Difference Between Branched Polymer and Linear Polymer

Main Difference – Branched Polymer vs Linear Polymer

A polymer is a type of macromolecule which is produced by polymerization of small units known as monomers. Polymers are a diverse group of macromolecules. Hence, there are a number of classifications to categorize polymers based on the origin (such as natural, synthetic polymers), properties (such as elastomers, thermosetting, thermoplastics), mechanism of polymerization (such as addition polymerization, condensation polymerization), structure, etc. Based on the structure of a polymer, it can be a linear polymer, branched polymer, or a network polymer. The main difference between branched polymer and linear polymer is that branched polymers have a branched structure whereas linear polymers have a linear structure.

Key Areas Covered

1. What is a Branched Polymer
– Definition, Different Forms, and Properties
2. What is a Linear Polymer
– Definition, Different Forms, and Properties
3. What are the Similarities Between Branched Polymer and Linear Polymer
– Outline of Common Features
4. What is the Difference Between Branched Polymer and Linear Polymer
– Comparison of Key Differences

Key Terms: Atactic, Branched Polymer, Isotactic, Linear Polymer, Macromolecule, Monomers, Polymer, Polymerization, Syndiotactic, Tacticity

What is a Branched Polymer

A branched polymer is a macromolecule made from the polymerization of monomers and has a branched structure. Branching of polymers occurs by replacing some atoms from the polymer chain by substituents. The properties of these polymers are mainly affected by the amount of branching. The substituent is another polymer chain composed of covalently bonded monomer units. These side chains can be either short chains or long chains.

Different Types of Branched Polymers

Graft Polymer

A graft polymer is a branched polymer having side chains composed of different monomers to that of the main chain. In other words, it is a segmented copolymer composed of a linear backbone substituted with branches of a polymer of a different type.

Difference Between Branched Polymer and Linear Polymer_Figure 1

Figure 1: A Graft Polymer

Comb Polymer

According to the IUPAC gold book, a comb polymer is a polymer composed of comb macromolecules. In other words, it is composed of side chains on the same side of the backbone, and the polymer then appears like a comb.

Brush Polymer

This form of polymers looks like a brush where the backbone is substituted with side chains from different points. The density is high in these polymers.

Star-shaped Polymer

This form is the simplest form of other branched polymer forms. This structure contains several linear polymer chains attached in a central core.

Main Difference - Branched Polymer vs Linear Polymer

Figure 2: Different Branched Polymer Structures

Some Examples of Branched Polymers

Branched polymers are often amorphous since they cannot pack tightly in a regular manner due to the presence of branches. Hence, the density is also less than linear polymers. They have lower melting points and boiling points as well.

What is a Linear Polymer

A linear polymer is a macromolecule made out of many monomer units arranged in a straight line. A linear polymer consists of a single continuous chain of repeating units. The atoms bonded to each other covalently forms the backbone of the polymer. A linear polymer can have side groups attached to the backbone. These side groups are called pendant groups. But these side groups are not side chains. If they were side chains, then the polymer is no longer linear; it is then a branched polymer.

In a linear polymer, the pendant groups can be arranged in different patterns. These patterns are described under the concept of tacticity – the relative regularity of a polymer chain. Tacticity is the stereochemical arrangement of the units in the main chain of a polymer. According to the tacticity of a polymer, these linear polymers can be divided into three main groups as isotactic polymers, syndiotactic polymers, and atactic polymers.

Tacticity in Polymers

Isotactic Polymers

Isotactic polymers have their pendant groups on the same side of the polymer chain. These polymers are usually semi-crystallization.

Syndiotactic Polymers

Syndiotactic polymers have their pendant groups in an alternating pattern. These are most of the times crystalline polymers.

Atactic Polymers

Atactic polymers have the pendant groups in a random manner. Atactic polymers are amorphous.

Difference Between Branched Polymer and Linear Polymer - 2

Figure 3: Tacticity in Polymers

The backbone can be made out of either the same monomer or from different monomers. If it is the same polymer, it is called a linear homopolymer. If the backbone is made out of different monomers, it is called a linear copolymer. These copolymers can be found in different forms such as alternating copolymers (where the polymer chain is composed of regular alternating monomers), periodic copolymers (where the monomers are arranged in a repeating sequence) and, block copolymers (where the blocks of different monomers are arranged in a linear chain).

Some Examples of Linear Polymers

Linear polymers are often semi-crystalline or crystalline; since there are no branches, the polymer chains can tightly pack. Hence the density is high. The melting point and boiling point are increased because a high energy is required to separate these polymer chains which are tightly packed.

Similarities Between Branched Polymer and Linear Polymer

Both are macromolecules.
Both are formed by polymerization of monomers.
Both have high molecular weights.
Both types have a backbone composed of atoms that are covalently bonded to each other.
Both show tacticity.

Definition

Branched Polymer: A branched polymer is a macromolecule made from the polymerization of monomers and has a branched structure.

Linear Polymer: A linear polymer is a macromolecule made out of many monomer units arranged in a straight line.

Structure

Branched Polymer: Branched polymers have linear polymer chain substituted with one or more polymer chains (either short or long polymer chains).

Linear Polymer: Linear polymers have a straight polymer chain that may or may not composed of pendant groups.

Branching

Branched Polymer: Branching is present in branched polymers.

Linear Polymer: Branching is absent in linear polymers.

Side Groups

Branched Polymer: Branched polymers have polymer chains as side groups.

Linear Polymer: Linear polymers have pendant groups as side groups. They are not polymer chains.

Packing

Branched Polymer: Branched polymers are loosely packed.

Linear Polymer: Linear polymer chains can tightly pack.

Density

Branched Polymer: The density of branched polymers is low.

Linear Polymer: The density of linear polymers is high.

Melting and Boiling Point

Branched Polymer: The melting and boiling point of branched polymers is lower than that of linear polymers.

Linear Polymer: The melting and boiling point of linear polymers is higher than that of branched polymers.

Complexity

Branched Polymer: Branched polymers have a complex structure.

Linear Polymer: Linear polymers have a simple structure.

Examples

Branched Polymer: Some examples of branched polymers include starch, glycogen, etc.

Linear Polymer: Some examples of linear polymers include Teflon, polypropylene, etc.

Conclusion

A polymer is a giant molecule containing a high number of repeating units. Polymers can be grouped as linear polymers and branched polymers depending on their structures. The main difference between branched polymer and linear polymer is that branched polymers have a branched structure whereas linear polymers have a linear structure.

References:

1. “Polymer Properties Database.” Branched Polymers, Available here.
2. Lazonby, John. “Polymers: an overview.” The Essential Chemical Industry online, Available here.
3. “Branching (Polymer chemistry).” Wikipedia, Wikimedia Foundation, 17 Oct. 2017, Available here.
4. “Tacticity.” Wikipedia, Wikimedia Foundation, 2 Dec. 2017, Available here.
5. International Union of Pure and Applied Chemistry. “Comb polymer.” IUPAC Gold Book – comb polymer, Available here.

Image Courtesy:

1. “Graft copolymer 3D” By Minihaa – Own work (CC0) via Commons Wikimedia
2. “RAFT Architecture” By Chem538w10grp4 – Own work (Public Domain) via Commons Wikimedia
3. “Polypropylene tacticity de” By Minihaa – Own work (CC0) via Commons Wikimedia