Metallocene

1
Metallocene
Organometallic coordination compounds in which
one atom of a transition metal such as iron,
ruthenium or osmium is bonded to and only to the
face of two cyclopentadienyl ?5-(C5H5) ligands
which lie in parallel planes
Ferrocene The First Metallocene
2
Homogeneous Catalysis

• Considerations
• Mechanistic considerations better understood
• using soluble systems
• 2. Catalyst requirements lower as the implicated
• amounts are totally available for activity
• 3. No possibility of catalyst deactivation as a
  result
• of polymer coating
• 4. Uniform molecular distribution as there is no
  or
• marginal change in available catalyst

3
Breslow and Newberg observed that when an orange
soln of Cp2TiCl2 in toluene is treated with two
moles of Et2AlCl2, there is an immediate colour
change to red and finally to blue. The operative
equilibrium was found to be
4
Active Species
Rearrangement of Active Species and Propagation
5
Reaction of Cp2TiCl2 with Al2Cl6 (AlCl3)
Reaction of Cp2TiCl2 with MeAlCl2
Chloride abstraction by aluminum!!!
6
Generation of Ti(III) Abrupt Colour Changes
Aluminum alkyls are reducing agents, and
therefore a reduction Ti(IV) to Ti(III)
inevitably takes place if the two components are
brought together.
7
Polymerization and Reduction?
In Cp2TiCl2, as well as in Cp2TiEtCl, the
titanium is present in an approximately
tetrahedral environment. On complex formation
with an aluminum alkyl, one of the ligands of the
Al also requires a place in the coordination
sphere of the Ti. We propose that this
requirement forces the Ti into an octahedral
environment (only tetrahedral and octahedral
complexes of Ti have so far been reported).
8
By this procedure the Ti-Et bond comes under the
trans-influence of the bridged aluminum and
presumably suffers weakening. This weakening is
responsible for the two phenomena, polymerization
and reduction. In the absence of ethylene only
the reduction reaction has to be taken into
account. The octahedral complex has one
coordination site empty. A ß-hydrogen atom of the
ethyl group of a second complex unit may occupy
this site. Subsequent transfer of this hydrogen
to the other ethyl group would lead to the
formation of ethane and ethylene, as has been
observed experimentally. As a consequence the
titanium is reduced to Ti(III).
9
Under polymerization conditions, ethylene can
coordinate to Ti at the sixth, so far empty,
coordination site.
Part of the electron density will be transferred
from the bonding orbital of the ethylene to the
metal, thus weakening the ethylene double bond
and making it susceptible to polymerization.
10
Kinetics of Ethylene Polymerization Catalyzed by
Cp2TiCl2 and Me2AlCl2
Results indicated a relationship Rp
kpCm where Rp is the rate of polymerization,
C and m are the concentrations of propagating
metal alkyl complex and monomer, respectively.
Increase in Polymer Yield with Time
11
At a fixed temperature and monomer pressure, the
polymer molecular weight depends mainly upon the
catalyst concentration.
Kinetic expression for chain termination -dC/
dt ktC2
Initiation was followed by using (C14H3)2AlCl2
and measuring the increment of C14 in high
polymers with time.
12
Variation of C14 activity in polymer samples with
time
13
Arrhenius Plots
14
Natta and Mazzanti Provided a Closer Look
TiCl4 and PhAlCl2 when mixed results in the
formation of an equilibrium mixture consisting
of (a) and (b)
15
Configuration of Active Species
16
It is difficult to distinguish whether the
polymer chain grows on the Al or Ti center. Hence
a partial ionic dissociation takes place as
indicated in the mechanism below.
17
Cationic Zr(IV) Benzyl Complexes
Structure of 4
18
1c
19
RZrMe AlMe3 ?
Characterized Intermediate
Mechanism
20
Alk-1-yne Oligomerization
21
Catalyst Construction Progress, Challenges and
Opportunities
22
Metallocene Synthesis
Cp2TiCl2/Cp2TiMe2
23
Cp2ZrCl2/Cp2Zr(CH2Ph)2
24
CpTiCl3/CpTiMe3
25
(No Transcript)
26
(No Transcript)
27
EBIH2
rac-(EBI)Zr(NMe2)2
28
(No Transcript)
29
(No Transcript)
30
H
SBIH2
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
Catalysts of Commercial Importance
36
Dow Elastomers Business
Constrained Geometry Catalyst System
37
(No Transcript)
38
Autoclave for CGC Polymerizations
39
Catalyst Structure-Polymer Microstructure
Relationship
40
Polymerization and Metallocene Symmetry
Metallocenes have earned enormous attention as a
clear corr- elation between metallocene symmetry
and polymer stereo- chemistry is unambiguously
established. In 2002 polymer literature
contained more papers on metallocenes than any
other subject. The most studied ligands are Cp
and substituted Cp, 1-indenyl (Ind),
4,5,6,7-tetrahydro-1-indenyl (H4Ind) and
9-fluorenyl (Flu)
Indenyl Tetrahydroindenyl
Fluorenyl
41
The metallocene initiators are termed single-site
