What a learner can do afterwards
- Draws the repeat unit of an addition polymer from its monomer, and the monomer back from the repeat unit
- Draws the repeat unit of a polyester and of a polyamide and names the small molecule lost
- Explains why condensation polymers can be hydrolysed while poly(alkenes) resist
- Links the forces between chains to the strength and melting behaviour of a named polymer
1 · Read
Some plastics grow when alkene monomers open their double bonds and link up. Picture each double bond popping open so both end carbons grab a neighbour instead. Nothing leaves, so the chain just repeats the monomer formula.
Take ethene joining into polythene, the stuff of carrier bags. You draw the repeat unit as -CH2-CH2- inside brackets, with an n after the brackets for the many repeats, and you can read it straight back to ethene. The monomer and the repeat hold the same atoms, only linked into a chain.
Other plastics join by squeezing out a small molecule at every link. You join a diacid to a diol to make a polyester, or to a diamine to make a polyamide, and each new link lets go of one water molecule. A polyester repeat unit holds one acid part and one diol part joined by an ester link, -COO-. That ester or amide link can be hydrolysed by heating with acid or alkali, so a polyester bottle can return to monomers while a polythene bag cannot.
You tell the two families apart by asking what was lost. If nothing left, the chain resists splitting with water. Strong attractions between chains, like hydrogen bonds in a polyamide, also lift strength and melting point above chains held only by weak London forces.
Addition chains open double bonds and lose nothing, while condensation chains lose water at each link and can split back.
Readings 'Polyesters and polyamides' and 'Why polythene lasts so long' are original to this dataset.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.