Why a Polymer Repeating Unit Is Not the Same as a Monomer

Two polymers can both belong to the polyester family without being the same polymer. And a repeating unit can contain atoms from two different monomers without being identical to either one.

This is exactly what happens with polyethylene terephthalate (PET). Its repeating unit contains parts of two starting molecules: ethane-1,2-diol and terephthalic acid. Those molecules react, form ester linkages, and become part of a continuous polymer chain.

The key distinction is that a monomer is a starting molecule, whereas a repeating unit is the structural pattern repeated along the finished polymer chain. The repeating unit describes the product, not necessarily a molecule that existed before polymerisation.

Understanding this difference also helps you distinguish a polymer family, such as polyesters, from one specific member of that family, such as PET.

01Four terms you need to keep separate

Consider four different ways of describing a polymer.

These terms answer different questions.

• Polyester tells you what type of chemical linkages occur in the polymer backbone.
• PET tells you which particular polymer you are discussing.
• The monomers tell you which starting molecules supply the atoms.
• The repeating unit tells you what structural arrangement occurs along the finished chain.

A useful analogy is a repeating pattern on a piece of fabric. The pattern describes what you see repeated across the fabric, but it isn’t necessarily one of the materials used to manufacture it.

The analogy has a limit: polymer repeating units aren’t separate patches placed next to each other. Their atoms are connected by covalent bonds to form one continuous molecule.

Polyester is a family, not one particular polymer

A polyester is a polymer with repeated ester linkages in its main chain.

The characteristic ester connection is:

\[ \ce{-C(=O)-O-} \]

PET is one member of this family. Another example is poly(butylene terephthalate), or PBT.

Both contain ester linkages, but their structures differ.

The important point is that identifying a polyester does not automatically identify PET.

You need to examine the actual structure before giving the polymer a specific name.

02Why the PET repeating unit is different from its monomers

Let’s look at the actual starting molecules.

Monomer 1: Ethane-1,2-diol

Ethane-1,2-diol is also commonly called ethylene glycol.

Its structural formula is:

\[ \ce{HO-CH2-CH2-OH} \]

It contains two alcohol functional groups, one at each end.

Monomer 2: Terephthalic acid

Terephthalic acid is also called benzene-1,4-dicarboxylic acid.

Its structural formula is:

\[ \ce{HOOC-C6H4-COOH} \]

The benzene ring has two carboxylic acid groups attached at opposite positions, positions 1 and 4.

Both monomers have two reactive ends. This allows them to connect repeatedly and produce a long chain.

What happens when they become a polymer?

An alcohol group can react with a carboxylic acid group to form an ester linkage.

During the standard condensation model:

• The carboxylic acid loses its \(\ce{-OH}\) group.
• The alcohol loses the hydrogen from its \(\ce{-OH}\) group.
• These atoms form a water molecule.
• The remaining alcohol oxygen connects to the carbonyl carbon of the acid.

The new ester connection is:

\[ \ce{-C(=O)-O-} \]

This process repeats, connecting the monomer-derived sections into a polymer chain.

The resulting PET repeating unit is:

\[ \boxed{ \ce{[-O-CH2-CH2-O-C(=O)-C6H4-C(=O)-]_{n}} } \]

The two carbonyl groups are attached at positions 1 and 4 of the benzene ring.

Notice what the repeating unit contains:

• The section \(\ce{-O-CH2-CH2-O-}\) comes from ethane-1,2-diol.
• The section \(\ce{-C(=O)-C6H4-C(=O)-}\) comes from terephthalic acid.
• The two sections are connected through ester linkages.

The repeating unit therefore contains residues from two different monomer types.

A residue is the part of a starting molecule that remains incorporated into the polymer after the reaction.

Neither starting monomer, on its own, is the complete PET repeating unit.

Why the square brackets matter

There is another detail that can cause confusion in HSC Chemistry questions.

The square brackets identify a repeating pattern. They do not represent the actual ends of a polymer molecule.

For PET, the end of one written repeating unit connects to the beginning of the next.

This means an ester linkage can occur across a bracket boundary.

In the structure:

\[ \ce{[-O-CH2-CH2-O-C(=O)-C6H4-C(=O)-]_{n}} \]

one ester connection appears within the written unit, while the other forms where the final carbonyl carbon connects to the oxygen at the beginning of the next unit.

There are therefore two ester linkages per ethylene glycol/terephthalate pair in an extended PET chain.

This is why you should never treat the square brackets as though they cut the polymer into separate molecules.

03Why the repeating unit has a different molecular formula

The difference becomes particularly clear when you compare the molecular formulas.

Values are calculated using standard rounded atomic masses.

