated MS.
Benzene-d6, toluene-d8, n-pentane, n-hexane, andtoluenewere dried over
Na/K alloy. B(C6F5)3 was obtained from Boulder Scientific Co. and was
purified by sublimation (40ꢀ60 °C, 10ꢀ5 Torr). Toluene for polymer-
ization runs (HPLC, Lab-Scan) was purified by passing it through a
mixed-bed activated Cu/A4 molecular sieves column in an MBraun SPS-
5 unit (final concentration of O2 and H2O <1 ppm). MAO (10% w/w
twice with dry toluene (2 ꢁ 5 mL). The combined filtrates were
evaporated under vacuum, affording the product as a white solid, which
was recrystallized from toluene/n-pentane and obtained in 85% final
yield (502 mg). 1H NMR (C6D6, 295 K, 400.13 MHz, J in Hz): δ 7.32
(d, 2H, 4J = 2.94, H7), 6.53 (d, 2H, 4J = 2.94, H6), 4.17 (d, 2H, 2J = 13.31,
H5), 3.67 (s, 6H, H2), 2.72 (d, 2H, 3J = 9.02, H4), 2.52 (d, 2H,
2J = 13.30, H5), 1.87 (s, 24H, H3 þ H1), 1.00 (d, 2H, 3J = 9.20, H4),
0.81 (s, 6H, ZrꢀCH3). 13C{1H} NMR (C6D6, 295 K, 100.55 MHz): δ
154.5, 152.5, 139.3, 126.0, 114.0 (C6), 113.9 (C7), 64.1 (C5), 55.7
(C2), 52.3 (C4), 45.6 (C1), 39.7 (ZrꢀCH3), 35.7 (ꢀC(CH3)3), 30.2
(C3). Anal. Calcd for C30H48N2O4Zr: C, 60.87; H, 8.17; N, 4.73.
Found: C, 61.05; H, 8.38; N, 4.55.
1
solution in toluene) was purchased from Chemtura. H NMR analysis
revealed it to contain 38% of Al as “free” AlMe3. TBP was purchased from
Aldrich. Propene (polymerization grade) was purchased from SON and
used as received.
NMR samples were prepared in oven-dried J. Young NMR tubes. 1H,
13C{1H}, 1H-COSY, 1H-NOESY/1H-EXSY, 1H-PGSE, 1H,13C-HMQC,
1H,13C-HMBC, 19F, and 19F,1H-HOESY NMR experiments were per-
formed on a Bruker Avance DRX 400 instrument equipped with a
GREAT 1/10 gradient unit and a QNP probe with a Z-gradient coil,
without spinning. Typical mixing times were 600 ms for the Overhauser
experiments and within the range 4ꢀ600 ms for EXSY experiments. 1H-
PGSE NMR data were treated as described in the literature.38
Synthesis of [N,N0-Bis(3-tert-butyl-5-methoxy-2-hydroxy-
phenylmethyl)-N,N0-dimethylethylenediamine] (LigH2). The
ligand was prepared as described in the literature, butwith minor changes.2a
A 441 mg portion of N,N0-dimethylethylenediamine (5 mmol), 749 μL of
formaldehyde solution (37 wt % in water, 10 mmol), and 1.8 g of 3-tert-
butyl-4-hydroxyanisole (10 mmol) were dissolved in 15 mL of methanol
and kept under reflux for 3 h. The precipitated product was filtered off,
washed with cold methanol, and dried in an oven, at 65 °C under vacuum
for 3 h. A second crop of product could be obtained by keeping the
methanol solution in a refrigerator for several days. Total yields were ca.
85% (2.05 g). 1H NMR(CDCl3, 295 K, 200 MHz, J in Hz):δ10.4 (br, 2H,
OH), 6.79 (d, 2H, 4JH7,H6 = 3.00, H7), 6.39 (d, 2H, H6), 3.74 (s, 6H, H2),
3.63 (s, 4H, H5), 2.60 (s, 4H, H4), 2.24 (s, 6H, H1), 1.38 (s, 18H, H3).
