General Methods of Polymerization

Polymers are large molecules formed by repeating units of a single unit or a molecule in a chained structure. They are often mentioned as macromolecules. There is a possibility that some monomers lose their chemical groups during polymerization. The single units or molecules repeated in the polymer are known as monomers.

The classification of polymers is based on:

1. Physical and chemical structures.
2. Depending on where they are obtained.
3. Preparation methods.
4. Physical properties. 
5. Applications.    

The polymers are formed by polymerization, where the monomers are added to form the polymer chain. Based on the functional groups present in the reacting monomers, the mechanism of polymerization reaction varies widely in complexity. A free radical reaction is the simplest way of generating polymers from alkenes.

The dangers of polymerization can be greatly increased if it doesn’t proceed at the right pace and is not sufficiently moderated. In terms of polymerization mechanisms, step-growth and chain-growth are the most common, with the former having the advantage of being easier to implement. At the same time, the latter is more reliable in producing high-molecular-weight polymers.

• Step Growth: A condensation polymer may also be classified as a step-growth polymer because a small molecule like water is lost during polymer chain lengthening. When the conversion rate is low, their molecular weight is low, but when the conversion rate is high, their molecular weight is moderately high.
• Chain Growth: As part of chain-growth polymerization (or addition polymerization), unsaturated monomers are joined together. In many aspects of our lives, chain polymerization products are used, including medical materials, food packaging, and many more.

Classification based on Method of Polymerization

Based on the method of polymerization, there are 2 types:

Addition polymerization (or) chain growth reaction.

It is a process in which the monomers are added one after the other in a linear chain-like model. The end product formed is in such a way that the linear chain formed contains the multiple units of that monomer. 

This reaction is used in industries mostly for the construction of polymers.

The reaction involves majorly 3 steps:

• Initiation Initiation involves finding the active site for the monomer to be added.
• There are initiators involved are peroxides, azo compounds, lewis acids, and organometallic compounds.
• The initiators should possess free radicals. The molecules should have unpaired electrons.
• The reaction should take place in an oxygen-free environment as the presence of oxygen inhibits the reaction.
• The initiator radical is added to the monomer for further attachment.

• Propagation

• In this step, the monomer is further added to its active site.
• This step continues till the termination or chain transfer.
• There are three possible ways of attachment: head to head, head to tail, and tail to tail.                                                                  

• Termination

• Termination can occur by the free radicals combining with the molecules themselves instead of the monomer. But this method does not control the reaction well.
• Therefore, two other methods are used: coupling and disproportion.

• In a few cases, there in chain transfer:

• Chain transfer involves transferring radical activity to other species; it can either be the polymer or monomer, etc.
• This step helps in the formation of other polymer species.

There are different polymerization techniques in addition  polymerization.   

1.Bulk polymerization

• This mode of polymerization is used to obtain the purest form of the required polymer. This method yields the most per-unit volume. 
• This method requires mainly an initiator, a monomer, and a chain transfer agent if required.
• The mixture is heated to the polymerization temperature.
• The heat is withdrawn later as it’s an exothermic reaction.
• The viscosity increases during the reaction.
• The heat generated is controlled by circulating water.

Example: polystyrene.

2.Solution Polymerization 

• In this method, the monomer is dissolved with the initiator, and the resulting polymer is soluble in the solvent.
• The process is started with a higher concentration of monomers using less amount of catalyst.
• As the reaction proceeds, more solvent is added to control the viscosity.
• The rate of the reaction is usually low compared to emulsion polymerization.
• The end product formed is of low molecular weight.
• Mostly, this type of polymerization is chosen when the polymer solution is used directly for solvent-based coatings and adhesives.

3.Suspension polymerization

• This method was developed to control the enormous heat released in bulk polymerization.
• In this method, monomers are in the form of dispersed minute particles.
• This system contains a dispersing medium, monomer, stabilizing agents, and an initiator soluble in monomer (liquid).
• The reaction kinetics is similar to that of bulk polymerization. 

4.Emulsion polymerization

• The use of emulsion polymerization is crucial.
• Polymerization of monomers such as vinyl acetate, vinyl chloride, chloroprene, acrylamide, and copolymers like acryl-butadiene-styrene(ABS) are done using this method. 
• The polymerization is done in the form of emulsions (colloidal particles.) 
• This type of polymerization differs from suspension polymerization due to the particle type, size, and initiator used for the polymerization.
• This method yields a higher degree than any other technique.
• The end product is called latex. It can be used directly without any modifications or separation.  
• There are 2 types of emulsion polymerization: oil-in-water and water-in-oil.

Condensation polymerization

A series of reactions take place in which any two species react to give large molecules. It primarily involves condensation, such as esterification, ester interchange, or amidization. Water is one such small molecule that is eliminated. This method is apt for low-weight molecules. Vacuum processing or azeotropic distillation may be employed to remove the by-products. 

Polymerization is essentially the process of making polymers. The polymerization process involves the chemical reaction between small molecules or monomers that result in larger molecules. The polymerization reaction is one of the critical reactions used in industries to make most of the plastics we use. It plays an essential role in the environment, as we, living beings, also have macromolecules in our body, such as lipids that have long chains. 

Additionally, the structure is altered throughout the polymerisation process, but in some cases, certain polymerization processes can directly produce finished shapes. Polymerization processes should therefore be understood at a basic level.