4.6.1 · D5Polymers
Question bank — Classification — natural vs synthetic; addition vs condensation; thermoplastic vs thermosetting
The three axes, before you start
The whole point of this bank is that a polymer answers three independent questions. Picture three separate dials, each set on its own — they do not move each other.
To hammer this home, here are real polymers dropped into a matrix of the two hardest-to-separate axes — mechanism (rows) versus thermal behaviour (columns). If the axes were secretly linked, one diagonal would be empty. It is not — all four cells are filled.
Recall Why the filled matrix matters
If "addition ⇒ thermoplastic" were a law, the addition + thermosetting cell would be empty. But vulcanised rubber sits there. A single counter-example in each cell destroys the false shortcut — this is the visual proof behind every "the axes are independent" answer below.
The words this page relies on (defined here, once)
Several answers below use words the parent note assumed. Build them here so you never have to leave the page.
True or false — justify
All condensation polymers are thermosetting
False. Mechanism and thermal behaviour are independent axes — nylon-6,6 and PET are condensation yet thermoplastic because their chains are linear, not cross-linked.
Every addition polymer conserves the monomer's empirical formula in its repeat unit
True. The C=C π-bond only opens to form new σ-bonds; no atom leaves, so the repeat unit has the same formula as the monomer (e.g. ethene C₂H₄ → repeat unit C₂H₄).
A polymer must have exactly one answer on each of the three axes
True. The axes are independent questions, so any polymer is (one source) × (one mechanism) × (one thermal type) — e.g. Bakelite = synthetic, condensation, thermosetting.
Natural rubber is a condensation polymer because it comes from a tree
False. Source (natural) says nothing about mechanism — natural rubber is an addition polymer of isoprene (a C=C monomer); no small molecule is lost.
Cellulose acetate (rayon) is a synthetic polymer
False. It is semi-synthetic: a naturally occurring polymer (cellulose) that has been chemically modified, not built from scratch in the lab.
Thermosetting plastics melt if you heat them enough
False. Their chains are locked by strong covalent cross-links; heat cannot break these before the whole molecule decomposes, so they char/decompose rather than melt.
Thermoplastics can be remoulded because their covalent bonds break and reform on heating
False. The covalent backbone stays intact — it is the weak intermolecular forces (van der Waals, H-bonds) between chains that loosen on heating and reform on cooling, allowing reshaping.
Losing a small molecule during synthesis always means the mechanism is condensation
True. Elimination of H₂O, HCl or NH₃ per bond formed is the defining signature of condensation (step-growth); addition eliminates nothing.
PVC and Bakelite share the same thermal classification
False. PVC is linear → thermoplastic; Bakelite is a 3-D cross-linked network → thermosetting. Same axis, opposite answers.
Semi-synthetic polymers are always biodegradable like their natural parent
False. Chemical modification (e.g. nitration to gun cotton) can strip the very groups microbes attack, so biodegradability is not guaranteed — see Biodegradable Polymers.
Spot the error
"Teflon is condensation because forming a big chain must release something." — find the error
Wrong. Teflon forms from CF₂=CF₂ by the double bond opening — nothing is released, so it is addition. "Big chain must release something" is a false intuition.
"Nylon-6,6 is addition because two monomers add together." — find the error
"Add together" in everyday speech ≠ addition polymerisation. Each amide link expels one H₂O, so it is condensation. Addition specifically means C=C opening with zero by-product.
"Bakelite is thermoplastic because plastics can be melted and reshaped." — find the error
Bakelite is a covalently cross-linked network; it sets permanently and chars on heating. Not all "plastics" soften — only thermoplastics do.
"Vulcanised rubber is thermoplastic because natural rubber is soft." — find the error
Vulcanisation adds sulphur cross-links, converting the soft rubber into a cross-linked network — that is thermosetting behaviour. See Natural Rubber and Vulcanisation.
