Step-growth polymerization

Step-growth polymerization forms polymers in a manner that is quite different from chain-growth polymerization.  The mechanism requires that at least two different monomers participate in the reaction.(1)  Before we consider the specific steps in step-growth polymerization, we’ll first consider the necessary functional groups required in the monomers for the reaction(s) to occur.

Each stage in step-growth polymerization involves a reaction between dissimilar chemical groups which are part of the monomer molecules.(2)  The chemical groups are termed “functional groups” due to the fact they provide a specific attribute (functionality) that allows them to react together in a specific way to give the desired resin properties. 

The most common type of reaction in forming resin via step-growth polymerization involves the formation of a new bond (covalent bond) between the two functional groups.   Simultaneously, a by-product is produced from the reaction (sometimes water – which is why this polymerization is sometimes termed condensation polymerization).  Let’s illustrate this by looking at a generic alcohol and acid:

 

(alcohol)   (acid)                                    (ester)       (water)
-C-O-H + H-O-C-      -------------->         -C-O-C- + H2O
                          ||                                      
                         O                          

Two other types of monomers that combine via step-growth polymerization are an amine and an acid to form an amide:

 

(amine)  (acid)                                      (amide)   (water)
-N-H + H-O-C-           --------------->         -N-C- + H2O
  |                   ||                                            |    ||   
 H                 O                                           H  O  

                                                               

Water is not the only by-product that might be produced, however.  Here’s an example of a step-growth polymerization where an alcohol by-product is produced:

(terephthalate)    (alcohol)                                 (PET)           (alcohol)
          H       
           |
-C-O-C-H + H-O-C-            -------------->            -C-C-O-H + H-O-C-H
           |
          H

 

A particular step-growth polymerization reaction of great historical and commercial significance is the polymerization of nylon.  This reaction uses two symmetric bifunctional monomers (hexamethylene diamine and adipic acid).  The resulting material is a polyamide, commonly known as nylon 6,6; the numerical designation in the name represents the number of carbons in each monomer. 

Typical resins polymerized via step-growth include: acetals, nylons, polycarbonate, and polyesters. 

Our next discussions will involve specific resins and how they perform both physically and mechanically.  The resins to be presented have all been formed via the step-growth or chain-growth polymerization method; which in turn will have an effect on its strengths and weaknesses.