Relative reactivity of cyclic ethers, including epoxide, oxetane, tetrahydrofuran, and tetrahydropyran, was evaluated with their basicity and free energy of polymerization. The logarithm of relative reactivity, 1/rn, ofm-membered ring ethers withisubstituents ton-membered ring monomers withjsubstituents can be expressed by a linear combination of the differences in basicity, Δ(pKb)m·i-n·j, and in free energy, Δ(ΔG)m·i-n·j + γ