Mechanistic Insights into Polar Monomer Insertion Polymerization from Acrylamides

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.