Polymer Synthesis CHEM 421 Odian Book Chapter 8.4 and 8.5 will cover the remaining 4 lectures.

Polymer Synthesis CHEM 421

Polymer Synthesis CHEM 421 Ziegler Natta Catalysts Too numerous to describe all variants Two Components –Transition metal compoundGroup IV to VIII metals “catalyst” –Organometallic compoundGroup I to III metals “co-catalyst” I II III IV V VI VII VIII I II III

Polymer Synthesis CHEM 421 Ziegler Natta Catalysts Prepared by mixing in a dry, inert solvent in absence of oxygen, usually at low temperatures Usually heterogeneous –Challenges…

Polymer Synthesis CHEM 421 Ziegler Natta Catalyst Example: Titanium catalyst

Polymer Synthesis CHEM 421

Polymer Synthesis CHEM 421 Mechanism of Ziegler Natta Polymerizations: Initiation and Propagation Catalyst accepts ethylene as a ligand Migratory insertion –Ethyl (alkyl) migration Cossee-Arlman Mechanism

Polymer Synthesis CHEM 421 Mechanism of Ziegler Natta Polymerizations: Termination

Polymer Synthesis CHEM 421 Chain Transfer

Polymer Synthesis CHEM 421 Polypropylene 1000 kg / g Ti / hr Heterogeneous catalysts Chiral sites believed to be fixed on the surface of the catalyst

Polymer Synthesis CHEM 421

Polymer Synthesis CHEM 421 Ziegler Natta Catalyst Better to use TiCl 3 directly rather than forming in situ by reduction – TiCl 3 4 different crystalline forms α, γ, δhighly stereoregular PP βatactic 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 Synthesis CHEM C NMR Characterization

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

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

Polymer Synthesis CHEM 421

Polymer Synthesis CHEM 421 Analysis of i-PP made by Ziegler Natta Catalyst 13 C NMR analysis shows: once an error occurs, corrected by the chirality of the fixed catalyst site At the pentad level mmmmhigh concentration three weak signals mmmr : mmrr : mrrm 2 : 2 : 1 …… m m m m m m m r r m m m m m m m m

Polymer Synthesis CHEM 421 Analysis of i-PP made by Ziegler Natta Catalyst 13 C 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 Synthesis CHEM 421 Analysis of syn-PP made by Ziegler Natta Catalyst 13 C 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