A short fragment of a polymer chain (three repeat units, end groups shown) is drawn. Compared with the cis isomer of the same constitution (natural rubber), which statement about the polymer shown is correct?

Atrans; chains pack closely, so harder and less elastic than natural rubber
Bcis; harder than natural rubber because of better packing
Ctrans; more elastic and softer than natural rubber
Dtrans; cannot be vulcanised because it has no C=C bond

Answer: A

All the double bonds are trans (E), so this is trans-1,4-polyisoprene, as in gutta-percha and balata. The straight trans chains pack closely and regularly, so the material is harder, more crystalline and much less elastic than natural rubber, whose cis geometry gives kinked, loosely packed chains.

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A short fragment of a polymer chain (three repeat units, end groups shown) is drawn. Compared with the cis isomer of the same constitution (natural rubber), which statement about the polymer shown is correct?

Answer: A

More questions on Geometry, tacticity and crosslinking in drawn chains