scispace - formally typeset
Search or ask a question
Journal ArticleDOI

Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts

16 Jun 1995-Angewandte Chemie (John Wiley & Sons, Ltd)-Vol. 34, Iss: 11, pp 1143-1170
TL;DR: In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous metallocene-based catalysts as discussed by the authors allow efficient control of regio-and stereoregularities, molecular weights and molecular weight distributions, and comonomer incorporation.
Abstract: Current studies on novel, metallocenebased catalysts for the polymerization of α-olefins have far-reaching implications for the development of new materials as well as for the understanding of basic reaction mechanisms responsible for the growth of a polymer chain at a catalyst center and the control of its stereoregularity. In contrast to heterogeneous Ziegler–Natta catalysts, polymerization by a homogeneous, metallocene-based catalyst occurs principally at a single type of metal center with a defined coordination environment. This makes it possible to correlate metallocene structures with polymer properties such as molecular weight, stereochemical microstructure, crystallization behavior, and mechanical properties. Homogeneous catalyst systems now afford efficient control of regio- and stereoregularities, molecular weights and molecular weight distributions, and comonomer incorporation. By providing a means for the homo- and copolymerization of cyclic olefins, the cyclopolymerization of dienes, and access even to functionalized polyolefins, these catalysts greatly expand the range and versatility of technically feasible types of polyolefin materials. For corrigendum see DOI:10.1002/anie.199513681

Summary (4 min read)

Sawhorse projection:

  • -4] the authors are witnessing the evolution of new generations of catalysts and polyolefin materials, which originate from studies on homogeneous, metallocene-based polymerization catalysts.
  • In the following, the authors will attempt to review some of these recent developmentsP -7].
  • Based on Natta's early ideas about the role of chiral surface sites in the formation of isotactic polyolefins, [6. 11] very adequate models were proposed to explain the induction of stereoregular polymer growth by the chiral environment of the catalyst centers[9b.
  • The relationship between the properties of a particular catalyst and the coordination geometry of its reaction centers, however, leaves many questions open, [8g. h) due to the notorious nonuniformity of active sites in these heterogeneous catalysts and the limited experimental access to their structural details.

2.1. Metallocene Cations in the Polymerization of Ethene

  • A useful concept for the description of alkylaluminum-activa ted metallocene catalyst systems goes back to kinetic studies in Reichert's groupPl].
  • During the growth of a polymer chain, each metal-polymer species appears to alternate between a "dormant" state and a state in which it actively grows.
  • Consecutive equilibria appear to convert aluminum and metallocene halides first into Lewis acid-base adducts equivalent to inner (or contact) ion pairs and then into dissociated (or separated) ion pairs.
  • At any rate, the limitation of homogeneous catalyst systems to the polymerization of only ethene was a crucial obstacle for progress in this field for many years.

2.2. Polymerization of Propene and Higher Olefins

  • Cationic metaliocene complexes, particularly those that arise by in situ activation of a stable zirconocene precursor,[56] yield catalysts with very high activities.
  • They are easily deactivated, however, probably by minute traces of impurities.
  • Addition of AIMe 3 or AIEt3 has been shown to stabilize these cationic metallocene catalysts [Ss, 56b] by formation of AIR3 adducts [S7d].
  • As shown by Ballard et a1. [61] and by Watson et al., [621 neutral complexes of the type Cp2MIlIR act as single-component catalysts for the oligomerization of a-olefinsJ63.
  • The results of these studies lent additional support to the concept that an analogous olefin insertion into the isoelectronic species [Cp2ZrR] + is responsible for the growth of polymer chains in zirconocene-based catalyst systems.

2.3. Kinetics and Mechanisms of at-Olefin Polymerization

  • Effects of the solvent and counterions, neglected so far in these theoretical studies, might be of more than peripheral concern.
  • It remains to be clarified which interactions induce the more useful course to polymers in the analogous condensed-phase reaction systems.

2.4. Molecular Weights and Chain Termination Reactions

  • While ethene is polymerized by MAO-activated or cationtype CP2Zr-based catalysts to give polymers with high molecular weights in the range of 1 00 000 to 1000 000,[39J propene polymers obtained at room temperature with these catalysts have rather low degrees of polymerization, with molecular weights in the range of 200~ 1000.
  • IH and 13C NMR analyses show that polypropene chains produced with zirconocene-based catalysts bear n-propyl and 2-propenyl end groupS.£91].
  • The 2-propenyl end groups could arise by transfer of a fJ-H atom from the polymer chain to the metal center (Scheme 8).
  • The Zr-H unit generated by this process can then react with propene to form a Zr-n-propyl unit, from which a new polymer chain can start to groW.[96].
  • Another conceivable chain termination mechanism is the transfer of a fJ-H atom from the Zr-bound polymer chain directly to the fJ-C atom of a coordinated olefin molecule.

f3-H transfer to metal:

  • 1-monomer concentration, also known as f3-H transfer to olefin.
  • Molecular weights of polyolefins obtained with MAO-activated zirconocene systems generally increase with the concentration of the MAO cocatalyst,£39.
  • 69b. 98] in parallel with increased rates of chain growth.
  • This might be due to dilution effects favoring the dissociated or olefin-separated ion pairs C* relative to their associated precursors C, and hence the rate of chain propagation over that of chain termination.[99].
  • As an alternative, advocate that a bimolecular reaction of the active complex with a second zirconocene species terminates growth of the polymer chain in a manner possibly related to the second-order deactivation process discussed in Section 2.3.

