Aids to chemistry: lesson 26: Polymerisation
Francine Taylor-Campbell, Contributor

IMPORTANT POINTS

• Polymerisation is the formation of a large molecule from smaller units called monomers.
• Monomers containing the C=C can add together to form polymers (addition) or two units can react together to eliminate a small molecule such as water (condensation).
• Polysaccharides such as starch can be broken down to simple sugars (monosaccharides) by enzymes (such as amylase) or during acid hydrolysis.
• Proteins, polyesters and polyamides are all formed from condensation polymerisation
• Another name for polymers is macromolecules.
• Polymers can be natural (proteins, fats and polysaccharides) or synthetic (polythene, nylon, and terylene).
• Hydrolysis is the breakdown of large molecules to small ones by reaction with water. This can be done in the presence of an acid.
• When sugars are fermented in the absence of oxygen, ethanol is produced.
• Fractional distillation can be used to produce pure ethanol from the products of fermentation.

Natural Macromolecules (Polymers)

1. Proteins are polymers of amino acids (NH2-X-CO2H).

H2N-X- COOH + H2N-Y-COOH + H2N-Z-CO2H

The amino acids combine by losing a mole of water (OH-H) from each linkage to produce a protein with the amide linkage.

X-C-N-Y

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O H

2. Fats are esters with the linkage -C-O. They are natural macromolecules.

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O

When fats are heated with sodium hydroxide, they produce soaps, which are sodium salts of the acids.

R'CO₂R'' + NaOH == R'CO₂Na + R''OH

Soap alcohol

3. Polysaccharides consist of a large number of sugar units joined together by oxygen atoms.

+ HO-X-OH + HO-X-OH + = -X-O-X-O-X- where X = C6H10O4

A molecule of water is eliminated at each linkage.

Hydrolysis of the polysaccharide produces the sugar or sugars that make it up.

Example boiling starch with dilute hydrochloric acid (acid catalysed):

(C₆H₁₀O₅)n + nH₂O ==(HCl)==== nC₆H₁₂O₆

Starch glucose

C₁₂H₂₂O₁₁ + H₂O ==HCl=== C₆H₁₂O₆ + C₆H₁₂O₆

Sucrose glucose fructose.

Sucrose is a disaccharide and glucose and fructose are monosaccharides and isomers.

NB. When starch is moistened by saliva the enzyme amylase works to break down the starch to monosaccharides units such as maltose. This an example of an enzyme catalysed hydrolysis.

Fermentation

In the presence of yeast, sugars when mixed with water can be converted to alcohols. yeast

C₆H₁₂O₆ (aq) ===== 2C₂H₅OH (aq) + 2CO₂ (g)

The yeast contains organic catalysts called enzymes.

Next week, we will look at the formation of some synthetic polymers.

Winners of the JPS Science and Technology Expo 2008, Norman Manley High School, display their solar-powered battery. - Contributed

Francine Taylor-Campbell is an independent contributor.