End-group evidence of zwitterionic species in the anionic polymerization of cyanoacrylates by Lewis bases

X-ray fluorescence (XRF) spectra of poly(butyl cyanoacrylate) initiated, in tetrahydrofuran, by triphenylphospine and terminated by SO₃, show the persistent presence of phosphorus and sulfur in the polymer, in near-equivalent proportions, with the sulfur content partially removable by anion-exchange. It is concluded that all chains have initial phosphonium groups, and, in abesence of any hydrolysis-formed H₂SO₄, are macrozwitterions with terminal  SO₃⁻. Acid-terminated chains should have proton-capped ends and initial phosphonium⁺ · HSO₄⁻ ion-pairs.