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