Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
In condensation polymerisation monomers join with the loss of a small molecule at every link: H2O when a carboxylic acid is used, HCl when an acyl chloride is used. Each monomer needs two reactive groups. A diol plus a dicarboxylic acid (or diacyl dichloride) gives a polyester, for example PET (Terylene) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid; a single hydroxycarboxylic acid such as lactic acid can also polymerise on its own, giving poly(lactic acid). A diamine plus a dicarboxylic acid or diacyl dichloride gives a polyamide, for example nylon-6,6 from hexane-1,6-diamine and hexanedioic acid, or Kevlar from aromatic monomers. Proteins are natural polyamides made from amino acids, joined by peptide (amide) links. The repeat unit contains one of each monomer residue, so its Mr is the sum of the monomers minus the small molecules lost: 116 + 146 − 2(18) = 226 for nylon-6,6.
In a condensation polymer the repeat unit contains one residue of each monomer and the links inside the chain: do not leave H on the N end and OH on the C=O end of a repeat unit. Every link releases one small molecule, so forming n repeat units of nylon-6,6 releases about 2n H2O, not n. With a diacyl dichloride the molecule lost is HCl, not water.
To deduce monomers from a condensation polymer, find each ester (–COO–) or amide (–CONH–) link, break the C–O or C–N bond, and add back OH to the carbonyl carbon and H to the O or N (or Cl to the carbonyl carbon if a diacyl dichloride is asked for). An addition polymer has a backbone of carbon atoms only; its monomer is found by taking a two-carbon repeat unit and putting back the C=C. A monomer with a C=C bond undergoes addition; monomers with two reactive groups (OH, NH2, COOH, COCl) undergo condensation. Properties follow from the intermolecular forces between chains: the N–H and C=O groups of polyamides form hydrogen bonds between chains, which makes nylon and Kevlar strong and high-melting.
When cutting an amide link, the C=O stays with the acid monomer and the N–H with the amine: do not split the C=O away from its carbon. A polymer with a C–C backbone and side groups such as –CN or –COOCH3 is still an addition polymer, even if the side group is an ester. Polyesters cannot hydrogen bond chain to chain, because they have no H on O or N.
Poly(alkenes) such as poly(ethene) contain only strong, non-polar C–C and C–H bonds, so they are chemically inert and non-biodegradable: they persist in landfill and the oceans. Polyesters and polyamides contain polar C=O groups in their ester and amide links, so they can be hydrolysed by hot aqueous acid, alkali or by enzymes in microorganisms; they are biodegradable. Acid hydrolysis gives the free acid and the alcohol or the ammonium salt of the amine; alkaline hydrolysis gives the carboxylate salt and the alcohol or free amine. Photodegradable polymers contain C=O groups that absorb ultraviolet radiation; the energy breaks nearby bonds in the chain so the polymer crumbles. Burning plastics releases CO2 and can release toxic gases such as HCl from PVC.
Poly(alkenes) are not biodegradable because their C–C and C–H bonds are non-polar and strong, not because they are "too big". Biodegradable and photodegradable are different: one needs hydrolysis (water, acid, alkali or enzymes) at ester or amide links, the other needs UV light absorbed by C=O groups. In alkaline hydrolysis the acid monomer appears as its carboxylate salt, and in acid hydrolysis of a polyamide the amine appears as its ammonium salt.