A short fragment of a polymer chain (four repeat units, end groups shown, methyl groups drawn with wedges and dashes) is drawn. Which description of the fragment is correct? Use meso for two neighbouring stereocentres of the same relative configuration and racemo for opposite.

AIsotactic; all three diads are meso
BSyndiotactic; all three diads are meso
CSyndiotactic; all three diads are racemo
DAtactic; two diads racemo and one meso

Answer: C

Adjacent stereocentres alternate in configuration, so each of the three diads is racemo and the methyl groups lie on alternate sides of the chain: this polypropylene fragment is syndiotactic. An isotactic fragment would have all diads meso, and an atactic one would mix them.

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A short fragment of a polymer chain (four repeat units, end groups shown, methyl groups drawn with wedges and dashes) is drawn. Which description of the fragment is correct? Use meso for two neighbouring stereocentres of the same relative configuration and racemo for opposite.

Answer: C

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