See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the polymer repeat unit: The polymer shown is [-CH2-CH(Ph)-]n, which is polystyrene. Each repeat unit consists of a -CH2-CH(Ph)- fragment, meaning one carbon bears a phenyl group and the adjacent carbon does not. This is the classic head-to-tail addition polymer of styrene. Step 2 - Determine the monomer by 'opening' the repeat unit: Addition polymers are formed by the opening of a pi bond (alkene) in the monomer. The repeat unit -CH2-CH(Ph)- corresponds to the monomer CH2=CH(Ph), which is styrene (vinylbenzene). Joining n styrene monomers through addition polymerization across the C=C double bond gives exactly the polymer shown. Step 3 - Match to the options: Option (b) shows Ph-CH=CH2, which is styrene (phenyl group attached to a vinyl group, i.e., CH2=CH-Ph). This is precisely the monomer that gives [-CH2-CH(Ph)-]n upon addition polymerization. Step 4 - Eliminate other options: - Option (a) is a saturated compound (Ph-butane/propane derivative) with no double bond, so it cannot undergo addition polymerization to give the observed backbone. - Option (c) is 1,4-diphenyl-2-butene, a di-phenyl internal alkene; polymerization would give a different repeat unit containing two Ph groups per unit. - Option (d) is a conjugated diene with two Ph groups; its polymerization would give a different structure entirely. Therefore, the correct answer is B.