Copolymers contain two or more different monomer units. Classified by arrangement:

Random copolymer: Monomers arranged randomly (-AABABBBA-). Example: Buna-S (random butadiene + styrene).

Alternating copolymer: Strict alternation (-ABABAB-). Example: nylon-6,6 (alternating diamine and diacid units in the chain).

Block copolymer: Long blocks of each monomer (-AAAA-BBBB-AAAA-). Example: styrene-butadiene-styrene (SBS) thermoplastic elastomer.

Graft copolymer: Branches of monomer B grafted onto a backbone of monomer A. Example: starch-polyacrylonitrile graft copolymer (superabsorbent in diapers).

Why make copolymers? To combine desirable properties of both monomers. Buna-S: butadiene provides elasticity, styrene provides strength/hardness → better tire rubber than either homopolymer. SBS: styrene blocks provide strength, butadiene blocks provide elasticity → no vulcanization needed.