= Incorporated composition versus feed composition in EVA
\b[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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Copolymerisation composition equation, entry 15391, definition and notes 1 and 3]{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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Monomer reactivity ratios, entry 15410]{ref} Steady-state balance between the two chain-end types gives the familiar binary-model expression
$$
F=\frac{r_Vf^2+f(1-f)}{r_Vf^2+2f(1-f)+r_E(1-f)^2}.
$$
For 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 $F=1/9\approx0.111$. 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 is
$$
\overline F=\frac{f_0-(1-X)f}{X},\qquad X>0.
$$
For $f_0=0.20$, $X=0.50$ 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.
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