Ethylene oxide polymerizes very well with anionic initiators. This reaction fails for 5 membered (i.e., THF) and larger ether rings. It does work for many other heterocycles, however.
Ethylene oxide is unusual in that it can be polymerized by cationic initiators, as well.
These two reactions nicely exemplify the ionic ring opening mechanisms. Other more complicated systems work by analogous mechanisms.
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