= Polymer recovery accounting basis
A \b[polymer recovery accounting basis] specifies the conserved inventory used to compare initial material with recovered fractions. Possible choices include conditioned dry mass, original repeat-unit equivalents or the amount of a particular element. These bases coincide only under suitable chemical and sampling assumptions. A <polymer fraction-recovery balance> needs that convention before its recovery percentage has a clear meaning.
For fractions $i$ with measured masses $m_i$, suppose $k_i$ converts one unit of that product mass into the chosen original-material equivalent. Recovery on that basis is
$$
r=\frac{\sum_i k_i m_i}{B_0},
$$
where $B_0$ is the initial inventory in the same units. Taking every $k_i=1$ is a chemical assumption: it requires that the measured masses are directly comparable. Different solvent contents, water incorporated by reaction, salts, fillers or filter residues can invalidate it.
An original <polyglycolic acid> example makes the distinction visible. Ignoring initial end-group contributions for a repeat-unit bookkeeping comparison, one millimole of $\mathrm{C_2H_2O_2}$ repeat inventory represents approximately 58.04 mg. Complete <ester hydrolysis> to one millimole of <glycolic acid>, $\mathrm{C_2H_4O_3}$, gives approximately 76.05 mg. The roughly 18.01 mg difference comes from incorporated <water>, not extra recovered original repeat inventory. The manufacturer's structural and decomposition diagrams identify the polymer and hydrolysis product.<Kuredux Polyglycolic Acid Resin technical brochure by Kureha>[March 2022 brochure, PDF p. 2, Molecular Structure and decomposition diagram]{ref}
Dividing the product's 76.05 mg directly by 58.04 mg would report approximately 131.0% recovery on mixed bases. The monomer-to-original-repeat conversion is instead $k=58.04/76.05\approx0.76318$, restoring 58.04 mg of original repeat equivalents. Exact finite-chain accounting must include the original end groups and count hydrolyzed bonds; the repeat-unit example deliberately omits that small-chain correction. Soluble oligomers contain a different number of end groups per repeat, so the monomer conversion is not universal for an unidentified soluble mixture.
<Carbon-basis closure for polymer degradation> offers another explicit inventory. Each repeat and each glycolic-acid molecule has two carbon atoms, so the example contains approximately 24.02 mg of carbon before and after the specified hydrolysis. If subsequent chemistry produces other carbon-containing products, those products belong in that inventory too. Carbon introduced by solvent or background contamination must be distinguished from specimen carbon; otherwise elemental closure can look satisfactory for the wrong reason.
An original fraction count begins with 1.00 mmol of repeat equivalents and recovers 0.60 mmol in the solid, 0.30 mmol in identified soluble material and 0.05 mmol in rinses. Recovery is 95%, regardless of whether the corresponding product masses differ through hydration. The remaining 0.05 mmol is unaccounted on this basis; the deficit alone does not establish whether it was chemically transformed, escaped collection or was underestimated analytically.
A useful experiment records conditioning and drying, collects filtrate and rinses, identifies the soluble products and measures blanks. Kureha's published test uses filtered, dried residue after timed exposure of material at 2% solids concentration to deionized water.<Kuredux Polyglycolic Acid Resin technical brochure by Kureha>[PDF p. 3, Degradability test method]{ref} That retained-residue mass is one well-defined observable. Inferring the full chemical recovery requires the other fractions and their conversion factors. Report those assumptions separately from the <molar mass distribution> of whichever fraction was analyzed.
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