The main difference between block and graft copolymer is that block copolymers consist of two or more distinct blocks linked together by covalent bonds, while graft copolymers consist of a main chain with side chains that are structurally distinct from the main chain.
Copolymers are a class of polymers composed of two or more different monomers chemically bonded together in a single polymer chain. Unlike homopolymers, which consist of repeating units of the same monomer, copolymers incorporate multiple monomer units, allowing for a combination of properties from each monomer. Within the category of copolymers, there are two important subtypes: graft copolymers and block copolymers.
Key Areas Covered
1. What are Block Copolymers
– Definition, Structure, and Properties
2. What are Graft Copolymers
– Definition, Structure, and Properties
3. Similarities Between Block and Graft Copolymer
– Outline of Common Features
4. Difference Between Block and Graft Copolymer
– Comparison of Key Differences
Key Terms
Block Copolymer, Graft Copolymer
What are Block Copolymers
Block copolymers are unique macromolecules composed of two or more homopolymer subunits that are linked together through covalent bonds. These subunits can be different polymers, giving rise to distinct chemical blocks within the copolymer structure. A junction block, which is a non-repeating subunit, may sometimes be needed to connect the homopolymer subunits.
The classification of block copolymers is based on the number of distinct blocks they possess. Diblock copolymers consist of two different blocks, while triblock copolymers have three blocks. In general, a block is a segment of a macromolecule that comprises numerous repeating units and possesses at least one distinguishing feature that sets it apart from the adjacent portions.
Figure 1: Copolymers
The synthesis of block copolymers involves the polymerization of different monomers. For instance, a common diblock copolymer, polystyrene-b-poly(methyl methacrylate) or PS-b-PMMA, is typically created by first polymerizing styrene and subsequently polymerizing methyl methacrylate (MMA) from the reactive ends of the polystyrene chains. This results in a copolymer with two distinct chemical blocks.
Various living polymerization techniques, including atom transfer free radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), ring-opening metathesis polymerization (ROMP), and living cationic or living anionic polymerizations, are employed to synthesize diblock copolymers. An emerging technique known as chain shuttling polymerization is also gaining attention in this field.
What are Graft Copolymers
Graft copolymers represent a distinct class of branched copolymers characterized by the presence of side chains that are structurally different from the main chain. Typically, the main chain is composed of one type of monomer (A), while the branches originate from another monomer (B). Alternatively, the side chains may possess constitutional or configurational features that differ from those present in the main chain.
In graft copolymers, the individual chains forming the branches can be either homopolymers or copolymers. It is important to note that the presence of different copolymer sequencing is sufficient to define a structural distinction. Consequently, an A-B diblock copolymer with A-B alternating copolymer side chains is appropriately classified as a graft copolymer.
Let’s consider the example of grafting polystyrene chains onto polybutadiene, a synthetic rubber that retains one reactive C=C double bond per repeat unit. In this process, polybutadiene is dissolved in styrene, and free-radical polymerization is initiated. The growing chains have the ability to add across the double bonds of the rubber molecules, resulting in the formation of polystyrene branches. The resulting graft copolymer consists of a mixture of ungrafted polystyrene chains and rubber molecules.
Figure 2: Graft Copolymer
Similar to block copolymers, graft copolymers exhibit properties that reflect the combination of their constituent “components.” In the above example, the presence of rubbery chains in the graft copolymer allows it to absorb energy upon impact, making it significantly less brittle than ordinary polystyrene. This particular product is commonly referred to as high-impact polystyrene (HIPS).
Similarities Between Block and Graft Copolymer
- Both block copolymers and graft copolymers are types of copolymers; this means they are composed of two or more different monomers chemically bonded together.
- They exhibit a combination of properties derived from their constituent monomers.
- Both copolymers have applications in diverse fields, such as materials science, polymer engineering, coatings, drug delivery systems, and biomaterials.
Difference Between Block and Graft Copolymer
Definition
Block copolymers are unique macromolecules composed of two or more homopolymer subunits that are linked together through covalent bonds, whereas graft copolymers are a distinct class of branched copolymers characterized by the presence of side chains that are structurally different from the main chain.
Structure
Block copolymers consist of two or more distinct blocks linked together by covalent bonds, while graft copolymers consist of a main chain with side chains that are structurally distinct from the main chain.
Main Chain
The main chain of block copolymers is formed from one type of monomer. However, graft copolymers have a main chain that can be either a homopolymer or a copolymer.
Branches
Block copolymers do not have branches, while graft copolymers possess branches that originate from another monomer or have distinct constitutional or configurational features.
Synthesis
The synthesis of block copolymers often requires the use of living polymerization techniques. Meanwhile, graft copolymers can be synthesized using free-radical polymerization or other methods.
Properties
Block copolymers combine the properties of their individual blocks, resulting in unique material properties. On the other hand, graft copolymers combine the properties of the main chain and the side chains, leading to distinct material characteristics.
Conclusion
Block copolymers consist of two or more distinct blocks linked together by covalent bonds. On the other hand, graft copolymers consist of a main chain with side chains that are structurally distinct from the main chain. Thus, this is the main difference between block and graft copolymer.
Reference:
1. “Copolymer.” Wikipedia. Wikipedia Foundation.
Image Courtesy:
1. “Common Polymer Architectures” By Prabhachatterji at ml.Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Graft Copolymer” By Chemystery22 – Own work (CC BY-SA 3.0) via Commons Wikimedia
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