Various ethers were used to mediate the polymerizations of 1,3-pentadiene (PD) initiated by AlCl3 and by allyl chloride (AllyCl)/AlCl3. The introduction of the ethers exert considerable effects on polymer yield and molecular weight due to its interaction with the propagating carbocation. The carbocation reactivity is reduced by this interaction which is subject to the ether’s nucleophilicity determined by the steric hindrance of groups adjacent to oxygen. The reduction of carbocation reactivity gives rise to a decrease of polymer yield owing to inhibition of propagation but results in an augmentation of molecular weight due to suppression of various side reactions such as terminations. By using suitably nucleophilic ethers such as diphenyl ether, the polymerization can be mediated to give an high molecular weight polymer in high yield.