In this video I am gonna talk about addition polymerization.
Which is also known as free radical polymerization and chain growth polymerization.
Polymers are made by joining monomers in a linear chain.
The process of making polymers from monomers is known as polymerization.
Based on reaction kinetics polymerization reactions can be classified into addition
and condensation polymerization.
Addition polymers form by successive addition of monomer molecules onto the reactive ends of a growing polymer.
This process is similar to adding links onto a chain, and therefore it is called chain-growth polymerization.
Addition polymerization involves mainly three steps
First step is initiation step, in this step, a small number of monomers reacts with initiator
molecule to form active monomer or free radical.
In propagation step, the active monomers react with a large no. of monomers and form a long
chain free radical.
And finally in termination step, two active chains react with each other to form a neutral
polymer chain.
There are four types of addition polymerizations 1. Bulk, 2. solution
3. Emulsion and 4. Suspension polymerization
Bulk and solution polymerizations fall under the category of homogeneous polymerization
and emulsion and suspension polymerizations are known as heterogeneous polymerization.
The term homogeneous or heterogeneous refers only to initial condition of the system.
For example emulsion and suspension polymerization involve hydrophilic and hydrophobic reactants
in reaction mixture which makes reaction mixture heterogeneous.
Whereas bulk and solution polymerization takes place in one phase therefore are called homogeneous
polymerization.
Let's start with bulk polymerization.
In this type of polymerization monomer is taken in liquid form.
The sphere represents individual monomers.
Then the initiator is added which is monomer soluble.
The mixture is then heated to a temperature at which initiator molecules break into free radicals.
These free radicals then react with monomers and start polymerization.
During propagation, monomers combine together and grow into a polymer chain radical.
Polymerization is carried out under continuous agitation to transfer heat and mass properly.
To terminate the reaction, chain transfer agent is added.
Finally two polymer chain radicals combine together and form a long neutral polymer chain.
This reaction is purely exothermic and a wide range of molecular masses are produced.
Advantage of this reaction is the system is very simple, the polymer obtained is pure
and large castings may be prepared directly.
The Disadvantage is heat transfer and mixing become difficult as the viscosity of reaction
increases which leads to broad molecular weight distribution.
Due to highly exothermic nature of the reaction, heat transfer is very difficult.
And this method gives very low molecular weights polymers.
This method is used to synthesize polystyrene, Poly vinyl chloride, polymethyl methacrylate
and low density polyethylene.
Solution polymerization is very similar to bulk polymerization except the fact that monomer,
initiator, and chain transfer agent are dissolved in a non-reactive solvent.
So the polymerization step remains the same.
First monomer is taken.
Then the initiator is added and the reaction mixture is heated to initiate the polymerization.
The reaction result in a polymer which is also soluble in the chosen solvent and the
heat released by the reaction is absorbed by the solvent which helps to dissipate the heat
Finally long chain polymers are formed by reacting two polymer chain radicals.
To obtain pure polymer excess solvent has to be removed.
Hence, solution polymerization is mainly used for application where the presence of a solvent
is desired e.g., varnish and adhesives.
It is not useful for the production of dry polymers because of the difficulty of complete
solvent removal.
So the advantage of this method is heat transfer is easy due to presence of solvent.
Solvent also help in reducing viscosity which makes process very simple.
The disadvantage is the process the process of removal of excess solvent
Polymers such as poly(acrylic acid) (PAA) and poly(acrylonitrile) (PAN) are synthesized
using solution polymerization
Although bulk and solution polymerization are very simple methods to make polymers but
these methods have certain limitations.
Like we can’t get very high molecular weight polymers ad also polydispersity is very high.
Bulk polymerization is exothermic and solution polymerization is not environmentally friendly.
Emulsion polymerization is one of the way to overcome the limitations of bulk and solution
polymerization.
In emulsion polymerization water is taken as continuous phase.
Which contains water insoluble or hydrophobic monomer like oil?
The emulsifier or surfactant is added to form monomer in water emulsion.
If we look in the emulsion we can see that the surfactant surrounds the monomer droplet
and form a micelle structure.
This is a schematic of micelle where monomer droplet is surrounded by surfactants.
Surfactants have hydrophilic head and hydrophobic tail part.
Their hydrophobic tail is facing monomer and hydrophilic head is in water.
Each monomer droplet contains many monomer molecules.
As the initiator enters in droplet through the aqueous phase, it initiates the polymerization
in each micelle.
In propagation step the monomers start combining and grow into a chain radical.
Once all the monomers convert into polymer chain radical, de-emulsifying agent is added,
which disperse surfactants in water.
Finally the polymer chain radicals combine together and form a neutral polymer chain.
The advantage of this method is the rate of polymerization is very high which leads to
High molecular weight polymers The continuous water phase is an excellent
conductor of heat, so heat is easily controlled.
Polymerization proceeds in micelles so viscosity of the system remains almost the same.
The disadvantage is polymer needs purification, removal of emulsifier and de-emulsifier
And monomers such as vinyl chloride, butadiene, chloroprene, vinyl acetate, and acrylates
are polymerized by this technique.
Which gives water based paints, adhesive and coating materials.
Suspension polymerization is similar to bulk polymerization, and it could be considered
“bulk polymerization within a droplet.”
This is one of the most widely used polymerization technique.
It is also called as bead or pearl polymerization.
In this method, initiator is dissolved in hydrophobic monomer.
Then the hydrophobic monomers containing initiator are mechanically suspended in water
in presence of suspending agent.
The polymerization is carried out inside the small droplets of liquid monomer.
Finally the polymer chain radicals undergo termination step and form bead like polymers.
The advantage of this method is no heat, mass, chain transfer problem.
The product obtained is in granular form which is convenient to handle and isolation is easy.
The disadvantage is only water insoluble monomers are used.
Emulsifier is needed to wash the suspending agent also it requires long time for high
conversion.
Expandable polystyrene beads, PMMA, PVC and PVA beads can be synthesized using suspension
polymerization.
So in this video we learnt about addition polymers.
All the monomers that form addition polymers or undergo free radical polymerization are alkenes
or functionally substituted alkenes
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