Resin Transfer Moldable Fluorinated Phenylethynyl-Terminated Imide Oligomers with High Tg: Structure-Melt Stability Relationship

Phenylethynyl-terminated aromatic polyimides meet requirements of resin transfer molding (RTM) and exhibits high glass transition temperature (T_g) were prepared. Moreover, the relationship between the polyimide backbones structure and their melting stability was investigated. The phenylethynyl-terminated polyimides were based on 4,4′-(hexafluorosiopropylidene)-diphthalic anhydride (6FDA) and different diamines of 3,4′-oxydianiline (3,4′-ODA), m-phenylenediamine (m-PDA) and 2,2′-bis(trifluoromethyl)benzidine (TFDB) were prepared. These oligoimides exhibit excellent melting flowability with wide processing temperature window and low minimum melt viscosities (<1 Pa·s). Two of the oligoimides display good melting stability at 280–290 °C, which meet the requirements of resin transfer molding (RTM) process. After thermally cured, all resins show high glass transition temperatures (T_gs, 363–391 °C) and good tensile strength (51–66 MPa). The cure kinetics studied by the differential scanning calorimetry (DSC), ¹³C nuclear magnetic resonance (¹³C NMR) characterization and density functional theory (DFT) definitely confirmed that the electron-withdrawing ability of oligoimide backbone can tremendously affect the curing reactivity of terminated phenylethynyl groups. The replacement of 3,4′-ODA units by m-PDA or TFDB units increase the electron-withdrawing ability of the backbone, which increase the curing rate of terminated phenylethynyl groups at processing temperatures, hence results in the worse melting stability.

Radical trans-Hydroboration of Alkynes with N-Heterocyclic Carbene Boranes

Hydroboration of internal alkynes with N-heterocyclic carbene boranes (NHC-boranes) occurs to provide stable NHC (E)-alkenylboranes upon thermolysis in the presence of di-tert-butyl peroxide. The E isomer results from an unusual trans-hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical-chain reaction involving addition of an NHC-boryl radical to an alkyne to give a β-NHC-borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC-borane provides the product and returns the starting NHC-boryl radical. Experiments suggest that the observed trans-selectivity results from kinetic control in the hydrogen-transfer reaction.

Thermal polymerization of arylacetylenes: spectroscopic and thermal studies

The reaction mechanisms and kinetics of endcapped ethynyl polyethers were studies in the temperature range 150-220C. The results obtained by '3C and 'H NMR, FrTIR, HPLC and DSC indicate reactivity differences and competing reactions. These materials were cured and their thermal properties evaluated.