4.7 Plastics and polymers

Addition polymerisation

In this type of reaction, monomer molecules are added to a growing polymer chain one at a time. (No small molecules are eliminated in the process).

Four major examples of addition polymers are polyethylene, polypropylene, polyvinylchloride (PVC) and polystyrene (Table 4.18). All four of these organic polymers are also plastics.

1. Polyethene (polyethylene)

   

   Earlier in this chapter we looked at the structure of a group of hydrocarbons called the alkenes. One example is the molecule ethene. The structural formula of ethene is shown in Figure 4.95. When lots of ethene molecules bond together, a polymer called polyethene (commonly called polyethylene) is formed. Ethene is the unsaturated monomer which, when joined to other ethene molecules through an addition reaction, forms the saturated polymer polyethene.

   Polyethene is the most common plastic with over 80 million metric tons produced each year. It is commonly known as polyethylene. It is cheap and is used to make squeeze bottles, plastic bags, films, toys and molded objects as well as electric insulation. It has a recycling number 4 which means that it is easy to process, has strength, toughness, flexibility, is easy to seal and has a barrier to moisture.

   

   A polymer may be a chain of thousands of monomers, and so it is impossible to draw the entire polymer. Rather, the structure of a polymer can be condensed and represented as shown in Figure 4.96. The monomer is enclosed in brackets and the n represents the number of repeating units (the saturated form of the monomer) in the polymer, where n is any whole number. What this shows is that the monomer is repeated an indefinite number of times in a molecule of polyethene.

   

   Video: 27QC

2. Polypropene (polypropylene)

   

   Another example of a polymer is polypropene (Figure 4.98). Polypropene (commonly known as polypropylene) is also a plastic, but is stronger than polyethene and is used to make crates, fibres and ropes as well as being used in textiles, stationery and car parts. In this polymer, the monomer is the alkene called propene.

   

   Video: 27QD

3. Polyvinyl chloride (PVC)

   

   Polyvinyl chloride or PVC (Figure 4.100) is formed from the monomer chloroethene, which is commonly known as vinyl chloride. PVC is used in construction, especially plastic piping. With the addition of a plasticiser it is also used in clothing and upholstery and to replace rubber. The role of the plasticiser is to increase the ability of a material to change shape without breaking.

   

   Video: 27QF

4. Polyvinyl acetate

   

   Polyvinyl acetate or PVA (Figure 4.102) is formed from the monomer ethenyl ethanoate, which is commonly known as vinyl acetate. PVA is used in various glues and adhesives (such as wood glue).

   Ethenyl is not the same as ethyl. Ethyl is an ethane molecule bonded to another compound (\(\color{red}{\textbf{R}}\)). Ethenyl is an ethene molecule bonded to another compound (\(\color{red}{\textbf{R}}\)).

   

   

5. Polystyrene

   

   Polystyrene is made from the monomer styrene which is a liquid petrochemical. Styrene consists of a benzene ring (a six membered ring with three double bonds) bonded to an ethene chain. Polystyrene is an aromatic polymer and has many uses including protective packaging, in trays, as plastic lids and bottles.

   Benzene can be represented with three double bonds:

   

   or with a circle inside the ring:

   

   

It is interesting to note that the polymerisation of a monomer leads to different physical properties of the polymer (Table 4.19). For the polymers the melting points are dependent on the polymer grade.

Addition polymers

Textbook Exercise 4.30

What trend is visible when comparing the melting points of monomers and their polymers?

Polymerisation increases the melting point OR while the monomers are in the gas or liquid phase the polymers are solid.

Name the monomer (where polymer is given) or polymer (where monomer is given) for the following:

The polymer is polypropene or polypropylene.

The monomer is ethene or ethylene.

The monomer is chloroethene or vinyl chloride.

Condensation polymerisation

In this type of reaction, two monomer molecules combine by means of a covalent bond, and a small molecule such as water is lost in the bonding process. Nearly all biological macromolecules are formed using this process. Polyesters are polymers that form through condensation polymerisation.

