Polymers Solutions: #9

9.* (1994 F 12) We return to Prof Waymouth’s research on soluble catalysts which produce stereoregular polypropylene. Two stereoisomeric precatalysts are shown below. When these Zr(IV) metallocene stereoisomers are activated with aluminum alkyl, one isomer produces isotactic polypropylene; the other isomer produces atactic polypropylene. You are not told which catalyst produces which polymer. Recall that isotactic polypropylene is a high-melting, stereoregular polymer whose microstructure is a polymeric coil. By contrast, atactic polypropylene has a disordered, stereorandom microstructure; the atactic form is lower melting.

Given the above facts and the structures depicted below for the two precatalysts predict which catalyst forms isotactic and which forms atactic polypropylene. (Hint: the polymerization process which determines stereoregularity occurs at the zirconium center.)

Stereoregularity can only occur if the catalyst is chiral… (chirality comes from chirality!)

--chiral catalyst (on the right) yields ISOTACTIC polymer

--achrial catalyst (on the left) yields ATACTIC polymer