A polymer (Greek poly = many, mer = unit) is a very high molecular mass macromolecule built by joining a large number of small repeating units called monomers through covalent bonds. The process of linking monomers into a polymer is polymerisation. For example, thousands of ethene (CH2=CH2) molecules join to give polythene, —(CH2—CH2)n—, where n (the degree of polymerisation) may run into thousands.
Polymers are classified along four useful axes.
1. By source. Natural polymers occur in plants and animals — proteins, cellulose, starch and natural rubber. Semi-synthetic polymers are chemically modified natural polymers, such as cellulose acetate (rayon) and cellulose nitrate (gun cotton). Synthetic (man-made) polymers are prepared in the laboratory or industry — polythene, PVC, nylon, Teflon and synthetic rubbers.
2. By structure. Linear polymers have monomers joined in straight long chains that pack closely, giving high density, high tensile strength and high melting points (e.g. high-density polythene). Branched polymers carry side chains on the main chain so the chains pack loosely, lowering density and strength (e.g. low-density polythene, amylopectin). Cross-linked (network) polymers have chains joined by covalent bridges into a three-dimensional rigid network (e.g. bakelite, melamine, vulcanised rubber).
3. By mode of polymerisation. Addition polymers form by repeated addition of unsaturated monomers with no loss of any small molecule (e.g. polythene from ethene). Condensation polymers form by repeated reaction between bi- or poly-functional monomers with elimination of a small molecule such as water or HCl (e.g. nylon-6,6, terylene).
4. By molecular forces. This classification, based on the strength of intermolecular forces between chains, decides mechanical behaviour. Elastomers have the weakest forces — coiled chains held by a few cross-links, so they stretch greatly and recoil (rubber). Fibres have the strongest forces — strong hydrogen bonding or dipole–dipole forces give close packing, high tensile strength and thread-forming ability (nylon, terylene). Thermoplastics have intermediate forces; they soften on heating and harden on cooling, and can be remoulded repeatedly (polythene, PVC, polystyrene). Thermosetting polymers are heavily cross-linked; on heating they set permanently into an infusible mass and cannot be remoulded (bakelite, melamine).
Molecular mass. Because chains in a sample differ in length, polymers have an average molecular mass. The number-average (M̅n) counts each molecule equally, while the mass-average (M̅w) weights heavier chains more; their ratio, the polydispersity index (PDI = M̅w/M̅n), measures the spread of chain lengths and equals 1 only for a perfectly uniform polymer.
Define the terms monomer, polymer and polymerisation, with one example.
Solution- A monomer is the small repeating unit (a low molecular mass molecule) from which a polymer is built, e.g. ethene, CH2=CH2.
- A polymer is a high molecular mass macromolecule formed by joining many monomers through covalent bonds, e.g. polythene.
- Polymerisation is the chemical process of linking monomers into a polymer.
Answer: Many ethene monomers polymerise to give the polymer polythene.
Classify the following as natural, semi-synthetic or synthetic polymers: cellulose, cellulose acetate, polythene, natural rubber, nylon-6,6.
Solution- Cellulose occurs naturally in plant cell walls → natural.
- Cellulose acetate is chemically modified cellulose → semi-synthetic.
- Polythene is made industrially from ethene → synthetic.
- Natural rubber is the latex of Hevea brasiliensis → natural.
- Nylon-6,6 is made from adipic acid and hexamethylenediamine → synthetic.
Answer: Natural — cellulose, natural rubber; semi-synthetic — cellulose acetate; synthetic — polythene, nylon-6,6.
Distinguish between thermoplastic and thermosetting polymers, giving one example of each.
Solution- Thermoplastics are linear or slightly branched polymers with intermediate forces; they soften on heating and harden on cooling and can be remoulded repeatedly, e.g. polythene, PVC.
- Thermosetting polymers are heavily cross-linked; on heating they set permanently into an infusible mass and cannot be remoulded, e.g. bakelite, melamine.
- Hence thermoplastics are recyclable by reheating whereas thermosets are not.
Answer: Thermoplastic — polythene (remouldable); thermosetting — bakelite (sets permanently).
Why is high-density polythene (HDPE) more rigid and denser than low-density polythene (LDPE)?
Solution- HDPE consists of essentially linear, unbranched chains.
- Linear chains pack closely together, increasing intermolecular contact and crystallinity.
- LDPE has many branches that keep chains apart, so packing is loose.
- Close packing in HDPE gives higher density, greater rigidity and a higher melting point.
Answer: HDPE chains are linear and pack closely, whereas LDPE is branched and packs loosely, so HDPE is denser and more rigid.
Arrange elastomers, fibres and thermoplastics in increasing order of intermolecular forces and explain.
Solution- Elastomers have the weakest forces — chains are coiled and held by only a few cross-links, allowing large reversible stretching.
- Thermoplastics have intermediate forces — enough to give shape but allowing softening on heating.
- Fibres have the strongest forces — strong hydrogen bonding or dipole interactions give close packing and high tensile strength.
Answer: Elastomers < thermoplastics < fibres (increasing intermolecular forces).
A polymer sample contains chains of widely differing lengths. Explain why its molecular mass is reported as an average and define number-average molecular mass.
Solution- During polymerisation, chain growth stops at random, so different molecules contain different numbers of monomer units.
- A single sample is therefore a mixture of molecules of many molecular masses, and no single value can describe it.
- The number-average molecular mass M̅n is the total mass of all molecules divided by the total number of molecules, giving each molecule equal weight.
Answer: Because chain lengths vary, an average is used; M̅n = (total mass of molecules) / (total number of molecules).