Condensation polymers — nylon-6,6, nylon-6, terylene (PET), bakelite, melamine, Kevlar: Foundations

Before you can follow nylon, PET (short for polyethylene terephthalate, the plastic in drink bottles — we spell it out in full in Section 8), or Kevlar, you must be able to read the shorthand the parent note uses without pausing. This page defines every symbol and idea from absolute zero, in build-order. Nothing here assumes you have met it before.

0. Reading a chemical drawing at all

Figure — Condensation polymers — nylon-6,6, nylon-6, terylene (PET), bakelite, melamine, Kevlar

Look at the figure: the single line is one handshake, the double line ( $C = O$ ) is a double handshake. That double bond appears in every linkage in this whole chapter, so lock it in now.

1. Condensed structural formulas — dashes and bonding parentheses

Before subscripts, meet the sideways way chemists write a molecule on one line — the condensed structural formula. The parent note uses it constantly, so define it now.

Definition Bonding parentheses — e.g.

$C (= O)$ Sometimes an atom has a branch hanging off the side, not along the line. We show that branch in round parentheses immediately after the atom it hangs from. So $C (= O)$ means: a carbon on the main line that also carries a double bond to an oxygen sticking out sideways. The parentheses here are structural — they say "this is bonded on as a side-branch", not "repeat this group".

Likewise $N (H)$ means "a nitrogen on the line that also carries one hydrogen off to the side". We could equally write the amine end as $- N H_{2}$ or $H_{2} N -$ (a nitrogen with two hydrogens); the parenthesis form $H - N (H) -$ is just those same two hydrogens drawn explicitly — one before the $N$ , one branching off it — so you can see each hydrogen separately when we track which one leaves.

2. Subscripts — the little numbers

3. What "polymer" even means

4. The subscript $n$ — "very many"

5. Functional groups — the "reactive ends"

A functional group is a small cluster of atoms that is the reactive part of a molecule — its "hands". The long boring $C H_{2}$ chain in between does nothing; the ends do all the chemistry.

In the figure each group is drawn as a coloured "hand". The leading dash $-$ in $- C O O H$ just means "this attaches to the rest of the molecule here".

6. "Bi-functional" — two hands per monomer

The three panels in the figure show exactly why hand-count decides whether you get a rope or a rigid mesh. Hold this picture: it is the difference between thermoplastic and thermosetting behaviour.

7. The condensation move itself

Now that dashes and structural parentheses are defined (Section 1), we can safely write the reaction on one line.

8. The arrow $⟶$ and the "over-arrow" conditions

9. Rings and the benzene hexagon

The figure contrasts a floppy $- (C H_{2})_{n} -$ chain (bends anywhere) with a rigid $- C_{6} H_{4} -$ rod. This rigidity plus Hydrogen bonding between chains is the whole secret of Kevlar's strength.

10. Hydrogen bonding — the weak "sticky note" between chains

Prerequisite map

The list below feeds the topic bottom-to-top; the same relationships are drawn in the diagram beneath it (if your reader doesn't render diagrams, the indented list is the map).

Atoms and bonds → feed skeletal drawings (hidden carbons) and functional groups ( $- C O O H$ , $- N H_{2}$ , $- O H$ ) and condensed formulas (dashes, structural parentheses).
Polymer (many parts in one) → feeds the subscript $n$ / repeat-unit notation.
Functional groups → feed the condensation move (loses water) and the hand-count (bi- vs tri-functional).
Condensation move → feeds the amide and ester linkages.
Hand-count → feeds chain vs 3-D network.
Atoms and bonds → also feed rings and the benzene hexagon → rigid aromatic chains → hydrogen bonding between chains.
All of these arrows converge on the Condensation polymers topic.

Read this top-down: atoms feed groups, condensed formulas and skeletal drawings, the idea of a polymer feeds the $n$ notation, groups feed the condensation move and the hand-count, and those plus rings and hydrogen bonding feed the whole parent topic.

Equipment checklist

Self-test: cover the right side and answer aloud.

What does a plain line (or a dash $-$ on the same line) between two atom letters mean? ::: One shared electron pair = one bond.
In a zig-zag skeletal drawing, what sits at each bare corner and line-end? ::: A carbon atom, with enough hidden hydrogens to give it 4 bonds.
What does the $=$ in $C = O$ mean? ::: A double bond (two shared pairs, stiffer/stronger).
In $C (= O)$ , what do the parentheses tell you? ::: A structural side-branch — an oxygen double-bonded off the side of that carbon (NOT a "repeat this" grouping).
How do you tell "repeat" parentheses from "side-branch" parentheses? ::: Look after the closing bracket: a trailing number means repeat ( $(C H_{2})_{4}$ ); no number means a bonded side-branch ( $C (= O)$ ).
If a letter has no subscript, how many of that atom are present? ::: One (blank defaults to 1).
How many carbons does $H O O C - (C H_{2})_{4} - C O O H$ have? ::: 6 — the 4 in the bracket plus the 2 carboxyl carbons.
What does $[\dots]_{n}$ mean? ::: Repeat the bracketed repeat unit a large number ( $n$ ) of times.
Which three functional groups drive this chapter? ::: $- C O O H$ (acid), $- N H_{2}$ (amine), $- O H$ (alcohol).
In the amide condensation, which atoms leave and from where? ::: The whole $- O H$ from the acid plus one $- H$ from the amine, combining into one water.
Why does that particular $H$ leave and not the $C = O$ oxygen? ::: The $C = O$ oxygen is locked in a strong double bond; only the single-bonded $- O H$ and a spare amine $- H$ can leave to form water.
Acid + alcohol gives which linkage and what leaves? ::: An ester $- C O - O -$ link, releasing water.
What is the difference between a symbol over the arrow and one after it? ::: Over the arrow = a condition you supply (heat, added water); after the arrow = a byproduct the reaction makes.
What does $Δ$ over an arrow mean? ::: Heat is applied.
What does the acronym PET stand for, and which linkage family is it? ::: Polyethylene terephthalate — a polyester (acid + alcohol → ester).
What does $- C_{6} H_{4} -$ represent? ::: A benzene hexagon bonded into the chain at two corners.
What is a hydrogen bond and why does Kevlar need it? ::: A weak $N - H \dots O$ attraction; millions of them zip chains into strong sheets.
What single feature separates condensation from addition polymers? ::: Condensation throws away a small molecule (water/HCl) at each bond; addition loses nothing.