Buna-S is a copolymer, since it is formed by:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Polymerization of two different monomers
A copolymer is a polymer made from more than one kind of monomer, whose units appear together in the same chain. Buna-S is made by copolymerising 1,3-butadiene and styrene:
\( n\,\ce{CH2=CH-CH=CH2} + n\,\ce{C6H5CH=CH2} \xrightarrow{\text{Na}} \ce{-[CH2-CH=CH-CH2-CH2-CH(C6H5)]_{n}-} \)
(“Bu” for butadiene, “Na” for the sodium used as catalyst, “S” for styrene.)
Why the other options are incorrect
Option (1) Polymerization of single monomer — Incorrect. That gives a homopolymer, e.g. polythene from ethene.
Option (3) Polymerization of homopolymers — Incorrect. A copolymer is made from monomers, not by joining finished homopolymers.
Option (4) Polymerization of molecules with double and triple bonds — Incorrect. Neither butadiene nor styrene has a triple bond; the type of multiple bond is not what defines a copolymer.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Addition Polymers – …
