MHT-CET Chemistry · Introduction to Polymer Chemistry

Classification of Polymers

Polymers are classed four ways — by source (natural, semisynthetic, synthetic), by chain structure (linear, branched, cross-linked), by the number of monomer kinds (homopolymer, copolymer) and by the strength of the intermolecular forces (elastomer, fibre, thermoplastic, thermosetting) — and the same polymer answers a different question under each.

All Introduction to Polymer Chemistry notes MHT-CET Chemistry strategy

Why this matters

22 PYQs, none HARD. Eleven are the force-based classes — pick the thermosetting one (urea-formaldehyde, bakelite), the thermoplastic (polythene, polyvinyls), the fibre (polyesters, nylon 6,6) or the elastomer (Buna-S, neoprene) from a mixed list, asked in one wording or another almost every sitting; eleven are the other three axes — copolymer or homopolymer (five times), linear or branched, semisynthetic, biodegradable, and the source of linen. Two cards.

Concept 1 of 2

By Source, by Chain Structure, by Number of Monomers

Intuition

Three quick axes. SOURCE: made by nature (rubber, cellulose, starch, proteins), made by modifying a natural polymer (cellulose nitrate, cellulose acetate, rayon — semisynthetic), or made from small molecules (everything else). STRUCTURE: a linear chain packs well (HDPE), a branched one cannot (LDPE), and a cross-linked network cannot move at all (bakelite, melamine). MONOMERS: one kind gives a homopolymer (polythene, PVC, PAN, thermocol), two kinds a copolymer (Buna-S, Buna-N, PHBV, glyptal, polycarbonate). The paper's convention for 'identify the copolymer' is the rubber — Buna-S is keyed over nylon 6,6 every time both appear.

Definition

Natural: natural rubber, cellulose, starch, proteins, wool, silk. Linen comes from the flax plant. Semisynthetic: cellulose nitrate, cellulose acetate, viscose rayon. Synthetic: nylon, dacron, polythene, PVC, Buna-S…
Linear: HDPE, PVC, nylon, polyesters. Branched: LDPE, glycogen. Cross-linked (network): bakelite, melamine-formaldehyde, urea-formaldehyde, vulcanised rubber.
Homopolymer (one monomer): polythene, PVC, PAN, polystyrene (thermocol), Teflon, PMMA, natural rubber, nylon 6. Copolymer (two or more): Buna-S (SBR), Buna-N, PHBV, glyptal, polycarbonate, dacron, nylon 6,6.
Paper convention: with Buna-S and nylon 6,6 in the same list, the keyed copolymer is Buna-S and the keyed polyamide/condensation polymer is nylon 6,6.
Biodegradable: PHBV, nylon 2-nylon 6 (glycine + ε-aminocaproic acid). Teflon, LDPE, PAN, terylene, nylon 6 and 6,6 are not.

Three axes

\text{source: natural / semisynthetic / synthetic};\quad \text{chain: linear / branched / cross-linked};\quad \text{monomers: homo- / co-polymer}

Worked example

Classify cellulose acetate, LDPE and Buna-N on the three axes.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Introduction to Polymer ChemistryMODERATE

Which among the following pairs of polymers contains both members as copolymers?

[Q82 · 11th May Shift 1 · 2024]

Nylon 6,6 as the copolymer

It is made from two monomers, and the paper still keys Buna-S when both are offered. Reserve nylon 6,6 for 'polyamide', 'condensation' and 'fibre'.

Drill 10 more on by source, by chain structure, by number of monomers

Concept 2 of 2

Elastomer, Fibre, Thermoplastic, Thermosetting: Ranked by Intermolecular Force

Intuition

Rank the forces between chains and the class falls out. Weakest: ELASTOMERS — chains coil and a few cross-links pull them back (rubbers: natural, Buna-S, Buna-N, neoprene). Strongest: FIBRES — hydrogen bonds or dipoles line the chains up into threads (nylons, polyesters/terylene, PAN). In between: THERMOPLASTICS — linear or lightly branched, soften on heating and can be remoulded (polythene, PVC, polystyrene, PMMA, Teflon). THERMOSETTING plastics are the odd class: heavily cross-linked on heating, they set once and cannot be remelted (bakelite, urea-formaldehyde, melamine-formaldehyde).

Definition

Elastomers (weakest forces): natural rubber, Buna-S, Buna-N, neoprene, vulcanised rubber.
Fibres (strongest — H-bonds/dipole): nylon 6, nylon 6,6, terylene/dacron/polyesters, polyacrylonitrile.
Thermoplastics (intermediate; soften and remould): polythene, polyvinyls (PVC), polystyrene, PMMA, Teflon, novolac.
Thermosetting (cross-linked on heating, cannot be remoulded): bakelite, urea-formaldehyde resin, melamine-formaldehyde.
NOT true of a thermoplastic: 'possesses extensive covalent cross-linking' — that is the thermoset.

Class	Intermolecular force	Heating	Examples
Elastomer	Weakest (a few cross-links)	Stretch and recover	Natural rubber, Buna-S, Buna-N, neoprene
Thermoplastic	Intermediate	Softens, remouldable	Polythene, PVC, polystyrene, Teflon, PMMA Thermoplastics have NO extensive cross-linking.
Thermosetting	Cross-linked network	Sets once, cannot remelt	Bakelite, urea-formaldehyde, melamine-formaldehyde
Fibre	Strongest (H-bond / dipole)	High tensile strength, drawn to thread	Nylon 6, nylon 6,6, terylene, PAN

The force ranking is elastomer < thermoplastic < fibre; thermosets are a structure, not a force level.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Introduction to Polymer ChemistryMODERATE

Which from following polymers is classified as fibres depending on inter molecular forces?

[Q68 · 14th May Shift 2 · 2024]

Neoprene as a fibre, bakelite as a thermoplastic

Anything called a rubber (Buna, neoprene) is an elastomer; anything ending in '-formaldehyde' or called bakelite is thermosetting. Fibres are the nylons, polyesters and PAN.

Drill 10 more on elastomer, fibre, thermoplastic, thermosetting: ranked by intermolecular force

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

By Source, by Chain Structure, by Number of Monomers
Three axes
$\text{source: natural / semisynthetic / synthetic};\quad \text{chain: linear / branched / cross-linked};\quad \text{monomers: homo- / co-polymer}$

Reference tables (1)

Elastomer, Fibre, Thermoplastic, Thermosetting: Ranked by Intermolecular Force4 rows

Class	Intermolecular force	Heating	Examples
Elastomer	Weakest (a few cross-links)	Stretch and recover	Natural rubber, Buna-S, Buna-N, neoprene
Thermoplastic	Intermediate	Softens, remouldable	Polythene, PVC, polystyrene, Teflon, PMMA Thermoplastics have NO extensive cross-linking.
Thermosetting	Cross-linked network	Sets once, cannot remelt	Bakelite, urea-formaldehyde, melamine-formaldehyde
Fibre	Strongest (H-bond / dipole)	High tensile strength, drawn to thread	Nylon 6, nylon 6,6, terylene, PAN

The force ranking is elastomer < thermoplastic < fibre; thermosets are a structure, not a force level.

Watch out for (2)

Nylon 6,6 as the copolymer
→ By Source, by Chain Structure, by Number of Monomers
Neoprene as a fibre, bakelite as a thermoplastic
→ Elastomer, Fibre, Thermoplastic, Thermosetting: Ranked by Intermolecular Force

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