Polymer SynthesisCHEM 421

• Odian Book

Chapter 8.4 and 8.5 will cover the remaining 4 lectures.

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421Ziegler Natta Catalysts

• Too numerous to describe all variants

• Two Components– Transition metal compound Group IV to VIII

metals“catalyst”

– Organometallic compound Group I to III metals“co-catalyst”

I II III IV V VI VII VIII I II III

Polymer SynthesisCHEM 421Ziegler Natta Catalysts

• Prepared by mixing in a dry, inert solvent in absence of oxygen, usually at low temperatures

• Usually heterogeneous–Challenges…

Polymer SynthesisCHEM 421Ziegler Natta Catalyst

• Example: Titanium catalyst

AlR2Cl + TiRCl3

AlR2Cl

TiCl4

TiCl4 AlRCl2 + TiRCl3

AlR3 TiRCl3 AlR2Cl + TiR2Cl2

TiRCl3 TiCl3 + R

TiRCl3 TiCl2 + RCl

2 TiCl3TiCL2

AlR3 +

+

+

+

.

TiCl4

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421

Mechanism of Ziegler Natta Polymerizations:

Initiation and Propagation

• Catalyst acceptsethylene as a ligand

• Migratory insertion– Ethyl (alkyl) migration

Cossee-Arlman Mechanism

Ti CH2CH3 Ti CH2CH3

H2C CH2

Ti

CH2

Polymer

Ti CH2CH3

CH2H2C

Polymer SynthesisCHEM 421

Mechanism of Ziegler Natta Polymerizations:

Termination

Ti

CH2

Polymer

Ti HH2

PolymerH+

Ti

CH2CH3

Ti H

Polymer SynthesisCHEM 421Chain Transfer

Polymer SynthesisCHEM 421Polypropylene

• 1000 kg / g Ti / hr

• Heterogeneous catalysts

• Chiral sites believed to be fixed on the surface of the catalyst

TiCl3 (CH3CH2)3Al+

H2C CH

CH3

Isotactic Polypropylene

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421Ziegler Natta Catalyst

• Better to use TiCl3 directly rather than forming in situ by reduction

– TiCl3 4 different crystalline formsα, γ, δ highly stereoregular PPβ atactic PP

• Tacticity– Mix of catalyst crystalline forms– Does not yield:

…m m m m r m m r m m m m m r m r m m m… – But rather:

…m m m m m m m m m m m m m m…mixed with …m r m m r r m r m m r m r r m r m r m r…

Polymer SynthesisCHEM 421

13C NMR Characterization

Polymer SynthesisCHEM 421Prochiral Relationships

• Reaction with an achiral reagent will give equal quantities of enantiomers

– Optically inactive product

• Each face is enantiotopic

“re” face

“si” face

1

1

2

2 3

3

H3C H

O

H CH3

O

Polymer SynthesisCHEM 421Prochiral Relationships

Racemic 1-deutero-ethanol

Acetaldehyde

R-1-deutero-ethanol

H3C H

O

PropyleneH3C H

CH2

H3C D

O

CH3CHDOH

NaBH4

CH3CHDOH

Enzyme

"re" face "si" face

O

H3C

H

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421

Analysis of i-PP made by Ziegler Natta Catalyst

• 13C NMR analysis shows:

• once an error occurs, corrected by the chirality of the fixed catalyst site

• At the pentad levelmmmm high concentration

three weak signalsmmmr : mmrr : mrrm 2 : 2 : 1

… …

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

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

Analysis of syn-PP made by Ziegler Natta Catalyst

• 13C NMR analysis shows:

… …

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

… …

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

Enantiomorphic Site Control

Chain - end Control