Chemistry lecture 31

POLYMER

A polymer is a large molecule (macromolecule) composed of repeating structural units. These subunits are typically connected by covalent chemical bonds.

The word ‘polymer’ is coined from two Greek words namely poly means many and mer means unit or part.  Polymers are defined as high molecular mass macromolecules, which are formed by joining of repeating structural units on a large scale.  The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds.  This process of formation of polymers from respective monomers is called polymerization.  

Classification of Polymers

There are several ways of classification of polymers based on some special considerations.  The following are some of the common classifications of polymers:

·         Classification Based on Source

1. Natural polymers

These polymers are found in plants and animals. Examples are proteins, cellulose, starch, resins and rubber.

2. Semi-synthetic polymers

Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usual examples of this sub category.

3. Synthetic polymers

A variety of synthetic polymers as plastic (polythene), synthetic fibers (nylon 6,6) and synthetic rubbers (Buna - S) are examples of manmade polymers extensively used in daily life as well as in industry.

·         Classification Based on Structure of Polymers

1. Linear polymers

These polymers consist of long and straight chains.  The examples are high density polythene, polyvinyl chloride, etc.

2. Branched chain polymers

These polymers contain linear chains having some branches, e.g., low density polythene.

3. Cross linked or Network polymers

These are usually formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various linear polymer chains.  Eg: Bakelite, melamine, etc.

·         Classification of Polymers in Chemistry Based on Molecular Forces

A large number of polymer applications in different fields depend on their unique mechanical properties like tensile strength, elasticity, toughness, etc.   These mechanical properties are governed by intermolecular forces, e.g., van der Waals forces and hydrogen bonds, present in the polymer.   These forces also bind the polymer chains.  Under this category, the polymers are classified into the following four sub groups on the basis of magnitude of intermolecular forces present in them.

1. Elastomers

These are rubber – like solids with elastic properties. In these elastomeric polymers, the polymer chains are held together by the weakest intermolecular forces.  These weak binding forces permit the polymer to be stretched. A few ‘cross links’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released as in vulcanised rubber.  

2. Fibers

Fibers are the thread forming solids which possess high tensile strength and high modulus.  These characteristics can be attributed to the strong intermolecular forces like hydrogen bonding.  These strong forces also lead to close packing of chains and thus impart crystalline nature.  The examples are polyamides (nylon 6, 6), polyesters (terylene), etc.

3. Thermoplastic polymers

These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling.   These polymers possess intermolecular forces of attraction intermediate between elastomers and fibers. Some common thermoplastics are polythene, polystyrene, polyvinyls, etc.

4. Thermosetting polymers

These polymers are cross linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and again become infusible. These cannot be reused. Some common examples are bakelite, urea-formaldehyde resins, etc.

Explanation to Organic Chemistry Polymers:

Addition Polymers:

These are the polymers formed by the addition of the molecules of monomer or monomers to form large molecule without elimination of anything. The process of formation of addition polymers is termed as addition polymerization. Some examples of addition polymers are as follows:

a) Polyethylene or Polythene: It is prepared by heating pure ethene at 460K to 480K temperature and under high pressure. It is used for insulation of electric wires and for manufacturing of pipes, toys, bags etc.

b) Polypropylene: It is manufactured by the polymerization of Propylene or Propene. It is used for making ropes, seat covers, carpets etc.

c) Synthetic rubber: It is prepared by polymerization of chloroprene (2 - chloro - 1,3 - butadiene). Neoprene is called as Neoprene synthetic rubber. It is used for insulating electric wires, conveyor belts and for manufacturing the containers.

d) Buna - S - Rubber: It is a co-polymer of 1,3 butadiene and styrene. It is prepared by polymerization of 1,3 Butadiene and styrene. It is used for making belts, rubber soles and tyres.

e) Teflon or Polytetra fluoroethylene (PTFE): It is prepared by polymerization of tetrafluoroethylene. It is used for making non-stick pans, seals, gaskets etc.

Condensation Polymers:

These polymers are formed by the combination of monomers with elimination of simple molecules such as water and alcohols.

a) Nylon: Nylon is a copolymer of two different monomers, a diamine and a dicarboxylic acid. An amine group - NH₃ on the first monomer reacts with carboxylic acid group (COOH) on the second monomer and makes a link between the two molecules. A water molecule is lost, each time a link is made. During this polymerization, an amide link is formed, so, nylon is also known as a polyamide. It is used for making bristles of tooth brush.

b) Polyester: Polyesters are condensation polymers that are made from two monomers. One monomer has -OH group at each end and the other molecule has -COOH group at each end.

Polyester is used for making fibers, clothes and conveyor belts.

Biopolymer

Biopolymers are polymers produced by living organisms. There are three main classes of biopolymers based on the differing monomeric units used and the structure of the biopolymer formed. Polynucleotides long polymers which are composed of 13 or more nucleotide monomers, Polypeptides short polymers of amino acids, and Polysaccharides which are often linear bonded polymeric carbohydrate structures. Cellulose is the most common organic compound and biopolymer on Earth. About 33 percent of all plant matter is cellulose. The cellulose content of cotton is 90 percent and that of wood is 50 percent.

TYPES OF POLYMERS: