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Polyethylene terephthalate, more explicitly poly(ethylene terephthalate), is a polyester made from ethylene-glycol and terephthalate-derived units. It is often called PET polymer in materials contexts. Its chain contains repeated ester connections and para-substituted aromatic rings; it is not polyethylene with a physically mixed additive. ACS identifies the material as a condensation polymer of ethylene glycol and terephthalic acid.^[ref]

An ideal repeat can be writtenwith the two aromatic attachments opposite one another. Counting this repeat gives ${\mathrm{C}}_{10}{\mathrm{H}}_8{\mathrm{O}}_4$. The same count follows from the monomer formulas:This is repeat-unit bookkeeping; exact finite-chain composition also includes the end groups.

Using standard rounded atomic masses, the repeat contributes about $192.17\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. An original ideal chain with 1000 such repeats therefore has about $192.17\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$ of repeat-unit mass before end-group corrections. A real sample has a molar mass distribution; the repeat count does not assign one molecular mass to every chain.

The ester connections are part of the polymer backbone. Their cleavage by hydrolysis can shorten chains before all products leave the specimen, so retained mass and number-average molar mass need separate observations. Polymer fraction-recovery balance is relevant when soluble fragments and retained material are sampled differently. Neither the polyester label nor a bottle's disappearance from view identifies a degradation mechanism.

An illustrative comparison takes two samples with identical repeat chemistry but different crystallization and processing histories. Their thermal and mechanical responses can differ without a change in the repeat formula. A proposed experiment compares thermal response and chain-distribution measurements under documented conditioning, while checking for hydrolytic change during preparation. This separates molecular identity, chain length and material organization. The derivations here describe an ideal structure; no universal processing temperature or degradation rate is inferred from it.

A 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 iswhere $B_0$ 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 ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{O}}_2$ repeat inventory represents approximately 58.04 mg. Complete ester hydrolysis to one millimole of glycolic acid, ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_3$, 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.^[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 $k=58.04/76.05\approx 0.76318$, 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.^[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.

Selective-fragment-loss bias in polymer molar mass arises when the chains retained or analyzed are not representative of the original molecular population. Preferential loss of short fragments can raise the measured number-average molar mass of the survivors even while the specimen loses mass and chain scission continues.

Let $M$ denote molar mass for a molecule drawn with equal probability from the original chain population. If its retention probability is $p(M)$, the number-average molar mass of retained chains isConsequentlyRetention favouring larger chains can make this difference positive. This is an original selection calculation; it does not assume a particular hydrolysis mechanism. If the intended measurand is explicitly the retained fraction, its correctly measured mean is not intrinsically erroneous. Bias appears when that mean is interpreted as the whole population's mean.

An original mixture has 90 chains at $1\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$ and ten at $100\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$. Its number-average molar mass is $10.9\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$. Removing all short chains leaves the survivor mean at 100, although only $90/1090\approx 8.26\%$ of the initial chain mass has left. The large numerical increase does not imply chain growth.

A weight-average molar mass is a different statistic: $M_w={\sum }_i{N}_i{M}_i^2/{\sum }_i{N}_i{M}_i$. The same initial mixture has $M_w\approx 91.83\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$, showing why an averaging convention must accompany a reported molecular-mass value. Equal molecule weighting and equal mass weighting answer different questions.

Experimentally, distinguish residue, released soluble fragments and the fraction actually injected into the analytical instrument. A polymer fraction-recovery balance can compare their recovered mass. The manufacturer's polyglycolic acid test measures filtered dry residue after water exposure; it does not directly supply the chain distribution in the liquid that passed through.^[ref]

Size-exclusion chromatography adds a further measurement model: IUPAC defines its separation chiefly through hydrodynamic volume in a porous, non-adsorbing material, not molecular mass alone.^[ref] Solubility, preparation losses, detector response and the mass calibration can therefore affect what distribution is inferred. Preserve the sampled fraction's identity instead of treating a cleaner chromatogram or a higher surviving mean as evidence that degradation reversed.