analysis of i -pp made by ziegler natta catalyst
DESCRIPTION
Analysis of i -PP made by Ziegler Natta Catalyst. 13 C NMR analysis shows:. …. …. m m m m m m m r r m m m m m m m m. Enantiomorphic Site Control. …. …. m m m m m m m r m m m m m m m m m. Chain - end Control. - PowerPoint PPT PresentationTRANSCRIPT
Polymer SynthesisCHEM 421
Analysis of i-PP made by Ziegler Natta Catalyst
• 13C NMR analysis shows:
… …
m m m m m m m r r m m m m m m m m
… …
m m m m m m m r m m m m m m m m m
Enantiomorphic Site Control
Chain - end Control
Polymer SynthesisCHEM 421
Soluble Metallocene CataystsPolyethylene
• Bis(cyclopentadienyl)zirconium dichloride
• Plus methylaluminoxane (MAO)
• 5,000 kg PE / g Zr / hr (indefinitely active)
H3C Al
CH3
CH3 HO Al
CH3
OHH2O+ 2 CH4
HO Al
CH3
O H
x
n
Zr Cl2
Polymer SynthesisCHEM 421
Soluble Metallocene CataystsPolypropylene
Polymer SynthesisCHEM 421
Soluble Metallocene Cataysts
• Used a mixture of meso and racemic ansa-titanocenes
– Stereo-rigid
– Ethylene-bridged
– Rac: chiral with C2-symmetric axis
– Meso: achiral
Polymer SynthesisCHEM 421
Rac Ansa Catalysts
Non-superimposable Mirror Images
Polymer SynthesisCHEM 421
Soluble Metallocene Cataysts
Ewen J. Am. Chem. Soc. 1984
Et[Ind]2TiCl2H2C CH
CH3
Mixture of:
+
56% chiral rac44% achiral meso
63% isotactic PP
37% atactic PP
Polymer SynthesisCHEM 421
PP from ansa-Metallocene Catalysts
• Methyl pentad region• Fractions of iso-PP versus atactic-PP tracks with
ratio of racemic to meso catalyst levels• MWs on each sample were similar indicating that
both stereo-isomers of the catalyst give rise to similar propagation and termination rate constants
Polymer SynthesisCHEM 421
Soluble Metallocene Cataysts
• Confirmed Ewen’s proposal that it was the chiral ansa-metallocene that was responsible for the formation of isotactic PP
– Made the zirconium derivative
– 7,700 kg PP / mol Zr / hr (!)
1985 Kaminsky and Britzinger
Et[Ind]2TiCl2H2C CH
CH3
+
100% chiral rac
isotactic PP
Polymer SynthesisCHEM 421
Mechanism for Isotactic PP
• Conformational modeling studies (Corradini 1991)
• The growing polymer chain is forced into the open region
• Chirality of the metallocene is relayed to the incoming monomer through the orientation of the methyl group
• Since C2-symmetry, both reactions sites are homotopic
– Therefore selective for the same olefin enantioface
» Giving rise to isotactic PP
Polymer SynthesisCHEM 421
Soluble Metallocene CataystsPolypropylene
CH2 CH
CH3
P
Polymer SynthesisCHEM 421
Polymer SynthesisCHEM 421
Rac Ansa Catalysts
C2 Axis of Symmetry
Binding Sites are Homotopic
Polymer SynthesisCHEM 421
H H H H H H
H H H H H H
H H HH
H
H
H H HH H
HHHHHHHH
H H H HH
Polymer SynthesisCHEM 421
Polymer SynthesisCHEM 421
Polymer SynthesisCHEM 421
(a) Cs symmetry (b) Cp of different size(c) Steric bulk flanking the metallocene wedge with an open
region in center
Polymer SynthesisCHEM 421
• Hemi-isotactic
• Hemi-syndiotactic
How to make this?
... ...
... ...
Polymer SynthesisCHEM 421
Polymer SynthesisCHEM 421
… …
m m r r m m m m r r r r m m m m r