Synthesis of Polypropylene glycol Polypropylene glycol is produced by ring-opening polymerization of
propylene oxide. The initiator is an alcohol and the catalyst a base,
usually potassium hydroxide. When the initiator is ethylene glycol or
water the polymer is linear. With a multifunctional initiator like
glycerine, pentaerythritol or sorbitol the polymer branches out.
Source: https://en.wikipedia.org/wiki/Polypropylene_glycol
Polypropylene glycols ... are formed by action of base on propylene oxide in the presence of 1,2-propanediol as an initiator.

I don't understand the meaning of an initiator here. My mechanism is the first the OH- adds to the propylene oxide giving $\ce{HOCH2CH2O-}$ which again adds to another molecule of the oxide and the chain continue and the polymer is formed.
My mechanism requires no role of the &quot;initiator&quot;. I want to know what is the requirement of the diol in initiating the polymerisation, whether my mechanism is correct and if wrong, what is the actual mechanism?
If you just use hydroxide, then the initiator is water.