Classification — natural vs synthetic; addition vs condensation; thermoplastic vs thermosetting: Worked examples

The scenario matrix

Every polymer question is really asking you to place the polymer at one point in a 3-D grid:

Axis	Possible answers	The clue you hunt for
Source	natural / synthetic / semi-synthetic	Does it grow in a plant or animal? Was it made in a lab? Is it a modified natural thing?
Mechanism	addition / condensation	Is the monomer a $C=C$ (addition) or a molecule with two functional groups that loses a small molecule (condensation)?
Thermal	thermoplastic / thermosetting	Are the chains separate (weak forces → remouldable) or cross-linked into one giant 3-D web (char, never melt)?

Because the three axes are independent (parent note, section 3), the number of possible labels is $3 \times 2 \times 2 = 12$ combinations. We do not need all 12 — we need to hit every distinct decision at least once, plus the awkward edge cases:

Cell to cover	Which example hits it
Source = natural	Ex 1 (cellulose), Ex 6 (rubber)
Source = synthetic	Ex 2, 3, 4, 5, 8
Source = semi-synthetic	Ex 7 (rayon)
Mechanism = addition	Ex 2 (PVC), Ex 6 (rubber)
Mechanism = condensation	Ex 3, 4, 5
Thermal = thermoplastic	Ex 2, 3, 8
Thermal = thermosetting	Ex 4 (Bakelite), Ex 6b (vulcanised)
Mass-loss numeric (condensation bookkeeping)	Ex 5
Zero / degenerate input (a monomer with no double bond and one functional group)	Ex 8b
Word problem (real-world material choice)	Ex 8
Exam twist (same monomer, two products; crosslinking flips thermal class)	Ex 6

Keep this table in view: as we finish each example we tick off its cells.

The 3-question decision flow

Before the examples, here is the exact order to ask the three questions. Following the arrows never fails.

Example 1 — a natural polymer (Source axis)

Worked example Statement

Classify cellulose (the stuff of cotton and wood) on all three axes. Cells: source = natural.

Forecast: Where does cotton come from? Is a plant "a lab"? Do you know cellulose's monomer? Guess all three before reading on.

Source. Why this step? It is the easiest axis and it is answered by asking "does it grow?". Cellulose is the main fibre in plant cell walls → natural.
Mechanism. Why this step? We check the monomer. Cellulose is many glucose units joined by losing a water molecule at each link (a glycosidic bond). Small molecule expelled ⇒ condensation.
Thermal. Why this step? We ask whether the chains are separate or netted. Cellulose chains are long and held to each other by many hydrogen bonds (weak forces, not covalent crosslinks) — see Intermolecular Forces. That makes it conceptually thermoplastic-like, but in practice cellulose chars before it softens because those H-bonds are so numerous it decomposes first. For classification we call it a natural condensation polymer; the thermal label is not the useful axis here.

Verify: natural (grows in plants) ✔, condensation (loses H₂O per link) ✔. This matches the parent note's list of natural polymers.

Example 2 — synthetic addition thermoplastic (the "default" case)

Worked example Statement

Classify PVC (poly-vinyl chloride), made from vinyl chloride $CH_{2} = CHCl$ . Cells: synthetic, addition, thermoplastic.

Forecast: Spot the double bond in the monomer — what does that instantly tell you about mechanism?

Source. Why? Vinyl chloride is an industrial chemical, not a plant product → synthetic.
Mechanism. Why? The monomer has a $C=C$ . The second (π) bond opens and reconnects to the next monomer with no atom removed — see Addition Polymerisation Mechanism. So it is addition. Check the mass: monomer $C_{2} H_{3} Cl$ and the repeat unit $- (CH_{2} -CHCl) -$ have the same formula $C_{2} H_{3} Cl$ .
Thermal. Why? PVC is made of long separate linear chains — no covalent bridges between them. Heating loosens the weak forces → it softens and can be remoulded → thermoplastic.

Verify: monomer mass $= 2 (12) + 3 (1) + 35.5 = 62.5$ ; repeat-unit mass $= 62.5$ . Equal ⇒ mass conserved ⇒ addition confirmed. ✔

Example 3 — synthetic condensation thermoplastic (Nylon)

Worked example Statement

Classify Nylon-6,6, from hexamethylenediamine $H_{2} N-(CH_{2})_{6} -NH_{2}$ + adipic acid $HOOC-(CH_{2})_{4} -COOH$ . Cells: synthetic, condensation, thermoplastic.