13C{1H} NMR (CDCl3, 295 K, 50 MHz): δ 151.6 (ꢀCOCH3), 150.5
(ꢀCOH), 137.9 (ꢀCC(CH3)3), 122.2 (ꢀCCH2N), 112.7 (C6), 111.0
(C7), 62.5 (C5), 55.6 (C2), 53.5 (C4), 41.4 (C1), 34.8 (ꢀC(CH3)3), 29.3
(C3). Anal. Calcd for C28H44N2O4: C, 71.15; H, 9.38; N, 5.93. Found: C,
71.40; H, 9.55; N, 5.82.
Synthesis of Complex 2. The complex was synthesized as described
in the literature, but with minor changes.2a A 473 mg portion of LigH2
(1 mmol) was weighed into a Schlenk flask and dissolved in dry toluene
(10 mL). The resulting solution was added to another Schlenk flask
49
containing a solution of Zr(benzyl)4 (455 mg, 1 mmol) in the same
solvent (10 mL). The mixture was kept at 65 °C for 2 h, and then the solvent
was removed under vacuum to give the product as a pale yellow powder.
Recrystallization from toluene/n-pentane afforded the product in 85% yield
(630 mg). 1H NMR (C6D6, 295 K, 400.13 MHz, J in Hz): δ 7.34 (d, 4H, 3
J = 7.21, H90),7.26 (d, 2H, 4J = 2.93, H7), 7.21 (t, 4H, 3J = 8.07, H900), 6.92
(t, 2H, 3J = 7.41, H9000), 6.42 (d, 2H, 4J = 2.93, H6), 3.75 (d, 2H, 2J = 13.41,
H5), 3.59 (s, 6H, H2), 2.99 (d, 2H, 2J = 10.48, H8), 2.66 (d, 2H, 2J = 9.19,
H4), 2.55 (d, 2H, 2J = 10.33, H8),2.37 (d, 2H, 2J = 13.78, H5), 1.86 (m,
24H, H3 and H1), 0.90 (d, 2H, 4J = 9.49, H7). 13C{1H} NMR (C6D6, 295
K, 100.55 MHz): δ 153.4, 152.7, 148.9, 139.2, 128.8, 127.5, 127.2, 126.5,
121.6, 113.8 (C6), 113.5 (C7), 69.1 (C8), 64.0 (C5), 55.3 (C2), 52.8 (C4),
45.6 (C1), 35.8 (C(CH3)3), 30.5 (C3). Anal. Calcd for C42H56N2O4Zr: C,
67.79; H, 7.59; N, 3.76. Found: C, 67.95; H, 7.78; N, 3.53.
Synthesis of Ion Pair 3a. A 10 mg portion of complex 1 (17 μmol)
and 12 mg of B(C6F5)3 (23 μmol) were dissolved in dry toluene
(0.5 mL). The solution became pale yellow, and a yellow oily phase
separated. The latter was washed with dry pentane and dried under
vacuum, affording the desired product as an oil that is poorly soluble
1
in benzene. H NMR (C6D6, 295 K, 400.13 MHz, J in Hz): δ 7.10
(d, 1H, 4J = 2.58, H7), 7.02 (d, 1H, 4J = 2.69, H7), 6.57 (d, 1H, 4J = 2.60,
4
2
H6), 6.47 (d, 1H, J = 2.34, H7), 3.80 (d, 2H, J = 14.28, H5), 3.60
(s, 3H, H2), 3.54 (s, 3H, H2), 3.07 (d, 1H, 2J = 14.74, H5), 3.02 (d, 1H,
2J = 14.65, H5), 2.78 (m, 2H, H4), 2.33 (d, 1H, 2J = 10.84, H4), 2.03
(d, 1H, 2J = 10.84, H4), 1.89 (s, 3H, H1), 1.68 (s, 3H, H1), 1.47 (br s,
3H, BꢀMe), 1.32 (s, 9H, H3), 1.30 (s, 9H, H3), 0.61 (s, 3H, ZrꢀMe).
19F NMR (C6D6, 295 K, 376.65 MHz, J in Hz): δ ꢀ132.11 (br
3
3
d, 6F, J = 20.7, o), ꢀ163.95 (t, 3F, J = 20.7, p), ꢀ166.69 (br t,
6F, 3J = 19.4, m). The solubility was too low to allow 13C NMR
measurements.
