1.2-polybutadiene complexes with d8 pseudo-square- planar transition-metal salts based on nickel(II), palladium(II), and platinum(II)

The effects of four d(8) transition-metal complexes from Group 10 on the thermal, mechanical, optical, and spectroscopic properties of atactic 1,2-polybutadiene are compared, in addition to their ability to induce gelation. Olefin coordination and subsequent metal-catalyzed chemical crosslinking occur much more quickly, and to a greater extent, at ambient temperature with PdCl2(CH3CN)(2) than with PtCl2(C6H5CN)(2). Alkene side groups in the polymer attack the pseudo-square-planar metal center (i.e., Pd2+ or Pt2+) from above or below the plane of the coordinatively unsaturated low-molecular-weight organometallic complex and displace neutral acetontrile or benzonitrile ligands via an associative mechanism. Gelation occurs much more quickly with Pd2+ than with Pt2+, and the ambient-temperature elastic modulus of solid polybutadiene/palladium. complexes increases significantly, without high-temperature annealing, so that a weak rubbery polymer is transformed into a glass via 3 mol % Pd2+. Alkene functional groups in the side chain of the polymer do not coordinate to bis(dimethyl)glyoximatonickel(II) at ambient temperature because (1) it is difficult to displace anionic dimethylgloxime ligands that are bidentate; (2) these planar nickel complexes with C-2h, symmetry are stacked along the c axis via interlocking methyl groups on adjacent molecules; and (3) there is a lack of pi back-bonding between d(xy) on Ni(II) and empty pi antibonding orbitals of C=C, which typically stabilizes olefin complexes with pseudo-square-planar d(8) metal centers. Pseudo-octahedral nickel(H) chloride hexahydrate does not form a complex with the polymer, in agreement with some macroscopic properties of these materials. The observed trend in the transition-metal-modified properties of atactic 1,2-polybutadiene in the solid state and in the gel state is Pd(II) > Pt(II) much greater than Ni(II). (C) 2004 Wiley Periodicals, Inc.