Classification

Polymers are giant molecules (macromolecules) built by joining many small repeating units called monomers through covalent bonds. The word 'polymer' comes from the Greek 'poly' (many) and 'meros' (parts). From the DNA in your cells to the plastic bottle on your desk, polymers are everywhere in both living organisms and synthetic materials. Understanding how they are classified helps chemists design materials with precisely tuned properties — flexibility, heat resistance, strength, and biodegradability. Polymers are classified in two major ways. The first classification is based on the mechanism of polymerisation: addition polymerisation (chain-growth) versus condensation polymerisation (step-growth). In addition polymerisation, monomers containing double bonds add across each other with no atoms lost, so the polymer has the same empirical formula as the monomer. In condensation polymerisation, monomers with two functional groups react repeatedly, eliminating a small molecule (usually water or HCl) at each step, so the polymer formula differs from the monomer formula. The second major classification is based on thermal behaviour: thermoplastics versus thermosetting polymers. Thermoplastics consist of linear or branched polymer chains held together by weak intermolecular forces; they soften when heated and can be reshaped repeatedly. Thermosetting polymers form irreversible three-dimensional cross-linked networks during curing — once set, they cannot be re-melted without decomposition. This article explores both classification systems in depth, providing the chemical detail you need to excel in your studies.

Key takeaways
  • Addition Polymerisation (Chain-Growth)
  • Condensation Polymerisation (Step-Growth)
  • Thermoplastics
  • Thermosetting Polymers
  • Comparison Summary and Memory Aids

Addition Polymerisation (Chain-Growth)

Addition polymerisation requires monomers that contain at least one carbon–carbon double bond (C=C). The mechanism proceeds in three stages: initiation (a free radical, cation, or anion opens the double bond to form a reactive intermediate), propagation (the reactive end adds successive monomer units, extending the chain), and termination (two growing chains combine or disproportionate). Because both carbons of the double bond become part of the backbone and nothing is expelled, the molecular formula of the repeat unit is identical to that of the monomer. Example: ethene (CH₂=CH₂) polymerises to form poly(ethene), —[CH₂—CH₂]ₙ—. Other addition polymers include poly(propene), poly(chloroethene) (PVC), poly(tetrafluoroethene) (PTFE/Teflon), and poly(styrene).

Condensation Polymerisation (Step-Growth)

Condensation polymerisation involves monomers with two or more functional groups (e.g., –OH, –COOH, –NH₂, –COCl) reacting pairwise to form a covalent bond while simultaneously eliminating a small molecule. Each bond-forming step can occur between any two reactive chain ends, so the average molecular mass grows slowly at first and increases sharply only at very high conversions. Key examples include: (1) Nylon-6,6 — formed from hexane-1,6-diamine and hexanedioic acid, eliminating water at each amide bond; (2) Dacron/Terylene (PET) — from ethane-1,2-diol and benzene-1,4-dicarboxylic acid (terephthalic acid), eliminating water at each ester bond; (3) Nylon-6 — from caprolactam (a single bifunctional monomer) via ring-opening condensation, losing water. Proteins and nucleic acids in living organisms are biological condensation polymers (peptide bonds and phosphodiester bonds, respectively).

Thermoplastics

Thermoplastics are polymers whose chains interact only through weak forces: van der Waals forces, dipole–dipole interactions, or hydrogen bonds. There are no covalent cross-links between chains. When heated above their glass transition temperature (Tᵍ) or melting point (Tₘ), chains gain enough kinetic energy to slide past one another, making the material soft and mouldable. On cooling, the chains re-entangle and the material re-hardens — this cycle is reversible and repeatable. Examples: poly(ethene), PVC, poly(propene), PET, nylon. Thermoplastics can be recycled by remelting. Their mechanical properties depend heavily on chain length (degree of polymerisation), branching, and the presence of polar groups or H-bonding.

Thermosetting Polymers

Thermosetting polymers are formed when monomers or pre-polymers react to create an extensive three-dimensional covalent network. This cross-linking is achieved by using monomers with three or more functional groups. Once the network has formed (the 'curing' step, which may require heat, UV light, or a chemical hardener), the structure is permanent. Heating such a polymer does not melt it; instead, excess heat degrades (burns) the polymer. This makes thermosets harder, more rigid, and more heat-resistant than most thermoplastics, but non-recyclable by conventional means. Classic examples: Bakelite (phenol–formaldehyde resin), epoxy resins (e.g., Araldite), vulcanised rubber (sulfur cross-links between polyisoprene chains), and urea–formaldehyde resins used in circuit boards and kitchen utensils.

