Copolymerization of Ethylene with Acrylate Monomers by Amide-Functionalized α‑Diimine Pd Catalysts

We report the ethylene
homopolymerization and ethylene/methyl-acrylate
(MA) and ethylene/acrylic-acid (AA) copolymerization behavior of a
series of (N,N′-diaryl-α-diimine)­Pd
catalysts that contain secondary amide (−CONHMe) or tertiary
amide (−CONMe2) substituents on the N-aryl rings,
including the “first-generation” catalysts {(2,6-iPr2-Ph)­NCMeCMeN­(2-CONHMe-6-iPr-Ph)}­PdMeCl (1a,a′) and {(2,6-iPr2-Ph)­NCMeCMeN­(2-CONMe2-6-iPr-Ph)}­PdMeCl (1b,b′) and the “second-generation”
catalysts [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2-CONHMe-6-iPr-Ph)]­PdMeCl (1d,d′) and [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2-CONMe2-6-iPr-Ph)]­PdMeCl (1e,e′). Activation of 1d,d′ and 1e,e′ by NaB­{3,5-(CF3)2C6H3}4 generates
active ethylene polymerization catalysts that produce highly branched
(77–81 br/1000 C) polyethylenes with number-average molecular
weights (Mns) in the range 26–60
kDa. The replacement of two isopropyl units in 1a,a′ and 1b,b′ with
benzhydryl groups in 1d,d′ and 1e,e′ leads to a significant improvement
in ethylene homopolymerization performance. The secondary amide-functionalized
catalyst 1d,d′ incorporates ca. twice
as much MA and ca. three times as much AA as the iPr-substituted catalyst [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2,6-iPr2-Ph)]­PdMeCl (1f,f′)
in copolymerization with ethylene. The reactions of 1a,a′ and 1b,b′
with metal salts that contain weakly coordinating anions lead to extrusion
of CH4 and the formation of [{(μ-κ2-N,N′,κ-O-α-diimine)­Pd}2(μ-CH2)]2+ complexes, in which
the amide carbonyl O atoms coordinate to Pd centers.