Polymerization is the method of joining tiny molecules to create a large molecule. These tiny molecules are referred to as monomers. The monomers are the fundamental unit of polymers. These monomers attach to one another to form polymers.
According to the chemical reaction involved, the polymerization method is split into two groups, namely, addition polymerization and condensation polymerization. Both these groups of reactions have some differences. But, before talking about the differences, first let us understand what these two types of polymerization reactions exactly mean.
Addition Polymerization
Addition polymerization, according to the name, consists of forming polymers when an addition reaction occurs. It is the method of recurring addition of monomers that have multiple bonds (double or triple) to form polymers.
The recurring monomers create a linear or branch framework based on the kind of monomer. The polymers formed are called addition polymers.
Homopolymers – The generation of addition polymers by the polymerization of a single polymer species is referred to as homopolymer, for instance, polypropylene ((C3H6) n).
Copolymers - The generation of addition polymers that takes place by the addition polymerization of two different monomers is referred to as copolymer, for instance, Buna-S, Buna-N, etc.
In the process of addition polymerization, the monomers realign themselves to create a new design. However, there is no loss of molecule or an atom. Addition polymerization is also known as chain-growth polymerization and consists of four types of processes which are as follows:
Free Radical Polymerization - In this process, the addition polymer is formed by the addition of atoms with a free electron in its valence shells. These are referred to as free radicals and take part in a subsequent chain during free radical polymerization.
Cationic Polymerization – In this method, a cation is formed which leads to a chain reaction. This further leads to formation of an extended chain of recurring monomers.
Anionic Vinyl Polymerization – In this process, the polymerization of specifically vinyl polymers takes place with a strong electronegative group to establish a chain reaction.
Coordination Polymerization – This technique was brought forth by two scientists – Natta and Ziegler – who received a Nobel Prize for their contribution. These two scientists created a catalyst that allows managing the free radical polymerization. The overall reaction gives rise to a polymer with higher strength and density.
Some techniques under addition polymerization include: Emulsion polymerization, solution polymerization, suspension polymerization and bulk polymerization. Furthermore, examples of addition polymers are Polyvinyl chloride (PVC), Buna rubbers, polyethylene, etc.
Addition polymerization consists of the following steps:
Chain initiation
Chain propagation
Chain termination
Chain initiation – It is the method of initially creating a chain carrier (intermediates like ions and radicals) in the polymerization reaction. Based on various ways of energy distribution, it consists of high energy initiation, thermal initiation, and chemical initiation. High energy initiation is the formation of chain intermediates by radiation. Thermal initiation is the method of acquiring energy and dissipated to homolytic division to create an active center by molecular thermal movement. Further, chemical initiation arises because of chemical initiators.
Chain propagation – According to the IUPAC (International Union of Pure and Applied Chemistry), chain propagation is referred to as an active center on the evolving polymer molecule that adds one monomer molecule in order to create a new polymer molecule that is one recurring unit bigger with a new active center.
Chain termination – In this process, the active center disappears, giving rise to the ending product or termination of chain propagation.
Condensation Polymerization
Also known as step-growth polymerization, condensation polymerization forms polymers via step growth process. In this method, the molecules of monomers react to create a chain that further substitutes specific molecules. These molecules are the by-products of the process, which in a majority of times, is a water molecule. Further, the polymers in this method are called condensation polymers.
The kind of polymers that are produced from the condensation polymerization process relies on the monomers. If the monomer consists of a single reactive group, the polymers formed have low molecular weight and monomers with two reactive end groups produce linear polymers. In addition, monomers with over two reactive groups give rise to polymers with a three-dimensional (3D) linking.
To understand better, let us take the example of Nylon 6, 6.
Nylon 6, 6 is a result of two monomers and both of them consist of 6 carbon (As such the name, Nylon 6, 6), hexamethylene and adipic acid.
Condensation polymers that occur naturally are cellulose, the polypeptide chains of poly (β-hydroxybutyric acid) and proteins, etc. Dacron (Polyester), and Nylon 6,6 (Polyamide) are some of the examples of synthetic condensation polymers. The techniques involved in condensation polymerization are solution polycondensation and melt polycondensation.
Properties of Condensation Polymers
Condensation polymers are usually low in molecular mass.
In general, the end groups on the polymer's chain stay active. In further phases of condensation polymerization, the groups of shorter chains combine to form bigger chains.
Condensation polymers take more time in getting prepared as compared to addition polymers and generally require lab settings such as the addition of heat.
The availability of polar functional groups on the polymer chains increases the chain-to-chain attraction, particularly in the case of hydrogen bonding. This gives rise to the tensile strength and crystalline behavior of the condensation polymers.
Let us find out the differences between addition polymerization and condensation polymerization:
Addition polymerization gives rise to homo-chain polymers, while condensation polymerization gives rise to hetro-chain polymers.
There are three individual steps in addition to polymerization, which are as follows: Initiation, propagation and termination. On the other hand, there is no such terminal step in condensation polymerization. The terminal groups stay reactive throughout the whole mechanism.
Monomers only attach to the active site of the chain in addition to polymerization, while in condensation polymerization, the molecules can react with each other.
The most notable difference is that there is no loss of atom or molecule in addition to polymerization, while loss in the form of molecules of ammonia, water, etc. takes place in condensation polymerization.
