D06. Distinguish between condensation and addition polymerization based on the composition change and the requisites of polymerization.

Condensation polymerization

This polymerization process is a special case of the step-growth polymerization. During a condensation process, a condensation product is formed. This condensation product can be water, but also other condensation products such as different alcohols may form.

The simplified reaction can be written as a condensation reaction between a diol and a dibasic acid:

HO-R-OH + HO-C(O)-Q-C(O)-OH => HO-R-O-C(O)-Q-C(O)-OH + H₂O  

R and Q represent different central structures.

Examples

In dentistry, we use two impression materials that set via condensation reactions. One of these materials is the polysulfide impression material that produces water when it sets (D05). The other impression material is the condensation-polymerized silicone (c-silicone) that produces ethanol when it sets. Because these condensation products can evaporate from the above impression materials, these two impression materials are known to shrink over time. The larger ethanol molecule as well as its tendency to evaporate faster than water explains why c-silicones shrink faster and more than the polysulfides over time.

This reaction shows a step growth reaction during which a diol reacts with an isocyanate group to form a urethane. As seen from this slide, no condensation product is formed despite the similarities with the reaction shown on the previous slide. Because of the similarities between the two reactions, both can be called step-growth polymerization reactions, while only the reaction shown in the previous slide can be called a condensation polymerization reaction.

Addition polymerization

To overcome the above problems with condensation products that evaporate and cause shrinkage over time, manufacturers of silicone impression materials have now developed so called polyvinylsiloxane impression materials. These materials contain carbon double bonds (vinyl groups) that during polymerization open up and link the monomer units together via an addition polymerization reaction, which explains why these silicones are called a-silicones. During the setting reaction, no by-product is formed, explaining why these materials retain their dimensional stability over time after they have set.

The simplified reaction can be described as:

I*+ C(H₂)=C(H₂)  => I-C(H₂)-C(H₂)*

I-C(H₂)-C(H₂)* + C(H₂)=C(H₂)  => I-C(H₂)-C(H₂)-C(H₂)-C(H₂)* 

I-C(H₂)-C(H₂)-C(H₂)-C(H₂)* +  C(H₂)=C(H₂)  => I-C(H₂)-C(H₂)-C(H₂)-C(H₂)-C(H₂)-C(H₂)*  and so on....

I* represents a free radical