What are polymers?

Polymers are large and very high-molecular-weight substances which consists of hundreds to thousands of repeating units. Polymers are also known as macromolecules. Number-average-molecular weight is the measuring system for the polymers.

Categories of polymers

Based on the nature of the repeat units, polymers can be-

1. Homopolymer
2. Copolymer

Homopolymer

The polymers in which repeating units are from only one monomer unit are known as homopolymers. The Molecular weight of the polymer is higher since it has numerous monomers. This process is a step-growth polymer.

An example of a homopolymer is the polymerization of an ethene molecule to give polythene. Here, ethene is the single monomer unit, which polymerizes to form polythene.

Copolymer

In copolymers, the repeating units are formed by the combination of two or more types of monomer units. In the formation of a copolymer, the removal of small molecules like water, ammonia, and a few more take place. It has a higher molecular weight than the respective monomers.

An example of the copolymer is the polymerization of hexamethylene diamine and adipic acid to give Nylon-6, 6 and removal of water in the presence of acid-catalyzed reaction.

Classification of polymers

Classification based upon source

Based on their source, polymers are:

1. Natural polymer
2. Semi-Synthetic polymer
3. Synthetic polymer

Natural polymer

These polymers occur naturally. Examples of natural polymers are DNA, cellulose, protein, silk, wool.

Semi-synthetic polymer

These polymers occur naturally and are modified by chemicals. Examples of semi-synthetic polymers are rayon threads, vulcanized rubber, and gun cotton.

Synthetic Polymer

These polymers are man-made, and these polymers are extensively used in daily life. Examples of synthetic polymers are fibers (nylon, polyester), plastics (polythene), and rubbers (polystyrene).

Classification based on the structure

Based on their structure, polymers are:

1. Linear polymer
2. Branched-chain polymer
3. Cross-linked or three-dimensional polymer

Linear polymer

In linear polymer, monomers are joined as a straight polymer chain. The linear polymer has a high-density, high melting point, and high tensile strength because of the close packing of the polymer chains. It is a long-chain polymer. It also has the properties of a copolymer. Examples of linear polymers are polyvinyl chloride, polyester, nylon.

Branched-chain polymer

In branched-chain polymers, monomer units are not only combined in the linear chain but are also attached through branches of different lengths along the main chain. Branched-chain polymers have a lower density, lower melting point, and lower tensile strength compared to linear chain polymers because branched-chain does not pack well. Examples of branched-chain polymers are amylopectin and glycogen.

Cross-linked or three-dimensional polymer

In cross-linked polymers, monomer units are initially formed as a linear polymer, and then these polymer chains join to form a three-dimensional network structure. They are hard, rigid, and brittle. The molecular weight of this polymer is very high. Examples of cross-linked polymers are Bakelite and urea-formaldehyde polymer.

Classification based on the mode of polymerization

Based on their polymerization, polymers are:

1. Addition polymers
2. Condensation polymers

Additional polymer

Additional polymers are produced by the addition of many same or different monomer units possessed by a double or triple bond. It has a high molecular weight.

An example of an addition polymer is polythene produced from a monomer unit that is ethene.

Condensation polymer 

Condensation polymer is formed through the condensation reaction between two functional groups of monomer units, which eliminates small molecules like water, ammonia, alcohol, hydrogen chloride, and a few more to produce a polymer. This process of polymerization is known as condensation polymerization.

An example of condensation polymer is Nylon-6, 6. Nylon-6, 6 is obtained from the monomer units of hexamethylene diamine, and adipic acid followed by condensation reaction.

Classification based on molecular forces

The application of polymer mainly depends upon mechanical properties like tensile strength, hardness, elasticity, and a few more. Mechanical properties mainly depend upon intermolecular forces of attraction like Vander Waal forces, dipole-dipole interaction, and hydrogen bonding.

Polymers that are classified based on their Molecular Forces are divided into four categories.

1. Elastomers
2. Fibers
3. Thermoplastics
4. Thermosetting polymers

Elastomers

Elastomers have the weakest intermolecular forces of attraction between the polymeric chains. The attraction between such polymers is a Vander Waal force, which is too weak to be present between the polymer chains. Examples of elastomers are vulcanized rubber, Buna-S, and Styrene.

