Condensation polymerization

Introduction

In polymer chemistry, a significant quantity of important and practical polymeric substances are formed by conventional transformations of functional groups of polyfunctional reactants. In such a reaction, a smaller molecule is produced as a byproduct.

This process generally involves the combination of two or more monomers with distinct structures. Nylon 6,6 is a condensation polymer synthesized by Wallace Carothers in 1935. Nylon 6,6 was the first laboratory synthesized condensation polymer. Nylon 6,6 is formed by condensing monomeric units of hexamethylenediamine and adipic acid. Wallace Carothers also contributed to the invention of neoprene. Other than synthetic polymers, there are various naturally occurring condensation polymers synthesized by plants and microorganisms like bacteria.

What is Condensation Polymerization?

Condensation polymerization is a process in which two or more monomeric units undergo a condensation reaction or combine to form a larger molecule or polymer. Unlike addition polymerization, this process is a type of step-growth polymerization. Generally, the step-growth polymers increase the chain length by forming bonds between carbon and heteroatoms (like O, N, S) present in monomeric units of the polymer. This indicates that the monomer unit generally constitutes two or more functional groups at the end of the chain, that is, they are bifunctional. In this process of polymerization, a small molecule (such as methanol, water, etc) is eliminated as the byproduct.

Some industrially important polymers like nylon 6,6, dacron, spandex, etc. are produced by this type of polymerization. The subclasses of condensation polymerization are used to produce commercially important products that we use in our day-to-day life. Polyester, polyamides, polyacrylamide, polycarbonates, and silicone are some subcategories of condensation polymers that are utilized in producing commercially significant materials such as bottles, clothes, tanks and aircraft parts, coverings, cosmetics etc.

Overview and Importance of Polymerization

Polymerization is a process that involves a chemical combination of monomers to produce a larger molecule called a polymer. The polymer produced can have a chain structure or a network framework. The monomeric unit can be numerous units of the same compound or they can be of different compounds. More than hundreds of monomer units are required to produce a polymer. Polymers have high elasticity, high tensile strength and the ability to form fibres.

Monomeric units are attached via covalent bonds. Polymerization is categorised into two categories based on the mechanism of reaction followed to produce polymers. The first category is condensation polymerization which involves the sequential condensation of monomers to form a polymer.

The second category is addition polymerization which involves the rapid addition of monomers without the elimination of small molecules. The addition polymerization occurs in the presence of a catalytic substance that can sometimes control the properties of polymer by dictating the structure. By structural distinctions, polymers are categorised as linear polymers and cross-linked polymers. Linear polymers can be crystalline solids or viscous liquids whereas cross-linked polymers are thermosetting resins.

Degree of Polymerization

The number of monomeric units present in a polymer is represented by the degree of polymerization. In industrial applications, the degree of polymerization exceeds thousands due to the large-scale production of polymeric material. The degree of polymerization can give a good idea about the number of repeating units. These repeating units are the same for homopolymers whereas copolymers can have different types of repeating units. The degree of polymerization is significantly important in altering the mechanical properties of a polymer. The degree of polymerization can alter the properties of a polymer dramatically. This is because as the length of the chain of the polymer increases the mechanical properties like hardness, strength, and ductility, increase suddenly but keep on decreasing eventually.

The degree of polymerization is further classified into two categories namely the number-average degree of polymerization and weight-average degree of polymerization. The number-average degree of polymerization is based on the mole fractions of the polymer or the number of molecules present in the polymer. This category employs the calculation of osmotic pressure to determine the degree of polymerization. The weight-average degree of polymerization is based on the weight fraction of the polymer or the gross weight of the polymer. Rayleigh light scattering is experimentally employed to determine this category of the degree of polymerization.

