Final answer:
Condensation silicones set via condensation polymerization, which involves the loss of small molecules like water. This differentiates them from polyethers, polysulfides, and polyvinyl siloxanes, which set through addition polymerization without producing by-products.
Step-by-step explanation:
The impression materials listed—polyethers, polysulfides, polyvinyl siloxanes, and condensation silicones—belong to the category of dental impression materials used for making accurate dental and gingival molds. These materials undergo a setting reaction to form a rubbery matrix that accurately captures the intended form. Among these, condensation silicones differ from the others, as they set by a condensation polymerization reaction. This polymerization process involves a stepwise reaction where monomers with two or more functional groups react with each other, resulting in the loss of small molecules like water. Thus, the answer to the question is condensation silicones (D), which set by a condensation polymerization reaction, unlike the others that set by an addition polymerization.
In contrast, polyethers, polysulfides, and polyvinyl siloxanes set by addition polymerization. This type of polymerization involves monomer molecules bonding to each other directly without the loss of any other atoms. These materials are formulated to initiate this reaction without resulting in the elimination of by-products. This makes them different from condensation silicones which follow the condensation mechanism, typically producing a molecule of water or another small molecule as a by-product of chain elongation.