The starting material used for the preparation of decron is

Dacron, also known as Terylene, is a well-known polyester.

It is synthesized by the condensation polymerization of ethylene glycol (ethane-1,2-diol) and terephthalic acid (benzene-1,4-dicarboxylic acid) at a temperature of $420$ to $460$ K in the presence of a catalyst such as zinc acetate-antimony trioxide.

The reaction involves the elimination of water molecules to form ester linkages.

Therefore, the starting materials for the preparation of Dacron are ethylene glycol and terephthalic acid.

Answer: Ethylene glycol and terephthalic acid