Chemistry » Organic Molecules » Plastics And Polymers

Condensation Polymerisation

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 the figure below, or a monomer that contains both a hydroxyl group and a carboxylic acid.

The propagation of the ester linkage in a polyester.

1. Polyethylene terephthalate (PET)

A collection of products made from polyethylene terephthalate: sailcloth and plastic squeeze bottles.

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 the figure below.

Fact:

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.

(a) Ethylene glycol (alcohol) and terephthalic acid (carboxylic acid) monomers react to form (b) the polymer poly(ethylene terephthalate) (PET). (c) A simplified representation of PET.

2. Polylactic acid (PLA)

Fact:

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’.

3-D printing using polylactic acid 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 (see figure below) 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.

Ester linkages between lactic acid monomers.

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 the figure below.

The polyester polylactic acid.

 Polymer name Some uses polyethylene terephthalate synthetic fibre plastic containers resins polylactic acid medical implants packaging textiles

Table: Some common uses of polymers formed through condensation reactions.

For polyethylene terephthalate there are two monomers ethylene glycol and terephthalic acid that join to form the polymer. Each monomer has its own unique properties (see table below).

 Name Monomer/polymer Melting point (℃) Boilingpoint (℃) Phase(at 25℃) $$\color{blue}{\text{terephthalic acid}}$$ monomer $$\text{300}$$ sublimes(solid $$\to$$ gas) solid $$\color{blue}{\text{ethylene glycol}}$$ monomer $$-\text{12.9}$$ $$\text{197.3}$$ liquid $$\color{blue}{\text{polyethylene}}$$$$\color{blue}{\text{terephthalate}}$$ polymer $$>\text{250}$$ decomposes solid $$\color{red}{\text{lactic acid}}$$ monomer $$\text{16.8}$$ $$\text{122}$$ liquid $$\color{red}{\text{polylactic acid}}$$ polymer $$\text{150}$$ – $$\text{160}$$ does not boil solid

Table: Different physical properties of condensation monomers and polymers.