"Condensation polymers lose exactly one water molecule total." — find the error
One small molecule is lost per bond formed, not per whole chain. For n diol + n diacid you form ≈2n links, losing ≈2n waters (mass bookkeeping figure above).
"Because addition conserves mass, an addition polymer's mass equals the sum of all monomer masses." — find the error
The statement is actually correct — addition eliminates nothing, so M(polymer) = n × M(monomer) since the small-molecule count is 0 in the bookkeeping equation. The "error" is the trap of doubting it.
"Starch is synthetic because it's processed in factories." — find the error
Processing does not change origin. Starch is produced by plants → it is a natural polymer regardless of how it is later handled.
Why questions
Why does opening a C=C double bond conserve mass
The π-electrons of the double bond become new σ-bonds between monomers; no atoms are removed, so the repeat unit keeps the monomer's formula.
Why do thermoplastics soften reversibly but thermosets do not
Thermoplastics are held between chains only by weak forces (van der Waals, H-bonds — defined above and in Intermolecular Forces) that loosen and reform; thermosets are locked by strong covalent cross-links that cannot re-form once broken.
Why is "addition vs condensation" independent of "thermoplastic vs thermosetting"
One axis describes the reaction mechanism (what happens to monomers); the other describes chain architecture (linear vs cross-linked). Neither dictates the other — see the filled matrix above.
Why must condensation monomers each carry two functional groups
A chain grows only if each unit can bond on both ends; with one reactive group the reaction stops after a single link (see Condensation Polymerisation).
Why do most natural polymers biodegrade while most synthetics do not
Natural polymers have bonds (ester, amide, glycosidic) that enzymes evolved to cleave; many synthetic C–C backbones have no matching enzyme, so microbes cannot break them.
Why does more cross-linking generally make a polymer harder and more heat-resistant
Cross-links tie chains into a rigid 3-D network so they cannot slide past one another, resisting both deformation and softening on heating.
Why can a copolymer's properties differ from either homopolymer
Combining two different monomers (e.g. Buna-S) blends and interrupts their packing and forces, giving properties neither pure polymer shows — see Copolymers.
Edge cases
A monomer with a C=C double bond AND two –OH groups — which mechanism
It depends on the reaction chosen: if the C=C opens with no loss it is addition; if the –OH groups condense with loss of water it is condensation. The functional groups present allow either path.
An addition polymer that is later cross-linked (e.g. rubber + sulphur) — its final thermal class
Once cross-linked it becomes thermosetting, even though it was built by addition. This proves the axes are truly independent.
A "polymer" made of just two monomer units (n = 2) — is it a polymer
Practically no; it is a dimer/oligomer. Polymer status requires a large repeating chain — the "giant molecule" definition of the parent note.
Heating a thermoplastic far above its softening point — what happens at the extreme
First it softens/melts (reversible); pushed hotter it eventually decomposes — so even a thermoplastic has an upper limit where the covalent backbone finally breaks.
A condensation reaction where the eliminated molecule is HCl instead of water — still condensation
Yes. Condensation is defined by loss of any small molecule (H₂O, HCl, NH₃), not water specifically.
A perfectly linear polymer with strong H-bonds between chains (e.g. nylon) — thermoplastic or thermoset
Thermoplastic. H-bonds are strong for intermolecular forces but still far weaker than covalent cross-links, so heating overcomes them and the polymer softens.
Zero small molecules eliminated but two different monomers used — what is this
Still addition by mechanism (no by-product), but using two monomers makes it a copolymer — the two labels describe different things and coexist.
Connections
- Addition Polymerisation Mechanism — why C=C opening loses no atoms.
- Condensation Polymerisation — bifunctional monomers and small-molecule loss.
- Natural Rubber and Vulcanisation — an addition polymer cross-linked into a thermoset.
- Intermolecular Forces — the weak forces behind thermoplastic softening.
- Biodegradable Polymers — why source affects breakdown.
- Copolymers — two monomers, one chain.