Ruthenium-catalyzed ring-opening polymerization syntheses of poly(organodecaboranes): New single-source boron-carbide precursors

Ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) of 6-R-B10H13 organodecaboranes containing strained-ring cyclic olefinic substituents has been found to be an important new method of Generating poly(organodecaborane) polymers with higher molecular weights than previously attainable. The monomers, 6-(5-cyclooctenyl)-B10H13 (1), 6-(5-norbornenyl)-B10H13 (2), and 6-(4-cyclohexenyl)-B10H13 (3), were synthesized via the titanium-catalyzed decaborane hydroboration of 1 equiv of 1,5-cycloctadiene, 2,5-norbornadiene, and 1,4-cyclohexadiene, respectively. The syntheses of the saturated, linked-cage compounds 6,6'-(1,5-cyclooctyl)-(B10H13)(2) (4) and 6,6'-(2,5-norbornyl)-(B10H13)(2) (5) were also achieved by either the titanium-catalyzed decaborane hydroboration of the remaining double bond of 1 or 2 or the titanium-catalyzed reactions of 1,5-cycloctadiene and 2,5-norbornadiene with an excess amount of decaborane. ROMP of 1 and 2 using either of the Grubbs catalysts, Cl2Ru(=CHPh)(PCy3)L, L = PCy3 (I) or H(2)IMes (II), afforded the poly(6-cyclooctenyldecaborane) (PCD, 6) and poly(6-norbornenyldecaborane) (PND, 7) polymers. Molecular weights with M-n in excess of 30 kDa were readily obtained with polydispersities between 1.1 and 1.8. Both polymers are stable powders that are soluble in polar organic solvents. Studies of the ceramic conversion reactions of 6 and 7 using TGA, XRD, DRIFT, Raman, SEM, elemental analyses, and density measurements showed that they convert to boron-carbide/carbon ceramics upon pyrolysis. In accordance with their higher boron-to-carbon ratios, studies of the ceramic conversion reactions of compounds 4 and 5 showed them to be excellent single-source molecular precursors to boron-carbide ceramics with little or no excess carbon.