the mer

The word polymer comes from Greek: poly means many, mer means part. So a polymer is literally many parts, and the mer is that one part — the small chunk of chain that repeats again and again. Think of a beaded necklace: the whole necklace is the polymer, and the mer is the single bead pattern that gets copied hundreds or thousands of times down the string. If you know what one bead looks like and how many there are, you know the whole necklace.

For polyethylene the mer is -CH2-CH2-, and the chain is simply that unit written out n times, (CH2-CH2)n. Knowing the mer lets you do real bookkeeping: the mer of polyethylene weighs 28 g/mol (two carbons at 12 plus four hydrogens at 1), so a chain of molecular weight 280,000 g/mol must contain 280,000 / 28 = 10,000 mers. Different plastics are just different mers repeated: propylene's mer carries a CH3 side group, vinyl chloride's carries a chlorine atom, styrene's carries a benzene ring.

The mer matters because a polymer's whole personality is set by which small unit repeats and how it is arranged. It is worth keeping the mer and the monomer straight, because beginners mix them up constantly: the monomer is the free reactive molecule you start with, while the mer is the unit as it sits locked inside the finished chain. For addition polymers the two have the same atoms; for condensation polymers the mer is lighter, because a small molecule was thrown off when the bond formed.