What is Repeat unit, degree of polymerisation?

Example 49.4 (A polyethene chain)

A chain of polyethene of molar mass $280\,000\,\mathrm{g}/\mathrm{mol}$ has $n = 280000/28.0 = 10000$ repeat units, so twenty thousand carbon atoms in a row. In a real sample the chains do not all have the same length: $n$ is an average.

Example 49.8 (PET, a polyester)

Ethane-1,2-diol, $\ce{HO-CH2-CH2-OH}$, carries two alcohol groups; benzene-1,4-dicarboxylic acid, $\ce{HOOC-C6H4-COOH}$, two acid groups. Each acid group forms an ester with an alcohol group and releases water: the chain grows from both ends. The polymer is poly(ethylene terephthalate), PET, the material of drink bottles and of polyester fibres. Its repeat unit, $\ce{C10H8O4}$, has a molar mass of $192.0\,\mathrm{g}/\mathrm{mol}$, that is the two monomers ($62.0 + 166.0$) minus two molecules of water ($36.0$).

Example 49.11 (Two polyethenes)

Polyethene made under very high pressure has many short branches: its chains pack badly, and it gives soft, transparent films for bags. Made with a catalyst at low pressure, its chains are nearly linear and pack into crystalline regions: it is denser and stiffer, and gives milk bottles and crates. Same monomer, same repeat unit, different architecture.