Comparison Summary and Memory Aids

A useful way to distinguish the two thermal classes: thermoplastics behave like chocolate — they melt on heating and reset on cooling. Thermosetting polymers behave like a boiled egg — once set, they cannot revert. For the polymerisation mechanisms, check for: (1) a C=C double bond in the monomer → likely addition polymerisation, no small molecule by-product; (2) two different functional groups (–OH + –COOH, or –NH₂ + –COCl) → condensation polymerisation, water or HCl released. Note that some polymers (like nylon-6 from caprolactam) involve a bifunctional single monomer in ring-opening condensation, so always look for the elimination of a small molecule as the diagnostic test.

The Key Equation

\cen(H2N−(CH2)6−NH2)+n(HOOC−(CH2)4−COOH)−>[−NH−(CH2)6−NH−CO−(CH2)4−CO−]n+2nH2O\ce{n(H2N-(CH2)6-NH2) + n(HOOC-(CH2)4-COOH) -> [-NH-(CH2)6-NH-CO-(CH2)4-CO-]_n + 2nH2O}
  • n — Degree of polymerisation (dimensionless): The number of repeating monomer units in one polymer chain; typically thousands for commercial nylon.
  • \ce{H2N-(CH2)6-NH2} — Hexane-1,6-diamine (HMDA) (—): A diamine monomer with six methylene groups; provides the –NH– unit in the nylon backbone.
  • \ce{HOOC-(CH2)4-COOH} — Hexanedioic acid (adipic acid) (—): A dicarboxylic acid monomer; provides the –CO– unit in the nylon backbone.
  • \ce{H2O} — Water (by-product) (—): Eliminated at each amide bond-forming step; 2n molecules total are produced per chain of degree n.

Worked Example

Problem. A student is given two unknown polymers, X and Y. Polymer X has the repeat unit —[CH₂–CHCl]ₙ— and is formed from a single monomer. Polymer Y is formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol; it softens at ~260 °C and can be remoulded. (a) Classify polymer X by polymerisation mechanism. Write the monomer formula. (b) Classify polymer Y by both mechanism and thermal behaviour. (c) State the small molecule eliminated in forming polymer Y and write the linkage formed.

\ceHOOC−C6H4−COOH+HO−CH2CH2−OH−>[−OC−C6H4−COO−CH2CH2−O−]n+H2O\ce{HOOC-C6H4-COOH + HO-CH2CH2-OH -> [-OC-C6H4-COO-CH2CH2-O-]_n + H2O}

Answer. (a) Addition polymerisation; monomer = chloroethene (CH₂=CHCl). (b) Polymer Y forms by condensation polymerisation and is a thermoplastic. (c) Water (H₂O) is eliminated; an ester linkage (–COO–) is formed.

Figure
Diagram
Polymer Classification OverviewBy Polymerisation MechanismADDITIONMonomer: C=C double bondNo by-productRepeat unit = monomere.g. PVC, PTFE, PECONDENSATIONMonomer: 2 functional groupsBy-product: H₂O or HClRepeat unit ≠ monomere.g. Nylon, PET, proteinsBy Thermal BehaviourTHERMOPLASTICLinear / branched chainsWeak intermolecular forcesSoftens on heating ✓Re-mouldable, recyclable ✓e.g. PE, PET, NylonTHERMOSETTING3D cross-linked networkCovalent cross-linksChars/decomposes on heat ✗Non-mouldable after curing ✗e.g. Bakelite, epoxy resin
Origins · 1 minWallace Hume Carothers and how this was discovered
WH
Wallace Hume Carothers
1896–1937 · American

Carothers was an organic chemist at DuPont who developed the theoretical framework for condensation polymerisation and used it to synthesise neoprene (1931) and nylon-6,6 (1935) — the first fully synthetic polyamide fibre. His work firmly established the distinction between addition and condensation mechanisms and showed that high-molecular-mass polymers were true covalent molecules, not mere aggregates. Nylon-6,6 was the first commercially successful synthetic polymer fibre, announced to the public in 1938. Carothers suffered from severe depression throughout his career. He patented his discoveries but never lived to see nylon stockings hit store shelves in 1940, having died in 1937 at age 41. The word 'nylon' was coined by DuPont after his death.

  • Addition Polymerisation: Mechanism — Explores the initiation, propagation, and termination steps of free-radical addition polymerisation in detail, building directly on the mechanism overview in this article.
  • Condensation Polymerisation: Nylon and Polyesters — Provides worked examples and equations for forming nylon-6,6, nylon-6, and PET, complementing the condensation section of this classification article.
  • Peptide Bonds and Protein Structure — Proteins are biological condensation polymers; understanding peptide-bond formation reinforces the condensation mechanism and connects synthetic polymers to biological macromolecules.
  • Functional Groups in Organic Chemistry — Recognising –OH, –COOH, –NH₂, and C=C groups is essential for predicting which type of polymerisation a monomer will undergo, as described in this article.
  • Intermolecular Forces and Physical Properties — The distinction between thermoplastics and thermosetting polymers hinges on the difference between weak intermolecular forces and covalent cross-links, a concept grounded in intermolecular-force theory.