A short fragment of the polymer chain (two repeat units, end groups shown) is drawn. The geometry of the C=C bonds is not specified. Which monomer among the structures shown gives this synthetic rubber by 1,4-addition?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The chain keeps one C=C per repeat unit and has Cl on the carbon of the double bond, so it is neoprene (polychloroprene). It forms by 1,4-addition of 2-chlorobuta-1,3-diene (chloroprene), giving -CH2-C(Cl)=CH-CH2- units. 1-Chloro-1,3-butadiene would put Cl on the terminal carbon of the double bond, and 2,3-dichlorobutadiene would give two Cl per unit.

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