The copolymerizations of maleic anhydride (MA) and propylene oxide (PO) were carried out in toluene, dioxane, diethoxyethane, or dimethyldigol solution in the presence of ethylzinc compounds EtZnX (X = Et- or substituted phenoxy group). On the basis of the1H-NMR data of copolymers and VPC of the products of their hydrolysis, it was found that donor solvents and X substituents involving CH3O-, Cl-, or CH3CO- donor groups in the ortho position of the aromatic ring (e.g., X = 2-CH3OC6H4O-, 2,6-(CH3O)2C6H3O-, 2-CH3COC6H4O-, etc.) caused an increase in the number of alternating sequences in copolymers chains. In the reaction of MA and PO in tetrahydrofuran (THF) solution, the above-mentioned zinc compounds gave highly alternating MA-PO-THF terpolymers.