Polymer
A polymer is a large macromolecule composed of many repeating structural units called monomers, linked together by covalent bonds through a process called polymerisation. Polymers can be natural (e.g., cellulose, proteins, DNA) or synthetic (e.g., polyethylene, nylon, PVC), and their physical properties are governed by chain length, branching, cross-linking, and monomer identity. They are indispensable in modern industry, biology, and materials science.
Key Formula
n(CH2=CH2) → [-CH2-CH2-]n (polyethylene, degree of polymerisation = n)
LaTeX: n\,\text{CH}_2{=}\text{CH}_2 \xrightarrow{\text{catalyst}} [\text{-CH}_2\text{-CH}_2\text{-}]_n
| Symbol | Meaning | Unit |
|---|---|---|
| n | Degree of polymerisation (number of monomer units) | dimensionless |
| M_n | Number-average molecular weight | g/mol |
| M_w | Weight-average molecular weight | g/mol |
Worked Example
Problem
A sample of polyethylene has a number-average molecular weight (Mn) of 56,000 g/mol. The molecular weight of the ethylene monomer is 28 g/mol. Calculate the number-average degree of polymerisation (DPn).
Solution
Step 1: Recall the relation: DPn = Mn / M0, where M0 is the monomer molecular weight. Step 2: M0 (ethylene, CH2=CH2) = 2(12) + 4(1) = 28 g/mol. Step 3: DPn = 56,000 / 28 = 2,000. Step 4: This means, on average, 2,000 ethylene units are linked per chain.
Answer
Degree of polymerisation = 2,000 repeat units per chain.
Common Polymers, Their Monomers, and Applications
| Polymer | Monomer | Type | Key Property | Application |
|---|---|---|---|---|
| Polyethylene (PE) | Ethylene | Addition | Flexible, chemically inert | Plastic bags, bottles |
| Polypropylene (PP) | Propylene | Addition | High melting point | Pipes, automotive parts |
| Nylon-6,6 | Hexamethylenediamine + Adipic acid | Condensation | High tensile strength | Textiles, gears |
| PVC | Vinyl chloride | Addition | Rigid or flexible | Pipes, cables, flooring |
| Natural rubber | Isoprene | Natural addition | Elasticity | Tyres, gloves |
| Cellulose | Glucose | Natural condensation | Structural rigidity | Paper, cotton, wood |
Interactive Tools
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Related Terms
Condensation Reaction
A condensation reaction is a type of chemical reaction in which two molecules combine to form a larger molecule with the simultaneous loss of a small molecule, most commonly water (H2O) but sometimes methanol, HCl, or ammonia. Condensation reactions are fundamental in the synthesis of polymers, esters, amides, and biological macromolecules including proteins, nucleic acids, and polysaccharides. The reverse process, in which the small molecule is reincorporated to break the bond, is called hydrolysis.
Peptide Bond
A peptide bond is a covalent amide linkage formed between the carboxyl group (–COOH) of one amino acid and the amino group (–NH2) of another, with the elimination of a water molecule in a condensation reaction. The resulting –CO–NH– bond has partial double bond character due to resonance delocalisation of the nitrogen lone pair into the carbonyl, making it planar and restricting rotation around the C–N bond. Peptide bonds are the primary covalent linkages in all protein chains.
Protein Structure
Protein structure refers to the hierarchical three-dimensional arrangement of a protein, described at four levels: primary (sequence of amino acids), secondary (local folding into alpha helices and beta sheets), tertiary (overall 3D fold of a single polypeptide), and quaternary (assembly of multiple polypeptide subunits). The structure of a protein is intimately linked to its function, and alterations in structure — through mutation, denaturation, or misfolding — can lead to loss of function or disease. Determining and predicting protein structure is one of the central challenges of biochemistry and structural biology.
From Greek 'polys' (many) and 'meros' (part). The term was coined by Swedish chemist Jöns Jacob Berzelius in 1833 to describe compounds with the same empirical formula but different molecular masses. Modern polymer science was largely founded by Hermann Staudinger in the 1920s.