Polymers Basics, Classification, Polymerization, Addition and Step Growth Polymerization

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.

  • Introduction to Polymers
  • Definitions in polymer subject
  • Classification of Polymers
  • Polymerization in polymers
  • Addition Polymerization
  • Step Growth Polymerization

Introduction to Polymers

"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.

Introduction to polymers and basics


Oligomer definition in polymer

The oligomer is a small molecule with a small number of repeating units but not large enough to consider as a polymer.


Monomer definition in 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

Polymerization is a process which make polymers by combining two or more monomers.



Repeating unit

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".

Degree of polymerization

Classification of Polymers

polymers can be classified in different ways.

  • Based on Origin of source
  • Based on backbone of the polymer chain
  • Based on behavior
  • Based on polymer composition
  • Based on polymer structure

Classification of polymers based on origin of source

There are three different classifications, natural, semi-synthetic, synthetic according to the origin of source.

Classification based on origin of source

Natural polymers

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.

  • Natural rubber: Natural rubber is an important polymer in the industry mainly consisting of cis-1,4-polyisoprene
  • Cellulose - Cellulose is a polysaccharide consisting of 3000 or more glucose units. It is a water-insoluble highly crystalline polymer. Plant cell wall's main substance is cellulose.
  • Protein - Proteins are formed by polymerization of amino acids. An amine group of one molecule connects with a carboxylic group of another molecule by peptide bonds to make a protein. Because of that protein can be called a polypeptide. There are 20 amino acids that have different R groups that makeup proteins in general.


Semi-synthetic polymers

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.

  • Vulcanized rubber - Vulcanized rubber is a chemically modified polymer by heating natural rubber with sulphur to form cross-links and get improved physical properties. Some of them are high tensile strength, high wear and tear resistance, and high elasticity. Vulcanized rubber is used to make many items. Such as tires, toys, and shoes.
  • Rayon (Cellulose acetate) - Rayon is the first semi-synthetic fiber that is prepared from cellulose with extensive chemical treatment. Rayon has very soft, comfortable, good absorbent, and versatile properties. In the textile industry rayon is used to make clothing.


Synthetic polymers

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.

Synthetic Polymers and their uses

Classification of polymers based on backbone of the polymer chain Classification of polymers based on backbone of the polymer chain

Organic polymers

Polymers that essentially contain carbon atoms in the backbone is called as organic polymers.


Inorganic polymers

Polymers that does not have carbon atoms in their backbone is called as inorganic polymers.



Classification of polymers based on behavior

Classification of polymers based on behavior

Fibers

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.


Plastics

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.

  • Thermoplastics - Thermoplastic polymers are softened on heating and stiffen on cooling. Because of that they can be reshaped and reused. They are formed by an addition reaction. Polyethylene, polyvinyl chloride are examples of thermoplastics.
  • Thermosetting - Thermosetting polymers cannot soften on heating and stiffen on cooling like thermoplastics. Because of that they cannot be reshaped and reused. They are formed by condensation polymerization. Bakelite, Polyurethane are examples of thermosetting.

Elastomers

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.



Classifications of polymers based on polymer composition

Classifications of polymers based on polymer composition

Homopolymers

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.


Copolymers

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.

Classification of Copolymers

Classifications of polymers based on polymer structure

Classifications of polymers based on polymer structure



Polymerization in polymers

Polymerization is a process which make polymers by combining two or more monomers. There are two basic types of polymerization methods.

  1. Addition Polymerization
  2. Step Growth Polymerization

Addition Polymerization

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

Addition polymerization requires initiators as a catalyst. Three categories of initiators are,

  1. Free radical initiators
  2. Cationic initiators
  3. Anionic initiators

According to the initiators, there are three categories of addition polymerization.

  1. Free radical polymerization
  2. Cationic polymerization
  3. Anionic polymerization

Free radical polymerization

Mechanism of free radical polymerization

Types of initiation

  • Thermal decomposition
  • Photolysis
  • Redox reactions
  • Persulfates
  • Ionizing radiation
  • Electrochemical


Cationic polymerization

Mechanism of cationic polymerization

Anionic polymerization

Mechanism of anionic polymerization


Step Growth Polymerization

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

Step growth polymerization can be divided into two types.

  1. Condensation Reaction
  2. Addition Reaction

Condensation Reactions

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.

Condensation Reaction

Step growth polymerization often occurs through condensation reactions.



Addition Reactions

In addition reactions, the removal of a small molecule, like condensation reactions, does not take place.

Addition Reaction in step growth polymerization


References

  1. Sciencedirect.com. 2021. Living Cationic Polymerization - an overview | ScienceDirect Topics. [online]. [Accessed 2 June 2021]
  2. Ebewele, R., 2000. Polymer science and technology. Boca Raton, Fla.: CRC Press.