A polymer is a large molecule, or macro-molecule, composed of repeating structural units called monomers. These monomers are chemically bonded together to form long chains or networks. Polymers can be either natural or synthetic and are used in a wide range of materials and products due to their diverse properties.

4.1 Introduction to polymers

The word polymer is derived from the Greek words’ ‘poly’ meaning many and ‘meros’ meaning part. This term is applies to giant molecules built up of large numbers of interconnected repeating units. A polymer is a large chain like molecule built up from many repeating smaller molecules.
A polymer is generally comprised of more than 100 monomers; structures with lower numbers of monomers are known as oligomers. These polymers or oligomers may be linear, slightly branched, or highly interconnected. In the branched case the structure develops into a large three-dimensional network. With few exceptions, the repeat unit in the polymer usually corresponds to the monomer from which the polymer was made.

The size of a polymer molecule may be defined either by its mass or by the number of repeat units in the molecule.
This latter indicator of size is called the degree of polymerization. The relative molar mass of the polymer is thus the product of the relative molar mass of the repeat unit and the degree of polymerization.

Examples of polymers are: polyethylene,wood (cellulose), protein,…etc

Monomers
The small molecules used as the basic building blocks for polymers are known as monomers. Mono means one.
Example of monomers are: Glucose(C₆H₁₂O₆) is a monomer for cellulose (wood).

4.2 polymerization reaction

The chemical change by which monomer units combine to form a polymer is known as polymerization reaction.

The essential feature of a monomer is to have two or more functional groups(poly functionality). This gives each monomer the capacity to form chemical bonds to at least two other monomer molecules.
 Bi-functional monomers can form only linear, chain like polymers. Monomers with higher functionality yield cross-linked, network polymeric product

Addition polymerization

Addition polymerization is a reaction in which a chain reaction adds new monomer units to the growing polymer molecule one at a time. This type of polymerization is also known as chain growth polymerization.
This type of polymerization reaction occurs by those involving unsaturated monomers.
For example, the addition polymerization of many ethylene molecules yields polyethylene and that of styrene molecules gives polystyrene.

Note that the molecular formula of a polymer is n times that of the monomer and upon polymerization, the double bond is converted to a single bond. When monomers with triple bonds are used it is converted to double bonds.
Note also that: If a polymer is given, you can identify the monomer unit that is repeated in it. If the mass of the polymer is known, the number of the monomer molecules is determined by dividing the molar mass of the polymer by the molar mass of the monomer.

Example: The addition polymer polyvinyl chloride (PVC) has the structure:

a. draw the structure of the monomer from which PVC is made.

b. How many monomer units are in a PVC polymer that has a molar mass of 1.33 x 10⁵ g/mol?

Solution:

We need to find the repeating unit from which this polymer chain is constructed

The repeating unit and the monomer have carbon-carbon double bond.

b. If the PVC had a molar mass of 1.33 × 10⁵ g/mol, how many monomer units (n) are there?

Given: Molecular mass of CH₂=CHCl (Vinyl chloride) is 63.5g/mol and molar mass of the PVC 1.33 × 10⁵ g/mol
Required: The number (n) of CH₂=CHCl
Relation: Number of monomer units (n)= Molar mass of the polymer/ Molar mass of the monomer

n= 1.33 x10⁵ gmol^-1 /62.5g mol^-1

n= 2128 vinyl chloride molecules

Steps of addition polymerization
The addition polymerization process takes place in three distinct steps.

1. Chain initiation step: it is a time when the polymerization process is started by means of a reactive
   initiator molecule. The initiator molecules can be a free radical, a cation or anion. These reactive centers
   react readily with other monomers without eliminating the active center.
2. Chain propagation step: It is the period when a monomer adds to the growing chain, with the free radical, cation or anion transferring to the chain end. High molar mass polymers are formed right from the beginning of the reaction.
3. Chain termination step: This is the step when the polymerization is stopped since chain reactions do not continue indefinitely. The radical, cation, or anion is ―neutralized‖ stopping the chain propagation. A polymerization reaction initiated by free radicals is called a free radical polymerization. The free radical route to chain polymerization is the most important step in polymerization reaction. polymerization reaction initiated by a cation is called a cationic polymerization. A polymerization reaction initiated by an anion is called an anionic polymerization.

Condensation polymerization
 Condensation polymerization involves the combination of monomer molecules, with the release of simple molecules like water or methanol as by product.
 These products are normally referred to as condensate.
 This polymerization is also known as step growth polymerization.
 In order to produce a condensation polymer, the monomers involved two or more functional groups. Typically, such reactions take place between reactive components, such as dibasic acids with dia-mines to give poly amides, or di-basic acids with diols to form polyesters, Proteins, starch and cellulose are examples of natural condensation polymers. Whereas, Polyester (Polyethylene terephthalate or Dacron) and the poly amide Nylon 66 represented below, are two examples of synthetic condensation polymers.

They can be produced by the condensation reaction of diacids and diamines. One of the most common polyamides is nylon 66 or nylon. It is a co-polymer and is prepared by the condensation of 1,6-diaminohexane (hexa methylene diamine) and 1,6-hexanedioic acid (adipic acid). The basic amine reacts with the acid to form a salt. Heating removes water and forms the amide bonds.
 Dacron is a co-polymer and is formed when equi molar amounts of 1,4-benzene dicarboxylic acid (terephthalic acid) and 1,2-ethanediol (ethylene glycol) react.

Uses of nylon include making parachutes,ropes,clothes,stockings, haircombs ,andrugs.

They are also used to reinforce automobiles tires.

Consider the reaction between an alcohol with two –OH groups, HO–R –OH, and a dicarboxylicacid,HOOC–R–COOH as represented below.In this case the ester formed still has a reactive group at both ends of the molecule.