The sequences of synthetic copolymers are notoriously difficult to control, and numerous complex protocols have been developed to help us do so. However, we lack a comprehensive understanding of what governs the sequential incorporation of different monomers into a chain for even the simplest of copolymerizations. To learn more, we performed coarse-grained modeling of a linear, step-growth copolymerization between two monomers. Our results demonstrate that transient associations and emergent self-organization among the reacting monomers and nascent oligomers can exert considerable influence over the sequential arrangement of monomers in the resulting chains, even when reaction barriers between the different monomer pairs are held constant. The magnitude of the effect we observe for differing co-monomer attractions can be explained by an emergent phase separation among the nascent oligomers – a phase separation that is both initiated and limited by the copolymerization.