Typical radical traps (galvinoxyl, TEMPO, DPPH) react with palladium hydrides, sometimes at rates competitive with those of palladium hydride catalyzed reactions that follow an insertion mechanism (for example, alkene isomerization). Thus, positive results for radical reaction tests can be misleading. The complexes with more polarizable (neutral complexes rather than cationic) and more accessible hydrides, and the less sterically protected radical traps, react faster.

The neutral palladium complex [Pd(C6F5)Br(NCMe)2] (1) was found to effect the polymerization of acrylates upon addition of 1 equiv of a monodentate phosphine or pyridine or an excess of halide. Methyl methacrylate was not polymerized, and furthermore, its addition stopped the progress of independently initiated methyl acrylate polymerization in the phosphine-based system. Addition of ethene also inhibited the polymerization of methyl acrylate. However, over 10 mol % incorporation of 1-hexene in the polymer was achieved when the latter was added together with methyl acrylate. The polymerizaion mechanism is discussed.

[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.

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