chemistry

Repeat unit

Repeat unit is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Repeat unit rather than just read about it. In short: A repeat unit or repeating unit , or mer, is a part of a polymer whose repetition would produce the complete polymer chain (except for the end groups) by linking the repeat units together successively along the chain, like the beads of a necklace. A repeat unit is sometimes called a mer (or mer unit) in polymer chemistry. "Mer" originates from the Greek word meros, which means "a part".

Key takeaways

Repeat unit belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Repeat unit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Repeat unit from memory before moving on to harder problems.

Reference excerpt

A repeat unit or repeating unit , or mer, is a part of a polymer whose repetition would produce the complete polymer chain (except for the end groups) by linking the repeat units together successively along the chain, like the beads of a necklace.

A repeat unit is sometimes called a mer (or mer unit) in polymer chemistry. "Mer" originates from the Greek word meros, which means "a part". The word polymer derives its meaning from this, which means "many mers". The mer is not the same thing as a monomer—a mer is a repeating unit within a larger molecule, whereas a monomer is an actual molecule that exists independently, either prior to polymerization or after decomposition.

Overview One of the simplest repeat units is that of the addition polymer polyvinyl chloride, -[CH2-CHCl]n-, whose repeat unit is -[CH2-CHCl]-. In this case the repeat unit has the same atoms as the monomer vinyl chloride CH2=CHCl. When the polymer is formed, the C=C double bond in the monomer is replaced by a C-C single bond in the polymer repeat unit, which links by two new bonds to adjoining repeat units. In condensation polymers (see examples below), the repeat unit contains fewer atoms than the monomer or monomers from which it is formed. The subscript "n" denotes the degree of polymerisation, that is, the number of units linked together. The molecular mass of the repeat unit, MR, is simply the sum of the atomic masses of the atoms within the repeat unit. The molecular mass of the chain is just the product nMR. Other than monodisperse polymers, there is normally a molar mass distribution caused by chains of different length. In copolymers there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.

Other vinyl polymers Polyethylene may be considered either as -[CH2-CH2-]n- with a repeat unit of -[CH2-CH2]-, or as [-CH2-]n-, with a repeat unit of -[CH2]-. Chemists tend to consider the repeat unit as -[CH2-CH2]- since this polymer is made from the monomer ethylene (CH2=CH2). More complex repeat units can occur in vinyl polymers -[CH2-CHR]n-, if one hydrogen in the ethylene repeat unit is substituted by a larger fragment R. Polypropylene -[CH2-CH(CH3)]n- has the repeat unit -[CH2-CH(CH3)]. Polystyrene has a chain where the substituent R is a phenyl group (C6H5), corresponding to a benzene ring minus one hydrogen: -[CH2-CH(C6H5)]n-, so the repeat unit is -[CH2-CH(C6H5)]-.

Condensation polymers: repeat unit and structural units

In many condensation polymers, the repeat unit contains two structural units related to the comonomers which have been polymerized. For example, in polyethylene terephthalate (PET or "polyester"), the repeat unit is -CO-C6H4-CO-O-CH2-CH2-O-. The polymer is formed by the condensation reaction of the two monomers terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH), or their chemical derivatives. The condensation involves loss of water, as an H is lost from each HO- group in the glycol, and an OH from each HOOC- group in the acid. The two structural units in the polymer are then considered to be -CO-C6H4-CO- and -O-CH2-CH2-O-.

References

Worked examples

Example 1 — a first encounter with Repeat unit

Start with the simplest possible case. Write down what Repeat unit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Repeat unit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Repeat unit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Repeat unit

In research: Repeat unit appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Repeat unit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Repeat unit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Polymer chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Repeat unit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Repeat unit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Repeat unit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Repeat unit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Repeat unit in simple terms?

Why does Repeat unit matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Repeat unit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Repeat unit.