Condensation polymers form a unique and essential class of polymers that evolve from reactions between functional groups present in monomers. These reactions result in the generation of a polymer chain and the release of a small molecule.
Formation of Condensation Polymers
Condensation polymers are developed through specific reactions between functional groups in the individual monomers. The process gets its name from the fact that when these monomers react, they 'condense', leading to the release of a small molecule, often water or methanol. This mechanism sharply contrasts with addition polymerisation, where there isn't any small molecule eliminated.
- Example: If you have a monomer with an alcohol group (-OH) and another with a carboxylic acid group (-COOH), their reaction would lead to the formation of an ester linkage (-COO-) with water as a by-product.
In this example, sulfuric acid catalyses the reaction of acetic acid (carboxylic acid group) and ethanol (alcohol group) to form ethyl acetate (ester linkage) and water.
Image courtesy of Ben Mills
Representation of Repeating Units
Polyamides
Polyamides are condensation polymers that result from the reaction between dicarboxylic acids and diamines. The functional groups involved are the amine group (-NH₂) and the carboxylic acid group (-COOH).
- Repeating Unit Formation: The amine group of one monomer reacts with the carboxylic acid group of another, forming an amide linkage (-CONH-) and releasing water.
Condensation polymerization of dicarboxylic acid and diamine.
Image courtesy of Calvero
Polyesters
Polyesters, on the other hand, form through the reaction between dicarboxylic acids and diols. The functional groups that participate are the alcohol group (-OH) of the diol and the carboxylic acid group (-COOH) of the dicarboxylic acid.
- Repeating Unit Formation: The alcohol group of one monomer reacts with the carboxylic acid group of another, resulting in an ester linkage (-COO-) and the release of water.
Condensation Polymerization to Polyester.
Image courtesy of MaChe
Biological Macromolecules and Condensation Reactions
All biological macromolecules, such as proteins, nucleic acids, and polysaccharides, form through condensation reactions.
- Proteins: Amino acids, the building blocks of proteins, join via peptide bonds. This bond is a result of a condensation reaction between the amine group of one amino acid and the carboxylic acid group of another, with water being the small molecule released.
Formation of peptide bond- a condensation reaction between the amine group of one amino acid and the carboxylic acid group of another.
Image courtesy of Renate90
- Nucleic Acids: The nucleotide monomers in RNA and DNA undergo condensation reactions, resulting in the formation of phosphodiester bonds. Water is again the by-product.
Structure of nucleotide within a molecule forming a phosphodiester bond.
Image courtesy of Kep17
- Polysaccharides: Sugars or monosaccharides link together through glycosidic linkages. This bond forms due to a condensation reaction between hydroxyl groups of two sugar molecules, releasing water.
Image courtesy of CNX OpenStax
Interestingly, the reverse of condensation, known as hydrolysis, is the process by which these biological macromolecules degrade. During hydrolysis, water is utilised to break down the macromolecule into its constituent monomers.
Identifying Functional Groups for Condensation Reactions
Recognising functional groups in monomers that can participate in condensation reactions is crucial. These groups determine the nature of the linkage and the type of polymer formed.
- Diamines: Compounds with two amine groups (-NH₂) at their terminals.
- Dicarboxylic acids: Molecules with two carboxylic acid groups (-COOH) at their ends.
- Diols: Compounds having two alcohol groups (-OH) at their terminals.
It's important to understand that not all monomers with these functional groups will form polymers. The reaction's success relies on factors like the spatial arrangement of functional groups, the nature of the monomer, and the reaction conditions.
FAQ
Polyamides, commonly known as nylons, are used in clothing because of their unique properties. They are exceptionally strong and resistant to wear and abrasion, making them ideal for durable clothing items. Furthermore, polyamides possess excellent elasticity and resilience, ensuring that garments retain their shape even after repeated wear. Additionally, they have good resistance to chemicals, oils, and UV radiation. Due to their hydrophobic nature, polyamide fabrics dry quickly, and they also have a soft and smooth texture, which enhances the comfort of the clothing.
The physical properties of condensation polymers can be quite different from addition polymers. Condensation polymers, like polyesters and polyamides, often have high melting and boiling points due to the strong intermolecular forces between polymer chains, attributed to the polar nature of their functional groups. They are also typically more resistant to chemical attack. Addition polymers, on the other hand, can range from being flexible to very rigid, depending on their structure and the nature of the monomer. Their resistance to chemicals and their melting and boiling points can also vary significantly based on the nature of the polymer and the presence or absence of branching.
Not all condensation polymers are biodegradable. While it's true that some condensation polymers, such as polyesters derived from natural resources, can be broken down by microorganisms, others, especially those derived from petrochemicals, are resistant to biodegradation. The biodegradability of a polymer depends on its chemical structure and the presence of functional groups that can be attacked by enzymes. It's essential to distinguish between biodegradable polymers and those that are merely bio-based (derived from biological sources but not necessarily biodegradable).
Functional groups in monomers play a pivotal role in determining the properties of the resulting polymers. For instance, the presence of polar functional groups, like hydroxyl (-OH) or carboxylic acid (-COOH), can increase the polarity of the polymer, leading to higher melting and boiling points due to stronger intermolecular forces. Functional groups can also influence the polymer's solubility in water or other solvents. Additionally, certain functional groups can make the polymer more susceptible to chemical reactions, allowing for modifications or cross-linking. In essence, the nature and arrangement of functional groups in monomers directly influence the chemical, physical, and mechanical properties of the resulting polymer.
Condensation polymers and addition polymers are both types of polymers, but they form through different reaction mechanisms. Condensation polymers form by the reaction between two different monomers, with each monomer having two functional groups. As these groups react, they form a covalent bond and release a small molecule as a by-product, commonly water or methanol. On the other hand, addition polymers form from the polymerisation of a single type of monomer containing a double bond. The double bond breaks and allows the monomers to join together, forming the polymer chain without releasing any by-products.
Practice Questions
Polyesters are formed through the condensation polymerisation between dicarboxylic acids and diols. The functional groups involved in this process are the carboxylic acid group (-COOH) from the dicarboxylic acid and the alcohol group (-OH) from the diol. When these two functional groups react, they form an ester linkage (-COO-) and release a molecule of water as the by-product. This mechanism aligns with the general principle of condensation reactions, where the reaction between two monomers leads to the formation of a polymer chain and the release of a small molecule, commonly water or methanol.
Biological macromolecules, such as proteins, nucleic acids, and polysaccharides, are formed through condensation reactions. In the case of proteins, amino acids react together; the amine group of one amino acid reacts with the carboxylic acid group of another. This forms a peptide bond, and water is released as a by-product. The process by which these biological macromolecules degrade is called hydrolysis. Hydrolysis is essentially the reverse of condensation. During this process, water is utilised to break down the macromolecule into its individual monomers. An example is the digestion of proteins into amino acids, which involves the hydrolysis of peptide bonds.
