a copolymer
A necklace made from a single colour of bead is one thing; a necklace strung from two different beads is another, and by choosing how you mix the colours you can create endless patterns and moods. A copolymer is a chain built from two (or more) different kinds of mer, rather than just one repeated unit. It is the polymer chemist's version of alloying: instead of settling for the properties of one monomer, you blend two monomers into a single chain to get a mix of both their virtues.
How the two mers are arranged along the chain gives distinct kinds of copolymer. In a random copolymer the two units are scattered irregularly; in an alternating copolymer they strictly take turns, A-B-A-B; in a block copolymer you get a long run of A then a long run of B; in a graft copolymer, chains of B are grown as side branches off a backbone of A. Real examples abound: ABS plastic combines acrylonitrile, butadiene, and styrene to be both tough and rigid, and SBR — styrene-butadiene rubber — is the synthetic rubber in most car tyres.
It matters because copolymerisation is one of the most powerful tuning knobs available: you can dial in toughness, flexibility, or clarity by choosing the second monomer and its proportion, without inventing a new molecule from scratch. One honest point to keep clear: a copolymer is a single chain that contains both mer types chemically bonded together. That is not the same as a polymer blend, which is just two different finished polymers physically mixed like oil stirred into water — the distinction matters because the two often behave quite differently.
ABS, the plastic in LEGO bricks and helmets, is a copolymer of three monomers: butadiene gives rubbery toughness, while styrene and acrylonitrile give rigidity and chemical resistance. No single one of them alone would do the job.
A copolymer blends two or more monomers into one chain — like alloying, but for plastics.
A copolymer is one chain containing two different mers chemically bonded together — not the same as a polymer blend, which is two separate finished polymers merely mixed physically.