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Mechanistic Insights into Polar Monomer Insertion Polymerization from Acrylamides

Friedberger, Tobias, Wucher, Philipp, Mecking, Stefan
Journal of the American Chemical Society 2012 v.134 no.2 pp. 1010-1018
X-ray diffraction, acetonitrile, acrylamides, additives, catalysts, chelates, dimethyl sulfoxide, electrospray ionization mass spectrometry, ethyl acetate, ethylene, methanol, polymerization, propionic acid
N-Isopropyl acrylamide (NIPAM), N,N-dimethyl acrylamide (DMAA), and 2-acetamidoethyl acrylate (AcAMEA) were copolymerized with ethylene employing [(P^O)PdMe(DMSO)] (1-DMSO; P^O = κ²-P,O-Ar₂PC₆H₄SO₂O with Ar = 2-MeOC₆H₄) as a catalyst precursor. Inhibition studies with nonpolymerizable polar additives show that reversible κ-O-coordination of free amide retards polymerization significantly. Retardation of polymerization increases in the order ethyl acetate ≪ methyl ethyl sulfone < acetonitrile < N,N-dimethylacetamide ≈ N-methylacetamide ≈ propionic acid < dimethylsulfoxide. Pseudo-first-order rate constants for the insertion into 1-DMSO were determined to increase in the order DMAA < AcAMEA < NIPAM < methyl acrylate. Exposure of 1-DMSO to NIPAM resulted in the formation of consecutive insertion products [(P^O)Pd(C₆H₁₁NO₂)ₙMe] (n ≤ 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 1-DMSO, [(P^O)Pd{η¹-CH(CONHiPr)CH₂CH₃}(κ¹-O-MeOD)] (2-MeOD), 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-C,O–C(CH₂CH₃)C(O)NMe₂}] (3) and [(P^O)Pd{κ²-C,O–CH₂C(CH₃)C(O)NMe₂}] (4). The four-membered chelate of 3 is opened by coordination of 2,6-lutidine (3 + 2,6-lutidine ⇌ 3-LUT) with ΔH° = −41.8(10.5) kJ and ΔS° = −115(37) J mol–¹ K–¹.