When two different monomers are copolymerized, which factor primarily determines the properties of the resulting copolymer?

When two different monomers are copolymerized, the overall properties of the copolymer are mainly governed by the relative amounts of each monomer that end up in the polymer chain, called the copolymer composition. Because each monomer contributes its own chemical structure, the proportion of each determines the stiffness, melting point, and chemical resistance of the final material. The reactivity ratios of the monomers affect how the composition develops during polymerization, but the final property is still set by how much of each unit is present. For example, a copolymer made of 70 % styrene and 30 % acrylonitrile will be much harder and more heat‑resistant than one that is 30 % styrene and 70 % acrylonitrile. Thus, the key factor is the composition ratio of the two monomers in the copolymer.