Polymers

What is it?

A polymer is a very large molecule made by joining many small molecules, called monomers, into a long chain. Plastics, rubber, nylon, DNA, proteins, starch and cellulose are all polymers.

• The repeat unit is the part of the chain that repeats over and over. It is drawn in brackets with an n, and the bonds at each end extend through the brackets.
• The degree of polymerization, n, is the number of repeat units in a chain, often thousands.

There are two ways to build a polymer:

• Addition polymerization: monomers with a C=C double bond join; the double bond opens and nothing else is formed.
• Condensation polymerization: monomers with two functional groups each join, and a small molecule (usually water) is released at every link.

$$n\,\ce{CH2=CH2} \longrightarrow \text{–[CH}_2\text{–CH}_2\text{]}_{n}\text{–}$$

Why does it matter?

• Materials. Poly(ethene) bags, PVC pipes, PTFE non-stick coatings, polyester and nylon clothing, and car tyres are polymers designed for particular properties.
• Life. Proteins (from amino acids), DNA (from nucleotides) and starch and cellulose (from glucose) are natural condensation polymers.
• Environment. Most plastics are not biodegradable, so their disposal, recycling and replacement are major environmental issues.

How does it work?

1. Common addition polymers

Monomer	Polymer	Repeat unit	Uses
ethene, $\ce{CH2=CH2}$	poly(ethene)	–[CH₂–CH₂]–	bags, bottles
propene, $\ce{CH2=CHCH3}$	poly(propene)	–[CH₂–CH(CH₃)]–	ropes, crates
chloroethene, $\ce{CH2=CHCl}$	poly(chloroethene), PVC	–[CH₂–CHCl]–	pipes, window frames
tetrafluoroethene, $\ce{CF2=CF2}$	PTFE	–[CF₂–CF₂]–	non-stick coatings

2. From monomer to repeat unit (and back)

1. Draw the monomer with the C=C in the middle and the other groups above and below.
2. Change the C=C to a C–C single bond.
3. Add a bond sticking out from each carbon, through a pair of brackets, and write n after the closing bracket.

To find the monomer from a repeat unit, reverse this: take the two main-chain carbons, remove the end bonds and put back the C=C.

3. Condensation polymers

Each monomer needs two reactive groups, one at each end, so the chain can keep growing in both directions:

• Polyesters (such as PET, used for drinks bottles and fleece): a dicarboxylic acid + a diol, joined by ester links (–COO–).
• Polyamides (such as nylon-6,6): a dicarboxylic acid + a diamine, joined by amide links (–CONH–):

$$\begin{aligned} &\small \ce{HOOC(CH2)4COOH + H2N(CH2)6NH2} \\[2pt] &\quad \small \ce{-> C12H22N2O2 + 2H2O} \end{aligned}$$

(Here $\ce{C12H22N2O2}$ is one repeat unit of the chain, with its two amide links.)

• Proteins: amino acids (each with an –NH₂ and a –COOH group) joined by amide links, called peptide links.

4. Properties and disposal

• Thermoplastics (poly(ethene), PVC) are made of separate chains that slide past each other when heated, so they soften and can be remelted and recycled.
• Thermosets have cross-links (covalent bonds between chains), so they do not melt; they char when heated.
• Addition polymers have only strong, unreactive C–C and C–H bonds, so they are not biodegradable and persist for centuries. Condensation polymers can be broken down by hydrolysis at their ester or amide links, and some are designed to be biodegradable.
• Incineration recovers energy but releases $\ce{CO2}$; burning PVC also releases toxic $\ce{HCl}$ gas.

Visualise it

Worked example

Common mistake

Remember this