3. Stereoregular Polymers from Chiral Metallocene Catalysts

  • The conformationally constrained indenyl and tetrahydroindenyl ligands give these complexes chiral structures (Fig. 3 ) which could be expected to be retained even under catalysis conditions.
  • When activated with MAO in the manner described above, these ansa-metallocenes f104 ] were indeed found, in independent studies by Ewen with (en)z TiCI 2 /MAOf 105 ] and by Kaminsky and KuIper with rQc-(en)2ZrClz/MAOy06] to polymerize propene and other IX-olefins to give highly isotactic polymers.
  • These findings led to extensive exploration of the mechanisms by which these catalysts control the stereochemistry of polymer growth and the effects of different metallocene structures on the tacticities and other properties of the polymers produced.

3.2. Mechanisms of Stereochemical Control

  • These a-agostic interactions can thus be considered to be the origin of the stereoselectivity of chiral ansa-metallocene catalysts.
  • The a-agostic model would thus describe the re or si orientation of an inserting olefin as ticularly high stereoselectivity [117h,;, 120b, d] thus appear to othe authors this property to their suppression of chain-end isomerization relative to olefin insertion,.

3.3. Activities of ansa-Zirconocene Catalysts

  • Puzzling is also the dependence of catalyst activities on the monomer concentration.
  • Whether these observations indicate the participation of more than one olefin molecule in the insertion transition state[76L a notion diverging from most of the current thinking about this reaction--or whether they arise from the participation of a second olefin in another reaction step that augments the overall activity of the catalyst, such as reactivation following a 2,1-insertion, [156.

3.4. Polymer Chain Lengths and Crystallinity

  • A polydispersity of Mw/M n ~ 1, which indicates a "living" polymerization system (i.e. polymer chains permanently attached to the metal centers on which they simultaneously start to groW), [90] are typically observed for ring-opening metathe-sisI159.160] and group-transfer polymerization catalysts,116Ia.b.
  • J but so far not with either heterogeneous Ziegler-Natta catalysts or with zirconocene-based catalysts for normal IX-olefin polymerization.
  • Heterogeneously produced isotactic polypropenes normally crystallize in a regular packing of parallel helices, the classicallX-modification.
  • Their crystallinity appears to correlate again with the length of uninterrupted syndiotactic chain segments; [174) a large proportion of an amorphous phase, together with a small crystallite size, appears to cause the high transparency of this material.

conc.

  • Rei. iX-olefins are taken up more readily by isospecific, chiral ansametallocenes than by unbridged complexes.
  • They first polymerized ethene with a samarocene catalyst and used this system then to initiate the polymerization of the polar monomer by group-transfer polymerization or by ring-opening polymerization of a lactone (Scheme 17).
  • End group distributions in ethene-propene copolymers were studied by Kashiwa and co-workers by 13C NMR spectroscopy.
  • As with heterogeneous Ziegler-Natta catalysts,t176b J a distinct comonomer effect is generally observed with MAO-activated zirconocenes.
  • Under otherwise identical conditions, the rate of copolymerization of ethene with higher IX-olefins often exceeds that of the homopolymerization of ethene.

4.3. Functionalized Polyolefins

  • In contrast to heterogeneous polymerization catalysts, which afford shorter propene oligomers only in the presence of H 2 , that is, only with saturated chain ends , metallocene catalysts give easy access to propene oligomers with olefinic end groups, which can be converted to various other functional groupsI222.
  • These polypropene block and graft copolymers are efficient blend compatibilizers.
  • By enhancing the dispersion of otherwise incompatible polymers and improving their interfacial adhesion, these copolymers allow the formation of " alloys" of isotactic polypropene, for example, with styrene-acrylonitrile or polyamides[224b.
  • They are obtained by way of the short isotactic polypropene chains with unsaturated end groups formed by the ansa-metallocene catalyst Me2Si(2-Me-4-tBu-CsH2)2ZrCI2!.

