Process of starting up a gas phase olefin polymerization reaction

A process of starting up a gas phase olefin polymerization reaction carried out in the presence of a catalyst in a fluidized bed reactor, comprising the steps of:

• (a) flowing a gas composition, comprising an olefin, hydrogen and an inert gas, through the fluidized bed reactor having a content of solid polymer particles lower than 1% of the maximum capacity of the fluidized bed reactor;
• (b) activating the catalyst in a pre-contacting pot;
• (c) after the step of activating, feeding the catalyst into the fluidized bed reactor;
• and
• (d) (i) after the step of feeding the catalyst, monitoring the olefin concentration in the gas composition,
• (ii) when the olefin concentration drops, starting the feeding of the olefin into the fluidized bed reactor, and
• (iii) controlling the olefin flow, thereby maintaining a concentration of olefin in the fluidized bed reactor within a specified range.