Stereoisomerism in Polymers

Which of the following polymers could not exist as isotactic and syndiotactic stereoisomers?
Enter letters (A - G), corresponding to your selection. Do not enter superfluous characters, since they will be counted as incorrect answers.

A   Polypropylene
B   Poly(methyl methacrylate)
C   Poly(vinylidene chloride)
D   Polyacrylonitrile
E   Poly(vinyl acetate)
F   Teflon
G   Polystyrene

Enter your selection here:  

This script written by William Reusch, Dept. of Chemistry, Michigan State University. Please send comments and corrections to