Polystyrene is made from a single monomer, styrene, with structure CH2=CH-C6H5 by an addition polymerisation. The monomer has IUPAC name ethenylbenzene, though phenylethene is a more common name.
Nylon 6 is also made from a single monomer, caprolactam, which is a cyclic amide (also called 2-azacycloheptanone). The reaction is a ring opening polymerisation. The product is a polyamide and is a condensation polymer, though it is not made by condensation polymerisation (it could be made this way from 6-aminohexanoic acid).