If you simply add the two monomer formulas, you get:

\[ \ce{C2H6O2 + C8H6O4} \]

\[ \ce{= C10H12O6} \]

But the repeating unit has the formula:

\[ \ce{C10H8O4} \]

Why are four hydrogen atoms and two oxygen atoms missing?

They account for two water molecules.

The idealised repeating-unit balance is:

\[ \ce{C2H6O2 + C8H6O4} \]

\[ \ce{-> C10H8O4 + 2H2O} \]

This is the bookkeeping for an internal repeating unit in a long polymer chain. The two water molecules correspond to the two ester connections associated with each monomer pair.

There is an important qualification: a real, finite linear chain has terminal groups. These remain unreacted, so the exact number of water molecules released depends on the number of ester bonds actually formed.

For long chains, the terminal-group correction becomes relatively small.

The central lesson is that the PET repeating unit has a different composition from either starting monomer because it contains contributions from both, with atoms removed during condensation.

Don’t turn this into a universal rule

Not every polymer repeating unit loses water during formation.

For example, ethene undergoes addition polymerisation:

\[ \ce{nCH2=CH2 -> [-CH2-CH2-]_{n}} \]

The monomer and repeating unit have the same overall atom composition, but they are still not the same chemical structure.

The carbon-carbon double bond in ethene becomes part of a chain of carbon-carbon single bonds.

So the distinction between a monomer and a repeating unit is not simply about having different molecular formulas. It is about their different roles and chemical structures.

05What can an experiment actually tell you?

Suppose a chemist receives an unidentified plastic sample and wants to determine whether it contains ester functional groups.

A useful analytical technique is infrared spectroscopy, particularly attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR).

The key question is:

If the material contains ester linkages, what should the instrument detect?

How the apparatus works

In an ATR-FTIR instrument, the polymer sample is pressed against a special crystal.

An infrared beam enters the crystal and reflects internally. At each reflection, a small electromagnetic field interacts with the material touching the crystal.

Some infrared frequencies are absorbed by molecular vibrations.

The remaining signal reaches a detector, which records the absorption at different wavenumbers.

The instrument produces a spectrum showing which frequencies were absorbed.

What should the spectrum show?

If the sample contains ester functional groups, two useful regions to examine are:

These are indicative ranges, not exact universal values. Actual absorption positions depend on the chemical environment and sample.

A strong carbonyl absorption, combined with suitable C-O absorption, supports the presence of ester groups.

However, there is a problem with the original prediction.

Finding an ester functional group does not automatically prove that the sample is PET.

Other polyesters contain ester linkages too. Some polymers also contain ester groups as side groups rather than in their main chains.

A better investigation would compare the full spectrum with reliable reference spectra and use additional structural evidence to establish the polymer’s connectivity.

For example, more detailed structural analysis can help distinguish a para-substituted terephthalate section from a differently substituted aromatic section.

What the experiment can and cannot establish

The important limitation is that an instrument measures a physical signal, not a polymer’s name.

You must interpret that signal alongside the molecular structure.

This is the difference between identifying a broad polymer family and establishing the identity of one particular polymer.

07Questions and solutions

Question 1

A polymer is produced from ethane-1,2-diol and terephthalic acid.

A student writes the following statement:

“The monomer of PET is \(\ce{C10H8O4}\).”

Explain why this statement is incorrect.

In your answer:

(a) Identify the two parent monomers.

(b) State the polymer family and the specific polymer name.

(c) Explain what \(\ce{C10H8O4}\) actually represents.

Solution 1

The statement is incorrect because \(\ce{C10H8O4}\) is the formula of the PET repeating unit, not either of its parent monomers.

(a) The two parent monomers are:

Ethane-1,2-diol:

\[ \ce{HO-CH2-CH2-OH} \]

Terephthalic acid:

\[ \ce{HOOC-C6H4-COOH} \]

(b) The polymer belongs to the polyester family.

Its specific name is poly(ethylene terephthalate), or PET.

(c) The formula \(\ce{C10H8O4}\) represents one idealised PET repeating unit.

It contains the combined structural contributions of both monomers after condensation polymerisation.

Two water molecules are removed in the ideal internal repeat-unit bookkeeping.

The tempting mistake is to assume that one repeating unit must be one monomer molecule. PET demonstrates why this is not true: one repeating unit contains residues from two different monomer types.

Question 2

A polymer has the following repeating structure:

\[ \ce{[-CH2-CH(C(=O)OCH3)-]_{n}} \]

A student argues that it must be a polyester because every repeating unit contains an ester functional group.

Evaluate this argument.

Identify:

(a) The type of linkage forming the main polymer backbone.

(b) Whether the material belongs to the polyester family.

(c) The monomer from which the polymer can be formed through addition polymerisation.

