Polymer Synthesis CHEM 421 Odian Book Chapter 8

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” Group IV to VIII metals – Organometallic compound “co-catalyst” Group I to III metals I II IV V VI VIII III

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: Polymer Synthesis CHEM 421 Initiation and Propagation • 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 Ti. Cl 3 directly rather than forming in situ by reduction – Ti. Cl 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 r m m m… – But rather: …m m m m… mixed with …m r m m r r m r m r…

13 C NMR Characterization Polymer Synthesis CHEM 421

Prochiral Relationships Polymer Synthesis CHEM 421 • Each face is enantiotopic 1 2 3 “si” face 1 “re” face 3 2 • Reaction with an achiral reagent will give equal quantities of enantiomers – Optically inactive product

Prochiral Relationships Acetaldehyde Polymer Synthesis CHEM 421 Propylene Racemic 1 -deutero-ethanol R-1 -deutero-ethanol

Polymer Synthesis CHEM 421

Analysis of i-PP made by Ziegler Natta Catalyst • 13 C Polymer Synthesis CHEM 421 NMR analysis shows: … … m m m m r r 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 : 1

Analysis of i-PP made by Ziegler Natta Catalyst • 13 C Polymer Synthesis CHEM 421 NMR analysis shows: … … m m m m r r m m m m Enantiomorphic Site Control … … m m m m r m m m m m Chain - end Control

Analysis of syn-PP made by Ziegler Natta Catalyst • 13 C Polymer Synthesis CHEM 421 NMR analysis shows: … … r r r r m m r r r r Enantiomorphic Site Control … … r r r r m r r Chain - end Control r r r