Abstract: The effect of tetramethyl ammonium, triethyl benzyl ammonium, tetrabutyl ammonium, tetrapentyl ammonium and tripentyl benzyl ammonium counterions on the block copolymerizations of a, a-dimethyl-β-propiolactone(PVL) initiated by poly(l,2-propandiol adipic acid ester) (PPA) terminating with carboxy pentanoyl end groups were studied. The average block efficiencies were obtained separately as 45% ,57%,7% ,87% and 78%. It was observed that the spectra of unblocked PPAfrom polymerization solutions were similar as that of the original PPAexcept small absorption bands of anhydride and carbon-carbon double bond at 1760 and 1610cm^-1.The mechanisms of these initiations were discussed. It was proposed that there existed two mechanisms of carboxylate ions attacking on alkoxy and acyl carbons of PVLat the same time in these initiations, the former mechanism gave the block copolymers and the latter gave the homopolymer through a chain transfer. Various quaternary ammonium counterions may vary the ratios of these two mechanism, thus may vary block efficiencies.