5. Perspectives

  • Polymers with properties distinctly different from those produced in homogeneous solution-with unusually high molecular weights-have recently been obtained by fixing a metallocene catalyst on unpretreated supports [233 l and by synthesizing covalently linked ansa-metallocenes directly on a Si0 2 supportJ234 l.
  • These observations are probably connected with the site-isolation effects, that is, with the strict suppression of all binuclear reaction intermediates.
  • Known to arise from linking catalyst centers covalently to a solid support.
  • [235 l If methods become available for a controlled synthesis of such covalently supported metallocenes and for their structural characterization, one could imagine another round of developments toward novel metallocene-based Ziegler-Natta catalysts that are heterogeneous, like their predecessors discovered forty years ago, yet endowed with wider process and product variability and with superior environmental properties.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts
Hans
H.
Brintzinger, * David Fischer, Rolf Miilhaupt, Bernhard Rieger,
and
Robert
M.
Waymouth
Dedicated
to
all those excellent graduate students
who
are the true heroes
of
this story
Current studies on novel, metallocene-
based catalysts for the polymerization
of
ex-olefins
have far-reaching implications
for the development
of
new materials as
well
as for the understanding
of
basic
reaction mechanisms responsible for the
growth
of
a polymer chain
at
a catalyst
center and the control
of
its stereoregu-
larity. In contrast to heterogeneous
Ziegler-
Natta
catalysts, polymerization
by
a homogeneous, metallocene-based
catalyst occurs principally
at
a single
type
of
metal center with a defined coor-
dination environment. This makes it
possible to correlate metallocene struc-
tures with polymer properties such as
molecular weight, stereochemical mi-
crostructure, crystallization behavior,
and mechanical properties. Homoge-
neous catalyst systems now afford effi-
cient control
of
regio- and stereo regular-
ities, molecular weights and molecular
weight distributions, and comonomer
incorporation.
By
providing a means for
the homo- and copolymerization
of
cyclic olefins, the cyclopolymerization
of
dienes, and access even to functional-
ized polyolefins, these catalysts greatly
expand the range and versatility
of
tech-
nically feasible types
of
polyolefin mate-
rials.
Keywords: alkenes . catalysis . metal-
locenes . polymerizations
1.
Introduction
Sawhorse
projection:
Modified
Fischer
projection:
Forty years after Karl Ziegler's invention
of
transition metal
catalyzed polyinsertion and Giulio
Natta's
discovery
of
the
stereoselective polymerization
of
ex-olefins'£!
-4]
we
are witness-
ing the evolution
of
new generations
of
catalysts and polyolefin
materials, which originate from studies on homogeneous, metal-
locene-based polymerization catalysts. In the following,
we
will
attempt to review some
of
these recent developmentsP -
7]
Research on metallocene-catalyzed olefin polymerization has
derived much
of
its impetus from the desire to model the reac-
tion mechanisms
of
heterogeneous polymerization catalysts. In
the evolution
of
Ziegler-
Natta
catalysis, an empirical approach
has proven highly successful. Modern MgClz-supported cata-
lysts have tremendous activities for the production
of
polypropene and other polyolefins;
at
the same time, they are
['J Prof. Dr. H. H. Brintzinger
Fakultat
fur Chemie
der
Universitiit
78434 Konstanz (Germany)
Telefax: ]nt. code
+(7531)
883137
Dr. D. Fischer
BASF
AG.
Ahteilung
ZKP
Ludwigshafen (Germany)
Prof. Dr.
R.
Mulhaupt
Institut fur
Makromolekulare
Chemie der Universitat Freiburg (Germany)
Dr.
B.
Rieger
Institut
fUr
Anorganische Chemie der U niversitiit Tubingen (Germany)
Prof. Dr. R. M.
Waymouth
Department
of
Chemistry. Stanford University (USA)
~
CH. CH.
1--
CH
I
..
"
...
~
..
~
......
r:-
atactic
----.
I
-----
CH.
CH.
~
CH.
~
CH.
~
..
'
..
~
......
~
...
CH.
CH.
CH.
synd
iotactic
----.
I
I
-----
CH.
CH.
CH. CH.
CH
I I I
....
~
...
~
...
~
..
~
..
r:-
isotactic
----.
-----
Conformation
of
chains
in
crystalline
isotactic
polypropene:
Scheme
1.
Structure
of
isotactic. syndiotactic,
and
atactic polypropene in sawhorse
and
modified Fischer projections [107] (top),
and
conformation
of
the chains in
crystalline isotactic polypropene, determined by
Natta
and
Corradini
[2]
(bottom).
1143
Ersch. in: Angewandte Chemie International Edition in English ; 34 (1995), 11. - S. 1143-1170
http://dx.doi.org/10.1002/anie.199511431
Konstanzer Online-Publikations-System (KOPS)
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-239301

