Copolymerization

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]