How are polymers classified by origin, structure, polymerisation type and composition?

In one line

Polymers are classified in several ways: by source (natural, semi-synthetic, synthetic), by chain structure (linear, branched, cross-linked), by method of polymerisation (addition, condensation), by composition (homopolymer, copolymer), by tacticity and by thermal behaviour.

Explained simply

Just as you can sort students by city, branch or height, polymers can be grouped by different questions: Where do they come from? What shape are their chains? How were they made? How many kinds of monomers are in them? How do they react to heat?

How it works

1. Origin:
   • Natural: cellulose, starch, proteins, natural rubber, DNA.
   • Semi-synthetic (chemically modified natural polymers): cellulose acetate, cellulose nitrate, vulcanised rubber.
   • Synthetic: polyethylene, PVC, nylon, polyester, Bakelite.
2. Structure of chains:
   • Linear: long straight chains, pack well (HDPE, PVC).
   • Branched: side branches, lower density and crystallinity (LDPE).
   • Cross-linked / network: chains joined by covalent links; hard and rigid (Bakelite, melamine-formaldehyde).
3. Type of polymerisation:
   • Addition (chain-growth): no by-product; from unsaturated monomers (PE, PP, PVC, polystyrene, PTFE).
   • Condensation (step-growth): small molecule like H₂O, HCl or CH₃OH is eliminated (nylon 6,6, polyester/Terylene, Bakelite).
4. Composition:
   • Homopolymer: one type of monomer (PE, PVC).
   • Copolymer: two or more types (Buna-S from butadiene and styrene; ABS). Can be random, alternating, block or graft.
5. Tacticity (arrangement of side groups along the chain): isotactic (all on the same side), syndiotactic (alternating sides), atactic (random). Isotactic polypropylene is crystalline and strong; atactic is soft and amorphous.
6. Thermal behaviour / application: thermoplastics, thermosetting plastics, elastomers and fibres.

Key formula

No formula. Example reactions: addition: \( n\,\mathrm{CH_2{=}CHCl \rightarrow -(CH_2{-}CHCl)_n-} \); condensation forms an amide or ester link with loss of H₂O.

Quick example

Nylon 6,6 is at the same time synthetic (origin), linear (structure), a condensation polymer made from two different monomers (hexamethylenediamine and adipic acid), and a thermoplastic used as a fibre. One polymer belongs to several classes at once, so always state the basis of classification.

Common mistakes

• Calling vulcanised rubber purely natural; it is chemically modified (semi-synthetic).
• Saying condensation polymers always need two different monomers; a single monomer with two different groups can also condense (e.g., an amino acid forming a polyamide).
• Forgetting examples; each class should have at least one.

How MU asks it

"Classify polymers on different bases with examples" (5 marks) or "Distinguish addition and condensation polymerisation" (3 marks). For full marks give 4–5 bases, sub-types under each, and one example per sub-type; a small sketch of linear/branched/cross-linked chains earns extra clarity.