Condensation Polymerization or Step Growth Polymerization

Now that we have learnt about polymers and their types, let us learn about polymerization. Polymerization is the process by which monomer molecule undergoes chemical reaction to form polymer chains. Here we will look at condensation polymerization and step growth polymerization.

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Addition Polymerization

Polymers can be classified into 2 types as per their preparation process they undergo – Addition polymerization and Condensation Polymerization. In addition polymerization, the entire monomer molecule becomes a part of the polymer.

For example, when ethylene is polymerized to make polyethene, every atom (present in the monomer) of ethylene becomes a part of the polymer.

condensation polymerization

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Condensation Polymerization

On the other hand, in Condensation polymerization, a part of the monomers is eliminated when the monomer becomes a part of the polymer. The part that is eliminated is usually a small molecule like water or HCl gas. Such small molecules (which are like by-products of a reaction) condense on things as tiny droplets, hence giving the name condensation polymerization.

For better understanding let us consider the preparation of Nylon 6,6:

Nylon 6,6 is prepared using two monomers,  each of which contains 6 carbon (thus giving the polymer the name as Nylon 6,6), adipoyl chloride and hexamethylene diamine.When these two compounds polymerize, the chlorine atoms from the adipoyl chloride along with one of the amine hydrogen atoms, are eliminated/expelled in the form of HCl gas.

Characteristics of Condensation Polymers

  • Condensation polymers form more slowly than addition polymers and often require suitable lab environments, like the addition of heat.
  • The polymers are generally lower in molecular weight.
  • Often, the terminal functional groups on the polymer’s chain remain active.In the late stages of polymerization, groups of shorter chains end up combining into longer chains.
  • The presence of polar functional groups on the polymer chains enhances the chain-to-chain attraction (especially in the cases of hydrogen bonding).This attributes to the crystallinity and tensile strength of the polymer.

Step-Growth and Chain Growth

The other system by which we can classify the type of polymerizations involved is divided into 2 types – Chain Growth Polymerizations and Step Growth Polymerizations. In simple terms, during the process of Chain Growth polymerization, monomers become a part of the polymer one at a time. The growth only happens at the end of the chains. It is the addition of monomers to an ever-growing chain.

The process of Step-growth polymerization is a little more complicated than chain-growth polymerization.Now let us look at an example for Step-Growth Polymerization for better understanding. Let us consider the process of making Polyester called polyethylene terephthalate.

  • The two monomers which are terephthaloyl chloride and ethylene glycol react to form a dimer-ester dimer. A dimer is two structurally same or similar monomers attached to each other by bonds that can be either strong, weak, covalent or intermolecular.
  • In this stage of a chain growth system, a third monomer would add to the dimer to form a trimer, then a fourth to form a tetramer, and continue so forth. But this is not the case when it comes to Step-growth polymerization.
  • The dimer in this instance during step-growth polymer can undergo different changes.  It can react with one of the monomers to form a trimer.
  • That was one possibility. The dimer can also attach to another dimer to form a tetramer:
  • The dimer may also bond with a trimer to form a pentamer.
  • Making things more complicated, these newly formed tetramers and pentamers can undergo reaction to form even bigger oligomers. This process may continue, resulting in growth of these oligomers and eventually the oligomers are big enough to be classified as polymers.

Difference Between Two Processes

In a step-growth reaction, the growing chains can react with each other to form longer chains. This holds true for chains of all lengths. But in an addition polymerization, only the monomers can react with growing chains. Two growing chains can’t join together the way they can in a step-growth polymerization.

An important point to understand is, polymerizations can step growth or chain growth, and they can be condensation or addition. All step-growth polymerizations may not necessarily condensation polymerization, while not all chain growth polymerizations are addition polymerization.The two systems should not be reconciled.

Solved Example for You

Q: Formaldehyde is the starting material for the manufacture of which of the following?

  1. Bakelite
  2. Caprolactum
  3. Rayon
  4. None of the above

Sol: The correct answer is option “A”. Bakelite is a trade name for various synthetic resins of which phenol-formaldehyde resins are amongst the most widely known.

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