Polymerization

Polymerization is the synthesis of polymers; giant molecules (macromolecules) that are built from repeating units called monomers. The addition of two monomers yields a dimer; adding more units yields a trimer, tetramer, and so on. Polymers can consist of thousands of monomer blocks, yielding chain-like molecules with a very high molecular weight.

There are many examples of natural polymers, such as proteins, polysaccharides, nucleic acids (DNA), and natural rubber. The original synthetic polymers were designed to mimic properties of natural polymers. The chemical industry is mainly focused on the synthesis of organic polymers, although there are examples of inorganic polymers.

The names of synthetic polymers are generally derived from their monomers. For instance, the simple polymer polyethylene is formed from the monomer ethylene:

3 CH2=CH2  →  -CH2-CH2-CH2-CH2-CH2-CH2-
ethylene      polyethylene (trimer)

Usually, the equations for polymerization are represented in the following format (where x represents a large number):

x CH2=CH2  →  -(-CH2-CH2-)-x

There are two common types of polymerization reactions:

1. Radical Polymerization (Addition Polymerization/Chain-Reaction Polymerization)
2. Condensation Polymerization (Step-Reaction Polymerization)

A Radical Polymerization is started by a catalyst, or an initiator such as O₂ or a peroxide. The resulting polymer is formed from an alkene by chain-propagation. The three steps in a radical polymerization reaction are:

1. Initiation - cleavage of the initiator into radicals
   

   ROOR → 2 RO·
   

2. Propagation - attack of the carbon-carbon double bond by a radical. This process yields a new radical that can attack another carbon-carbon bond double bond (X is any organic or inorganic side-group)

                    H
                    |
RO· CH2=CH → RO-CH2-C·
        |           |
        X           X


       H                     H     H
       |                     |     |
RO-CH2-C·  CH2=CH  →  RO-CH2-C-CH2-C· (etc.)
       |       |             |     |
       X       X             X     X

3. Termination - chain growth is halted when two radicals meet (disproportionation)

         H     H                H H
         |     |                | |
   ~-CH2-C· + ·C-CH2-~  → ~-CH2-C-C-CH2-~
         |     |                | |
         X     X                X X
   

A Condensation Polymerization takes place between two polyfunctional molecules, with the elimination of a smaller molecule such as water. For instance, a difunctional carboxylic acid can react with a difunctional alcohol:

                    H    H                    H    H
   O    O           |    |           O    O   |    |
HO-C-R1-C-OH  +  HO-C-R2-C-OH  →  HO-C-R1-C-O-C-R2-C-OH  + H2O
                    |    |                    |    |
                    H    H                    H    H

The polyester building block can continue growing by esterification with other polyester molecules. This type of polymerization is also possible one single monomer unit (a carboxylic acid containing an -OH hydroxyl group.)

Pol`y*mer`i*za"tion (?), n. Chem.

The act or process of changing to a polymeric form; the condition resulting from such change.

 

© Webster 1913.