highly stereoselective, forming practically only the isotactic,
most useful stereoisomer
of
the polymer[8] (Scheme 1). Still un-
satisfactory, however,
is
our
understanding
of
the reaction
mechanisms
that
are the basis
of
this advanced catalyst technol-
ogy.
Plausible hypotheses concerning these mechanisms
have certainly been advanced, most notably Cossee's model
of
polymer chain growth by cis-insertion
of
ct-olefins into a
Ti-C
bond on the surface
of
crystalline TiCI
3
.[9.10]
Based
on
Natta's
early ideas
about
the role
of
chiral surface sites in the
formation
of
isotactic polyolefins,[6.
11]
very adequate models
were proposed to explain the induction
of
stereoregular polymer
growth by the chiral environment
of
the catalyst centers[9b.
12]
(Scheme 2).
H.-H. Brintzinger
D. Fischer R. Miilhaupt
The relationship between the properties
of
a particular cata-
lyst and the coordination geometry
of
its reaction centers, how-
ever, leaves many questions open,
[8g.
h)
due to the notorious
nonuniformity
of
active sites in these heterogeneous catalysts
and the limited experimental access to their structural details.
In
a review written in 1980, Pino and Miilhaupt summarized this
shortcoming:
[4b]
"Up
to
now, there
is
no direct
proof
for the
structures proposed; most
of
them are the result
of
speculations
or derived from indirect experimental
indications."
In
this situation, related developments in other fields
of
catal-
ysis nourished the hope that homogeneous organometallic cata-
lysts capable
of
stereoselective olefin polymerization might
eventually allow more direct observations
on
the catalyst species
B.
Rieger
R.
M. Waymouth
Hans-Herbert Brintzinger, born
in
Jena, Germany,
in
1935, studied chemistry
in
Basel, where
he
received his Ph.D.
in
1960 with
Hans Erlenmeyer and his
Habilitation
in
1964. Afier teaching chemistry at the University
of
Michigan for several years, he
moved to the University
of
Constance
in
1972, where he holds a professorship for General Chemistry and Coordination
Chemistry. His fields
of
interest are organometallic chemistry and reaction mechanisms
in
homogeneous catalysis.
David Fischer, born
1963
in
Cologne, Germany, studied chemistry at the University
of
Cologne. After earning his diploma
in
physical chemistry,
he
joined
Rolf
Miilhaupt
's
group at the University 0/ Freiburg as a graduate student.
He
received his Ph.D.
in
1992for studies
on
propene polymerization with homogeneous zirconocene catalysts. After a postdoctoral period with Robert
Waymouth at Stanford,
he
joined the plastics laboratory
of
BASF
AG
in
1993, where
he
is
still working
on
stereospecific propene
polymerization with metallocene catalysts.
Rolf
Miilhaupt
was
born
in
Waldshut-Tiengen, Germany,
in
1954. From 1973 to 1978 he studied chemistry
in
Freiburg; he
received his
Ph.D.
in
1981 with Professor Pino at the Laboratory for Technical Chemistry at the
ETH
in
Ziirich. Following
industrial research positions at the Central Research and Development Experimental Station
of
Dupont
in
Wilmington,
Delaware,from 1981
to
1985 and at Ciby-Geigy AG
in
Marly, Switzerland,from 1985 to 1989, he
was
appointed professor
of
Macromolecular Chemistry at the University
of
Freiburg
in
1989. Since 1992 he has been director
of
the Material Research
Center
in
Freiburg. His research interests include, besides transition metal catalyzed polymerization, new polymer structural
materials, material compounds, dispersions and specialty polymers.
Bernhard Rieger
was
born
in
Augsburg, Germany,
in
1959. He studied chemistry at the University
of
Munich and received his
Ph.D.
in
1988 for studies
on
enantioselective hydrogenations with rhodium-phosphane catalysts. After research appointments
at the Institutefor Polymer Science and Engineering
of
the University
of
M assachuselts at Amherst and
in
the plastics laboratory
of
BASF
AG, he received his Habilitation
in
1995
at
the University
of
Tiibingen,
It'here
he presently holds a faculty position
in
chemistry.
Robert M. Waymouth was born
in
Warner Robins, Georgia
(USA)
,
in
1960. He received bachelor's degrees
in
mathematics and
chemistry at Washington and Lee
University and his Ph.D.
from
California Institute
of
Technology
in
1987, where he worked
u'ith Professor
R.
Grubbs. Following a year's postdoctoral appointment at the
ETH
in
Switzerland with Professor Pino, he joined
thefaculty at Stanford
in
1988, where he
is
now
an
Associate Professor
of
Chemistry. His research interests are
in
stereoselective
catalysis.
1144

