A short fragment of a polymer chain (five repeat units, end groups shown, substituents drawn with wedges and dashes) is drawn. Which statement about the fragment and its likely bulk properties is correct?

AAtactic; the irregularity gives the highest melting point
BAtactic; irregular chains pack poorly, so low crystallinity
CIsotactic; regular chains give a highly crystalline polymer
DSyndiotactic; alternating groups give a rubbery solid

Answer: B

The diads are meso, racemo, racemo, racemo (found by comparing neighbouring configurations), so there is no regular pattern and the fragment is atactic. Such irregular chains cannot pack closely, so the material has low crystallinity and is soft with a low softening point. Isotactic or syndiotactic chains pack regularly and give crystalline, higher-melting polymers.

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A short fragment of a polymer chain (five repeat units, end groups shown, substituents drawn with wedges and dashes) is drawn. Which statement about the fragment and its likely bulk properties is correct?

Answer: B

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