Mechanistic Insights into Polar Monomer Insertion Polymerization from Acrylamides

<i>N-</i>Isopropyl acrylamide (NIPAM), <i>N</i>,<i>N-</i>dimethyl acrylamide (DMAA), and 2-acetamidoethyl acrylate (AcAMEA) were copolymerized with ethylene employing [(P^O)­PdMe­(DMSO)] (<b>1-DMSO</b>; P^O = κ²-<i>P</i>,<i>O</i>-Ar₂<i>P</i>C₆H₄SO₂<i>O</i> with Ar = 2-MeOC₆H₄) as a catalyst precursor. Inhibition studies with nonpolymerizable polar additives show that reversible κ-<i>O</i>-coordination of free amide retards polymerization significantly. Retardation of polymerization increases in the order ethyl acetate ≪ methyl ethyl sulfone < acetonitrile < <i>N</i>,<i>N</i>-dimethylacetamide ≈ <i>N</i>-methylacetamide ≈ propionic acid < dimethylsulfoxide. Pseudo-first-order rate constants for the insertion into <b>1-DMSO</b> were determined to increase in the order DMAA < AcAMEA < NIPAM < methyl acrylate. Exposure of <b>1-DMSO</b> to NIPAM resulted in the formation of consecutive insertion products [(P^O)­Pd­(C₆H₁₁NO₂)<i>_n</i>Me] (<i>n</i> ≤ 3), as determined by electrospray ionization mass spectrometry. The solid-state structure of the methanol adduct of the 2,1-insertion product of NIPAM into <b>1-DMSO</b>, [(P^O)­Pd­{η¹-<i>C</i>H­(CONHiPr)­CH₂CH₃}­(κ¹-<i>O-</i>MeOD)] (<b>2-MeOD</b>), was determined by single crystal X-ray diffraction. Both 2,1- and 1,2-insertions of DMAA into the Pd–Me bond of a [(P^O)­PdMe] fragment occur to afford a ca. 4:1 mixture of chelates [(P^O)­Pd­{κ²-<i>C</i>,<i>O–C</i>(CH₂CH₃)­C­(<i>O</i>)­NMe₂}] (<b>3</b>) and [(P^O)­Pd­{κ²-<i>C</i>,<i>O–C</i>H₂C­(CH₃)­C­(<i>O</i>)­NMe₂}] (<b>4</b>). The four-membered chelate of <b>3</b> is opened by coordination of 2,6-lutidine (<b>3</b> + 2,6-lutidine ⇌ <b>3-LUT</b>) with Δ<i>H</i>° = −41.8(10.5) kJ and Δ<i>S</i>° = −115(37) J mol^–1 K<sup>–1