Question

Buna-N, a synthetic rubber is a copolymer of:
(A) $C{{H}_{2}}=CH-CH=C{{H}_{2}}$ and ${{C}_{6}}{{H}_{5}}-CH=C{{H}_{2}}$
(B) ${{H}_{2}}C=CH-NC$ and $C{{H}_{2}}=CH-CH=C{{H}_{2}}$
(C) $C{{H}_{2}}=CH-CCl=C{{H}_{2}}$and ${{H}_{2}}C=CH-CH=C{{H}_{2}}$
(D) $C{{H}_{2}}=CN-CN$and ${{H}_{2}}C=CH-CCl=C{{H}_{2}}$

Answer 1

The actual scientific name of this polymer contains the clues about the constituent compounds. The common name can also be used to guess its base polymer.

Complete answer:
The Buna-N is the common name for the polymer known as Acrylonitrile Butadiene Rubber or simply Nitrile rubber. Let us first discuss the common name. The Buna-N was actually one of the first base polymers patented back in 1934. This name has been stuck since then although the compounds used today are drastically different.
The name Acrylonitrile Butadiene Rubber gives three significant clues:
- One of its constituents is acrylonitrile (${{H}_{2}}C=CH-CN$)
- The other constituent is butadiene ($C{{H}_{2}}=CH-CH=C{{H}_{2}}$)
- The compound has properties of a rubber
The formula of acrylonitrile can be guessed even if we have never cared to memorise it. The term “Acrylo” comes from the acyl group of the acrylic acid ($C{{H}_{2}}=CH-COOH$) and the term nitrile is the opposite of the cyanide group ($-CN$) which means “$-NC$”. This makes the overall formula of acrylonitrile to be (${{H}_{2}}C=CH-CN$).
The butadiene is the base polymer here. We can derive the formula of this compound even if you do not have a memory of it. “but” refers to four carbons and “diene” refers to two double bonds. Hence it is a compound with four carbons and two double bonds, something like this “$C{{H}_{2}}=CH-CH=C{{H}_{2}}$”.
So we conclude from the above discussion that the answer to the question at hand is option (B)

Additional information:
- The compounds in option (A) are Butadiene ($C{{H}_{2}}=CH-CH=C{{H}_{2}}$) and Styrene (${{C}_{6}}{{H}_{5}}-CH=C{{H}_{2}}$) which make up the polymer Buna-S.
- The compound in the third option ($C{{H}_{2}}=CH-CCl=C{{H}_{2}}$) is 2-chloro-1,3-butadiene or chloroprene. Upon polymerization it becomes neoprene ${{\left[ C{{H}_{2}}-CH=CCl-C{{H}_{2}} \right]}_{n}}$.

Note:
Copolymer is always derived from two different monomer species. So any option containing only one compound should straight away be eliminated.
In the last option there are two cyanide groups in the first compound. Remember acrylonitrile has only one of them.