Polymerization of acrylates by neutral palladium complexes.: Isolation of complexes at the initial steps

The polymerization of methyl acrylate by pentafluorophenyl. complexes [Pd-2(mu-X)(2)(C6F5)(2)L-2] (L = tetrahydrothiophene (tht), X = Cl, 2; L = tht, X = Br, 3; L = AsPh3, X = Br, 4) gives atactic polymers in good yields. Mechanistic studies reveal that the polymerization of methyl acrylate starts by insertion of methyl acrylate in the Pd-aryl bond of the precatalyst to give the alkyl complexes trans- [Pd-2(mu-Cl)(2){CH(CO2Me)-CH2C6F5}(2)(tht)(2)] (5) and trans-[Pd-2-(mu-Cl)(2){CH(C6F5)CH2CO2Me}(2)(tht)(2)] (6). These complexes can be isolated, and the X-ray crystal structure of 5 has been determined. Complexes 5 and 6 decompose mainly by beta-H elimination but also by homolytic cleavage of the Pd-C bond in the light. In the presence of methyl acrylate, insertion of MA in hydrido-Pd species produces the alkyl complex trans[Pd-2(mu-Cl)(2){CH(CO2Me)CH3}(2)(tht)(2)] (9). Then a radical polymerization is initiated by small amounts of the radicals generated from these complexes (5, 6, or 9). Formation of 9 is the regeneration pathway of radicals after a termination reaction has occurred by recombination of the growing radical with palladium and beta-H elimination. The success of the polymerization requires a slow but steady supply of radicals by slow decomposition of alkyl complexes (5 and 6) or by slow generation of Pd-H species that provide new alkyl complexes (9), as well as an efficient recycling of the Pd-H generated in the termination step to 9.