Forecast: Two different monomers, each with two handles — does that smell like addition or condensation?

Source. Why? Both monomers are lab-made → synthetic.
Mechanism. Why? Each monomer is bifunctional ( $- NH_{2}$ at both ends, or $- COOH$ at both ends). When an $- NH_{2}$ meets a $- COOH$ they form an amide bond and kick out one $H_{2} O$ . Small molecule lost ⇒ condensation — see Condensation Polymerisation.
Thermal. Why? The chains are long but not cross-linked — held by H-bonds between amide groups. Weak forces loosen on heating ⇒ thermoplastic (nylon can be melt-spun into fibre, which is only possible because it melts!).

Verify: it is condensation and thermoplastic at once — proving the two axes are independent, exactly the parent note's warning. ✔

Example 4 — synthetic condensation thermosetting (Bakelite, the exam favourite)

Worked example Statement

Classify Bakelite, from phenol + formaldehyde. Cells: synthetic, condensation, thermosetting.

Forecast: It is hard and cannot be remelted. Which thermal label does that force?

Source. Why? Man-made → synthetic.
Mechanism. Why? Phenol and formaldehyde link with loss of $H_{2} O$ each step → condensation.
Thermal. Why? Here is the twist: phenol can bond at three positions (two ortho, one para). Bonding in three directions builds a 3-D covalent network — every chain is stapled to its neighbours. Heat cannot break covalent crosslinks, so it chars, never melts → thermosetting.

Verify: compare with Ex 3 (nylon = condensation but thermoplastic). Same mechanism axis, opposite thermal axis → confirms independence, and confirms crosslinking is the deciding factor. ✔ (Mnemonic: "Bakelite is Baked Brittle".)

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

Example 5 — the mass-loss numeric (condensation bookkeeping)

Worked example Statement

A polyester is made from $n$ molecules of ethylene glycol ( $HO-CH_{2} -CH_{2} -OH$ , $M = 62$ ) and $n$ molecules of terephthalic acid ( $HOOC-C_{6} H_{4} -COOH$ , $M = 166$ ). This is PET. If $n = 100$ , how much mass is lost as water when one single long chain forms, and what is the chain's mass? Cell: condensation numeric.

Forecast: How many ester links form when you alternate 100 + 100 monomers into one chain? Guess the number of waters.

Count the bonds. Why? The parent's mass formula needs the number of small molecules lost. Alternating $A B A B \dots$ with $100$ of each gives $200$ beads in a row, hence $200 - 1 = 199$ links. Each link expels one $H_{2} O$ .
Water mass lost. Why? $M (H_{2} O) = 18$ . Total lost $= 199 \times 18 = 3582$ .
Chain mass. Why? Apply $M_{polymer} = \sum M_{monomers} - (bonds) \times M_{small molecule} .$ $\sum M = 100 (62) + 100 (166) = 6200 + 16600 = 22800.$ $M_{polymer} = 22800 - 3582 = 19218.$

Verify: For large $n$ the " $- 1$ " hardly matters, so $\approx 2 n = 200$ waters lost — the parent note's " $\approx 2 n$ " rule. Units: all in g mol⁻¹, consistent. $19218 + 3582 = 22800$ = total starting mass ✔ (mass is conserved overall — water just left the chain).

Example 6 — the exam twist: one monomer, two thermal fates (rubber)

Worked example Statement

(a) Classify natural rubber (polymer of isoprene, $CH_{2} = C(CH_{3}) -CH=CH_{2}$ ). (b) Now it is vulcanised (heated with sulphur). How does the classification change? Cells: natural + addition (a); thermosetting (b) — the twist.

Forecast: Rubber has $C=C$ bonds and grows in a tree. Then we add sulphur bridges — what happens to the thermal axis?

(a) Source. Why? It bleeds from the rubber tree → natural.
(a) Mechanism. Why? Isoprene has $C=C$ double bonds that open and add → addition. So natural rubber is a rare natural addition polymer.
(a) Thermal. Why? Raw rubber chains are separate and only weakly linked → soft, sticky → essentially thermoplastic (it goes gummy when warm).
(b) Vulcanisation. Why this step? Sulphur atoms form $- S-S -$ covalent bridges between chains — see Natural Rubber and Vulcanisation. Those bridges are crosslinks. Adding crosslinks converts the material to a 3-D network → thermosetting (hard, springy, no longer remouldable).

