Grignard reagents react with oxetane, a four-membered cyclic ether, to yield primary alcohols, but the reaction is

Question:

Grignard reagents react with oxetane, a four-membered cyclic ether, to yield primary alcohols, but the reaction is much slower than the corresponding reaction with ethylene oxide. Suggest a reason for the difference in reactivity between oxetane and ethyleneoxide.