Syntheses and Structures of Bulky Monophosphine-Ligated Methylpalladium Complexes: Application to Homo- and Copolymerization of Norbornene and/or Methoxycarbonylnorbornene

Several new bulky monophosphine-ligated methylpalladium complexes were synthesized. Treatment
of Pd(cod)MeCl (cod = cyclooctadiene) with bulky phosphines, PtBu3, or P(o-tol)3 afforded monophosphine-ligated methylpalladium chloride complexes 1a and 1b, which were isolated as crystalline solids
in excellent yields. The structures depended on the shape of the ligand:  a mononuclear three-coordinate
structure for 1a and a chloride-bridged dinuclear structure for 1b. Absorption of chloride from
methylpalladium chloride complexes 1a and 1b afforded the corresponding methylpalladium triflate
complexes 2a and 2b, which were structurally characterized by NMR spectroscopy and X-ray
crystallography. In the presence of NaB[3,5-(CF3)2C6H3]4, 1a and 1b catalyzed the addition homopolymerization of norbornene or methoxycarbonylnorbornene. The activity of the catalyst was found to be
dependent on the structure of the complexes and the counteranions. The catalyst system 1a-NaB[3,5-(CF3)2C6H3]4 was also active for the copolymerization of norbornene and methoxycarbonylnorbornene
to form a block-type copolymer. The obtained copolymer has the highest ester content probably due to
the two vacant sites on the palladium. The molecular weight distribution reached the narrowest value,
indicating the long lifetime of the catalyst.