A study was conducted to investigate the differences in reactivity between ethyl cyanoacrylate (ECA) with phosphines and amines, which contain different alkyl substituents. It was found that when an equimolar amount of dimethylphenylphosphine and ECA react, a stable zwitterion is formed. This is the first time the proposed initiating species for alkyl cyanoacrylate polymerization has been sufficiently stable to be isolated and fully characterized spectroscopically. In contrast, triphenylphoshine reacts with ECA to form polymer, regardless of the molar ratio between the monomer and initiator. The reactivity between primary, secondary, and tertiary amines and ECA also exhibit significant differences. Tertiary amines initiate rapid ECA polymerization with a strong exotherm to produce high molecular weight polymers. In contrast, the reaction of ECA with primary or secondary amines occurs at a much slower rare resulting in low molecular weight oligomers or polymers. After a H-1 NMR and LR spectroscopic study, it was demonstrated that intramolecular proton transfer occurs after the initial Michael-type addition of the primary or the secondary amine to the ECA double bond to form aminocyanopropionate esters. The differences in reactivity among the three classes of amines with ECA can now be attributed to the initial formation of aminocyanopropionate esters for primary and secondary amines and only polymer for tertiary amines. (C) 2001 Elsevier Science Ltd. All rights reserved.