Polymers Solutions: #6

6.* (1995 F 16) Professor Waymouth at Stanford has invented a remarkable homogeneous catalyst which, when activated by an aluminum alkyl derivative, produces a block copolymer from propylene. This copolymer has alternating isotactic and atactic regions; it is an elastomer even though there are no covalent cross links between individual polymer chains. The active catalyst exists in two conformations (shown below); these slowly interconvert during the catalytic reaction.

A. Briefly discuss the stereochemical properties of these two conformers. Explain why one conformer produces isotactic polymer whereas the other produces atactic polymer.

The conformer on the left is chiral, so it produces the isotactic (chiral) blocks. The conformer on the right is achiral (note the internal mirror plane), so it produces the atactic (achiral) blocks.

B. Discuss the structural features of this copolymer which make it a useful elastomer.

•Atactic blocks: These are disordered with many more conformational degrees of freedom than the isotactic blocks. These regions account for the elastomeric properties.

•Isotactic blocks: These regions associate to form microcrystalline domains in the polymer, which serve as physical constraints. Without these, the polymer would flow.