Scheme
2.
Model for the stereospecific polymerization
of
propene
at
a chiral Ti
center on the edge
of
a TiCl, crystal. as proposed
by
Corradini and co-workers
[12
bJ.
The growing polymer chain occupies the open sector
of
the chiral coordina-
tion sphere; the olefin adopts that enantiofacial orientation which places the olefin
substituent
lrans to the polymer chain
at
the incipient
C-C
bond (left); a cis orien-
tation
of
olefin substituent and polymer chain
is
disfavored (right).
involved and, hence,
on
the mechanisms
of
polymer growth
and
its stereochemical control,l13] This sentiment was expressed, for
example, by Boor in the concluding
chapter
of
his
book
on
Ziegler-
Natta
catalysts.[3c]
Particularly attractive as model systems
of
this kind ap-
peared alkylaluminum-activated metallocene complexes
of
the
Group
4 transition metals, which have been known since 1955
to catalyze the polymerization
of
ethene[14.
IS]
and
have since
been the topic
of
many
studies. The simple coordination geome-
try
of
Group
4 metallocene
complexes-only
two reactive,
cis-
positioned ligand
sites-promised
opportunities for identifying
and
characterizing essential intermediates
of
homogeneous
polymerization catalysts
and
providing
more
direct experimen-
tal verification for some
of
the hypotheses in the field
of
Ziegler-
Natta
catalysis.
2. Activation
of
Metallocene Halides
for
IX-Olefin
Polymerization
In
1957, patents
and
publications
by
Breslow and Newburg(14]
at
the Hercules Research
Center
and
by
Natta,
Pino,
and
their
co-workers
[15]
reported
that
homogeneous reaction mixtures
of
dicyclopentadienyltitanium dichoride
(CPz
TiCl
z
)
and
diethyl-
aluminum chloride (EtzAlCI) catalyze the formation
of
poly-
ethene under conditions similar to those used with hetero-
geneous Ziegler catalysts.
Numerous
subsequent studies were
aimed
at
the identification
of
reaction intermediates
and
reac-
tion mechanisms
of
this homogeneous catalysis. The results
obtained have been summarized previously;
(4"
b.
16]
some are
particularly noteworthy in the context
of
the following discus-
sion.
Formation
of
(alkyl)titanocene complexes Cpz
TiRCl
(R = Me
or
Et) by ligand exchange with the alkylaluminum
cocatalyst, polarization
of
the Cpz Ti
-CI
bond
by Lewis-acidic
aluminum centers in an
adduct
of
the type
CpzTi(R)-
CI
. AIRCl
z
,
and
insertion
of
the olefin into the
CpzTi-R
bond
of
this
(or
some closely related) electron-deficient species,
had
been ded uced by 1960 from spectroscopic, kinetic,
and
isotope
labeling studies
done
at
the Hercules laboratories by Breslow,
Newburg,
and
Long
and
by Chien
[17,181
(Scheme 3). These ear-
Scheme
3.
Polymerization
of
ethene by cis-insertion into the
Ti-C
bond
of
an
alkylaluminum halide activated titanocene complex, as proposed by Breslow, New-
burg, and Long [17].
Iy
metallocene studies contributed to the ideas
put
forth by
Cossee[9a
l
with regard to the mechanisms
of
heterogeneous
Ziegler-
Natta
catalysis.
2.1. Metallocene Cations
in
the Polymerization
of
Ethene
An
interesting question remained unresolved by this early
research: does olefin insertion occur in a bimetallic species, in
which
an
alkyl
group
or
a halogen bridges the titanium
and
aluminum centers, as advocated by
Natta
and
his group,(19] by
Patat
and
Sinn,[ZO]
by Reichert
and
co-workers,[ZI]
and
by Hen-
rici-Olive
and
Olive?[Z2]
Or
does it require the formation
of
a
truly ionic species
[CpzTiR]+ by abstraction
of
a halide anion
and
its
incorporation
into an anion R
x
CI
4
_
x
A[
-,
as proposed by
Shilov, Dyachkovskii,
and
Shilova?[23]
Crystal structures
of
complexes
that
were occasionally isolat-
ed from reaction mixtures containing
Cpz TiCl
z
and
an
alkylalu-
minum chloride[24,
25]
were
not
conclusive in this regard, as they
represented degradation products, which require either reoxida-
tion[17]
or
renewed alkyl exchange with alkylaluminum cocata-
lysts[4a] for reactivation.
In
1986, however,
Jordan
and
co-work-
ers isolated the tetra phenyl borate salts
of
cations such as
[CP
Z
ZrCH
3
'THFj+
and
[CpzZrCHzPh'
THFt
and
demon-
strated their capability to polymerize ethene without addition
of
any activator.[26] These
and
related findings in the groups
of
Bochmann,[Z7] Teuben,[28]
and
Taube[29] and by Hlatky
and
Turner
at
Exxon
[30]
lent general credence to the proposal
that
(alkyl)metallocene cations are crucial intermediates in homoge-
neous polymerization catalysis.
A useful concept for the description
of
alkylaluminum-acti-
va ted metallocene catalyst systems goes back to kinetic studies
in Reichert's
groupPl]
During
the growth
of
a polymer chain,
each
metal-polymer
species appears to alternate between a
"dormant"
state
and
a state in which it actively grows. This
"intermittent-growth"
model was further elaborated by
Fink
[31]
and
by Eisch
[3Z]
and
their co-workers in extensive ki-
netic
and
reactivity studies. Consecutive equilibria
appear
to
convert alkylaluminum
and
(alkyl)metallocene halides first into
Lewis
acid-base
adducts
equivalent
to
inner (or contact) ion
pairs
and
then into dissociated (or separated) ion pairs.
In
these
highly dynamic equilibria, only the cation
of
a separated ion
pair
appears
to be capable
of
interacting with an olefin substrate
and, hence,
of
contributing to polymer
growthP3]
Contact
ion
pairs, which
appear
to
dominate
in these equilibria,
can
then be
termed
"dormant"
in this regard(34.
351
(Scheme
4).
This model would explain the inability
of
metallocenes acti-
vated by alkylaluminum halides to catalyze the polymerization
of
propene
and
higher oiefins[4a,
361
by the insufficient capability
of
the more weakly coordinating, substituted
IX-olefins
to form
1145

