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The dehydrocoupling of the fluorinated secondary phosphine-borane adduct R2PH.BH3 (R = p-CF3C6H4) at 60 degrees C is catalyzed by the rhodium complex [{Rh(mu-Cl)(1,5-cod)}2] to give the four-membered chain R2PH-BH2-R2P-BH3. A mixture of the cyclic trimer [R2P-BH2]3 and tetramer [R2P-BH2]4 was obtained from the same reaction at a more elevated temperature of 100 degrees C. The analogous rhodium-catalyzed dehydrocoupling of the primary phosphine-borane adduct RPH2.BH3 at 60 degrees C gave the high molecular weight polyphosphinoborane polymer [RPH-BH2]n (Mw = 56,170, PDI = 1.67). The molecular weight was investigated by gel permeation chromatography and the compound characterized by multinuclear NMR spectroscopy. Interestingly, the electron-withdrawing fluorinated aryl substituents have an important influence on the reactivity as the dehydrocoupling process occurred efficiently at the mildest temperatures observed for phosphine-borane adducts to date. Thin films of polymeric [RPH-BH2]n (R = p-CF3C6H4) have also been shown to function as effective negative-tone resists towards electron beam (e-beam) lithography (EBL). The resultant patterned bars were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS).

The polyphosphinoboranes [PhPH−BH2] n (1), [iBuPH−BH2] n (2), [(p-nBuC6H4)PH−BH2] n (3), and [(p-dodecylC6H4)PH−BH2] n (4) were prepared from the corresponding phosphine−borane adducts RPH2·BH3 (R = Ph, iBu, p-nBuC6H4, p-dodecylC6H4) via a rhodium-catalyzed dehydrocoupling procedure at elevated temperatures (ca. 90−130 °C). Samples of polymers 1 and 2 and the new materials 3 and 4 were characterized by multinuclear NMR spectroscopy, and the molecular weights were determined by light scattering methods in THF or CH2Cl2 solutions. The absolute weight-average molecular weight of 2 was determined by static light scattering and found to be M w = 13 100, and values of M w of ca. 20 000 were estimated for the sample of polymers 1 and the new material 3 using dynamic light scattering (DLS). The molecular weights of polymers 3 and 4 were also analyzed by gel permeation chromatography using polystyrene standards, and values up to M w = ca. 80 000, M n = ca. 10 000 were determined for 3 and M w = ca. 168 000, M n = ca. 12 000 for 4. The chemical stability of 1 in THF toward HNEt2 or nBu3P was demonstrated using NMR spectroscopy and DLS analysis, which indicated that no significant polymer degradation occurred. WAXS analysis of 1 and 3 showed that the polymers are amorphous. The glass transition temperatures (T g) of polymers 2, 3, and 4 were analyzed by DSC and were detected at ca. 5, 8, and −1 °C, respectively. TGA analysis on 1−3 revealed T 5% values (temperature for which 5% or the weight is lost) of 240 °C for 1 and ca. 150−160 °C for 2 and 3. After heating to 1000 °C, ceramic yields in the range of 35−80% were obtained. The high ceramic yield for 1 (75−80%) indicates that this material is of interest as a pyrolytic precursor to boron phosphide-based solid-state materials.

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