JEE Advanced levelSingle correct optionDifficulty 3 of 3pl252
A short fragment of a polymer chain (four repeat units, end groups shown, substituents drawn with wedges and dashes) is drawn. Counting each pair of neighbouring stereocentres as one diad (meso if the relative configuration is the same, racemo if opposite), which description is correct?
A1 meso and 2 racemo diads; not stereoregular
B3 racemo diads; syndiotactic
C2 meso and 1 racemo diads; not stereoregular
D3 meso diads; isotactic
Answer: C
The three neighbouring pairs are meso, racemo, meso, so there are 2 meso and 1 racemo diads. This is neither all-meso (isotactic) nor all-racemo (syndiotactic), so the fragment is not stereoregular (atactic-type) PVC.