catalysts as each metal center has the same
coordination environment. The resultant polymer
has narrower distributions of molecular
wt, regiochemistry and stereochemistry.
Stereoselective polymerizations with high
reaction rates occur for metallocene catalysts
that are both chiral and stereorigid. Chiral and
stereorigid metallocenes have appropriately
substd ?5-Cp ligands that are linked together by
a bridging group. These are also referred to as
ansa metallocenes. The bridging groups may be
CH2CH2, CH2, SiMe2 or CMe2. The unbridged
catalysts donot achieve highly stereoselective pol
ymerization as free rotation of the ?5-Cp ligand
results in achiral environment at the active
site. The bridge locks the symmetry of the
active sites.
42
The group 4 metallocene has two active sites (the
two R- groups on the metal). The
stereoselectivity of each of the two coordination
sites on the metal may be enantioselective
or nonselective. The relationship between the
stereoselectivities of the two active sites of a
metallocene catalyst (homotopic, enantiotop- ic,
diastereotopic) determines the type of
stereocontrol (chain end or site end). Group 4
metallocenes have the following general geometry
The angle between the ligands, ß, is called
bite angle is in the range 60-75 deg. The metal
is pseudotetrahedral and ? is in the range
115-125 deg. ? is few degrees less than 90. The
plane of the two ligands are not parallel
and hence these are called bent metallocenes.
43
C2v Symmetry
Examples unsubstituted bis Cp catalysts,
Me2Si(Flu)ZrCl2 These catalysts are achiral,
and both the coordination (active) sites are
chiral and homotopic. Atactic polymer is
formed with chain end control
44
C2 Symmetry
Examples rac-Me2SiInd2ZrCl2
Meso fraction separated by fractional
crystallization!
The two sites are equivalent (homotopic) and
enantioselective for the same monomer
enantioface. As a result, there is isoselective
polymerization.
45
The steric environment at the active site
determines which enantioface of the incoming
monomer is coordinated to the transition metal.
The chiral active site forces the
propagating polymer chain to assume an
orientation that minimizes the steric
interaction with one of the ?5-ligands and this
results in discrimination between two faces of
the monomer. There is precedence of catalyst site
control as the mode of propagation. The
structural variables on the ligand plays an
important role in determining the course of
polymerization by altering the bite angle as a
result the stereorigidity of the ligand is
altered. If the bite angle is too large,
stereorigidity is lowered and the degree of
isotacticity decreases.
46
Interrelation between Structural Parameters
1. Ti metallocenes are less active and less
stereoselective than Zr and Hf. Zr metallocenes
are most useful as these are most active in
comparison with their Ti and Hf analogues. These
have been optimized by various structural
variations to yield very high stereoselectivity
along with high molecular weight. Hf metallocenes
produce higher molecular weights but not better
stereoselectivities as compared to Zr
analogues. 2. Substituents at the 3- and 4-
positions of the Cp ring have maximum effect in
increasing activity, isoselectivity and molecular
weight. Substitu- ents at the 2- and 5- positions
have a positive but lesser effect. The
6- membered ring plays the role of 4- and 5-
substituents in Ind and H4Ind ligands.
47
H4 Ind ligands generally increase isoselectivity
with some decrease in activity. 3. The effect
of bridge between ligands depends upon the type
of the ligand. The bite angle and stereorigidity
are affected by the type of the bridge depending
upon the type of the ligand. 4. The presence of
heteroatoms into the ligands via alkoxy or
trisubstituted amino groups generally
deteriorates catalyst. 5. Bisfluorenyl
zirconocenes generally are neither highly active
nor isoselective.
48
Cs Symmetry
Popular examples include
(2)
(1)
(1) produces a highly atactic polymer even higher
than best C2 metallocene. (2) produces highly
syndiotactic polymer
49
C1 Symmetry
Some popular examples include
There are no elements of symmetry. Each site is
in a chiral environment. These exhibit a range of
stereo specificity depending on the choice of
ligand.
50
Schematic Representation of Various classes of
Polyolefins
Decreasing stereoregularity
51
Non-Metallocene Synthesis
52
ORTEP diagram of 1,8-C10H6(NSiMe3)2ZrCl2 dimer
53
(No Transcript)
54
ORTEP diagram of 1,3-C3H6(NSi(i-Pr)3)2ZrCl2
55
(No Transcript)
56
(No Transcript)
57
(No Transcript)
58
(No Transcript)
59
(No Transcript)
60
(No Transcript)
61
(No Transcript)
62
(No Transcript)
63
(No Transcript)
64
(No Transcript)
65
Status of Non-Metallocene Research
66
Example
300 MHz Spectra in CD2Cl2
67
Performance Requirement Driven Product Design
Logic