The word “Polymer” is a Greek word which literally means “Many parts”. A polymer is that chemical molecule which has a long set of identical building blocks, all linked with a bond, be it ionic or covalent. The building block through which any polymer molecule is built is known as a monomer. These monomers are generally very reactive molecules. The process in which monomers are all linked together to form the long-chain polymers is called a polymerization reaction or polymerization process.
The classification of the polymerization process is done by taking a look at the mechanism of the reaction. The mechanism here refers to how the transformation of reactants into products happens. So, there are two types of polymerization reaction listed below:
Like the name “Addition” in this process, the polymers are formed when the corresponding monomers are added to each other. The structure of the polymer, i.e. if the polymer would branch or develop a long chain would depend upon three things, the catalyst used, the reaction conditions and the monomers used. There is no loss of atoms in the process. So, the law of conservation of mass and stoichiometry both are applicable in the addition polymerization process. Usually, the monomers which are used in this process are unsaturated molecules (the carbon molecules which contain double or triple covalent bonds). Additionally, there are four more sub-types of addition polymerization process:
a. Free Radical Polymerization
Free radicals are those atoms which have only one free electron in their valence shell. So, the polymerization reaction classified on the basis of free radicals in the initiation process is called free radical polymerization reaction. The formation of free radical is done in the presence of a particular catalyst known as the free radical generating initiator. Benzol peroxides or peroxides, in general, are perfect examples of the free radical generating initiator. The free radicals which are formed are highly reactive (because of just one unpaired electron instead of a proper negative or positive charge). So, when they are created, they rapidly react with other free radicals present to form longer chains of the carbon molecules known as polymers.
Polymerization reactions classified on the basis of using cations with the reacting monomers are known as Cationic Polymerization. In this polymerization, both the initiation and propagation steps occur with the help of a cation. So, the chain growth occurs when the cation transfers its charge to the reacting monomers to make them reactive. These reactive monomers now react with other monomers in a similar fashion to form the polymer.
Anionic Vinyl Polymerization
In this process, the anions are introduced to make the monomers reactive. These reactive anions (Usually formed when a strong Lewis base or a nucleophile transfers its charge to the reacting monomers) then react with other monomers resulting in the propagation of the chain and forming the required polymers.
This is a special kind of free radical polymerization which happens in the presence of a particular catalyst known as the Ziegler-Natta catalyst (it is a metal complex). This catalyst allows us to control the free radicals and how they react, leading to the formation of polymers which are denser and more robust. The initiation occurs when this catalyst adds the monomer to itself. The propagation of the chain occurs when more monomeric molecules are added to the metal complex or the catalyst. The termination occurs when the added molecules of the monomers leave the metal complex as the required polymers.
In this type of reaction, the polymers are formed by elimination of simpler molecules (mainly water and small chain alcohols), thus the name condensation. The reactants are also different for the condensation polymerization reaction. Here the reactants must contain two different functional groups at the two ends of the molecule. At each step of this reaction, the molecules formed also include the same two functional groups at the two ends of the intermediate. This allows the response to further continue, and the formation of long-chain carbon molecules or polymers is completed. Also, at each step of the way, there is the elimination of smaller molecules or condensation.
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A straightforward example of the condensation reaction is displayed above. In this reaction, the carboxylic acid group and the amine group at the ends of the two reactants, react with each other to form the long chain carbon molecule or polymer. Two molecules of water are also eliminated in the process as evident from the reaction above.
1. What are the Main Differences Between the Addition and Condensation Polymerization Process?
In addition polymerization process, the reacting monomers can only attach themselves to the active sites of the molecules. In condensation polymerization, any molecule can react with any molecules at any given time.
There are three distinct steps in the addition polymerization process. The first step is initiation. In this step, the reactive intermediate state is formed. The next step is propagation. In this step, the reactive intermediate reacts, and the formation of a chain takes place. The third and final step is termination. The condensation polymerization reaction, however, has no termination step. The reactants remain active throughout the entire process.
2. What is the Role of a Catalyst?
A catalyst in the polymerization process has three roles. One of its functions is to reduce the activation energy of the reaction by providing an alternative pathway for the reaction to occur. Secondly, because of this reduction in activation energy, it speeds up the entire process. The third action of the catalyst is to sometimes determine the nature of the final product.