Condensation Polymerisation and Biodegradability

The Mechanics of Condensation Polymerisation

Condensation polymerisation differs fundamentally from addition polymerisation. While addition polymerisation involves the opening of $C=C$ double bonds in unsaturated monomers, condensation polymerisation occurs between monomers that possess at least two functional groups. When these groups react, they form a chemical bond and release a small byproduct molecule, typically water ($H_2O$). Because each monomer is bifunctional, the reaction can occur at both ends of the molecule, allowing a long chain to grow.

Formation of Polyesters

Polyesters are produced when dicarboxylic acids react with diols. A dicarboxylic acid contains two $-COOH$ groups, and a diol contains two $-OH$ groups. During the reaction, the $-OH$ group from the carboxylic acid and the $-H$ from the alcohol combine to form water, leaving behind an ester link ($-COO-$) that joins the monomer units.

For example, the polymerisation of a generic diol ($HO-R-OH$) and a generic dicarboxylic acid ($HOOC-R'-COOH$) can be represented as:

$n HO-R-OH + n HOOC-R'-COOH \rightarrow -[O-R-O-CO-R'-CO]_n- + 2n H_2O$

A well known example is Terylene (PET), formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Polyamides and the Structure of Proteins

Polyamides are formed when a dicarboxylic acid reacts with a diamine (containing two $-NH_2$ groups). The reaction between the $-COOH$ group and the $-NH_2$ group creates an amide link ($-CONH-$) and releases water.

Proteins are naturally occurring polyamides. They are formed from amino acid monomers, which are unique because they contain both an amine group and a carboxylic acid group on the same molecule. This allows one amino acid to react with another, forming a long chain known as a polypeptide. The amide link between amino acids is specifically referred to as a peptide link.

The formation of a protein from amino acids follows this pattern:

$n H_2N-CH(R)-COOH \rightarrow -[NH-CH(R)-CO]_n- + n H_2O$

Identifying Repeating Units and Monomers

To succeed in the ESAT, you must be able to switch between the structures of monomers and polymers.

1. To find the monomers from a polymer: Identify the repeating unit and locate the ester or amide link. To 'reverse' the reaction, add the elements of water back. Add an $-OH$ to the carbonyl ($C=O$) to restore the carboxylic acid, and add an $-H$ to the oxygen or nitrogen to restore the alcohol or amine.

2. To find the repeating unit from monomers: Remove the $-OH$ from the carboxylic acid end and the $-H$ from the amine or alcohol end. Connect the remaining fragments and place them in square brackets with the bond extending through the brackets to show the chain continues.

Biodegradability and Environmental Impact

The chemical structure of a polymer determines its environmental persistence.

Non-biodegradable polymers are typically addition polymers like poly(ethene). Their backbone consists of strong, non-polar $C-C$ bonds that are chemically inert. Microorganisms do not have the enzymes necessary to break these bonds, so they persist in the environment for hundreds of years.

Biodegradable polymers include many condensation polymers. Because polyesters and polyamides contain polar ester and amide links, they can be broken down by hydrolysis. In this process, water (often aided by enzymes or acidic/alkaline conditions) reacts with the polymer to break the links, turning it back into its constituent monomers. This makes these materials much easier to dispose of through composting or natural decay.