Tailored cationic palladium(II) compounds as catalysts for highly selective dimerization and polymerization of vinylic monomers: Synthetic and mechanistic aspects

Article abstract of DOI:10.1021/om00028a066

The electrophilic palladium(II) compounds Pd(2,6-di-tert-butylpyridine)₂(CH₃NO₂) ₂(BF₄)₂ (1), Pd(PPh₃)₂(BF₄)₂ (2), and Pd(Ph₂PCH₂CH₂PPh₂)(BF ₄)₂ (3) were found to be highly selective catalysts for the acyclic dimerization of vinyl monomers (styrene, ethylene, and propylene) and the linear polymerization of p-divinylbenzene, the reactions proceeding through a carbocationic mechanism. One manifestation of the high selectivity was the ability to catalyze the dimerization of ethylene in the presence of propylene and styrene in the presence of α-methylstyrene even though the second member of each pair was normally significantly more reactive in reactions involving carbocationic intermediates. The linear polymerization of p-divinylbenzene involved in step-growth mechanism. The synthesis of a telechelic polymer through cross-coupling between the terminal vinyl groups of linear poly(p-divinylbenzene) and the vinyl group of a functionalized styrene derivative was also achieved. The reaction rates for the linear dimerization of styrene and the linear polymerization of p-divinylbenzene were found to be first order in the monomer concentration and fractional order in the catalyst concentration. Mathematical modeling indicated that the fractional order in the catalyst concentration was due to preequilibria involving anion dissociation from the metal center, and actually, the catalytic species in the case of 2 was found to be the dication, Pd(PPh₃)₂²⁺.