"The impact of initiator choice and design of initiator cocktails on polymer properties in high-pressure polymerization processes indicate that the often-taken approach of quick mixing and ideal reactor behavior does not hold for any case of operation conditions. This challenge to inspect how further insight can be achieved either by lab-scale experiment or simulation. At the hand of CFD simulations and lab-scale experiments it will be discussed what state of the art exists to establish and identify temperature gradients in high-pressure experiments and visualization of mixing characteristics utilizing model setups. Moreover, studies on initiator cocktail are shown that demonstrate the initiators in cocktails interfere and do not act independently. Finally a pathway will be outlined how CFD simulations can help identifying hot-spots in reactor operation that may lead to decompositions. "