What Is a Polymer?

Introduction

A plastic carrier bag, a strand of wool, the rubber in a car tyre and the DNA in every one of your cells share one feature: each is made of enormous molecules built by joining small units end to end. These giant molecules are polymers , from the Greek poly (many) and meros (parts). Polymer chemistry asks how such chains form, how long they grow, how they pack together and why that gives materials that can stretch, flow, soften or stay rigid. This page sets out the vocabulary used throughout the unit.

Core explanation

Monomers and polymers. A monomer is a small molecule with at least two reactive sites (or a reactive double bond, which behaves as two sites) so that it can link to neighbours on both sides. Ethene, CH₂=CH₂, is a monomer; when thousands of ethene molecules add together, the product is poly(ethene), usually called polyethylene. A polymer is the resulting macromolecule: a long molecule whose structure consists of many repetitions of a simple unit.

The repeat unit. The repeat unit is the smallest fragment that, repeated, reproduces the chain. For polyethylene it is –CH₂–CH₂– (often written –[CH₂–CH₂]ₙ–). The repeat unit is not always identical to the monomer: in addition polymerisation the atoms are the same but a C=C double bond has become two single bonds to neighbouring units; in condensation polymerisation a small molecule such as water is lost, so the repeat unit contains fewer atoms than the monomers that formed it.

Degree of polymerisation. The number of repeat units in one chain is the degree of polymerisation , X (also written DP or n). The molar mass of a chain is then

M = X × M₀ (+ the mass of the two end groups)

where M₀ is the molar mass of the repeat unit. For long chains the end groups contribute a negligible fraction and are usually ignored. A polyethylene chain with X = 10 000 has M ≈ 10 000 × 28.05 g mol⁻¹ ≈ 2.8 × 10⁵ g mol⁻¹.

Scale. Typical commercial polymers have molar masses from about 10⁴ to 10⁶ g mol⁻¹, and ultra-high-molar-mass polyethylene exceeds 3 × 10⁶ g mol⁻¹. A fully stretched chain of X = 10 000 would be roughly a micrometre long but only a few tenths of a nanometre thick — a ratio like a strand of spaghetti hundreds of metres long. In reality the chain coils up on itself.

Why size matters. Short chains (oligomers) behave like ordinary molecular substances: a C₈ alkane is a volatile liquid and a C₃₀ alkane is a waxy solid. Once chains are long enough to entangle with one another, a threshold typically reached somewhere above a few hundred backbone atoms, the material gains strength, toughness and the ability to form fibres and films. Properties such as tensile strength rise steeply with chain length and then level off.

Chains are not all the same length. Almost every synthetic polymer is a mixture of chains with different values of X, so its molar mass is described by averages. That idea is developed fully later in the unit.

Step-by-step reasoning

To find the degree of polymerisation of a sample:

1. Identify the repeat unit and draw it with open bonds at each end. 2. Add up the atomic masses in the repeat unit to get M₀. 3. Divide the chain molar mass by M₀: X = M ÷ M₀. 4. Round to a whole number and check that the answer is plausible (hundreds to tens of thousands for a useful polymer).

Visual explanation

Imagine a single bead labelled –CH₂–CH₂–, then thousands of identical beads threaded onto one string. Now crumple that string into a loose ball and mix it with thousands of other similar strings. The picture shows the repeat unit, the chain, the random-coil shape and the entanglements that hold the material together.

Real-world analogy

A paperclip chain is a good model. Each paperclip is a repeat unit, the finished chain is the polymer, and the number of clips is the degree of polymerisation. A handful of loose clips pours like a liquid; long chains of clips tangle into a mass that you can lift as one piece.

Real-world example

Paraffin wax and polyethylene are both made of –CH₂– units. Wax has chains of roughly 20 to 40 carbon atoms and crumbles easily. Polyethylene for milk bottles has chains with thousands of carbon atoms and is tough enough to survive being dropped. Same chemistry, different chain length, completely different material.

Why?

Why does chain length change properties so dramatically? Intermolecular forces between chains add up along their length, and long chains become entangled so that one chain cannot slide past another without dragging its neighbours. Short molecules cannot entangle, so they separate easily and the solid is weak.

Common misconception

"A polymer is just a very big molecule of one fixed size." In a synthetic polymer the chains have a spread of lengths, and the sample is described by average molar masses rather than a single exact formula.

Worked example

Question: A sample of poly(propene), repeat unit –CH₂–CH(CH₃)–, has chains of molar mass 2.10 × 10⁵ g mol⁻¹. Find the degree of polymerisation.

Reasoning: The repeat unit is C₃H₆, so M₀ = 3(12.01) + 6(1.008) = 42.08 g mol⁻¹. X = 2.10 × 10⁵ ÷ 42.08 ≈ 4990.

Answer: About 5.0 × 10³ repeat units per chain.

Quick check

1. What is the degree of polymerisation of a polyethylene chain of molar mass 56 100 g mol⁻¹? Answer: X = 56 100 ÷ 28.05 = 2000 repeat units.

Exam focus

Be precise with vocabulary: monomer, repeat unit and polymer are different things. Show M₀ explicitly in calculations, state that end groups are neglected, and remember that the repeat unit of a condensation polymer has lost the small molecule eliminated in each linking step.

Advanced insight

IUPAC calls the smallest repeating fragment the constitutional repeating unit (CRU). For polymers made from two complementary monomers, such as nylon-6,6, the CRU contains one residue of each monomer, so a chain of X structural units corresponds to X/2 CRUs. Care is needed over which definition of X a textbook or data table is using.

Summary

Polymers are macromolecules made of many repeat units derived from monomers. The degree of polymerisation X counts the repeat units, and chain molar mass is M ≈ X × M₀. Useful polymers have molar masses of about 10⁴ to 10⁶ g mol⁻¹. Long chains entangle, which gives strength and toughness absent in short-chain analogues such as wax.

Practice questions

1. Distinguish between a monomer and a repeat unit, using poly(ethene) as your example. Answer: The monomer is ethene, CH₂=CH₂, a separate molecule with a double bond; the repeat unit is –CH₂–CH₂–, the fragment within the chain joined by single bonds to its neighbours. 2. Calculate the molar mass of a poly(chloroethene) (PVC) chain with X = 1500. The repeat unit is –CH₂–CHCl–. Answer: M₀ = 2(12.01) + 3(1.008) + 35.45 = 62.49 g mol⁻¹, so M ≈ 1500 × 62.49 ≈ 9.37 × 10⁴ g mol⁻¹. 3. Explain why a C₂₀ alkane is a weak waxy solid whereas polyethylene is tough. Answer: The C₂₀ chains are too short to entangle and have limited total intermolecular attraction, so they slide apart easily; polyethylene chains are long enough to entangle and have large cumulative dispersion forces. 4. Why are end groups usually ignored when calculating the molar mass of a polymer chain? Answer: There are only two end groups per chain, so for thousands of repeat units their mass is a negligible fraction of the total.