Kinetics of Anionic 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. Anionic polymerization reaction is one of two processes of Ionic polymerization to convert a monomer molecule into a polymer. "Anionic polymerization is defined as a type of addition ionic polymerization in which the active centres are anions (or negative ions)". In this, the anionic initiator is used to transfer the negative charge onto the monomer molecule so that it becomes reactive. This reactive monomer (or monomer anion) reacts with other monomer molecules to form a polymer in the same way that a free radical or cationic polymerization does. Examples of polymers formed by anionic polymerization are Synthetic Polybutadiene rubber, styrene butadiene rubber (SBR), Thermoplastic Styrene elastomer, etc. 

                The monomer molecules used in ionic polymerization are styrene, diene, epoxide, aldehyde, cyanoacrylates, acrylates, etc. Whereas ionic initiators are generated from strong bases such as Grignard reagent, sodium hydroxide (NaOH), sodium amide (NaNH₂), C₂H₅Na, KNH₂, etc. 

Kinetics–

Since, anionic polymerization is a type of addition polymerization reaction. Therefore, this will also happen in three steps namely initiation, propagation and termination just like free radical polymerization and cationic polymerisation. Let's look at these three steps one by one.

Chain Initiation

In this, the anionic initiator (I⁻) obtained from the strong base used, reacts with the monomer molecule and forms a new monomer anionic initiator.

–––––––––––––––(1)

Chain Propagation

Once a reactive intermediate (monomer anionic 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 anionic 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 "Anionic polymerization" at which the monomer molecule disappears. It is clearly visible from equation (6) that the rate of "Anionic polymerization" is directly proportional to the first power of anionic initiator concentration and second power of the monomer concentration.