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The copolymer composition equation, also called the Mayo-Lewis equation, relates the instantaneous monomer-consumption composition to the current reacting mixture under the terminal model of copolymerization. For binary mixture mole fractions $f_1+f_2=1$, let $F_1$ be the fraction of type-1 units among the units being incorporated at that instant. Thenwhere $r_1,r_2$ are monomer reactivity ratios.^[ref] The equation concerns molar incorporation, not the mass fraction or cumulative composition of a finished specimen.

A short derivation identifies the steady-chain-end assumption. Let $c_i$ be monomer concentrations and $E_i$ concentrations of active ends. Cross-additions interconvert the end types; balancing them gives $k_{12}{E}_1{c}_2=k_{21}{E}_2{c}_1$. The two positive consumption rates areSubstitution yields $v_1/v_2=c_1(r_1{c}_1+c_2)/[c_2(c_1+r_2{c}_2)]$. Normalizing $F_1=v_1/(v_1+v_2)$ gives the stated equation. This neglects additional sources of end-type changes and uses a terminal kinetic description.

For an original example with $r_1=r_2=2$ and $f_1=0.20$, the numerator is 0.24 and the denominator is 1.68, so $F_1=1/7\approx 0.143$. The instantaneous polymer is poorer in type 1 than the available mixture. At equimolar feed the same ratios give $F_1=1/2$; this composition equality does not imply independent sequence placement.

For a finite closed batch, conversion bookkeeping for binary copolymers relates the changing monomer pool to cumulative incorporated composition. Treating an initial $f_1$ as constant after substantial selective consumption can therefore misinterpret the final sample. The IUPAC recommendation addresses conversion and integrated composition data explicitly.^[ref] Useful experiments distinguish current feed, conversion and product composition instead of treating all three as the same label.

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]

Frederick M. Lewis, with Frank R. Mayo, coauthored the 1944 paper establishing a quantitative basis for comparing monomers in copolymerization, with styrene and methyl methacrylate as its named chemical system.^[ref] The published name is retained without expanding the middle initial or assigning individual experimental steps to either coauthor.

The scientific problem is that incorporated composition can differ from the reacting-mixture composition. In the terminal model of copolymerization, two monomer reactivity ratios describe relative self-addition and cross-addition preferences. Inferring both ratios requires observations that distinguish their effects, rather than one measurement that happens to match the feed.

An original identifiability example uses the copolymer composition equation at equimolar feed. It reduces toAn observation $F_1=1/2$ establishes $r_1=r_2$ within this model, but leaves their common value unknown. Pairs $(0.2,0.2)$, $(1,1)$ and $(2,2)$ all satisfy it. They need not predict the same sequence correlations or behavior at another feed.

At a second feed $f_1=0.20$, the three candidate pairs instead predict $F_1\approx 0.3684$, $0.2000$ and $0.1429$, respectively. This invented comparison explains the information gained by varying the monomer mixture. It is not a reconstruction of the original authors' measured constants.

Modern IUPAC recommendations likewise use several starting compositions together with conversion and product-composition observations.^[ref] Lewis's verified contribution belongs to the experimental and theoretical development of that comparison problem. Composition data test the kinetic model and its parameters; they do not acquire a unique interpretation merely because one fitted curve passes through one point.

Incorporated composition versus feed composition in EVA distinguishes the monomer mixture available to react from the units actually entering ethylene-vinyl acetate chains. Let $f$ be the mole fraction of vinyl acetate in the reacting monomer mixture, and $F$ its instantaneous fraction among newly incorporated units. Neither quantity is the mass fraction reported on a product datasheet. The copolymer composition equation relates instantaneous composition to the reacting mixture and monomer reactivity ratios; IUPAC explicitly notes that composition generally changes during the reaction.^[ref]

In the terminal model of copolymerization, the identity of the last unit controls addition-rate constants. Define $r_V=k_{VV}/k_{VE}$ and $r_E=k_{EE}/k_{EV}$, where the first subscript identifies the chain end and the second the incoming monomer. IUPAC defines these monomer reactivity ratios as homopropagation-to-cross-propagation rate-constant ratios.^[ref] Steady-state balance between the two chain-end types gives the familiar binary-model expressionFor an original illustration, stipulate $r_V=0.5$, $r_E=2$ and $f=0.2$. The numerator is 0.18 and denominator 1.62, so $F=1/9\approx 0.111$. These illustrative ratios are not measured kinetic constants for a commercial EVA grade. If both ratios were instead one, this model would give $F=f$ at every reacting-mixture composition.

