Odian Book Chapter 8.4 and 8.5 will cover the remaining 4 lectures.
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Transcript of Odian Book Chapter 8.4 and 8.5 will cover the remaining 4 lectures.
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