Step 1: Understanding the Concept:
Buna-S (SBR - Styrene Butadiene Rubber) is a copolymer. The name provides a hint: Bu stands for 1,3-butadiene, na is for the sodium catalyst used, and S stands for Styrene. Step 2: Key Formula or Approach:
Identify the monomers:
1. 1,3-Butadiene: $CH_2=CH-CH=CH_2$
2. Styrene: $CH_2=CH(C_6H_5)$ Step 3: Detailed Explanation:
When these two monomers polymerize, the double bonds rearrange to form a long chain:
\[ n(CH_2=CH-CH=CH_2) + n(CH_2=CH(C_6H_5)) \rightarrow \text{Buna-S} \]
The repeating unit contains the 4-carbon chain from butadiene (with a shifted double bond) and the 2-carbon chain from styrene (carrying the phenyl group, $-C_6H_5$). This matches the structure in Option (1). Step 4: Final Answer:
The structure is $[CH_2-CH=CH-CH_2-CH(C_6H_5)-CH_2]_n$.