>=::
>=::
>=::
C*-P
n
-
1
C'-P
n
C'-P
n
+
1
AI,1l
AI,.
AI,1l
AI
"
AI,
tAl,.
C-P
n
-
1
C-P
n
C-Pn+l
Scheme
4.
"Intermittent-growth" model involving equilibria between polymer-
bearing,
but inactive primary complexes
(C
- p"l and active catalyst species
(C*
- p"l,
generated
by
excess alkylaluminum halide, as proposed by Fink and co-workers
[311.
AI, = (AIEtCl
2
1
2
,
Al~
= unknown,
p"
= polymer chain withn monomer units;
C-P"
here Cp,TiP"C!'"
AICI
2
Et.
reactive, olefin-separated ion pairs by displacement
of
an
alumi-
nate
anion
from the metal center.
At
any rate, the limitation
of
homogeneous catalyst systems to the polymerization
of
only
ethene was a crucial obstacle for progress in this field for
many
years.
Fortunately,
this impediment was overcome by a series
of
serendipitous observations,I37-39] which led,
around
1980, to
the discovery by Kaminsky, Sinn,
and
co-workers
that
metallocenes
are
activated for the catalytic polymerization
of
propene
and
higher olefins by methyl aluminoxanes,I4a,
39
1
2.2. Polymerization
of
Propene
and
Higher Olefins
Water, which
had
long been considered to be a
"poison"
for
Ziegler-
Natta
catalysts, was first
reported
by Reichert
and
Meyer to cause a surprising increase in the rate
of
ethene poly-
merization by the catalyst system Cpz TiEtCI/AlEtCI
2
,I37J
Sub-
sequent studies by Long
and
Breslow
on
the effects
of
water in
the otherwise inactive system Cpz TiClz/MezAIClled
to
the no-
tion
that
formation
of
a dime ric aluminoxane,
CIMeAI-O
- AI-
CIMe, by partial hydrolysis
of
MezAICI might generate an ex-
ceptionally strong Lewis acid and, hence, a
potent
activator for
CP2
TiMeCI
toward
ethene polymerization.
I381
While studying halogen-free systems such as
CpzZrMe
z
/
AIMe
3
, Sinn
and
Kaminsky noticed
that
addition
of
water im-
parts
to this otherwise inactive reaction system a surprisingly
high activity for ethene polymerization which was, furthermore,
unprecedentedly
constant
over extended reaction times.[39a,
b]
Sinn
and
Kaminsky observed
that
an interaction between
Cp2ZrMeZ
and
AIMe
3
occurred only when water
had
been
added. The suspected formation
of
methyl aluminoxane (MAO)
by partial hydrolysis
of
AIMe
3
was subsequently
supported
by
its direct synthesis
and
characterization as a mixture
of
oligomers
of
approximate composition (MeAIO) •. Activation
of
Cp2ZrMeZ
and
CpzZrCJ
z
with preformed
MAO
did indeed
yield exceedingly active catalysts for the polymerization
of
ethene,[39b] Similar activities were obtained with MAO-activat-
ed
CP2
TiCI
2
; however,
at
temperatures above 0 °C this catalyst
system is rapidly deactivated,
most
likely by reduction to the
TiIll
stage.I
40
]
Sinn, Kaminsky,
and
co-workers noticed furthermore
that
MAO-activated homogeneous metallocene catalysts
were-in
contrast
to previously studied metallocene catalysts activated by
aluminum
halides--capable
of
polymerizing propene
and
higher 0Iefins.I
4a
,39c-
gJ
Although the achiral metallocene
catalysts were still lacking the stereo selectivity
of
heterogeneous
Ziegler-
Natta
systems, aluminoxane-activated metallocene
catalysts
now
came
to be
most
promising
as
model sys-
tems.
1146
While oligomeric alkyl aluminoxanes have been known for
more
than
30
years, for example as initiators for the polymeriza-
tion
of
oxiranes,I41
1
their exact composition
and
structure are
still
not
entirely clear.
When
the hydrolysis
of
AIMe
3
, which
is
highly exothermic (and indeed potentially dangerous[39
j
,42b
J
),
is
conducted
under
controlled conditions, it appears to generate
mostly oligomers
MezAI-[O-AIMe]n-OAIMez with n
~
5-
20.[39j]
Investigations in quite a
number
of
research groups by
cryoscopy,
UV, vibrational
and
NMR
spectroscopy, chro-
matography,
and
other
means
I38
,
39j,
42
-
50cJ
yield the following
picture for aluminoxane solutions. Residual AIMe
3
in
MAO
solutions[42b,
43]
seems to participate in equilibria
that
intercon-
vert different
MAO
oligomers[42b.
43
-461
and
possibly also
cyclic
and
branched oligomers.[3ge-
j
,46]
Cross-linking by
methyl-free oxoaluminum centers has been proposed to gener-
ate a microphase with
an
AlxOy
core.[
47
1 Aluminoxane clusters
[RAI(,u
3
-O])n' with R =
tert-butyl
and n =
4,
6,
or
9,
have been
isolated
and
structurally characterized by
Barron
and his
group.I
48aJ
Complexes with four-coordinate
AI
centers seem to
predominate in
MAO
solutions[48a.49J
and
might
contain
in-
tramolecular
AlzO ---AI
or
AI-CH3
---AI bridges.[4zb
J
The pres-
ence
of
three-coordinate
AI
centers in
MAO
solutions has been
deduced by Siedle
and
co-workers from
27
Al
NMR
data.[50b.c
J
While species
of
exceptional Lewis acidity are certainly present
in
MAO
solutions, their exact composition and structure
is
still
not
adequately understood.
I511
When toluene solutions
of
CpzZrCl
z
are treated with MAO,
a fast, initial ligand exchange reaction generates primarily the
monomethyl complex CP2ZrMeCl;[39i,44bJ excess
MAO
leads
to
CpzZrMe
z
.I
39
;j
These systems become catalytically active
when the concentration
of
excess
MAO
is
raised to AI:
Zr
ratios
of
about
200: 1
or
higher.
I39J
The ways in which excess
MAO
induces this activity have been investigated largely by spectro-
scopic methods.[39i,
50-
52.
53aJ
It
is
generally assumed
that
some
of
the
AI
centers in
MAO
have
an
exceptionally high propensity
to
abstract
a CH;- ion from
CpzZrMe
z
and
to sequester it in a
weakly coordinating ion
CH
3
-MAO-.
A fast, reversible trans-
ferof
13CH3
groups from
CpzZrMe
z
to the
AI
centers
ofa
MAO
activator was observed by Siedle et
aJ.l50b,
oj
Barron
and
co-
workers[48bJ obtained
NMR
spectroscopic evidence
that
CpzZrMe
z
and
alumoxane clusters like (u3-0)6AI6tBu6 form
complexes
of
the type [Cp2ZrMe +
...
(f.l3-0)6AI6(tBu)6Me
-]
in
[Dsltoluene solution, which polymerize ethene. The tendency
of
four-coordinate Al centers in these aluminoxane clusters to ab-
stract a methyl anion
is
ascribed by these
authors
to the relief
of
ring strain
upon
formation
of
the methyl complex.
91Zr
and
13C
NMR
spectra
of
CpzZrMez/MAO
solu-
tions[50b,cJ
and
solid-state
XPSI52J
and
13C
NMRI53a
J
studies
indicate formation
of
a cation [Cp2ZrR]+, which
is
most likely
stabilized by coordinative contact with its
CH
3
-
MAO - counter-
ion, for example
through
bonding like
that
in AlzO
---
Zr
or
AI-CH3
---
Zr. These contacts
appear
to
give way, in the pres-
ence even
of
substituted olefins, to olefin-separated ion pairs
[CpzZrR( olefin)] +
CH
3
-
MAO
- , the presumed prerequisite for
olefin insertion into the
Zr-
R bond. This
hypothesis-that
the
unusually low coordinating capability
of
the anion A - in the ion
pair
[CpzZrMe]+
A-is
crucial for catalytic activity [50e]_led
to
the discovery
of
a series
of
highly active cationic metallocene

