Tributing 1 monster is a cost to activate “Chthonian Polymer”.
You can only activate this card when the opponent Fusion Summons a monster, including with a Special Summon that is treated as a Fusion Summon, like “Miracle Fusion” or “Power Bond”. You cannot activate it when they Special Summon a Fusion Monster with a card effect, like “Metamorphosis” or “Cyber-Stein”.
You gain control of the Fusion Monster for as long as it remains on the field.