Difference Between Block and Graft Copolymer

The key difference between block and graft copolymer is that a block copolymer has blocks of repeating units whereas a graft copolymer has branches of repeating units.

A polymer is a macromolecule that has many repeating units linked to each other via covalent chemical bonds. And, these repeating units represent the monomers that are used in the polymerization process to make this polymer. Thus, there are many classifications for polymers depending on the structure, morphology, properties, etc. A copolymer has a different arrangement of monomers than other polymers. In this arrangement, more than one monomers involved in the formation of the polymer. Block and graft copolymers are two forms of such polymers that come under the classification of polymers according to the structure.

CONTENTS

1. Overview and Key Difference
2. What is Block Copolymer
3. What is Graft Copolymer
4. Side by Side Comparison – Block vs Graft Copolymer in Tabular Form
5. Summary

What is Block Copolymer?

A block copolymer is a copolymer that forms when two monomers cluster together and form ‘blocks’ of repeating units. The properties of this type of polymer materials depend on the distribution of the sequence of blocks, chemical nature of these blocks, average molecular weight, and molecular weight distribution.

Often, we can prepare these copolymers via sequential monomer addition. There, the two different monomers get polymerised in such a way that one monomer polymerizes first. After that, the second monomer attaches to the “living” polymer chain of the first monomer. There, the two monomers undergo copolymerization and form a block copolymer.

Examples of block copolymers include SBS rubber, a material that we use to make automobile tires. Moreover, the chemical name of this material is acrylonitrile butadiene styrene. The blocks in SBS rubber are polystyrene and polybutadiene. Moreover, nitrile and ethylene-vinyl acetate are some other examples for block copolymers.

What is Graft Copolymer?

Graft polymers are segmented copolymers with a linear backbone of one monomer and randomly distributed branches of another monomer. Here, the side chains are structurally different from the main chain of the polymer. However, although they are structurally different from each other, the individual grafted chains may be homopolymers or copolymers.

For instance, high impact polystyrene is a graft copolymer material. It is because the polymer contains a polystyrene backbone with polybutadiene grafted chains. Furthermore, most of the graft copolymers are useful as impact resistant materials, thermoplastic elastomers, and compatibilizers.  Another usage of graft copolymer is as an emulsifier in the preparation of stable blends or alloys.

What is the Difference Between Block and Graft Copolymer?

Polymers are macromolecules. According to the types of monomers used in the formation of the polymer, there are two types as homopolymers and copolymers. Among the two, copolymers contain at least two types of monomers in their structures. Comparing the two, the key difference between block and graft copolymer is that a block copolymer has blocks of repeating units whereas a graft copolymer has branches of repeating units.

As another important difference between block and graft copolymers, we can say that the major route of formation of the block copolymer is via sequential monomer addition while we can produce graft copolymers via atom transfer radical polymerisation. Furthermore, there is a difference between block and graft copolymers in the preparation method too.

Summary – Block vs Graft Copolymer

A copolymer is a polymer material having two or more monomers in its structure. Moreover, there are several types of copolymers as block copolymers, graft copolymers, alternating copolymers and random copolymers. However, the key difference between block and graft copolymer is that a block copolymer has blocks of repeating units whereas a graft copolymer has branches of repeating units.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “Block Copolymer Definition (Chemistry).” ThoughtCo, Jun. 7, 2018. Available here 
2. Paul F. Rempp, Pierre J. Lutz, in Comprehensive Polymer Science and Supplements, 1989
3. C. Hagiopol, in Reference Module in Materials Science and Materials Engineering, 2016

Image Courtesy:

1.”Block copolymer 3D”By Minihaa – Own work (CC0) via Commons Wikimedia  
2.”Graft copolymer 3D”By Minihaa – Own work (CC0) via Commons Wikimedia