What are Polymers?

Polymers are naturally occurring or synthetically produced large molecules or compounds which are made up of series of repeating units called monomers. They are joined together by chemical bonds. The term polymer is coined from Greek roots meaning 'many parts' and are found almost everywhere from the plastics bags to the rubber of our tires to our DNA strands.

Introduction

Polymers are formed by the process of polymerization in which monomers or 'single units' are linked together through a variety of chemical reaction mechanisms to form 3D networks, i.e., polymer chains.

They are usually composed of hydrogen and carbon atoms with carbon bonded together in long chains making it the backbone of polymers. Such polymers are called organic polymers. Examples include cellulose or DNA. Those which possess nitrogen, oxygen, sulphur or phosphorus backbones, like polysiloxanes, polyphosphazenes are called as inorganic polymers. 

The study of polymers, mainly synthetic polymer materials such as plastics and elastomers, is called polymer science.

Classification of polymers

Polymers can be classified in numerous ways due to their various structures, behaviours and occurrences. A few simple ways of classification of polymers are listed below:

Based on source

Under this category, there are three types of polymers: Natural, semi-synthetic and synthetic polymers.

1. Natural polymers : These polymers are isolated from natural materials, such as plants and animals. Examples are cellulose, starch, DNA, proteins, resins and natural rubber.
2. Semi-synthetic polymers : These polymers are derived by the chemical modification of natural products. Examples are cellulose nitrate, cellulose acetate (rayon), etc.
3. Synthetic polymers : These polymers are man-made and synthesized artificially in lab. Examples are synthetic fibres (nylon 6,6), plastics (polythene), synthetic rubber (BUNA-S), polyester, polystyrene, etc.

Based on Structure

Under this category, there are three types of polymers: Linear, branched-chain and cross-linked polymers.

1. Linear polymers : These polymers have straight and long chains with no side chain. They have high density, tensile strength and melting point. For example, high-density polyethylene, PVC (polyvinyl chloride), etc.
2. Branched-chain polymers : These polymers consist of linear chain like structure with some branching. They have low density, tensile strength and melting point due to irregular packing. For example, low density polyethylene.
3. Cross-linked polymers : These polymers are made of bi-functional and tri-functional monomers. Their monomers have strong covalent bonds. They are hard, rigid and brittle. For example, bakelite, melamine, etc.

Based on the modes of polymerization

Addition polymers or chain-growth polymers

They are formed by repeated addition of unsaturated monomeric units, that is, monomers with double or triple bonds. Alkenes and conjugated dienes mostly form such polymers. Nitrile rubber, BUNA-N, polytetrafluoroethylene (teflon), polyethene, etc. are examples of addition polymers.

Condensation polymers or step-growth polymers

They are formed by the combination of different monomers (carbon-carbon bond formation) whereby small molecules such as water, alcohol or ammonia are eliminated. Mostly organic compounds with bi-functional groups such as diols, diamines, dials and dicarboxylic acids form such polymers. Polyesters like polyethylene terephthalate (PET), polyamide like nylon, urethanes, polyethers, etc. are condensation polymers.

Based on monomers

Polymers are not just restrained to monomers with similar chemical composition, structure or molecular weight. Depending on the type of monomer, the following two polymers fall under this category. 

1. Homo-polymers : Polymers made of one kind of monomeric units are called homo-polymers. For example, nylon 6, polyethene and polypropylene.
2. Co-polymers : Polymers made of two or more kinds of monomeric units are termed as co-polymers. For example, Nylon 6,6.

Based on molecular forces

Depending on the extent of intermolecular forces present in them, polymers are classified into the following subgroups:

• Elastomers : They are solid, large, rubber-like polymers having both elasticity and viscosity. The polymer chains are held together by weak intermolecular forces, allowing them to be stretched easily. For example, polyisoprene and natural rubber.

• Fibres : They are solid, thread-like polymers. They have strong intermolecular forces like hydrogen bonding making them less elastic and increase their tensile strength. They are tough, strong and have high melting points. For example, terylene, nylon 6,6.

