Step Growth Vs Chain Addition Polymerization: 10 Differences (With Comparison Chart)

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What Is Step Growth Polymerization?

A step-growth polymerization is a stepwise reaction between bi-functional or multi-function monomers in which a higher-molecular weight polymer is formed after a large number of steps. Step-growth polymerization is greatly applied in the production of many natural and synthetic polymers including polyesters, polyethers, urethanes, epoxies and polyamides.  In order to achieve high molecular weight polymers through step-growth polymerization, the reaction has to proceed for a long time. In organic chemistry, the two common examples of step-growth reaction are:

  • The reaction of dicarboxylic acids with diamines to form polyamides (Nylon).
  • The reaction of organic diacids with alcohols to form polyesters, like polyethylene terephthalate (PET).

What You Need To Know About Step Growth

  1. Step growth polymerization takes place by reaction between monomers, oligomers and polymers.
  2. A polymer chain is formed at the beginning by the attachment of one monomer at a time.
  3. In step growth polymerization, monomers exist through the reaction, but large quantities of monomers are consumed early in the reaction.
  4. In step growth polymerization, all molecules that is, polymer, oligomer and monomer can react with any other molecule.
  5. Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length.
  6. Oligomers exist throughout the reaction, with the length distribution broadening and shifting to higher molecular weight with increasing reaction time.
  7. The reaction speed does not depend on the concentration of initiator and co-initiator.
  8. Long reaction time is necessary for production of high molecular weight polymers.
  9. The end groups of the oligomers and polymers are reactive throughout the polymerization process. There is usually no termination step.
  10. Similar steps are repeated throughout reaction process.

What Is Chain Addition Polymerization?

Chain addition polymerization also referred to as chain-growth polymerization technique is a chain reaction in which the growth of a polymer chain proceeds exclusively by reaction(s) between monomers(s) and active site(s) on the polymer chain with regeneration of the active site(s) at the end of each growth step. In other words, addition of each monomer unit regenerates the active site.  

The steps of chain addition polymerization include chain initiation, chain propagation and chain transfer. Chain termination does not always occur as is the case with step growth polymerization. Common polymers that can be obtained through chain addition polymerization include polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC), polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate etc.

What You Need To Know About Chain Addition Polymerization

  1. Chain addition polymerization takes place by repeated additions of monomers to the growing polymer chain.
  2. Oligomers are formed at the beginning and are later combined together, forming the polymer chain.
  3. Monomers exist throughout the reaction, its concentration decreases steadily with time.
  4. In chain addition polymerization, only monomers react to the active sites at the end of the growing chain.
  5. Molar mass of backbone chain increases rapidly at early and remains approximately   the same throughout the polymerization process.
  6. The mixture contains primarily monomers and polymers and only small amounts of growing polymer chains.
  7. The reaction speed does not depend on the concentration of initiator and co-initiator.
  8. Long reaction times do not have much significant effect on the average molecular weight, though long reaction time guarantees high degree of conversion.
  9. There are normally two distinctive mechanisms during polymerization, these include propagation and initiation. Often, there is also a termination step.
  10. Different steps operate at different stages of mechanism. They include initiation, propagation, termination and chain transfer.

Also Read: Difference Between Additional Polymerization And Condensation Polymerization

Difference Between Step Growth And Chain Addition Polymerization In Tabular Form

BASIS OF COMPARISON STEP GROWTH POLYMERIZATION CHAIN ADDITIONAL POLYMERIZATION
Description Step growth polymerization takes place by reaction between monomers, oligomers and polymers.   Chain addition polymerization takes place by repeated additions of monomers to the growing polymer chain.  
Formation A polymer chain is formed at the beginning by the attachment of one monomer at a time.   Oligomers are formed at the beginning and are later combined together, forming the polymer chain.  
Concentration Of Monomers In The Reaction In step growth polymerization, monomers exist through the reaction, but large quantities of monomers are consumed early in the reaction.   Monomers exist throughout the reaction, its concentration decreases steadily with time.  
Molecules Involved In The Reaction In step growth polymerization, all molecules that is, polymer, oligomer and monomer can react with any other molecule.   In chain addition polymerization, only monomers react to the active sites at the end of the growing chain.  
Molecular Weight Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length.   Molar mass of backbone chain increases rapidly at early and remains approximately   the same throughout the polymerization process.  
Reaction Mixture Oligomers exist throughout the reaction, with the length distribution broadening and shifting to higher molecular weight with increasing reaction time.   The mixture contains primarily monomers and polymers and only small amounts of growing polymer chains.  
Speed Of Reaction The reaction speed does not depend on the concentration of initiator and co-initiator.   The reaction speed does not depend on the concentration of initiator and co-initiator.  
Reaction Time Long reaction time is necessary for production of high molecular weight polymers.   Long reaction times do not have much significant effect on the average molecular weight, though long reaction time guarantees high degree of conversion.  
Termination Step The end groups of the oligomers and polymers are reactive throughout the polymerization process. There is usually no termination step.   There are normally two distinctive mechanisms during polymerization, these include propagation and initiation. Often, there is also a termination step.  
Reaction Steps Similar steps are repeated throughout reaction process.   Different steps operate at different stages of mechanism. They include initiation, propagation, termination and chain transfer.  

Also Read: Difference Between Amine And Amide