Solution 2

The student is incorrect. The polymer contains ester side groups, but its main backbone consists of carbon-carbon bonds, so it is not classified as a polyester.

(a) The main chain contains repeating carbon-carbon single bonds:

\[ \ce{-CH2-CH-CH2-CH-} \]

The ester groups are attached to the carbon backbone rather than forming connections along it.

(b) It does not belong to the polyester family under the usual backbone-based classification.

The presence of an ester group somewhere in a polymer is not sufficient. The ester linkages must occur repeatedly in the main chain.

(c) The corresponding monomer is methyl acrylate:

\[ \ce{CH2=CHCOOCH3} \]

During addition polymerisation, its carbon-carbon double bond opens, allowing the monomers to connect.

The ester group remains as a side group.

This question exposes an important distinction: identifying a functional group and identifying the structure of a polymer family are related tasks, but they are not identical.

Question 3

A sample of poly(butylene terephthalate), PBT, has a mass of 2.20 g.

Its repeating-unit formula is:

\[ \ce{C12H12O4} \]

Assume that:

• The average polymer chain contains 200 repeating units.
• All chains are linear.
• Terminal-group contributions are negligible.

Calculate:

(a) The molar mass of one repeating unit.

(b) The amount, in moles, of repeating-unit equivalents in the sample.

(c) The amount, in moles, of each parent monomer equivalent represented in the sample.

(d) The amount, in moles, of polymer molecules in the sample.

Use \(C=12.01\), \(H=1.008\), and \(O=16.00\).

Solution 3

The sample contains approximately \(9.99\times10^{-3}\) mol of repeating-unit equivalents and \(5.00\times10^{-5}\) mol of polymer molecules.

(a) Calculate the molar mass of one repeating unit.

\[ \begin{aligned} M_{\text{repeat}}&=12(12.01)+12(1.008)+4(16.00)\\ &=144.12+12.096+64.00\\ &=220.216\ \mathrm{g\,mol^{-1}} \end{aligned} \]

Therefore:

\[ M_{\text{repeat}}=220.22\ \mathrm{g\,mol^{-1}} \]

(b) Calculate the amount of repeating-unit equivalents.

\[ n=\frac{m}{M} \]

\[ \begin{aligned} n_{\text{repeat}}&=\frac{2.20}{220.22}\\ &=0.009990\ \mathrm{mol}\\ &\approx9.99\times10^{-3}\ \mathrm{mol} \end{aligned} \]

(c) Determine the monomer equivalents.

PBT is formed from:

• Butane-1,4-diol.
• Terephthalic acid.

Each repeating-unit equivalent contains one residue from each monomer.

Therefore:

\[ n(\text{butane-1,4-diol})=0.00999\ \mathrm{mol} \]

\[ n(\text{terephthalic acid})=0.00999\ \mathrm{mol} \]

(d) Calculate the amount of polymer molecules.

Each polymer molecule contains 200 repeating units.

Therefore:

\[ n_{\text{chains}}=\frac{n_{\text{repeat}}}{200} \]

\[ \begin{aligned} n_{\text{chains}}&=\frac{0.009990}{200}\\ &=4.995\times10^{-5}\ \mathrm{mol} \end{aligned} \]

Thus:

\[ n_{\text{chains}}\approx5.00\times10^{-5}\ \mathrm{mol} \]

The important distinction is that the amount of repeating units is 200 times the amount of polymer molecules.

A polymer sample contains relatively few individual chains compared with the total number of repeating units incorporated into those chains.

Question 4

An unknown polymer is analysed using ATR-FTIR spectroscopy.

The following illustrative absorption data are obtained:

A technician concludes that the unknown material must be PET.

Evaluate this conclusion.

In your response:

(a) Explain what the absorption data suggest about the functional groups present.

(b) Explain why the data do not uniquely identify PET.

(c) Propose an additional type of structural evidence that could help establish the specific polymer.

Solution 4

The technician’s conclusion is too strong. The spectrum supports the presence of ester functional groups, but it does not independently prove that the polymer is PET.

(a) The strong absorption around \(1730\ \mathrm{cm^{-1}}\) is consistent with carbonyl \(\ce{C=O}\) stretching.

The absorptions around \(1240\) and \(1100\ \mathrm{cm^{-1}}\) are consistent with C-O stretching.

Together, these observations support the presence of ester functional groups.

(b) Several different polymers contain ester linkages.

For example, PET and PBT are both polyesters, but they have different repeating structures.

Furthermore, some polymers contain ester groups as side groups rather than as part of their main backbone.

The spectrum therefore does not automatically establish:

• The identity of the parent monomers.
• Whether the ester linkages form the polymer backbone.
• The exact arrangement of atoms in the repeating unit.
• Whether the polymer is PET rather than another polyester.

