Yeah as Jazz said, I think it's called an initiator instead. The initiator is essentially an organic peroxide such as benzoyl peroxide.
Basically an initiator molecule 'splits' to form free radicals. These free radicals are highly reactive due to its unpaired electron and can react readily with the electron dense double bond of an ethylene monomer to form an initiator-ethylene radical. This is considered the first step of the LDPE production process called 'initiation'.
Just remember that catalysts speed up the reaction rate and aren't used up in the net reaction (like Jazz also said). The initiator benzoyl peroxide is different to a catalyst since it allows the polymerisation process to occur in the first place instead of speeding it up and are technically used in the reaction (since I think the initiator atoms ends up appearing at the ends of the polymer chain once LDPE formation is finished - I'm not fully sure of this though haha).