Dual Behavior of Cationic Palladium Pentafluorophenyl Complexes as Catalysts for the Homopolymerization of Acrylates and of Nonpolar Olefins

Abstract

[Pd(Pf)(NCMe)(N−N)]BF4, (Pf = C6F5; N−N = ethylenediamine (en, 1), 2,2′-bipyridine (2), (2,6-iPrC6H3)NC(Me)C(Me)N(2,6-iPrC6H3) (diimine, 3)) are very efficient initiators of the radical polymerization of alkyl acrylates. Mechanistic studies reveal that the polymerization of methyl acrylate with 1 starts by the insertion of the monomer into the Pd−aryl bond of the catalyst to give [Pd{CH(CO2Me)CH2Pf}(N−N)(NCMe)]BF4, (N−N = en (5), diimine (7)). In contrast to the case for other cationic palladium complexes, the presence of a Pf group does not allow the formation of C,O-palladacycles with a suitable ring size. This allows complex 5 (or 7) to decompose not only by β-H elimination but also by homolytic cleavage of the Pd−C bond in the light. The radical species generated in this process start the polymerization. Complex 1 is also a very efficient catalyst in the vinyl addition polymerization of norbornene by an insertion mechanism. Complex 3 polymerizes ethylene. Although complexes 1 and 3 are versatile polymerization initiators by two different mechanisms, neither copolymerization of methyl acrylate and norbornene with 1 nor that of methyl acrylate and ethylene with 3 were successful, leading to either very low yields or mixtures of homopolymers.

Bis(β‐ketoamino)copper(II)/methylaluminoxane systems for homo‐ and copolymerizations of methyl acrylate and 1‐hexene

“…This explains our inability to detect the radical by EPR. Similar pathway for radical generation has been observed in cobalt‐mediated radical polymerization45 and palladium‐mediated radical polymerization 11, 39, 46–49. The actual polymerization occurs by successive addition of acrylate monomer to the alkyl radical.…”

“…Authors presented that homopolymerization of MA and copolymerization of MA and 1‐hexene proceed by the insertion‐triggered radical polymerization 48, 49. Microstructures of PMA and MA‐hexene copolymer produced by palladium‐based system11, 39, 46–49 are very similar to those produced by bis(β‐ketoamino)copper(II)/MAO system. Therefore, it is believable that homopolymerization and copolymerization of MA with 1‐hexene proceed through insertion‐triggered radical mechanism.…”

“…Radical mechanism of the palladium‐based system for acrylate monomer polymerization has been proved 46–49. Albeniz and coworkers monitored the insertion of MA into the PdC 6 F 5 bond and homolytic cleavage of the new PdC bond with NMR.…”

Palladium(ii) complexes with pentafluorophenyl ligands: structures, C6F5 fluxionality by 2D-NMR studies and pre-catalysts for the vinyl addition polymerization of norbornene

“…1- 3 Some examples of palladium(II) complexes bearing N,Nchelate ligands have been reported for the vinyl polymerization of norbornene. 1,2,3,5,11, 12 At the same time, the pentafluorophenyl group with the 19 F nucleus is an excellent probe for molecular dynamics and for pre-catalyst activation mechanism by 19 F-NMR spectroscopy. 13 Generally, the species in MAO-activated Ni-or Pd-complexes and initiation steps for norbornene polymerization are not well known.…”

Geometry Constrained N-(5,6,7-Trihydroquinolin-8-ylidene)arylaminopalladium Dichloride Complexes: Catalytic Behavior toward Methyl Acrylate (MA), Methyl Acrylate-co-Norbornene (MA-co-NB) Polymerization and Heck Coupling

“…Polymerization was totally hindered in the absence of ambient light, confirming the free radical mechanism of our catalytic system. According to the mechanistic studies of Albéniz et al, free radicals were produced by light induced-homolytic cleavage of the Pd-C bond after the incorporation of one MA unit into the Pd-C 6 F 5 bond of 1 [60]. A similar mechanistic pathway was also proposed by Ye et al [35].…”