• Thermoplastics polymers : These polymers have linear or slightly branched structures. They become soft on heating and harden on cooling. They have intermediate forces of attraction. For example, poly vinyl chloride, polythene, polystyrene, etc.

• Thermosetting polymers : These polymers are highly cross-linked and branched which hardens on heating. They improve polymer's mechanical properties. They are not reusable. For example, bakelite, polyester, urea formaldehyde.

Based on tactility

Tacticity means deposition of side groups in space.

• Atactic polymers : The side groups are deposited randomly around the main chain.
• Syndiotactic polymers : The side groups are deposited in alternate fashion.
• Isotactic polymers : The functional group are all deposited on the same side of the chain.  

Some other Important Polymers

Biodegradable polymer

Biodegradable polymer molecules are large molecules or macromolecules which get decayed and broken down under aerobic or anaerobic conditions by different types of bacteria or enzymes. These polymers can be very useful in making capsule coatings, surgical bandages etc. For example, poly β-hydroxybutyrate-co-β-hydroxyvalerate (PHBV).

High-temperature polymers

These polymers remain stable even at very high temperatures. They cannot be destroyed even at high temperatures due to their high molecular weight. These polymers are majorly used in healthcare industries. 

Types of Polymerization

Free radical

In this type, the polymerized product is formed by free radicals like benzoyl peroxide, acetyl peroxide via free-radical mechanism, i.e., chain initiation, propagation and termination. For example, low-density polymer (LDP).

Cationic

In this type, a charge can be transferred to a monomer by using a cationic initiator, like aluminium chloride with water (AlCl₃+H₂O) or boron trifluoride with water (BF₃+H₂O). For example, polyisobutylene.

Anionic

A type of polymer-chain growth polymerization wherein the vinyl monomers are surrounded by electronegative groups like nitrile (-CN) and chloride (-Cl); such process is called anionic polymerization.

The catalyst used are triphenylmethyl sodium [(C₆H₅)₃C-Na], grignard’s reagent, Na/liq. NH₃. This type is mostly used for the production of synthetic rubbers, styrene solutions, etc.

Coordination

In this type, the chemical reaction is catalyzed by transition-metals. The most important class is the use of Ziegler--Natta catalyst. These reactions are used to polymerize terminal alkenes. For example, ethene polymerizes to give the most widely used plastics, polyethylene. Another example is propene polymerized to polypropylene. 

Characteristics of Polymers

• They usually have high melting and boiling points.
• They are either crystalline or amorphous in nature.
• Polymers possess high molecular masses since they are made of many monomers. 
• Their tensile strength increases with increase in their chain length and cross-linking. 
• They do not decompose; they just change from crystalline to semi-crystalline. 
• Regular polymers such as polyethylene are electrical insulators, however the polymers with π-conjugated bonds, such as polythiophene, acts as polymer-based semiconductors.
• They have the glass transition temperature, primarily the amorphous polymers, which is the temperature range where the polymer substrate changes from rigid glassy material to soft material.

Uses of Polymers

• Polytetrafluoroethene are used as non-stick coatings for pans, containers for laboratory substances.
• Polychloroethene are used as insulators in electrical wires, windows, pipes, gutters etc.
• Polyvinyl chloride (PVC) is used in manufacture of clothing, furniture and sewage pipes. It is also used for vinyl flooring.
• Polythene is mostly used plastics as carrier bags, shampoo bottles, plastics tiffin boxes, food wraps.
• Urea-formaldehyde resins are used for making unbreakable containers, adhesives, laminated sheets, etc.
• Glyptal is used for making paints, coatings and lacquers.
• Bakelite is used to make jewellery articles, toys and various kitchen products like frying pans. Due to their insulation properties, it is used to make switches and other electrical appliances.
• Polystyrene is a versatile plastic used for making food packaging and laboratory ware, air conditioners, microwaves. Polystyrene is used to make car parts like knobs, door panels. Commonly used cups, plates and sandwich containers are made of foam polystyrene.

Context and applications

This topic is useful for undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.

Practice Problems

Problem 1: Differentiate between condensation and addition polymerization.

Solution:

Problem 2: Differentiate between chain-growth and step-growth polymerisation.

Solution: