Several alkali metal salts that activated 18-crown-6 were applied as initiators of beta-butyrolactone anionic ring-opening polymerization in tetrahydrofuran at room temperature. Some of them, i.e., Ph2PK, Ph3HBK, (Me3Si)(2)NK, and t-BuOK, deprotonate the monomer which results in macromolecules with trans-crotonate starting group. Other salts, for example, MeOK, i-PrOK, PhCH2OK, and CbK (Cb denotes carbazolyl group), deprotonate monomer or open its ring in the acyl-oxygen position. After ring-opening KOH forms as intermediate and initiates further polymerization. Ph3CK and monopotassium salt of ethylenediaminetetraacetic acid also deprotonate monomer and open its ring, however, in the alkyl-oxygen position. Monopotassium salts of glycolic acid, diglycolic acid, or malonic acid initiate polymerization mainly by ring opening in the alkyl-oxygen position. It results in the formation of polymers with two reactive terminal groups. The salts used react with the monomer as strong bases, nucleophilic bases, or nucleophiles. It determines unsaturation of the polymers obtained in the wide range of 2-100 mol%.