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Copolymerization is polymerization that forms a copolymer: a polymer containing monomeric units derived from more than one monomer species. The different units occur within polymer molecules. Merely mixing two separately prepared polymers produces a polymer blend, which does not by itself establish copolymerization.^[ref]

Reacting-mixture composition and incorporated composition are different quantities. In an original material-balance example, start with $100\mathrm{mmol}$ each of monomers $A$ and $B$. Suppose $60\mathrm{mmol}$ of $A$ and $20\mathrm{mmol}$ of $B$ have entered the growing chains, with no other consumption or material loss. The cumulative mole fraction of $A$ units in the product iswhile the remaining monomer mixture contains $40\mathrm{mmol}$ of $A$ and $80\mathrm{mmol}$ of $B$, givingThe initial mixture was equimolar, but neither current value is $1/2$. Conversion bookkeeping for binary copolymers keeps these inventories distinct. The instantaneous copolymer composition equation predicts incorporation at a particular mixture composition under the terminal model of copolymerization; it does not automatically give the cumulative composition over a changing feed.

Composition also does not determine sequence. The schematic eight-unit segments $\mathrm{AAAAAABB}$ and $\mathrm{AAABAAAB}$ both contain six $A$ units and two $B$ units but have different arrangements. Neither is equivalent to placing an all-$A$ chain and an all-$B$ chain together in a polymer blend. Sequence architecture, overall mole fraction and the growth mechanism are separate specifications.

Experimentally, measuring the initial feed alone cannot identify monomer reactivity ratios. The IUPAC recommendation combines conversion and incorporated-composition observations at several starting compositions to constrain the kinetic model.^[ref] The historical 1944 paper by Frank R. Mayo and Frederick M. Lewis explicitly joined a theoretical comparison of monomer behavior with the copolymerization of styrene and methyl methacrylate.^[ref]

A polymer blend is a macroscopically homogeneous mixture of two or more different polymer species. The terminology does not require molecular-scale mixing: distinct microscopic phases may remain. IUPAC also treats the components as, in principle, separable by physical means.^[ref]

This distinguishes blending from making a copolymer. In a polymer blend, the different chains remain different molecular populations. In a copolymer, chemically different units occur within the same macromolecule. A blend can itself contain a copolymer as one component, so these descriptions answer different structural questions.

Composition also needs a stated denominator. For an original binary example, mix 30 g of one polymer with 70 g of another. The first component has mass fraction $w_1=0.30$. If their densities are stipulated as ${\rho }_1=1.2$ and ${\rho }_2=1.0\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, and component volumes are additive, its volume fraction is insteadThe total modeled volume is 95 cubic centimetres. Nonadditive mixing volumes would require a different conversion. Neither 30 mass percent nor 26.32 volume percent is a chain-number fraction without information about the chain molar masses.

Polymer blend miscibility asks whether the components form one equilibrium phase at a specified temperature, pressure and composition. A macroscopically uniform specimen can still have microscopic domains: the IUPAC definition deliberately does not equate a blend with a single phase. Conversely, chemical similarity alone does not establish complete mixing at every temperature.

An informative experiment therefore combines a stated composition and thermal history with observations at a stated length scale. Microscopy can reveal resolved domains, while light scattering can reveal composition fluctuations on other scales. An image with no resolved domains establishes only the limits of that observation. It does not prove that different chains have been covalently joined, or that a specimen is a single equilibrium phase. The distinction makes the term useful for describing a material before assigning a particular microscopic structure.