Verify: The source and mechanism axes did not change (still natural, still addition) — only crosslinking changed the thermal axis. This is the cleanest proof that the three axes are independent, and that crosslinking, not mechanism, decides thermoplastic vs thermosetting. ✔

Example 7 — the semi-synthetic edge case (rayon)

Worked example Statement

Classify cellulose acetate (rayon). Cell: source = semi-synthetic.

Forecast: It is made from cotton pulp, but chemists react it first. Is that "natural" or "synthetic"?

Source. Why this step? It is the tricky axis. Start from natural cellulose, then chemically replace some $- OH$ groups with acetate groups. A natural polymer that has been chemically modified is neither purely natural nor built-from-scratch synthetic — it is semi-synthetic.
Mechanism. Why? The backbone is still cellulose, built by condensation (water loss). So the underlying polymer is condensation.
Thermal. Why? Separate chains, weak forces ⇒ can be spun/moulded ⇒ thermoplastic-like.

Verify: matches the parent's definition: "semi-synthetic = a natural polymer chemically modified, e.g. cellulose acetate/rayon". ✔ This is the only cell that is easy to misfile as "synthetic" — the giveaway word is modified.

Example 8 — real-world word problem + degenerate input

Worked example Statement

(a) An engineer needs a handle for a hot frying pan: it must not soften even when very hot. Should she choose a thermoplastic or a thermosetting plastic, and why? (b) A student claims "ethanol ( $CH_{3} CH_{2} OH$ ) can be polymerised by addition because it contains carbon." Is that possible? Cells: word problem (a); degenerate/zero input (b).

Forecast: (a) Which class keeps its shape at high temperature? (b) Count ethanol's double bonds and functional groups.

(a) Match property to class. Why? The requirement is "stays rigid when hot". Only thermosetting plastics keep their shape at high temperature (they char rather than soften); thermoplastics would go soft and deform. So choose a thermosetting plastic (e.g. Bakelite / melamine).
(a) Sanity check. Why? Real pan handles are made of Bakelite/melamine — the theory matches the shop. ✔
(b) Test the degenerate input. Why this step? Addition needs a $C=C$ ; ethanol has none (all single bonds). Condensation needs two functional groups; ethanol has only one ( $- OH$ ). With zero double bonds and only one handle, a monomer cannot chain up at all.
(b) Conclusion. Why? "Contains carbon" is irrelevant — polymerisation needs the right reactive site, not just carbon. Ethanol is a dead end for both mechanisms.

Verify: (a) thermosetting is the physically correct choice; (b) ethanol has $0$ double bonds and $1$ functional group, failing the requirement for both addition (needs $\geq 1$ C=C) and condensation (needs $\geq 2$ groups). ✔ This is the "zero/degenerate input" cell: always ask does the monomer even have a reactive handle?

Scorecard — every cell hit

Recall Did we cover the whole matrix?

Natural ::: Ex 1 (cellulose), Ex 6 (rubber) Synthetic ::: Ex 2, 3, 4, 5, 8 Semi-synthetic ::: Ex 7 (rayon) Addition ::: Ex 2 (PVC), Ex 6 (rubber) Condensation ::: Ex 3, 4, 5, 7 Thermoplastic ::: Ex 2, 3, 8a-alt Thermosetting ::: Ex 4 (Bakelite), Ex 6b (vulcanised), Ex 8a Numeric mass-loss ::: Ex 5 (199 waters, chain mass 19218) Degenerate/zero input ::: Ex 8b (ethanol — no C=C, one group) Real-world word problem ::: Ex 8a (pan handle) Exam twist (crosslinking flips thermal class) ::: Ex 6

Connections

Addition Polymerisation Mechanism — the π-bond opening used in Ex 2 and 6.
Condensation Polymerisation — the water-losing links of Ex 3, 4, 5, 7.
Natural Rubber and Vulcanisation — the crosslinking twist of Ex 6.
Intermolecular Forces — why thermoplastics soften (Ex 2, 3).
Biodegradable Polymers — follow-up to the natural sources in Ex 1.
Copolymers — two-monomer systems like the nylon of Ex 3.