catalysts for the polymerization
of
propene
and
higher
a-olefins.
Even for large, weakly
coordinating
anions such as
(C6Hs)4B-
and
C2B9H~2
fairly
strong
interactions have been
observed with cationic (alkyl)zirconocene species.[30.
53h.
583]
Reaction systems containing [Cp2ZrMe] + together with
(C6Hs)4B-,
C2B9H~2'
or
other
carborane
anions
thus
polymer-
ize
propene
only
at
low rates,
if
at
all.[26f. g.
j.
27
-
30]
A
breakthrough
in this regard was the
introduction
of
perflu-
orinated
tetraphenylborate
as a
counterion
by
Hlatky
and
Turn-
er[54]
and
by
Marks
and
co-workers.[53b]
An
ion
pair
[Cp;ZrMet
(C6Fs)4B - (where
Cpx
is
some substituted
Cp
or
indenyl ligand)
is
formed by reaction
of
Cp~ZrMe2
with
dimethylanilinium tetrakis(perfluorophenyl)borate
or
by ab-
straction
of
CH;
from a (dimethyl)zirconocene complex by
trityl tetrakis(perfluorophenyl)borate[55.56] (Scheme
5). These
Cpx2ZrMe2 + [NHMe2Ph Y
[B(
CsF
s).r
j - CH,.
NMe2Ph
1 - Ph,CMe
Scheme
5.
Alternative ways
10
generate a
propene-polymerizing (alkyl)zirconocene
cation (associated with the weakly
coordi-
nating
B(C,F,);
anion) as reported
by
Hlatky. Upton. and Turner [54J. by
Marks
and co-workers
[53
bJ.
by
Ewen and Elder
[55]. and by Chien and co-workers [56].
Cp'
represents a variety
of
substituted and/Of
bridged cyclopentadienyl and indenyl
lig-
ands (see Section 3.1).
were
the
first weli-de-
fined zirconocene cata-
lysts capable
of
polymer-
izing
propene
and
higher
olefins
at
high rates with-
out
addition
of
a further
activator. Similar activi-
ties for
propene
polymer-
ization were subsequent-
ly
observed also with
other
base-free
or
weakly
stabilized[53.
57.
58]
(al-
kyl)metallocene cations.
Cations
obtained
by ab-
straction
of
CH;
from
a (dimethyl)zirconocene
complex by the power-
ful Lewis acid
B(C6FS)3
were likewise found
to
be highly active catalysts for a-olefin
polymerization.
[59.601
Crystal structures
obtained
by
Marks
and
co-workers,[60] for
example
that
of
[(Me2CsH3)2ZrCH:
...
H3C-
B(C6FS);J, re-
veal residual coordinative
contacts
between the cationic
Zr
cen-
ter
and
its
counterion
(Fig. 1). This
contact
appears
to resemble
those yet unidentified interactions
that
stabilize
an
(alkyl)-
metallocene
cation
in
contact
with a
H3C-MAO-
counterion.
Fig.
1.
Crystal structure
of
Ihe zirconocene catalyst
[(Me,C
,H
,),ZrCH~
...
H,C-
B(C,F,).~J.
as determined by Marks and co-workers [60]. The bridging
Zr···
CH,B
bond
is
substantially longer (255 pm) than the terminal
Zr-CH3
bond (225 pm).
In
both
cases, a fast methyl exchange occurs between the cation-
ic
and
anionic complex moieties;[50.60]
most
importantly,
both
types
of
contacts
appear
to
be weak
enough
to allow an a-olefin
to displace the
anion
from its
coordination
site
at
the
Zr
center.
Cationic metaliocene complexes, particularly those
that
arise
by in situ activation
of
a stable zirconocene precursor,[56] yield
catalysts with very high activities. They are easily deactivated,
however,
probably
by
minute
traces
of
impurities. Addition
of
AIMe
3
or
AIEt3 has been shown
to
stabilize these cationic
metallocene
catalysts[Ss,
56b]
by formation
of
AIR3 adducts[S7d]
Of
even greater simplicity,
at
least conceptionally, are cata-
lysts based
on
an
(alkyl)metallocene complex containing a Sc
IlI
,
ym,
or
a trivalent
lanthanide
center. As shown by Ballard
et
a1.[61]
and
by Watson
et
al.,[621
neutral complexes
of
the type
Cp2MIlIR act as single-component catalysts for the oligomeriza-
tion
of
a-olefinsJ63.
64]
While generally
more
difficult
to
prepare
and
to
handle
than
the
Group
4 metallocene catalyst systems
described above, catalysts such as (CsMes)2ScR
(R
= Me, Et)
provided Bercaw
and
his co-workers detailed information on
the
rates
and
mechanisms
of
individual olefin insertion steps.[63]
The
results
of
these studies lent additional
support
to the con-
cept
that
an
analogous
olefin insertion into the isoelectronic
species [Cp2ZrR]
+
is
responsible for the growth
of
polymer
chains in zirconocene-based catalyst systems.
2.3. Kinetics
and
Mechanisms
of
at-Olefin Polymerization
Because
of
many
practical advantages, activation
of
zir-
conocene dichloride derivatives by methyl aluminoxane still ap-
pears
to
be the generally favored
route
to
homogeneous poly-
merization catalysts. Substantial efforts have been made,
therefore,
to
identify intermediates arising in the resulting, rela-
tively complex reaction systems
and
to clarify the kinetics
of
the
polymer
chain
growth
they induce.
At
propene
pressures
of
1-2
bar
and
ambient temperatures
these reaction systems
produce
roughly 100
-1
000 kg
polypropene
per
hour
and
mol
of
CP2ZrClz' This corresponds
to
about
2000
-1
0 000 olefin insertions per
hour
at
each
Zr
cen-
ter. T
is
equivalent
to
the
production
of
500-
5000 polymer
chains with average molecular weights on the
order
of
M n =
200-
2000. Ethene
is
polymerized by these catalyst sys-
tems with still higher
turnover
numbers
of
10-100
insertions
per
second, which
approach
those
of
C-C
bond
forming en-
zymes.
[39b.
c.
g]
Attempts
to
identify the species involved in the rate-determin-
ing step
of
the
polymerization process by kinetic methods (i.e.
by determining
the
rate
of
polymer formation as a function
of
the
concentrations
of
zirconocene, MAO,
and
olefin reagents)
have been hampered
by
the complex time-dependence
of
the
catalytic reaction. Relatively high initial values, reached shortly
after the
components
are
mixed, decrease to much lower steady-
state
values.[6S-69] This decrease occurs within minutes
at
tem-
peratures
of
40-60
DC;
at
lower temperatures, it can take
hours
until the steady-state rate
is
reached.
In kinetic studies, Fischer
and
M iilhaupt
[69a.
bI describe the
steady-state activity
of
a CP
z
ZrCI
2
/MAO
catalyst system as a
sequence
of
reversible
and
irreversible processes. A reactive spe-
cies C*
appears
to
be generated in a fast equilibrium reaction
1147

