Everything in the parent classification note silently assumes you can decode a fistful of symbols. Below we earn each one — plain words, then a picture, then why the topic cannot do without it. Read top to bottom; each rung stands on the one below it.
Look at figure s01. Two balls (atoms) are pulled together by a spring — the spring is the shared electron pair. Chemists are lazy artists, so instead of drawing the electrons they just draw the spring as a single line.
Why the topic needs it. Every polymer chain is literally a row of these lines. When the parent note writes CH2−CH2, the little dash – between the two carbons is one of these single bonds. You cannot count what is lost or kept in a reaction until you can see each line as one bond.
In figure s02 the double bond is drawn as two springs side by side. The lower spring (σ) is short and tight. The upper spring (π) is stretched and floppy — it is the one that snaps first when the molecule reacts.
Why the topic needs it. The parent note says addition monomers "usually have a C=C double bond." That "=" is not decoration — it is the reason those monomers can join without losing any atoms (§7 below). See Addition Polymerisation Mechanism for the full snap-and-grab story.
So CH2 = one carbon carrying two hydrogens. H2O = two hydrogens plus one oxygen = water. NH2 = one nitrogen with two hydrogens.
Why the topic needs it. To check "did the polymer lose mass?" you add up atoms. You literally cannot do the mass bookkeeping in the parent's [!formula] callout without reading subscripts correctly.
Figure s03 draws each group as a little hand at the end of a rod (the rod = the rest of the molecule).
Why the topic needs it. Condensation is defined by these hands shaking and squeezing out a small molecule. Without knowing what −OH and −COOH are, the parent's polyester equation is just noise. Deep-dive: Condensation Polymerisation.
When the parent writes HO–R–OH, read it as "an O–H hand, then some chain we're not fussy about, then another O–H hand." Using R lets us talk about the pattern of the reaction without drawing every atom.
Why the topic needs it. It keeps the general reaction readable. If we drew all six carbons of a real diol every time, the important part — the hands and what they do — would be buried.
Figure s04 lines up a monomer, then shows the same shape clipped inside a bracket, then the bracket stamped n times into a long chain. The bracket is a "copy me" stamp.
Why the topic needs it. The entire distinction "repeat unit = monomer (addition) vs repeat unit < monomers (condensation)" lives in this bracket. It is the topic's core comparison.
So when the parent says addition "conserves mass," it means: nothing left the scene, so polymer weight = n × monomer weight. When condensation "loses mass," it means: some small molecules (the squeezed-out water etc.) walked away from the chain, so the chain weighs less than the bricks did — but if you weigh the chain plus the escaped water, the total is still conserved.
Why the topic needs it. This one line is how you tell addition from condensation on paper. Everything in §3 and §4 exists to let you read it.
Now the thermal axis. Two more pictures finish the toolkit.
Picture the difference like this: chains held only by weak intermolecular forces are like uncooked spaghetti strands lying loosely together — warm them and they slide apart (that is thermoplastic, remouldable). Chains welded by strong covalent cross-links are one giant rigid net — heat cannot un-weld them, so they char instead of softening (thermosetting).
Why the topic needs it. "Thermoplastic vs thermosetting" is entirely the question "is this chain held together by the weak between-glue or the strong cross-link glue?" You cannot answer it without these two words. See how sulphur does this on purpose in Natural Rubber and Vulcanisation.
Read it as: the plain single line at the top holds up everything; double bonds and functional groups branch into the two mechanism axes; cross-links branch into the thermal axis; all three axes meet in the classification topic. Related detail lives in Copolymers and Biodegradable Polymers.
What does one straight line between two atoms represent?
One shared pair of electrons — a single (σ) bond.
What extra thing does a double line "=" have, and why does it matter?
A second, weaker π-bond pair; it is the breakable pair that "opens" to start addition polymerisation.
In CH2, what does the small "2" count?
The number of hydrogen atoms on that one carbon (a subscript counts atoms in one molecule).
What is the difference between a subscript and a coefficient like 2H2O?
Subscript counts atoms inside a molecule; a coefficient in front counts whole molecules.
Why must a condensation monomer be bifunctional (two functional groups)?
So it can grab a neighbour on both sides and extend the chain in two directions.
What does −(CH2−CH2)−n mean?
The unit in brackets repeated n (a large whole number of) times.
Is the repeat unit always identical to the monomer?
No — same formula for addition, but lighter than the monomers for condensation because a small molecule was lost.
What does "addition conserves mass" mean in atom terms?
No atoms leave the chain, so polymer mass = n × monomer mass.
What is a cross-link?
A covalent bond tying one chain sideways to a neighbouring chain, forming a rigid network.
Which glue is strong and which is weak: covalent bond vs intermolecular force?
Covalent (inside/cross-links) is strong; intermolecular (between chains) is weak.
Why can thermoplastics be remoulded but thermosets cannot?
Thermoplastics are held by weak intermolecular forces that loosen on heating; thermosets are welded by strong covalent cross-links that heat cannot break.