(c) A more detailed structural investigation is required.

For example, NMR spectroscopy can provide information about the different chemical environments of atoms in the polymer.

This could help establish whether the structure contains the ethylene glycol-derived section and the para-substituted terephthalate section characteristic of PET.

Comparison with an authenticated reference spectrum would also strengthen the identification.

The key lesson is that evidence for an ester functional group identifies a chemical feature, not necessarily a specific polymer.

Question 5

An unknown polymer sample undergoes a detailed structural investigation.

The results indicate that:

• Its average repeating-unit formula is \(\ce{C10H8O4}\).
• It contains ethane-1,2-diol-derived sections.
• Its aromatic dicarboxylic acid components consist of 70.0 mol% terephthalic acid and 30.0 mol% isophthalic acid.
• The overall molar ratio of aromatic dicarboxylic acid components to ethane-1,2-diol components is 1:1.
• Spectroscopic evidence supports the presence of ester linkages.

A researcher concludes:

“The polymer must be PET because its repeating-unit formula is identical to PET and its diol-to-diacid ratio is 1:1.”

Evaluate the researcher’s conclusion.

Your answer must address:

(a) Whether the material can be identified as pure PET.

(b) Why the repeating-unit formula does not distinguish the two aromatic acid components.

(c) Whether the evidence establishes that both types of aromatic residue occur in the same polymer chains.

(d) What further evidence would be needed to distinguish a copolyester from a physical blend of two separate polyesters.

Solution 5

The researcher cannot conclude that the material is pure PET. The results establish an overall polyester composition containing both terephthalate and isophthalate components, but they do not establish how those components are arranged among the polymer chains.

This problem has several layers.

Step 1: Examine the aromatic acid components.

Terephthalic acid and isophthalic acid are positional isomers.

Both have the molecular formula:

\[ \ce{C8H6O4} \]

However, their carboxylic acid groups occupy different positions on the benzene ring.

• Terephthalic acid has a 1,4-substitution pattern.
• Isophthalic acid has a 1,3-substitution pattern.

Their molecular formulas are identical, but their structures are different.

Consequently, both can produce repeating-unit structures with the same overall formula when combined with ethane-1,2-diol.

Step 2: Interpret the 1:1 monomer ratio.

The total amount of aromatic dicarboxylic acid components is equal to the amount of ethane-1,2-diol components.

This is consistent with the expected overall composition of a linear polyester formed from bifunctional diols and dicarboxylic acids.

However, the ratio does not prove that every chain contains only terephthalate residues.

It describes the overall composition of the sample.

Step 3: Consider the two possible polymer arrangements.

There are at least two reasonable explanations for the results.

The first possibility is a copolyester.

Both terephthalate and isophthalate residues occur within the same polymer chains.

The second possibility is a physical blend.

Some chains are PET, while other chains are poly(ethylene isophthalate). The two different polymers are simply mixed together.

Both possibilities can produce the same overall 70:30 aromatic acid composition and the same average repeating-unit formula.

Step 4: Explain why the available evidence is insufficient.

Bulk composition tells us which structural components are present and their relative amounts.

It does not necessarily tell us which components are covalently connected within individual polymer chains.

The distinction is important because a physical mixture of two polymers is not chemically identical to a copolymer containing both structural units in the same chains.

The two materials can have different properties despite sharing the same overall composition.

Step 5: Identify the additional evidence required.

A technique that provides information about the intact polymer’s molecular connectivity is needed.

For example, suitable NMR analysis or carefully designed separation and fractionation studies could help determine whether the different aromatic residues occur within the same polymer chains or in separate polymer populations.

To identify pure PET, the structural evidence must establish the ethylene glycol-derived section and the 1,4-substituted terephthalate section without the additional isophthalate component.

The researcher’s original argument is tempting because the molecular formula and monomer ratio match those expected for PET.

But matching composition is not the same as matching molecular structure.

This is the final distinction to remember: a polymer’s family, repeating-unit formula, exact connectivity, and chain architecture are different pieces of structural information. You need enough evidence to establish each one before assigning a specific polymer name.

08Where to take this next

When an HSC Chemistry question gives you an unfamiliar polyester structure, work through the identification in layers:

1. Locate the ester linkages and establish whether they occur in the main backbone.
2. Identify the structural sections that make up the repeating unit.
3. Determine which parent monomer molecules could supply those sections.
4. Check the exact connectivity before naming the specific polymer.

For more practice reconstructing parent monomers from unfamiliar structures, see How to Find Polyester Monomers from a Repeating Unit.

For the separate task of drawing the complete PET structure from its starting molecules, see How to Draw PET from Ethylene Glycol and Terephthalic Acid.