Polyesters have a number of characteristics which make them very useful. They are resistant to stretching and shrinking, they are easily washed and dry quickly, and they are resistant to mildew. It is for these reasons that polyesters are being used more and more in textiles. Polyesters are stretched out into fibres and can then be made into fabric and articles of clothing. In the home, polyesters are used to make clothing, carpets, curtains, sheets, pillows and upholstery.

Polyesters are a group of polymers that contain the ester functional group in their main chain. This bond is called an ester linkage. For a polyester however there needs to be continuation of the chain. This requires a -diol (two alcohol functional groups) and a diacid (two carboxylic acid functional groups) as shown in Figure 4.105, or a monomer that contains both a hydroxyl group and a carboxylic acid.

1. Polyethylene terephthalate (PET)

   

   Although there are many forms of polyesters, the term polyester usually refers to polyethylene terephthalate (PET). PET is made from ethan-1,2-diol (ethylene glycol, an alcohol) and terephthalic acid (a carboxylic acid). In the reaction, a hydrogen atom is lost from the alcohol, and a hydroxyl group is lost from the carboxylic acid. Together these form one water molecule which is lost during condensation reactions. A new bond is formed between an oxygen and a carbon atom. This bond is called an ester linkage. The reaction is shown in Figure 4.107.

   Polyethylene terephthalate (PET) is not just a textile. PET is in fact a plastic which can also be used to make plastic drink bottles. Many drink bottles are recycled by being reheated and turned into polyester fibres. This type of recycling helps to reduce disposal problems.

   

   Video: 27QK

2. Polylactic acid (PLA)

   3D printing is the process of printing a solid, three-dimensional object. If you have a 3D printer it is possible to print objects ranging from fashion items (e.g. cell phone covers), to human organs, although organs require living cells as the 'ink'.

   

   Despite the name, polylactic acid (PLA) is actually a polyester (look for the ester linkage we spoke about before). It is an interesting polymer because the monomer used for this polymer (Figure 4.109) comes from the biological fermentation of plant materials, while most monomers used in plastics come from petroleum. As a result PLA is biodegradable and has low carbon dioxide (\(\text{CO}_{2}\)) emissions.

   

   PLA is mostly used for packaging material and, because it is biodegradable, it has the potential to alleviate land-fill disposal problems. The PLA polymer structural repeat unit is given in Figure 4.110.

   

Plastics and the environment

Although plastics have had a huge impact globally, there is also an environmental price that has to be paid for their use. The following are just some of the ways in which plastics can cause damage to the environment.

1. Waste disposal

   Plastics are not easily broken down by micro-organisms and therefore most are not easily biodegradeable. This leads to waste disposal problems.

2. Air pollution

   When plastics burn, they can produce toxic gases such as carbon monoxide, hydrogen cyanide and hydrogen chloride (particularly from PVC and other plastics that contain chlorine or nitrogen).

3. Recycling

   It is difficult to recycle plastics because each type of plastic has different properties and so different recycling methods may be needed for each plastic. Some plastics can be remelted and re-used, while others can be ground up and used as a filler. One of the problems with recycling plastics is that they have to be sorted according to plastic type. A list of some of the different types and their identification codes are given in Table 4.22. Alternatively, plastics should be re-used. In many countries, including South Africa, shoppers must now pay for plastic bags. This encourages people to collect and re-use the bags they already have.

PIC	Plastic type	Properties	Uses	Recycling
	polyethylene terephthalate	very clear high strength barrier to gas barrier to moisture	water bottles peanut butter jars mouthwash bottles	can recycle most places
	high-density polyethylene	very stiff high strength barrier to moisture allows gas through	yoghurt tubs detergent bottles cereal box liners	can recycle most places
	polyvinyl chloride	can be blended easily high strength very toughness	detergent bottles medical equipment piping	very rarely recycled
	low-density polyethylene	very flexible high strength barrier to moisture	clothing squeeze bottles bread bags	not often recycled
	polypropylene	high strength resistance to heat barrier to moisture	syrup bottles plastic caps straws	fairly easy to recycle
	polystyrene	many uses very clear is easily formed	egg cartons disposable cups take-away boxes	fairly easy to recycle
	other	dependent on combination of polymers	large containers sunglasses computer cases	often not recycled

Table 4.22: The plastic identification code (PIC), type, properties and uses of various commonly used plastics.