Fibers

Fibers have the strongest intermolecular forces of attraction between the polymeric chains. Interaction that takes place between polymer chains is hydrogen bonding or dipole-dipole interaction. These interactions are strong, so fibers have strong intermolecular forces of attraction. Examples of fibers are Nylon-6, 6, and polyacrylonitrile.

Thermoplastics

The polymers having intermolecular forces of attraction in between the elastomers and fibers between polymeric chains are called thermoplastics. These polymers are rigid at room temperature. Examples of Thermoplastics are Polythene, Teflon, and a few more.

Thermosetting polymers

These polymers are semi-fluid and semi-crystalline substances and have low molecular masses. To increase the hardness of the polymer then applies cross-linking between different polymers. Examples of thermosetting polymers are Bakelite, urea-formaldehyde, and some more.

Preparation of polymers

Preparation of nylon 6, 6

Nylon-6, 6 is manufactured through the condensation reaction taking place between the monomer units of hexamethylene diamine and adipic acid. Nylon fibers have high tensile strength, tough and elasticity. It is used in manufacturing carpets, bristles brushes, and textile fibers.

Preparation of nylon 6, 10

Nylon-6, 10 is manufactured through the condensation reaction taking place between the monomers of hexamethylene diamine having six carbon atoms and sebacic acid having ten carbon atoms. Nylon fibers are much stronger than other natural materials. It is used as a substitute for metals in bearings and gears.

Preparation of nylon 6

Nylon-6 is made from a single monomer which is an organic compound, caprolactam. When caprolactam is heated, water is released with ring-opening and E-aminocaproic acid is formed, then continued heating is provided for the polymerization process to form nylon 6.

It is used for the manufacturing of ropes, fabrics, and tire cords.

Preparation of teflon

Teflon is manufactured through an addition reaction in between the monomers of tetrafluoroethylene in the presence of high pressure and catalyst. Teflon is flexible and has great chemical inertness. It is used to manufacture non-stick utensils. It is also used in making pump packing, valves, filters, and clothes.

Preparation of polyacrylonitrile (PAN)

Preparation of polyacrylonitrile follows additional polymerization. It is prepared from the monomer unit of acrylonitrile in the presence of a peroxide catalyst. It is hard and has a high melting point. It is a chain-growth polymer. 

Preparation of Terylene or Dacron

It follows a condensation mechanism. The condensation polymerization of the monomer unit of ethylene glycol and terephthalic acid then elimination of water in the presence of a catalyst at 420-460 K temperature forms Terylene or Dacron. It is used in the manufacturing of wear fabrics, seat belts, sails, and tire cords.

Context and Applications

Practice Problems

1. Which of the following is a synthetic polymer?

1. 1. Cellulose
   2. Polyvinyl chloride
   3. Proteins
   4. Nuclei

Correct answer- b

Explanation: Synthetic polymers are man-made polymers that are derived from oil, gas, or coal. Cellulose, proteins, and nuclei are natural polymers whereas polyvinyl chloride is a man-made polymer synthesized by the polymerization of ethene molecule.

2. Which of the following polymers has high density, high melting point, and high tensile strength?

1. Linear Polymer
2. Branched-chain polymer
3. Cross-linked or three-dimensional polymer.
4. Polymer molecules

Correct answer- a

Explanation: Linear polymer is a straight-chain polymer that stacks over one another and forms a well-packed structure. Thus, the linear polymer has a high density, high melting point, and high tensile strength.

3. Which of the following polymers has the strongest intermolecular forces of attraction?

1. Elastomers
2. Fibers
3. Thermoplastics
4. Thermosetting polymers

Correct answer- b

Explanation: Fibers have the strongest intermolecular forces of attraction. It is due to intermolecular hydrogen bonding and dipole-dipole interaction. Fibers also possess high tensile strength and high modulus.

4. What are semi-synthetic polymers?

1. Man-made polymer
2. Occurs in nature
3. Prepared from both petroleum and natural products.
4. Non-covalent polymer

Correct answer- c

Explanation: Semi-synthetic polymers are prepared from both petroleum and natural products. For example, rayon.

5. Which of the following polymers has the weakest intermolecular forces of attraction?

1. Elastomers
2. Fibers
3. Thermoplastics
4. Thermosetting polymers

Correct answer- a

Explanation: Elastomers have the weakest intermolecular forces of attraction as the polymer chains are held together by weak van der Waals' forces.