Synthesis of Ion Pair 3c. A 10 mg portion of complex 1 (17 μmol)
and 20 mg of [C(C6H5)3][B(C6F5)4] (22 μmol) were dissolved in dry
toluene (0.5 mL). The solution became bright yellow, and a yellow oily
phase separated. The latter was washed twice with dry toluene (about
0.8 mL) and dried under vacuum, affording the desired product as an oil
that is poorly soluble in benzene (<10ꢀ4 M). 1H NMR (C6D6, 295 K,
Synthesis of Complex 1. A 473 mg portion of LigH2 (1 mmol)
was dissolved in dry toluene (20 mL) and cooled to 0 °C. To this
solution was added 1.3 mL of a n-BuLi solution (1.6 M in hexane,
2 mmol) dropwise, and the reaction mixture was warmed to room
temperature. The resulting solution was added to a stirred suspension of
233 mg of ZrCl4 (1 mmol) in dry toluene (10 mL) prepared previously.
The reaction mixture was stirred at 80 °C for 2.5 h and then cooled to
room temperature. To the resulting suspension was added 2.5 mL of
MeMgBr solution (3.0 M in diethyl ether, 7.5 mmol), and the mixture
was stirred overnight at room temperature.
3
400.13 MHz, J in Hz): δ 7.47 (br, 2H, H9a0), 7.35 (t, 3H, J = 7.03,
H9a00), 7.04 (m, overlapped with solvent), 6.77 (d, 2H, 3J = 7.50, H90),
6.56 (br s, 1H, H6), 6.53 (br s, 1H, H6), 3.62 (br s, 3H, H2), 3.60 (5H,
H2 and H5), 2.97 (m, 3H, H8), 1.78 (s, 3H, H1), 1.55 (br s, 12H, H1
and H3), 1.49 (s, 9H, H3), 1.36 (m, H4), 0.62 (s, 3H, H8). 19F NMR
(C6D6, 295 K, 376.65 MHz, J in Hz): δ ꢀ132.09 (m, 6F, o), ꢀ162.42 (t,
3F, 3J = 21.1, p), ꢀ166.45 (m, 6F, m). The solubility was too low to allow
13C NMR measurements.
Synthesis of Ion Pair 4b. A 20 mg portion of complex 2 (27 μmol)
and 16 mg of B(C6F5)3 (31 μmol) were dissolved in dry toluene
(0.5 mL). The resulting solution was stirred, and the solution became
pale yellow. The addition of pentane led to the formation of a dark yellow
The dark mixture was then evaporated under vacuum. To the residue
was added fresh dry toluene (10 mL), and undissolved LiCl was
removed by filtering the suspension through Celite that was washed
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oily phase. The latter was separated, washed with dry pentane, and dried
under vacuum, affording the desired product as an oil. 1H NMR (C6D6,
295 K, 400.13 MHz, J in Hz): δ 7.37 (d, 2H, 3J = 7.29, H9a0), 7.22 (s, 1H,
H7), 7.14 (m, 3H, H9a00 and H7), 7.00 (m, 5H, H9a000, H9000 and H900),
6.83 (d, 2H, 3J = 7.09, H90), 6.64 (s, 1H, H6), 6.45 (s, 1H, H6), 3.7(s, 3H,
H2), 3.59 (m, 3H, H2 and H8a), 3.22 (d, 1H, 2J = 12.17, H5), 3.19 (d,
1H, 2J = 12.17, H5), 2.82 (d, 1H, 2J = 14.56, H5), 2.68 (d, 1H, 2J = 11.72,
H8), 2.54 (m, 2H, H4 and H5), 2.30 (d, 1H, 2J = 12.01, H4), 2.16 (d, 2H,
2J = 11.60, H4), 2.03 (s, 3H, H1), 1.56 (s, 9H, H3), 1.53 (s, 9H, H3), 1.51
(s, 3H, H1). 13C{1H} NMR (C6D6, 295 K, 100.55 MHz): δ 155.4, 151.9,
150.1, 148.1, 139.1, 137.7, 136.1, 131.1, 129.4, 129.0, 128.4, 128.1, 127.8,
126.8, 124.4, 123.6, 123.1, 116.3 (C6), 115.2 (C6), 112.9 (C7), 112.0
(C7), 72.7 (C8), 65.4 (C4), 62.2 (C5), 57.4 (C2), 55.2 (C2), 42.8 (C1),
38.2 (C1), 35.6 (2C, C(CH3)3), 30.3 (C3), 30.1 (C3). 19F NMR (C6D6,
295 K, 376.65 MHz, J in Hz): δ ꢀ130.71 (br d, 6F, 3J = 22.6, o), ꢀ163.63
(t, 3F, 3J = 21.1, m), ꢀ166.62 (br t, 6F, 3J = 21.1, p).
