Polymer is a very large molecule made from many small structural units called monomers connected by covalent bonds. Polymerization is a process which make polymers by combining two or more monomers. In this tutorial, we will learn basics of polymers, polymerizations , polymerization methods and more.
Written by: Deshani Gunathilaka, Department of Polymer Science, Faculty of Applied Sciences, Univerity of Sri Jayewardenepura, Sri Lanka, last update: 10-06-2021
In this tutorial, we will learn followings.
"Polymer" is a Greek word. "Poly" - many and "mer" - parts. Polymer is a very large molecule made from many small structural units called monomers connected by covalent bonds.
The oligomer is a small molecule with a small number of repeating units but not large enough to consider as a polymer.
Monomer is a small molecule that can make polymers.
It should contain two or more bonding sites. Therefore monomers have double bonds or two or more functional groups. If monomers have two functional groups it makes a linear polymer. If they have more than two functional groups it makes a branched polymer.
Polymerization is a process which make polymers by combining two or more monomers.
Repeating unit is the basic unit which repeatedly placed in the polymer chain. It is closely similar to the monomer. The length of the polymer chain is decided by the number of repeating units in the polymer chain. That is called "Degree of polymerization".
polymers can be classified in different ways.
There are three different classifications, natural, semi-synthetic, synthetic according to the origin of source.
Natural polymers are polymers that occur naturally from a biological process. That means they are produced from plants, animals, and microbes. And also they can degrade easily by a biological process. Natural polymers are readily and abundantly available, environmentally friendly nontoxic products and several example are given below.
Semi-synthetic polymers are chemically modified polymers by using natural polymers. The physical properties of natural polymers are improved here. These polymers are a combination of synthetic and natural polymers and several example are given below.
Synthetic polymers are made artificially using chemical processes from low molecular weight compounds. Mainly petroleum based raw materials are used. There is a wide range of properties and uses of synthetic polymers. But not environmentally friendly. The first synthetic fiber is nylon.
Polymers that essentially contain carbon atoms in the backbone is called as organic polymers.
Polymers that does not have carbon atoms in their backbone is called as inorganic polymers.
Long thin polymers that having strong forces of attractions are fibers. They cannot stretch or be reshaped once formed. Fibers length is at least 100 times of their diameter.
Polymers that can stretch and flex more than fibers and can be molded into the desired shape. There are two categories of plastics according to their thermal behavior.
Elastomers can be stretch and flex more than fibers and plastics. So much so that it can be restored to its original shape after being stretched to great extents. It is called elastic property. The very first elastomer is rubber. Styrene-butadiene rubber (SBR) is the most important synthetic elastomer.
Polymers that polymerized by addition reactions using one type of monomer or single kind of repeating unit are known as homopolymers. Often they have a simple structure. Polyethylene, polyvinyl chloride and polyester are examples of homopolymers.
Polymers that polymerized by condensation reactions using more than one type of monomer or more than single kind of repeating unit are known as homopolymers. Often they have a complex structure. Styrene-butadiene rubber (SBR) and Bakelite are examples of homopolymers. Based on their distribution they can categorize into four types.
Polymerization is a process which make polymers by combining two or more monomers. There are two basic types of polymerization methods.
Addition polymerization is a process that forms polymers by monomers with one or more unsaturated bonds via rearrangement of bonds in an addition reaction without loss of any small molecule. In this polymerization, chain growth occurs only at one end. Therefore addition polymerization is also called chain-growth polymerization.
Addition polymerization requires initiators as a catalyst. Three categories of initiators are,
According to the initiators, there are three categories of addition polymerization.
Types of initiation
Step growth polymerization is a process that forms polymers by monomers with two or more functional groups via condensation or addition reaction.
Here a functional group of one monomer reacts with a functional group of another monomer and makes dimers. Similarly trimmers, tetramers are also made. Finally, these oligomers form a polymer. Therefore this is a stepwise reaction. Because of that, this polymerization is called step-growth polymerization.
Step growth polymerization can be divided into two types.
In condensation reactions, when reacting two monomers to form a polymer, there will be an elimination of a small molecule like H2O, HCl as a by-product.
Step growth polymerization often occurs through condensation reactions.
In addition reactions, the removal of a small molecule, like condensation reactions, does not take place.