The composition accumulated over a finite reaction interval is different again. Conversion bookkeeping for binary copolymers follows directly from conservation of monomer units. Normalize the initial total to one mole of units, let $X$ be the total molar conversion, and let $f_0$ and $f$ denote initial and remaining vinyl acetate fractions. Assuming no side losses and incorporation of every consumed monomer, the cumulative fraction isFor $f_0=0.20$, $X=0.50$ and remaining $f=0.30$, the polymer has $\overline{F}=0.10$. It is enriched in ethylene-derived units while the unreacted mixture becomes richer in vinyl acetate. At complete conversion, the overall incorporated fraction must return to $f_0$ under these assumptions, even if individual chains or earlier growth intervals differed.

An informative experiment therefore measures reacting-mixture composition, conversion and product composition separately. A low-conversion approximation needs evidence that composition drift is negligible over the actual interval; an initial feed label alone does not establish the chain composition. Convert incorporated mole fractions to mass fractions only after identifying which composition and reaction interval the measurement represents.

A monomer reactivity ratio compares two propagation rate constants for the same terminal chain type in a binary copolymerization. With $k_{ij}$ denoting addition of monomer $j$ to an end of type $i$,These are homopropagation-to-cross-propagation ratios, as defined in the IUPAC terminology.^[ref] Each is dimensionless when numerator and denominator use matching rate conventions. Neither is simply the ratio of the two monomers' total consumption rates.

A chain-end addition probability depends on available concentrations as well as on the rate constants. For a type-1 end at fixed mixture mole fractions, the terminal model of copolymerization givesFor stipulated $r_1=0.5$, an equimolar mixture gives $p_{11}=1/3$. At $f_1=0.8$ it gives $p_{11}=2/3$, whereas at $f_1=0.2$ it gives $p_{11}=1/9$. Even though $k_{11}<k_{12}$ in each case, sufficient abundance of monomer 1 can make self-addition more frequent. These are illustrative kinetic parameters, not measured constants for a named commercial polymer.

The pair $(r_1,r_2)$ determines both rows of the fixed-feed transition matrix. When $r_1{r}_2=1$, its two rows coincide for nonzero feed fractions, giving independent additions within the idealized fixed-feed model. That does not require the incorporated fraction to equal the feed fraction. For example, $r_1=2$, $r_2=1/2$ and $f_1=1/2$ give probability $2/3$ of adding type 1 from either end.

Experimentally, a ratio is associated with the specified monomer pair, reaction conditions and kinetic model. It is not a universal index belonging to an isolated monomer. The IUPAC recommendation uses several starting compositions together with conversion and incorporated-composition measurements to estimate the two ratios.^[ref] One measured copolymer composition at one feed generally cannot identify two unknown ratios. Joint uncertainty and possible composition drift belong to the interpretation.

The terminal model of copolymerization represents a growing copolymer chain's addition kinetics using only the identity of its final monomeric unit. For two incoming monomers, there are two end types and four chain-growth propagation rate constants $k_{ij}$, where $i$ identifies the existing end and $j$ the incoming monomer. Earlier sequence history is omitted from this kinetic state description.^[ref]

At a fixed reacting-mixture composition, addition rates give a two-state Markov chain. With $f_1+f_2=1$ the monomer mole fractions and $r_1=k_{11}/k_{12}$, $r_2=k_{22}/k_{21}$ the monomer reactivity ratios, its transition matrix isRows label the current terminal unit; columns label the newly added unit. Each row sums to one. This interprets a sequence at fixed feed and neglects initiation, termination and feed drift over the sampled run.

For an original example, take $f_1=f_2=1/2$ and $r_1=r_2=2$. Then $P_{11}=P_{22}=2/3$ and both cross-addition probabilities are $1/3$. The stationary distribution is $(1/2,1/2)$, so the long-chain mean composition is equimolar, while equal consecutive units are favored. A run of type 1 has a geometric distribution with stopping probability $1/3$ and mean length 3. Independent equimolar placement would instead give mean run length 2. Mean composition alone does not determine sequence correlations.

The copolymer composition equation concerns the instantaneous incorporated composition implied by this model. Applying it over a finite conversion interval requires following changes in the reacting mixture. The IUPAC recommendation explicitly bases ratio estimation on conversion and composition observations at different starting feeds.^[ref]

Historically, Frank R. Mayo and Frederick M. Lewis published a 1944 theoretical comparison of monomers with a study of styrene and methyl methacrylate.^[ref] A successful terminal-model fit is evidence about a chosen kinetic representation, not proof that every chemical influence vanishes beyond the last unit.