Chain and Step growth polymerization


Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers.

Chain-growth polymerization or chain polymerization is a polymerization technique where unsaturated monomer molecules add onto the active site of a growing polymer chain one at a time.

Chain-growth polymerization can be understood with the chemical equation.

chain

Step-growth

Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers, usually containing hetero atoms such as nitrogen or oxygen.

In step-growth polymerization, first two monomers of A and B react with each other through their terminal functional groups to form AB molecule.

Example

H2N-C6H12-NH2 and

HOOC-C4H8-COOH H2N-C6H12-NHOC-C4H8-COOH releasing one molecule H2O.

In next step AB molecules react with each other and with present A and B to form long chains of ABABABABA...ABAB.

Chain-growth

Chain-growth polymerization involves the linking together of molecules incorporating double or triple carbon-carbon bonds. These unsaturated monomers (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form a repeating chain, whose backbone typically contains only carbon atoms.

The chain length of polymer can be controlled by terminating the propagation step through addition of a compound to react with growing free radicals and stop the reaction. The polyethylene is the product of monomers which is used to produce many products like plastic shopping bag.

The difference between step growth and chain growth polymerization as follows:

Step growth polymerisation Chain growth polymerisation
All molecular species are present at any stage. Reaction mixture contains only a monomer, polymer and growing chain.
Long reaction time is required to get high molecular mass polymer. Increase in reaction time increases the yield but molecular weight is affected a little.
The molecular mass of polymer increases throughout the reaction. There is very little change in the molecular mass throughout the reaction.
The polymer is formed in gradual steps. Reaction is fast and polymer is formed at once.
Any two species present can react. Only one repeating unit is added at a time.
Concentration of monomers decreases steadily throughout the reaction. Concentration of monomers disappears early in the reaction.

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