Syntheses and properties of star- and dumbbell-shaped POSS derivatives containing isobutyl groups

Dumbbell-shaped isobutyl-substituted C2-linked polyhedral oligomeric silsesquioxane (POSS) ((1,2-bis(heptaisobutyl-T8-silsesquioxy)ethane) IBDE), C3-linked POSS ((1,3-bis(heptaisobutyl-T8-silsesquioxy)propane) IBDP) and C6-linked POSS ((1,6-bis(heptaisobutyl-T8-silsesquioxy)hexane) IBDH) were prepared by corner capping of heptaisobutyltricycloheptasiloxane trisilanol (IB7-OH) with 1,2-bis(trichlorosilyl)ethane, 1,3-bis(trichlorosilyl)propane and 1,6-bis(trichlorosilyl)hexane, respectively. The star-shaped POSS derivative ((octakis[3-(heptaisobutyl-T8-silsesquioxy) propyldimethylsiloxy]-Q8-silsesquioxane) 9POSIB) was prepared by hydrosilylation of heptaisobutylallyl-T8-silsesquioxane (IB7A1) and octadimethylsiloxy-Q8-silsesquioxane using Karstedt's catalyst. The star and dumbbell structures of the obtained compounds were confirmed by H-1-, C-13- and Si-29-nuclear magnetic resonance and matrix-assisted laser desorption ionization time-of-flight mass spectrometry analyses. With spin coating and subsequent baking at 120 degrees C, the star-shaped POSS compound formed an optical transparent film, but all of the dumbbell-shaped POSS compounds formed opaque whitish films. The refractive index of the transparent film was 1.4567, which was higher than that of a corresponding random silsesquioxane. 9POSIB showed significantly higher thermal stability than did the dumbbell-shaped POSS compounds. Thermogravimetric analysis of the star-shaped POSS derivative showed a clear baseline shift at 24 degrees C, indicative of the glass-transition temperature. However, IBDE, IBDP and IBDH showed no clear baseline shift. The melting points of IBDE, IBDP, IBDH and 9POSIB were observed at 231, 231, 239 and 96 degrees C, respectively. The lower melting point and appearance of the glass-transition point in 9POSIB suggested its lower crystallinity, which promotes the formation of a transparent film. Polymer Journal (2012) 44, 340-346; doi:10.1038/pj.2011.133; published online 21 December 2011