What a polymer is — Polymers

A polymer is a molecule built by joining many small units end to end. The small unit is the monomer, the repeating piece inside the chain is the repeat unit, and the number of repeats is the degree of polymerization.

Nothing in this chapter is a new reaction. Polymers are made by the reactions you already know, run over and over on a molecule that has two reactive ends instead of one. That is the only structural requirement, and it is the whole idea.

Two ways to build a chain

Every polymerization in this course is one of two kinds, and the difference is whether anything is left over.

	Addition (chain-growth)	Condensation (step-growth)
Monomer	An alkene	Two functional groups per monomer
Small molecule lost	None	Water or an alcohol
Repeat unit vs monomer	Same atoms	Monomer minus what was lost
How chains grow	One active end, adding fast	Any two pieces can join
Examples	Polyethylene, PVC, polystyrene	Nylon, polyester, Kevlar

The fastest way to tell them apart is to count atoms. Compare the repeat unit with the monomer: if the repeat unit is lighter, something was condensed out and it was a step-growth. If the formulas match, nothing was expelled — and for an alkene monomer that means the π bond simply opened, so it was an addition. The one case that breaks the shortcut is a ring-opening polymerization, where a cyclic monomer such as caprolactam becomes nylon 6 with no change of formula at all; check whether the monomer has a C=C before you trust it.

Addition polymerization: one active end

The count that runs the whole chapter. One reactive site makes a single bond and stops; two extends a line; three or more ties the lines to each other in every direction, which is what a network is. Count one functional group per site for a step-growth monomer and one C=C for a chain-growth one — counted that way, a vinyl monomer has two sites and divinylbenzene, with two C=C, is the cross-linker.The consequence reaches all the way to the end of the material’s life. Separate chains are held to each other by intermolecular forces, so heat lets them slide and a thermoplastic can be melted and remolded. A network is one covalent molecule, so heating it breaks bonds rather than loosening them — and a thermoset cannot be recycled by melting at all.

An alkene polymerizes by the mechanism you already have from the addition chapter. A radical, cation or anion adds to the double bond, and the new reactive center adds to the next monomer, and so on.

Radical — an initiator such as a peroxide starts it. This is how low-density polyethylene and polystyrene are made. Most of the world's polyethylene is not radical-made: HDPE and LLDPE come from coordination catalysts, which the next section takes up.
Cationic — works when the alkene bears donating groups, because the growing cation must be stabilized.
Anionic — works when the alkene bears withdrawing groups, for the mirror-image reason.

Regiochemistry follows the same rule it always has: whichever end of the alkene gives the more stable intermediate is where the chain attaches. For a monosubstituted alkene that gives head-to-tail linking, with all the substituents on alternating carbons.

Condensation polymerization: everything joins everything

Here each monomer carries two functional groups, and any two pieces can react with each other — monomer with monomer, monomer with dimer, dimer with trimer. Chains grow slowly at first and the molecular weight climbs steeply only at very high conversion.

That is a real practical difference. In an addition polymerization long chains exist from the start; in a step-growth, 99% conversion still gives a degree of polymerization of only about 100, so the last fraction of a percent matters enormously.

A monomer needs two reactive sites to give a chain. One site gives a small molecule and stops; three or more gives a cross-linked network rather than a chain, which is what makes a thermoset. Counting the reactive sites predicts the material.

Why the properties follow the structure

Chain length raises melting point and strength, because longer chains have more surface in contact and entangle more.
Regularity lets chains pack into crystalline regions. A stereoregular polymer is stronger and denser than the same polymer made randomly.
Branching prevents packing. Low-density polyethylene is branched and floppy; high-density polyethylene is linear and rigid, and they are the same repeat unit.
Cross-links tie chains to each other permanently. A few make rubber elastic; many make a rigid thermoset that cannot be melted at all.

That last distinction has a name worth knowing. A thermoplastic has separate chains held together by intermolecular forces, so heating lets them slide and it can be melted and remolded. A thermoset is one covalent network, so heating destroys it rather than softening it.

What carries forward

Count the reactive sites to know whether you get a chain or a network. Count the atoms to know whether it was addition or condensation. And read the chain's regularity and branching to predict whether the material is a bag or a bottle — because polyethylene is both, and the difference is not chemistry but architecture.