Kinetics of Addition Polymerization
"Polymerization technique is defined as the fundamental process by which low molecular weight compounds are converted into high molecular weight compounds (means, polymers)".
"Chain-growth polymerization reaction" also known as "Addition polymerization reaction". In this polymerization reactions, monomer molecules bond to each other to form a larger molecule called a polymer, without removing any of the smaller molecules as a byproduct. An addition polymer is a very long organic molecule formed as a result of an addition reaction between monomers with unsaturated carbon-carbon bonds.
☛ The kinetics of addition polymerization proceeds according to the chain polymerization mechanism that occurs in three basic steps;
1) Chain initiation
2) Chain propagation
3) Chain termination
Since, addition polymerization occurs in four ways (free radical/cationic/anionic/co-ordination Polymerization) therefore all its types follow the same kinetic behavior.
Kinetics of Free radical polymerization or addition polymerization–
The intermediate formed in this is the free radical. Let's look at its kinetics. Consider a monomer molecule 'M', an initiator 'I' and a free radical 'R'.
☆Initiation
A typical free radical polymerization begins with the thermal decomposition of the initiator. Here, an active species arises as a "free radical".
Thus, the rate of disappearance of initiator molecule is given as;
Once free radicals are formed, they react with the monomer molecule. In polymer chemistry, the first free radical reacts with a monomer molecule to form a new free radical species in the initiation step.
Since " ki >> kd ", the rate of decomposition of the initiator is the rate determining step (or because of non-measurable quantities, [M•] ).
Here, not all the radicals that are formed initiate the reaction (or react with the monomer molecule to form a monomer free radical). Therefore, we add a factor 'f' to the initiation step, which represents the fraction of free radicals that actually lead to the formation of monomer free radicals. Thus the rate of initiation is given as;
☆Propagation
From the initiation step, the radical reacts with other monomer molecules to extend the polymer chain and form new free radicals. Thus, new monomer free radicals are formed in this propagation step.
☆Termination
This is the final step of the chain reaction cycle. In this step, all monomer radicals disappear. Termination can happen through a combination (or coupling) or disproportionation. Here, we will look at the termination by coupling of two growing polymer chains;
Here,
" M• " represent the growing polymer chain
" kt " represent the termination rate constant
The rate of addition polymerization or free radical polymerization is defined as the rate at which the monomer molecule disappears. Thus, the rate of addition polymerization;
Here, the propagation step is the rate controlling step for the production of high molecular mass molecules, therefore "Ri <<< Rp ". Thus, from equation (4) ;
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.
Put the value of monomer radical concentration from equation(7) into equation(6) and we get;
Here, equation (8) shows the rate of addition polymerization (or free radical polymerization) at which the monomer molecule disappears.
It is clearly visible from equation (8), that the rate of addition polymerization is directly proportional to the square root of the initiator concentration and the monomer concentration. Thus,
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