Synthesis of Polypropylene glycol

Question

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 "initiator". 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?

Answer 1

If you just use hydroxide, then the initiator is water.

Answer 2

There are two broad classes of polymerisation, there are the condensation polymerisations and the addition polymerisations. The classic addition polymerisation is the reaction of a free radical with an alkene (olefin) such as styrene to form a macromolecular radical which is a little larger. The growing polymer chain has a reactive site at the end which reacts with more of the monomer thus making a longer polymer chain.

In the case of a hydroxide or alkoxide anion, it can react with ethylene oxide or propylene oxide to ring open the epoxide and form a new alkoxide which has a higher formula mass than the original anion.

This process can be repeated again and again to form either polyethylene oxide (polyethylene glycol) or polypropylene oxide.

The hydroxide anion (or an alkoxide) can start the polymerisation reaction, while many polymerisation initiators are compounds such as tert-butyl peroxide, benzoyl peroxide and AIBN which are able to create radicals which in turn start (initiate) the polymerisation these radical forming initiators are a subset of polymersation initiators.

Another example of a non radical initiator would be a bis-cyclopentadienyl methyl zirconium compound with a vaccent site on the metal. This will be able to start the formation of high density polyethylene (HDPE) if ethylene is added to this compound. This is an example of a Zieger Natter polymersation.

I think that you need to accept that a polymerisation initator is any substance which can start the formation of a polymer chain from monomer molecules by means of an addition polymerisation.