For the purpose of this work, each reactor module was used to
screen precatalysts 1 and 2 in combination with MAO, MAO/TBP,
[CPh3][B(C6F5)4]/TIBA, and B(C6F5)3/TIBA (TIBA = triisobutyl-
aluminum) according to the following procedure. The cells were fitted
with a preweighed glass vial insert and a disposable stirring paddle. The
reactor was closed, and then the scavenger was injected into each cell
through a valve: 150 or 100 μL of a 0.020 M solution of MAO (MAO,
Chemtura, 10% w/w solution in toluene) or TIBA (TIBA, Chemtura) in
toluene (toluene HPLC, Lab-Scan, 99.8%) and 3.25 (or 4.15) mL of
toluene. The reactor temperature was set to 60 °C, and stirring was started
at a speed of 800 rpm. The reactor was pressurized at 5.5 bar (80 psi) with
propene.
In an array of 18 ꢁ 1.2 mL glass vials positioned on a vortexer,
precatalysts and activators were premixed immediately prior to addition
into the cells (for activation with MAO and MAO/TBP, [Al]/[Zr] = 100
and [TBP]/[Al] = 0.5, for activation with [CPh3][B(C6F5)4]/TIBA,
[B]/[Zr] = 1.2, for activation with B(C6F5)3/TIBA, [B]/[Zr] = 1.0; in
the last two cases, no further TIBA was used apart from that used as
scavenger). Aliquots of the catalyst system solutions containing the
desired amount of catalyst (typically 50ꢀ100 mg corresponding to a
reaction time of 10ꢀ60 min) were then injected into the cells. The
polymerization was run at constant temperature and propene partial
pressure until the targeted propene uptake was reached, at which point
the reaction was quenched with air at 3.4 bar (50 psi) overpressure. The
reactor was opened, and the glass inserts were unloaded from the cells,
transferred to a centrifuge/vacuum drying unit (Genevac EZ-2 Plus),
and dried to constant weight, after which the polymer samples were
recovered and weighed on a Bohdan BA-100 Balance Automator unit.
The reproducibility of polymer yields on experiments in duplicate
turned out to be better than þ20%.
Synthesis of Ion Pair 4c. A 20 mg portion of 2 (27 μmol) and
28 mg of [C(C6H5)3][B(C6F5)4] (31 μmol) were dissolved in dry
toluene (0.5 mL). The resulting solution was stirred, and the solution
became red. The addition of pentane led to the formation of a dark oily
phase. The latter was separated, washed twice with dry toluene and once
with pentane, and dried under vacuum, affording the desired product as
an oil. 1H NMR (C6D6, 295 K, 400.13 MHz, J in Hz): δ 7.21 (s, 1H, H7),
7.14 (s, 1H, H7), 7.02 (t, 2H, 3J = 7.40, H900), 6.92 (t, 1H, 3J = 7.12, H90),
6.81 (d, 2H, 3J = 7.20, H900). 6.61 (s, 1H, H6), 6.42 (s, 1H, H6), 3.66 (s,
3H, H2), 3.57 (m, 3H, H2), 3.22 (d, 1H, 2J = 13.41, H5), 3.14 (d, 1H, 2
J = 13.17, H5), 2.79 (d, 1H, 2J = 14.49, H5), 2.66 (d, 1H, 2J = 11.50, H8),
2.55 (m, 2H, H4 and H5), 2.27 (d, 1H, 2J = 14.22, H4), 2.13 (m, 2H,
H4), 2.03 (s, 3H, H1), 1.59 (s, 9H, H3), 1.52 (s, 9H, H3), 1.50 (s, 3H,
H1). 19F NMR (C6D6, 295 K, 376.65 MHz, J in Hz): δ ꢀ132.13 (m,
6F, o), ꢀ162.31 (t, 3F, 3J = 21.1, p), ꢀ166.32 (m, 6F, m). The solubility
was too low to allow 13C NMR measurements.
