Exercises — Classification — natural vs synthetic; addition vs condensation; thermoplastic vs thermosetting

Ask one question per item: did nature make it, did a factory make it from scratch, or did we take a natural polymer and chemically change it?

• Natural (nature made it whole): cellulose, starch.
• Synthetic (built in lab from small monomers): PVC, Teflon.
• Semi-synthetic (natural backbone, then chemically modified): rayon (cellulose acetate), gun cotton (cellulose nitrate).

Why rayon is not "natural": the cellulose came from nature, but we bolted acetate groups onto it — a chemical change. Modified natural = semi-synthetic.

The tell-tale sign is what the monomer carries.

• (a) Monomer has a C=C double bond and only that. A double bond can just "open" and reconnect — nothing gets ejected → addition.
• (b) Two bifunctional monomers ($-{\text{NH}}_2$ and $-\text{COOH}$). When $-{\text{NH}}_2$ meets $-\text{COOH}$ they form an amide bond and squeeze out water → condensation.

Remouldable = thermoplastic = held by weak forces only.

• Remouldable: polythene, PVC (linear chains, weak van der Waals forces).
• Not remouldable (thermosetting, 3-D covalent network, chars instead): Bakelite, melamine.

• Source: man-made in industry → synthetic.
• Mechanism: the monomer has a C=C double bond; it opens to form σ-bonds with the next unit, no small molecule leaves → addition.
• Thermal: the chains are long and linear with no crosslinks, held only by weak forces, so it softens on heating → thermoplastic. Answer: synthetic, addition, thermoplastic.

Both monomers are bifunctional (each has two reactive ends). An $-\text{OH}$ reacting with a $-\text{COOH}$ makes an ester bond and ejects water → condensation, small molecule = H₂O. This is PET. Its chains are linear with no crosslinks → thermoplastic (which is why PET bottles can be melted and recycled).

The C=C opens; the repeat unit is $-({\text{CH}}_2-\text{CHCl})-$. This is addition, so mass is conserved. Check: monomer ${\text{C}}_2{\text{H}}_3\text{Cl}$ has molar mass $=2(12)+3(1)+35.5=62.5$. The repeat unit is also ${\text{C}}_2{\text{H}}_3\text{Cl}$ = $62.5$. Identical → no atoms lost. ✔

• Mechanism is fixed at birth: the chain was built by addition of isoprene's double bonds. Vulcanisation happens afterward, so it cannot rewrite how the chain was originally assembled → still addition. (See Natural Rubber and Vulcanisation.)
• Thermal class changes because crosslinks appear: vulcanisation inserts sulphur bridges (strong covalent crosslinks) between chains. Before: only weak forces → soft, thermoplastic-like. After: a 3-D covalent network → thermosetting (can't remould, chars). Lesson: the mechanism axis and the thermal axis are independent. Adding crosslinks after synthesis changes only the thermal behaviour.

• Step — what holds it together: a dense 3-D network of strong covalent C–C / C–O bonds links every chain to its neighbours.
• Step — what melting would require: to flow, chains must slide past each other. That means breaking the crosslinks.
• Why heat can't do it reversibly: covalent crosslink energies are huge compared to the weak forces in a thermoplastic. Before you supply enough heat to break them cleanly, the whole network decomposes (chars). Conclusion: no soft, flowable state exists between "solid Bakelite" and "burnt Bakelite" → thermosetting, sets forever.

Student B is right. Mechanism (addition/condensation) and thermal class (thermoplastic/thermosetting) are independent axes. Counterexample: vulcanised natural rubber is built by addition yet, once sulphur-crosslinked, it is thermosetting. So "addition ⇒ thermoplastic" is false.

• Line up the monomers alternately: diol–diacid–diol–diacid… For $n$ diol + $n$ diacid = $2n$ total monomer units in one chain, the number of links (bonds) between adjacent units is $2n-1$.
• Here $n=5$, so $2n=10$ units → $2n-1=9$ ester bonds.
• Each ester bond eliminates one water, so 9 water molecules are lost.
• General rule: for $2n$ units in one chain, waters lost $=2n-1$; for large $n$ this is $\approx 2n$.

• Sum of monomer masses: $5\times 62+5\times 90=310+450=760\text{g/mol}$.
• Water lost: $9\times 18=162\text{g/mol}$.
• $M_{\text{polymer}}=760-162=598\text{g/mol}$. Sanity check: in addition the bond count in the formula would be $0$, giving $M_{\text{polymer}}=760$ (no loss). The $162$ shortfall is the fingerprint of condensation.

Addition ⇒ zero small molecules eliminated (the π-bond just opens). $M_{\text{polymer}}=6\times 28-0=168\text{g/mol}.$ The repeat unit $-({\text{CH}}_2{\text{CH}}_2)-$ has formula ${\text{C}}_2{\text{H}}_4$, same as the monomer — mass conserved.

• Synthetic (phenol + formaldehyde, made industrially) → predicts we control its purity and can mass-produce it cheaply; it will not biodegrade (contrast Biodegradable Polymers).
• Condensation (an amide/ether-type link forms while H₂O is expelled) → predicts the process needs to remove water and that mass is lost vs monomers.
• Thermosetting (dense 3-D covalent crosslinks) → predicts it is hard, heat-resistant, an electrical insulator — which is exactly why it's used for switchboards and plug bodies.

The claim is wrong in spirit. A true thermosetting polymer does not have a melting point at all in the ordinary sense.

• Its chains are locked by a covalent 3-D network; there is no temperature at which they soften and flow reversibly.
• Heat it enough and the covalent bonds break irreversibly → it chars/decomposes, it does not melt to a liquid you can remould. Correct statement: melamine is thermosetting; on strong heating it decomposes/chars rather than melting. Set = set forever.

• Source: man-made → synthetic.
• Mechanism: both monomers carry C=C double bonds that open with no atom loss → addition.
• Thermal: the un-vulcanised material is essentially thermoplastic-like (weak forces); once vulcanised (crosslinked) it becomes thermosetting — same lesson as Q3.1.
• Extra axis: because it is built from two different monomers, it is a copolymer (see Copolymers). "Homopolymer vs copolymer" is a fourth independent way to classify.

• $n=1$: one diol + one diacid = $2$ units → $2n-1=1$ bond → 1 water. This is just a single ester (a dimer), the smallest possible "chain." Correct and non-degenerate.
• $n\to \infty$: waters $=2n-1\to 2n$; the "$-1$" becomes negligible, so the mass-loss fraction approaches a constant $\frac{2n\cdot 18}{\sum M_{\text{monomers}}}$. This is why chemists quote "$\approx 2n$ small molecules" for long chains. Reconciliation: the exact law is always $2n-1$; the $\approx 2n$ shortcut is its large-$n$ limit.

Give me any polymer and I answer three independent questions — who built it (natural/synthetic), how the bonds formed (opened a double bond = addition, glued-with-a-drop-lost = condensation), and what heat does (soft & remouldable = thermoplastic, locked & chars = thermosetting). If it has two monomers, I add a fourth label: copolymer.