Polymer SynthesisCHEM 421

Analysis of i-PP made by Ziegler Natta Catalyst

• 13C NMR analysis shows:

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m m m m m m m r r m m m m m m m m

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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?

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Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421

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m m r r m m m m r r r r m m m m r