Transcript Polymer Synthesis CHEM 421 Ziegler Natta Catalyst
Polymer Synthesis CHEM 421
• Odian Book Chapter 8.4 and 8.5 will cover the remaining 4 lectures.
Polymer Synthesis CHEM 421
Ziegler Natta Catalysts
Polymer Synthesis CHEM 421
• Too numerous to describe all variants • Two Components – Transition metal compound “catalyst” – Organometallic compound “co-catalyst” I II III IV V VI VII VIII I II III Group IV to VIII metals Group I to III metals
Ziegler Natta Catalysts
Polymer Synthesis CHEM 421
• Prepared by mixing in a dry, inert solvent in absence of oxygen, usually at low temperatures • Usually heterogeneous –Challenges…
Ziegler Natta Catalyst
• Example: Titanium catalyst
Polymer Synthesis CHEM 421
Polymer Synthesis CHEM 421
Mechanism of Ziegler Natta Polymerizations: Initiation and Propagation Polymer Synthesis CHEM 421
• Catalyst accepts ethylene as a ligand • Migratory insertion – Ethyl (alkyl) migration
Cossee-Arlman Mechanism
Mechanism of Ziegler Natta Polymerizations: Termination Polymer Synthesis CHEM 421
Chain Transfer Polymer Synthesis CHEM 421
Polypropylene
Polymer Synthesis CHEM 421
• 1000 kg / g Ti / hr • Heterogeneous catalysts • Chiral sites believed to be fixed on the surface of the catalyst
Polymer Synthesis CHEM 421
Ziegler Natta Catalyst
Polymer Synthesis CHEM 421
• Better to use TiCl 3
situ
by reduction directly rather than forming
in
– TiCl 3 4 different crystalline forms α, γ, δ highly stereoregular PP β 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…
13 C NMR Characterization Polymer Synthesis CHEM 421
Prochiral Relationships
Polymer Synthesis CHEM 421
• Each face is enantiotopic 1 “si” face 2 3 1 “re” face 3 2 • Reaction with an achiral reagent will give equal quantities of enantiomers – Optically inactive product
Prochiral Relationships
Polymer Synthesis CHEM 421
Acetaldehyde Propylene Racemic 1-deutero-ethanol R-1-deutero-ethanol
Polymer Synthesis CHEM 421
… Analysis of i-PP made by Ziegler Natta Catalyst
• 13 C NMR analysis shows:
Polymer Synthesis CHEM 421 … m m m m m m m r r m m m m m m m m
• once an error occurs, corrected by the chirality of the fixed catalyst site • At the pentad level mmmm high concentration three weak signals mmmr : mmrr : mrrm 2 : 2 : 1
… Analysis of i-PP made by Ziegler Natta Catalyst Polymer Synthesis CHEM 421
• 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 …
Analysis of syn-PP made by Ziegler Natta Catalyst Polymer Synthesis CHEM 421
• 13 C NMR analysis shows:
… … r r r r r r r m m r r r r r r r r Enantiomorphic Site Control … … r r r r r r r m r r r r r r r r r Chain - end Control