Citations
More filters
Journal ArticleDOI
TL;DR: The graph below shows the progression of monoanionic and non-monoanionic ligands through the history of synthesis, as well as some of the properties that have been identified since the discovery of R-Diimine.
Abstract: B. Anionic Ligands 302 IX. Group 9 Catalysts 302 X. Group 10 Catalysts 303 A. Neutral Ligands 303 1. R-Diimine and Related Ligands 303 2. Other Neutral Nitrogen-Based Ligands 304 3. Chelating Phosphorus-Based Ligands 304 B. Monoanionic Ligands 305 1. [PO] Chelates 305 2. [NO] Chelates 306 3. Other Monoanionic Ligands 306 4. Carbon-Based Ligands 306 XI. Group 11 Catalysts 307 XII. Group 12 Catalysts 307 XIII. Group 13 Catalysts 307 XIV. Summary and Outlook 308 XV. Glossary 308 XVI. References 308

2,369 citations

Journal ArticleDOI
TL;DR: Hydroamination of Alkenes and Alkynes under Microwave Irradiation and Nitromercuration Reactions 3878 9.8.4.5.
Abstract: 8.4.5. Nitromercuration Reactions 3878 9. Hydroamination of Alkenes and Alkynes under Microwave Irradiation 3878 * To whom correspondence should be addressed. Phone: +49 241 8

1,685 citations

References
More filters
Book
01 Jan 1995
TL;DR: In this paper, Free-Radical Chain-Growth Polymerization (FRCG) and Ionic chain-growth polymers (Ionic chain growth polymers) are discussed.
Abstract: Physical Properties and Physical Chemistry of Polymers.- Free-Radical Chain-Growth Polymerization.- Ionic Chain-Growth Polymerization.- Ring-Opening Polymerizations.- Common Chain-Growth Polymers.- Step-Growth Polymerization and Step-Growth Polymers.- Naturally Occurring Polymers.- Reactivity and Chemical Modifications of Polymers.- Polymeric Materials for Special Applications.

2,239 citations

01 Jan 1990

1,232 citations