Main Difference – Chain Growth Polymerization vs Step Growth Polymerization
Polymers are giant molecules made of small units known as monomers. Monomers are molecules having either unsaturated bonds or at least two functional groups per molecule. Monomers bind together to form polymer chains. Interactions between polymer chains cause the formation of giant polymer molecules. The formation of a polymer is called polymerization. Polymerization occurs mainly in two ways: chain growth polymerization and step growth polymerization. The main difference between chain growth polymerization and step growth polymerization is that chain growth polymerization has a growth a polymer chain at its ends whereas step growth polymerization has a combination of oligomers to form a polymer chain.
Key Areas Covered
1. What is Chain Growth Polymerization
– Definition, Explanation, Types
2. What is Step Growth Polymerization
– Definition, Explanation
3. What is the difference between Chain Growth and Step Growth Polymerization
– Comparison of Key Differences
Key Terms: Addition Polymerization, Anionic Polymerization, Cationic Polymerization, Chain Growth Polymerization, Condensation Polymerization, Monomer, Polymer, Polymerization, Radical Polymerization, Step Growth Polymerization
What is Chain Growth Polymerization
Chain growth polymerization is the process in which polymers are formed from unsaturated monomers. Chain growth polymerization is also called addition polymerization because monomers are added to the ends of polymer chains. The monomers are attached to the chain in the active site of the growing polymer chain, one monomer at a time. The growth of polymer chains occurs only at the ends. The steps of chain polymerization include,
- Initiation
- Propagation
- Termination
Figure: The polymerization of Ethylene. It is a chain growth polymerization.
According to the initiator used in the chain growth polymerization process, there are three types of chain growth polymerization.
- Radical Polymerization – the initiator is a radical
- Cationic Polymerization – the initiator is an acid/cation
- Anionic Polymerization – the initiator is a nucleophile
The propagation site of radical polymerization is a carbon radical whereas propagation site of cationic polymerization is a carbocation and the propagation site of anionic polymerization is a carbanion.
What is Step Growth Polymerization
Step growth polymerization is the formation of a polymer from bi-functional or multi-functional monomers. Step growth polymerization is also known as condensation polymerization. Unlike in chain growth polymerization, the polymer chains here are not formed at the beginning. First, dimers, trimmers, and tetramers are formed. Then these oligomers combine with each other forming long polymer chains. Therefore, the monomers are not attached to the ends of polymer chains as in chain growth polymerization.
Figure 2: The step growth polymerization begins with the formation of oligomers and then they are combined to form polymer chains.
Step growth polymerization has oligomers instead of polymer chains at the beginning because the monomers involved in this process are composed of at least two different functional groups. Therefore, the monomers are able to attach to each other at any time.
Figure 3: Chain Growth vs Step Growth Polymerization
The above graph shows the relationship between the two polymerization processes and the rate of the formation of the polymer.
Difference Between Chain Growth and Step Growth Polymerization
Definition
Chain Growth Polymerization: Chain growth polymerization is the formation of polymers from unsaturated monomers.
Step Growth Polymerization: Step growth polymerization is the formation of a polymer from bi-functional or multifunctional monomers.
Chain Growth
Chain Growth Polymerization: In chain growth polymerization, a polymer chain is formed at the beginning by the attachment of one monomer at a time.
Step Growth Polymerization: In step growth polymerization, oligomers are formed at the beginning and are later combined together, forming the polymer chain.
Types of monomers
Chain Growth Polymerization: Monomers used in chain growth polymerization are unsaturated monomers.
Step Growth Polymerization: Monomers used in step growth polymerization are bi-functional or multifunctional monomers.
Loss of Monomers
Chain Growth Polymerization: In chain growth polymerization, there is no rapid loss of monomers at the beginning.
Step Growth Polymerization: In step growth polymerization a rapid loss of monomers at the beginning can be observed.
Active Site
Chain Growth Polymerization: In chain growth polymerization, an active site can be observed at the end of the polymer chain.
Step Growth Polymerization: In step growth polymerization, all the monomers are active themselves.
Initiation
Chain Growth Polymerization: Chain growth polymerization requires initiators to break the double bond in monomer molecule.
Step Growth Polymerization: Step growth polymerization does not require initiators.
Termination
Chain Growth Polymerization: Chain growth polymerization shows no growth of polymer chain after termination.
Step Growth Polymerization: Step growth polymerization does not show termination.
Reaction Mixture
Chain Growth Polymerization: Only monomers and polymers are observed in chain growth polymerization.
Step Growth Polymerization: Any type of molecules (such as dimers, trimmers, tetramers or oligomers) can be observed in step growth polymerization.
Conclusion
Polymerization is an important process in so many industries. Polymerization can be done in two ways; chain growth polymerization and step growth polymerization. The main difference between chain growth polymerization and step growth polymerization is that the chain growth polymerization involves the growth a polymer chain at its ends whereas the step growth polymerization involves the combination of oligomers to form a polymer chain.
References:
1. Libretexts. “Step-Growth Polymers—Condensation Polymers.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 23 June 2017.
2. “Polymer Properties Database.” Step-growth Polymerization. N.p., n.d. Web. Available here. 23 June 2017.
3. “Polymerization.” Encyclopædia Britannica. Encyclopædia Britannica, inc., 06 Apr. 2016. Web. Available here. 23 June 2017.
Image Courtesy:
1. “RadicalPolymerizationEthylene” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Step-growth polymerization” By Chem538grp5w09 – Own work (Public Domain) via Commons Wikimedia
3. “Comparison between SG and CG” Chem538grp5w09 – Üleslaadija oma töö (Avalik omand) via Commons Wikimedia
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