(O Level Chem) Macromolecules

1. We have seen in the chapter on Alkenes that ethene gas molecules CH₂=CH₂ can be made to undergo addition polymerization to form the addition polymer, poly(ethene) or polythene, [-CH₂-CH₂]_n where n = 2000 to 20000. There is a sharp increase in the relative molecular mass from that of ethene to that of poly(ethene). There is also a sharp increase in the melting and boiling points because the van der Waals forces between the long-chained poly(ethene) molecules require much more energy to break than the weaker van der Waals forces between the small ethene molecules.

2. Depending on the conditions used, we can produce poly(ethene) of different density. For example, to produce low density (0.91-0.94 g/cm³) poly(ethene) which is used in plastic carrier bags and cling films, a temperature of 2000 ºC and a pressure 2000 atm with a small amount of oxygen impurity are used. To produce high density (0.95-0.97 g/cm³) poly(ethene) which is used to make milk bottles, a temperature of 60 ºC, a pressure of a few atm and catalysts containing titanium compounds are used.

3. A polymer is a macromolecule or large molecule that is made up of many monomers chained up together. A monomer is a molecule that is able to bond in long chains. The monomer of poly(ethene), for example, is ethene. The linking up of monomers to form polymers is called polymerization. There are two ways of linking monomers, namely, addition polymerization and condensation polymerization.

4. Addition polymerization occurs due to the presence of C=C functional group in monomers. One example of addition polymers, other than poly(ethene), is polystyrene which is used to make food containers. The monomer of polystyrene is phenylethene or styrene which contains the C=C bond. Other examples include PVC (polyvinyl chloride or poly(chloroethene)) which is used to make water pipes, PTFE (poly(tetrafluoroethene)) which is used as the non-stick coating on cooking utensils, and PP (poly(propene)). Draw the structures of the monomers of PVC, PTFE and PP. All these monomers contain the C=C bond.

5. Addition polymers contain saturated C-C bonds. The strong C-C covalent bonds require a lot of energy to break making addition polymers generally unreactive. The advantage of unreactivity is that addition polymers are safe and durable. Unfortunately, addition polymers are non-biodegradable which means that they cannot be decomposed naturally by bacteria. If addition polymers are irresponsibly disposed of, they would stay for many years causing land and water pollution, and harming wildlife. Addition polymers are usually disposed of by burying in landfills, burning in incinerators and recycling. All these ways of disposing addition polymers have their disadvantages. Landfills take up precious land, incineration releases carbon dioxide and toxic gases and recycling is difficult and expensive.

6. In condensation polymerization, the monomers have two functional groups at both of its ends. The condensation polymers that we are learning are nylon and Terylene which are man-made fibres used in clothing, curtain materials, fishing line, parachutes and sleeping bags. Carbohydrates, proteins and fats are natural condensation macromolecules which are not in the syllabus.
   

7. There are two monomers of nylon, namely a dicarboxylic acid and a diamine. The dicarboxylic acid has one -COOH functional group on each of its two ends. The diamine has one -NH₂ on each of its two ends. The carboxylic acid and amine functional groups react to form an amide linkage -CONH₂- and a water molecule. Nylon is said to be an example of polyamides because of the amide linkages. Because a small molecule is formed as a by-product, the reaction is described as a condensation reaction. For your information, proteins also have amide linkages but their monomers, called amino acids, have one -COOH group on one end and one -NH₂ group on the other end.

8. Nylon is tough, lightweight and waterproof. Because nylon does not let water vapor through, nylon waterproof clothing traps sweat, making it unpleasant to wear. Is nylon biodegradable? Nylon takes 30 to 40 years to biodegrade but this is still less than 500 years for poly(ethene).

9. There are two monomers of Terylene. One of the monomers is also a dicarboxylic acid, as in nylon. The other monomer is a diol which contains two alcohol -OH groups on both of its ends. We have seen esterification in the chapter on Carboxylic Acids and Alcohols in which an alcohol and a carboxylic acid react to form an ester and a water molecule. Similarly, in the formation of Terylene, ester linkages -COO- and water molecules are formed. Terylene is said to be an example of polyesters because of the ester linkages.