Mechanistic insights into polar monomer insertion polymerization from acrylamides

Abstract

Figure Persented: 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 = κ 2-P,O- Ar 2PC 6H 4SO 2O with Ar = 2-MeOC 6H 4) 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(C 6H 11NO 2) 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{η 1-CH(CONHiPr)CH 2CH 3} (κ 1-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{κ 2-C,O-C(CH 2CH 3)C(O)NMe 2}] (3) and [(P∧O)Pd{κ 2-C,O-CH 2C(CH 3)C(O)NMe 2}] (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. © 2011 American Chemical Society.