Diels-alder reaction cyclopentadiene and malic anhydride forming cic-norbronene-5,6-endo-dicarb… anydride

Cyclopentadiene exists as a dimer (Diels-Alder reaction of itsself). By distilling it you "cracked" (retro-Diels-Alder reaction) and you kept it cool to slow the Diels-Alder reaction with its self once distilled. If you do not have cyclopentadiene, but instead have the dimer the chemistry will not work and you will not have as much monomer as you think and wish.