Characteristics of Condensation Polymerization

The formation of condensation polymers is a much slower reaction than addition polymerization. This process progresses often in the presence of heat energy. The products of condensation polymerization or condensation polymers generally have molecular weight lower than addition polymers. The monomers of condensation polymers constitute multiple functional groups. The presence of these functional groups increases the intermolecular attraction and therefore affects the tensile strength and crystalline properties of the molecule. The products of condensation polymerization have amide linkage or ester linkage in the polymer framework. The condensation polymers that are made up of different monomeric units usually have characteristics of both the monomeric units.

The production of condensation polymers involves the elimination of a water molecule. The main characteristic of this type of polymerization is it is a type of step-growth polymerization. This means the product formation takes place in a stepwise manner to produce target condensation polymers. The rate of increment in the molecular weight of condensation polymers is very slow. Therefore, the product of this type of polymerization does not reach very high values of molecular weights.

Examples of Condensation Polymerization

There are many important examples of condensation polymers.

• Polyesters are formed by condensation of diols and dicarboxylic acid that involves the removal of water molecules in the process. An example of such a polyester is terylene.

• Polyamides are formed by condensation of diamine and dicarboxylic acid. Polyamides have amide linkages in the structure of the polymer. A dioyl dichloride can be used in the place of dicarboxylic acid in this process.

• Nylon 6,6 is also a polyamide produced synthetically. It is made up of 1,6-diamino hexane and hexane-1,6-dioic acid. The name is derived from the structure of monomers as the monomers have 6 carbon atoms.

• Nylon 6 another variation of Nylon 6,6 polyamide is a homopolymer of caprolactam. The condensation polymerization proceeds with the ring-opening of caprolactam which further reacts with other units of caprolactam. This variation of nylon also has an amide linkage in the skeletal structure of the polymer.

• Kevlar are condensation polymers that have numerous hydrogen bonds in their polymer backbone. This condensation polymer is a strong and flexible polymer that has flame-resistant properties. Therefore, it is used in bullet-proof vests. It is made up of 1,4-diaminobenzene and benzene-1,4-dicarboxylic acid.

Conclusion

In a nutshell, the process of condensation polymerization leads to the production of some commercially and industrially important compounds that have their application in modern-day society. Polyesters are used in making bottles, clothes, paints, umbrellas, musical instruments, etc. Polyamides such as nylon are used in making clothes, toothbrushes, screws, wheels, industrial filaments, etc. Polyacrylamides such as Kevlar are used in making bulletproof vests, parts of aircraft and tanks, seat belts, etc. Polycarbonates are a type of thermoplastics used in making armoured glass and lenses. Silicone, a very important and versatile polymer, is the most common polymer used in everyday life. It is highly heat resistant and a good insulator of heat and electricity.

FAQs

1. How is chain-growth polymerization different from step-growth polymerization?

Chain-growth polymerization	Step-growth polymerization
It is the formation of polymers from unsaturated monomers.	It is the formation of polymers from multifunctional monomers.
Separate attachment of monomers takes place	Oligomers are formed initially and polymerization of oligomers proceeds.
Unsaturated monomers	Multifunctional monomers
No rapid loss of monomers	Rapid loss of monomers initially
The active site is present at the end of the polymer chain	All monomers are active site
An initiator is required to break the double bond of monomers	No initiator required
No chain growth takes place after termination	Absence of termination

2. Proteins are formed by which type of polymerization reaction?

Proteins are formed by condensation polymerization of amino acid monomers. Amino acids consist of an amine and a carboxylic acid group. Therefore, the product formation of protein synthesis takes place by the formation of peptide linkages.

3. What are the monomers of cellulose?

Cellulose is made up of multiple monosaccharide units.

4. Which equation is used for calculating the degree of polymerization of step-growth polymerization?

Carothers equation is used for calculating the degree of polymerization of step-growth polymerization.

$$\mathrm{\underline{X_n}=\frac{1}{1-p}}$$

5. What is PET?

Polyethylene terephthalate is a polymer formed by condensation polymerization of ethylene glycol and terephthalic acid.