Copolymerization
= Copolymerization
\b[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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Copolymerization, C01336, definition; copolymer, C01335, more than one monomer species]{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 is
$$
\overline F_A=\frac{60}{60+20}=0.75,
$$
while the remaining monomer mixture contains $40\,\mathrm{mmol}$ of $A$ and $80\,\mathrm{mmol}$ of $B$, giving
$$
f_A=\frac{40}{40+80}=\frac13.
$$
The 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.<Radical Copolymerization Reactivity Ratios by IUPAC>[Official recommendation announcement: conversion/composition measurements at three or more starting feeds]{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.<Copolymerization. I. A Basis for Comparing the Behavior of Monomers in Copolymerization; The Copolymerization of Styrene and Methyl Methacrylate by Frank R. Mayo and Frederick M. Lewis>[Publisher title and byline, Journal of the American Chemical Society66, pp.1594–1601]{ref}