| Abstract
| - Polymerization of methyl acrylate (MA) by [Pd2(μ-X)2(C6F5)2(tht)2] (tht = tetrahydrothiophene) follows a radical mechanism and starts by insertion of MA to give 5 and 6. Pd−H species formed in the termination step reenter the catalytic cycle by insertion of MA to give a new alkyl derivative (9). These intermediates have been identified by NMR and, in the case of 5, by X-ray diffraction and afford the radicals initiating the polymerization.
- The polymerization of methyl acrylate by pentafluorophenyl complexes [Pd2(μ-X)2(C6F5)2L2](L = tetrahydrothiophene (tht), X = Cl, 2; L = tht, X = Br, 3; L = AsPh3, X = Br, 4) givesatactic polymers in good yields. Mechanistic studies reveal that the polymerization of methylacrylate starts by insertion of methyl acrylate in the Pd−aryl bond of the precatalyst togive the alkyl complexes trans-[Pd2(μ-Cl)2{CH(CO2Me)-CH2C6F5}2(tht)2] (5) and trans-[Pd2(μ-Cl)2{CH(C6F5)CH2CO2Me}2(tht)2] (6). These complexes can be isolated, and the X-raycrystal structure of 5 has been determined. Complexes 5 and 6 decompose mainly by β-Helimination but also by homolytic cleavage of the Pd−C bond in the light. In the presence ofmethyl acrylate, insertion of MA in hydrido−Pd species produces the alkyl complex trans-[Pd2(μ-Cl)2{CH(CO2Me)CH3}2(tht)2] (9). Then a radical polymerization is initiated by smallamounts of the radicals generated from these complexes (5, 6, or 9). Formation of 9 is theregeneration pathway of radicals after a termination reaction has occurred by recombinationof the growing radical with palladium and β-H elimination. The success of the polymerizationrequires a slow but steady supply of radicals by slow decomposition of alkyl complexes (5and 6) or by slow generation of Pd−H species that provide new alkyl complexes (9), as wellas an efficient recycling of the Pd−H generated in the termination step to 9.
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