Synthesis of "meso"-and "racemic"-like diastereomers of Me2Si(3-MeInd)(Ind)ZrCl2 (5 and 6, respectively) was achieved through either metathetical reactions between the dianion of Me2(3-MeIndH)-(IndH) (4) with ZrCl4 or via amine elimination reactions, followed by fractional crystallization. Propylene polymerizations using meso-5 in the presence of methyl aluminoxane under a variety of conditions leads to the formation of low molecular weight, semicrystalline, low tacticity, poly(propylene) (PP). The dominant chain transfer mechanism in this case is shown to involve -H transfer to monomer. In contrast, rac-6 provides higher molecular weight, semicrystalline, elastomeric poly(propylene) (elPP) under a variety of conditions; chain transfer in this case involves, predominantly, -H transfer to Zr. The properties of elPP produced using catalyst 6 show a gradual change from a lightly, cross-linked elastomer to a poorly crystalline thermoplastic, depending on both polymer molecular weight and crystallinity as revealed by differential scanning calorimetry and tensile testing. In particular, more crystalline material exhibits a higher initial modulus, yielding behavior and lower strain to break than less crystalline material of equivalent molecular weight. These findings further define polymer properties for the synthesis of flexible elastomers using this class of catalysts.

The synthesis and characterization, including crystallographic analysis, of the bifunctional boranes RCHdC[B(C 6 F 5 ) 2 ] 2 (R ) t-C 4 H 9 , 1a; C 6 H 5 , 1b; C 6 F 5 , 1c) by regioselective hydroboration of the corresponding 1-boraalkynes using HB(C 6 F 5 ) 2 are reported herein. Compounds 1a and 1b have been screened as cocatalysts for ethylene polymerization in the presence of Cp 2 ZrMe 2 (3) under a variety of conditions. NMR spectroscopic studies indicate that Cp 2 -Zr[η 2 -Bu t CtCB(C 6 F 5 ) 2 ] (4a), Cp 2 ZrMe(C 6 F 5 ), the organoborane Me 2 BC 6 F 5 , and methane gas are the final products formed from reaction of 1a with 3 in toluene solution at room temperature. The stoichiometric mechanism for this transformation has been elucidated through variable-temperature NMR studies. Complex 4a and MeB(C 6 F 5 ) 2 (7) were prepared independently and screened as ethylene polymerization catalysts and cocatalysts, respectively. Compound 4a is inactive for ethylene polymerization, either alone or in the presence of additional 1a. However, the combination of Cp 2 ZrMe 2 and 7 gives rise to the species [Cp 2 ZrMe] + [Me 2 B(C 6 F 5 ) 2 ] -(8), which although unstable at room temperature in solution (decomposing over a period of 60 min to give Cp 2 ZrMe(C 6 F 5 ) and the organoborane Me 2 -BC 6 F 5 ), is active for ethylene polymerization. From a comparison of activity and MW data, it is concluded that the putative ion pairs formed from 1a (or 1b) and 3 lack sufficient thermal stability at conventional polymerization temperatures and that the polymerization activity observed can be interpreted as arising from species 8.