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Transcript 
Propylene polymerization cycle on metallocene catalyst
Metallocenes-Factors responsible for
Stereo regulation
Orientation/insertion of monomer- 1,2 or 2,1
Configuration of the active site/structure-Site control
Ligand- monomer interactions
Substituents on ligands
Symmetry of the catalyst
Orientation of chain end group- Chain end control
Intramolecular interactions
Agostic interactions
Chain back-skip
Metallocenes with specific structure
Four possible modes of
Propylene co-ordination
Propylene- Modes of insertion
Primary 1,2 Insertion.
Carbon w/o un-substituted
alkene carbon gets attached
Secondary 2,1 insertion
Mode of insertion of propyleneFormation of transition state
Site specific stereo regulation – Interaction with ligands
a
b
Orientation of in-coming monomer- b is preferred over a
due non-bonding interactions between monomer & ligand
Effect of substituents in ligands on stereo specificity
Interplay of electronic & geometric factors
Model for chain end control- Pathways
Chain-end controlled Syndiotactic polymer formation
Chain-end control of monomer insertion
Metallocene- Binding of Hydrogen & Propylene
Sharing of electrons between α Hydrogen
and metal is called α-agostic association.
This leads to stabilization of the complex
This stabilization is crucial for the formation
of the transition state during adsorption &
activation of in-coming monomer
Significance of agostic interaction- a) Preferred orientation of Me w.r.t.P b) Rotation
around Zr-C bond can make P & Me in cis position. c) Agostic interaction keeps
Zr-C bond rigid and orientation of Me is retained
Mode of formation of Isotactic PP
Activation of Metallocene by MAO
Substitution of Cls with methyl groups followed by ion-pair formation &
creation of a vacant site
Stepwise insertion of Propylene in metallocene catalysts - 1
Step.1 Adsorption & activation of
1st propylene molecule on metal
site
Step.2. Electronic rearrangement
Leading to formation of
transition state
Step.3. Insertion of 1st propylene
Molecule into the metal-alkyl bond
Stepwise insertion of Propylene in metallocene catalysts - 2
Step.4 . Approach & bonding
of 2nd propylene molecule from
the opposite side wr.r.t 1st
Step.5. Electronic rearrangements followed by transition
state and insertion of 2nd
propylene molecule so that
all methyl groups are on same
side of the chain
Step.6 3rd propylene molecule
has to approach similar to the
1st one
Formation of Isotactic Polypropylene –Site controlled
Typical metallocene catalyst structures
For Isotactic PP
For Syndiotactic PP
Propylene can approach from one direction (Me upwards) only
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