Kinetics of Cationic Polymerization

"Ionic polymerization is defined as a type of addition polymerization in which the active centres (or intermediates) are ions or ion pairs". It can be considered as an alternative to free radical polymerization. Cationic polymerization reaction is one of two processes of Ionic polymerization to convert a monomer molecule into a polymer. 

"Cationic polymerization is defined as a type of addition ionic polymerization in which the active centres are cations (or positive ions)". 

In this, the cationic initiator is used to transfer the positive charge onto the monomer molecule so that it becomes reactive. This reactive monomer reacts with other monomer molecules to form a polymer in the same way that a free radical does. 

Examples of polymers formed by Cationic polymerization are Poly-isobutylene (PIB), Poly-butene and Butyl rubber, etc.

The types of monomers required for cationic polymerization are limited to "alkanes" with "electron donating substituents". 

These polymerization reactions are very sensitive to the type of solvent used as these are the solvents that provide us with the cationic initiator to initiate the polymerization reaction. 

The solvent used in cationic polymerization is strong and Lewis acids like- Sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), Aluminium chloride (AlCl₃), BF₃, titanium tetrachloride (TiCl₄), SnCl₄, etc.

   

Kinetics–

Since, cationic polymerization is a type of addition polymerization reaction. Therefore, this will also happen in three steps namely initiation, propagation and termination. 

Let's look at these three steps one by one.

Chain Initiation

In this, the cationic initiator (I⁺) obtained from the solvent used, reacts with the monomer molecule and forms a new monomer cationic initiator.

 –––––––––––––(1)

Chain Propagation

Once a reactive intermediate (monomer cationic initiator, M⁺) is generated, it can react with other stable molecules to form new reactive intermediates. The propagation step involves the formation of new intermediate ions.

 ––––––––––––––(2)

Chain Termination

In this phase, all intermediates (propagating centers, M⁺) generated from the first two steps disappear and no intermediate or reactive sites remain.

 –––––––––––––(3)

Here, I⁺ is the initiator, M is the monomer, M⁺ is the propagating centre and ki, kp, Kt are the reaction rate constants for initiation, propagation and termination steps respectively. 

The rate of Cationic Ionic polymerization is defined as the rate at which the monomer molecule disappears. Thus, the rate of Cationic polymerization reaction is;

 ––––––––––––––(4)

Here, the propagation step is the rate controlling step for the production of high molecular mass molecules, therefore "Ri <<< Rp ". Thus, in equation (4);

 ––––––––––––––(5)

Since, "M⁺ " is not a measurable quantity therefore we will convert it to a measurable quantity by applying the law of steady state approximation (SSA). 

➛From steady state approximation,

The change in the concentration of the reacting groups or intermediates is equal to zero. Thus,

Put the value of monomer cation concentration  into equation (5) and we get;

–––––––––––––(6)

Here, equation (6) shows the rate of "Cationic polymerization" at which the monomer molecule disappears. 

It is clearly visible from equation (6) that the rate of "Cationic polymerization" is directly proportional to the first power of initiator concentration and second power of the monomer concentration.

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