Computational Details. Density fuctional calculations were per-
formed with the Turbomole program50 (version 5.8) in combination
with the OPTIMIZE routine of Baker and co-workers.51 All geometries
were fully optimized at the restricted RI52-BP8653 level, using the
SV(P)54 basis set (small-core pseudopotential on Zr55). For each
structure the analytical frequencies were calculated, in order to check
that no imaginary values were present. Thermal corrections (enthalpy
and entropy) were calculated for the gas phase, 273 K, 1 bar, using the
standard formulas of statistical thermodynamics.56
Single-point solvent corrections were calculated using the conductor-
like screening model (COSMO)57 with ε = 2.37 to model an apolar
solvent (e.g., toluene). All energies mentioned in the text include this
solvation correction, unless noted otherwise. Initial geometries and
reasonable starting Hessians were obtained from PM3 computations
with the Spartan package from Wavefunction Inc.58
Synthesis of Ion Pair 6b. A 15 mg portionof ion pair 4b (12 μmol)
was dissolved in dry toluene (0.5 mL) in the presence of an excess of dry
tetrahydrofuran. After stirring, the solvent was removed by vacuum. The
1
resulting solid was washed with pentane and dried under vacuum. H
NMR (C6D6, 295 K, 400.13 MHz, J in Hz): δ 7.31 (m, 4H, H90 and
3
H9a0), 7.21 (m, 2H, H7), 7.12 (t, 2H, J = 7.66, H900), 7.08 (t, 2H,
3J = 7.50, H9a00), 6.93 (m, 2H, 9000 and 9a000), 6.59 (d, 1H, 4J = 2.73, H6),
6.54 (d, 1H, 4J = 2.78, H6), 7.08 (t, 2H, 3J = 7.50, H9a00), 4.29 (d, 1H,
2J = 14.19, H5), 3.70 (s, 3H, H2), 3.67 (s, 3H, H2), 3.52 (m, 6H, H8a and
Ha), 3.46 (d, 1H, 2J = 14.19, H5), 2.89 (m, 4H, H4, H5, and H8), 2.83
(d, 1H, 2J = 14.93, H5), 2.68 (m, 1H, H8), 2.00 (s, 3H, H1), 1.97 (s, 3H,
H1), 1.60 (s, 9H, H3), 1.45 (s, 9H, H3), 1.28 (m, 5H, Hb and H4).
13C{1H} NMR (C6D6, 295 K, 100.55 MHz): δ 154.7, 154.3, 150.8,
150.6, 149.2, 139.3, 136.2, 134.5, 133.1, 130.6, 129.8, 129.3, 128.8, 128.3,
127.7, 126.5, 126.1, 125.6, 123.4, 115.7 (C6), 115.0 (C6), 113.9 (C7),
113.2 (C7), 76.5 (THF, a), 64.0 (C4), 55.4 (2C, C2), 51.9 (C5), 47.3
(2C, C1), 35.4 (C(CH3)3), 35.3 (C(CH3)3), 30.5 (C3), 30.3 (C3), 25.6
(THF, b). 19F NMR (C6D6, 295 K, 376.65 MHz, J in Hz): δ ꢀ130.62
(br d, 6F, 3J = 22.5, o), ꢀ164.02 (t, 3F, 3J = 21.0, p), ꢀ166.89 (br t, 6F,
3J = 20.6, m).
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving DFT-optimized
b
structures of all the species considered. This material is available
free of charge via the Internet at http://pubs.acs.org.
Polymerization Runs. Polymerization experiments were carried
out with a high-throughput parallel reactor setup (PPR24, available from
Freeslate, Inc.) with three reactor modules, each containing eight
reaction cells (6 mL working volume per cell). The whole system is
housed in a triple MBraun LabMaster glovebox maintaining a pure
nitrogen atmosphere (oxygen and water levels <1 ppm). The monomer
gas and quench gas lines were plumbed directly into the reactors and
controlled by automatic valves; propene was fed after purification by
passing through columns containing a mixed bed of 4A molecular sieves
(3.2 mm pellets) and an activated copper catalyst (BASF R 3-11G).
Liquid reagents were robotically added to individual cells by syringes.
Solvents were previously purified in an MBraun SPS unit.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: alceo@unipg.it (A.M.); budzelaa@cc.umanitoba.ca
(P.H.M.B.).
’ ACKNOWLEDGMENT
This work is part of the Research Programme of the Dutch
Polymer Institute, Eindhoven, The Netherlands (project no.
#633).
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