D05. Describe the two classifications of step-growth (or condensation) polymerization using polysulfide impression material and polyurethane polymer as examples.

Step-growth (or "condensation") polymerization 

Traditionally, we used to differentiate between condensation and addition polymerization. However, since the term "condensation" implies that a condensation product (e.g., water) is formed, the term "condensation polymerization" is too narrow. For example, an -OH group can react with an O=C=N-group to form an -O-CO-NH- group (urethane group). Such a reaction belongs to the same reaction group as the typical condensation reactions, but no condensation product is formed. Therefore, to broaden the definition of condensation polymerization to include the urethane, the term "step-growth" polymerization should be used to include both the traditional condensation polymerization process as well as polymerization process similar to the formation of urethane groups.

Typical condensation reaction

Step-Growth polymerization without the formation of a condensation product

Addition polymerization

Addition polymerization is a free radical reaction during which carbon double bonds are opened and allowed to react with each other to form long chains.

Reaction rates

The step-growth polymerization reaction is slower than the addition polymerization reaction process.

Polymerization shrinkage

Most polymers shrink during the polymerization reaction. The reason is simply that the monomer molecules come closer during the polymerization reaction.

Practical example

Polysulfide impression materials set via a condensation reaction. During the setting reaction, lead peroxide reacts with the -SH groups. When two such -SH groups are close to each other, the lead peroxide donates an oxygen atom. This oxygen atom reacts with the two adjacent H atoms and water is formed. At the same time the two S atoms link the chains together and the material starts setting. During this reaction, water is formed as a by-product. This is a typical condensation reaction.