Polymerisation
The process of joining monomers into large macromolecules via addition or condensation reactions.
### Introduction to Polymers
Polymers are large molecules, also known as macromolecules, constructed from many small, repeating chemical units called monomers. The process of chemically joining these monomers to form a polymer is called polymerisation. Polymers are fundamental to modern life, forming the basis of plastics, fibres, and many biological molecules.
There are two main types of polymerisation that you need to understand for the Cambridge A Level Chemistry syllabus: addition polymerisation and condensation polymerisation.
### Addition Polymerisation
Addition polymerisation is a process in which monomer units add to one another in such a way that the polymer is the only product formed. The key characteristic of a monomer undergoing addition polymerisation is the presence of a C=C double bond.
The Process:
The reaction is typically initiated by a radical. The mechanism involves the breaking of the weaker π-bond (pi-bond) in the alkene monomer, allowing the monomers to link together to form a long saturated chain. The empirical formula of the polymer is identical to that of the monomer.
The structure of the polymer can be represented by its repeating unit. To find the repeating unit:
Examples of Addition Polymers:
Reaction: n(CH₂=CH₂) → [-CH₂-CH₂-]n
Reaction: n(CH₂=CHCH₃) → [-CH₂(CH₃)CH-]n
Reaction: n(CH₂=CHCl) → [-CH₂CHCl-]n
### Condensation Polymerisation
Condensation polymerisation involves a reaction between monomers that have two functional groups. When the monomers link together, a small molecule, typically water (H₂O), is eliminated for each bond formed. This is why it is called 'condensation'.
Unlike addition polymers, the repeating unit of a condensation polymer is not the same as the monomers because atoms are lost during the reaction.
There are two main types of condensation polymers: polyesters and polyamides.
1. Polyesters:
Polyesters are formed when monomers are linked by ester linkages (-COO-). This typically involves the reaction between a dicarboxylic acid (a molecule with two -COOH groups) and a diol (a molecule with two -OH groups).
Example: Terylene (also known as PET)
Terylene is formed from the monomers benzene-1,4-dicarboxylic acid and ethane-1,2-diol. A molecule of water is eliminated for each ester link formed.
Monomers: HOOC-C₆H₄-COOH + HO-CH₂-CH₂-OH
Polymerisation: ... → [-OC-C₆H₄-CO-O-CH₂-CH₂-O-]n + 2n H₂O
Terylene is widely used in textiles (as polyester fabric) and for making plastic bottles.
2. Polyamides:
Polyamides are formed when monomers are linked by amide linkages (-CONH-), also known as peptide links in biological contexts. This involves the reaction between a dicarboxylic acid and a diamine (a molecule with two -NH₂ groups).
Example: Nylon 6,6
Nylon 6,6 is formed from hexanedioic acid (a 6-carbon dicarboxylic acid) and 1,6-diaminohexane (a 6-carbon diamine). The name '6,6' comes from the number of carbon atoms in each monomer.
Monomers: HOOC-(CH₂)₄-COOH + H₂N-(CH₂)₆-NH₂
Polymerisation: ... → [-CO-(CH₂)₄-CO-NH-(CH₂)₆-NH-]n + 2n H₂O
Nylon is a strong, resilient fibre used for clothing, carpets, and ropes.
### Properties and Disposal of Polymers
The properties of polymers are directly related to their chemical structure, which also dictates their environmental impact.
* Addition Polymers: The backbone of an addition polymer consists of strong, non-polar C-C single bonds. These bonds are chemically inert and not easily attacked by microorganisms. As a result, addition polymers are non-biodegradable, leading to significant problems with plastic waste in landfills and oceans.
* Condensation Polymers: The ester and amide linkages in condensation polymers are polar and can be attacked by nucleophiles. They can be broken down by hydrolysis (reaction with water), a process that is much faster in acidic or alkaline conditions. This means condensation polymers are, in principle, biodegradable, although the process can still be very slow.
Disposal Issues:
Key Points to Remember
- 1Polymers are macromolecules formed from repeating monomer units.
- 2Addition polymerisation joins alkene monomers by breaking the C=C π-bond, forming only the polymer product.
- 3The repeating unit of an addition polymer (e.g., poly(ethene)) has the same empirical formula as its monomer.
- 4Condensation polymerisation joins monomers with two functional groups, eliminating a small molecule like H₂O.
- 5Polyesters (e.g., Terylene) contain ester linkages (-COO-), while polyamides (e.g., Nylon) contain amide linkages (-CONH-).
- 6Addition polymers are typically non-biodegradable due to their strong, non-polar C-C backbone.
- 7Condensation polymers are susceptible to hydrolysis at their ester or amide links, making them biodegradable.
- 8Polymer disposal poses environmental issues, including landfill accumulation and the release of toxic gases (e.g., HCl from PVC) upon incineration.
Pakistan Example
PET Plastic and the Textile Industry in Pakistan
In Pakistan, the condensation polymer Polyethylene terephthalate (PET), known commercially as Terylene, plays a dual role in the economy and environment. The country has a massive textile industry, one of the largest in the world, which heavily relies on polyester fibres (Terylene) for manufacturing clothing and fabrics. Simultaneously, the beverage and mineral water industries extensively use PET to produce plastic bottles. This widespread use creates a significant environmental challenge related to plastic waste. However, it also presents a major opportunity for recycling. Companies in Pakistan are increasingly involved in collecting used PET bottles and recycling them, either into new bottles or by converting them into polyester staple fibre, which is then used by the textile industry. This creates a circular economy model, linking the concepts of condensation polymerisation directly to Pakistan's industrial manufacturing and its growing environmental awareness.
Quick Revision Infographic
Chemistry — Quick Revision
Polymerisation
Key Concepts
Formulas to Know
C=C π-bond, forming only the polymer product.PET Plastic and the Textile Industry in Pakistan
In Pakistan, the condensation polymer Polyethylene terephthalate (PET), known commercially as Terylene, plays a dual role in the economy and environment. The country has a massive textile industry, one of the largest in the world, which heavily relies on polyester fibres (Terylene) for manufacturing clothing and fabrics. Simultaneously, the beverage and mineral water industries extensively use PET to produce plastic bottles. This widespread use creates a significant environmental challenge related to plastic waste. However, it also presents a major opportunity for recycling. Companies in Pakistan are increasingly involved in collecting used PET bottles and recycling them, either into new bottles or by converting them into polyester staple fibre, which is then used by the textile industry. This creates a circular economy model, linking the concepts of condensation polymerisation directly to Pakistan's industrial manufacturing and its growing environmental awareness.