RESUMO
Poly(acrylonitrile) (PAN) fibers have two essential drawbacks: they are usually processed by solution-spinning, which is inferior to melt spinning in terms of productivity and costs, and they are flammable in air. Here, we report on the synthesis and melt-spinning of an intrinsically flame-retardant PAN-copolymer with phosphorus-containing dimethylphosphonomethyl acrylate (DPA) as primary comonomer. Furthermore, the copolymerization parameters of the aqueous suspension polymerization of acrylonitrile (AN) and DPA were determined applying both the Fineman and Ross and Kelen and Tüdõs methods. For flame retardancy and melt-spinning tests, multiple PAN copolymers with different amounts of DPA and, in some cases, methyl acrylate (MA) have been synthesized. One of the synthesized PAN-copolymers has been melt-spun with propylene carbonate (PC) as plasticizer; the resulting PAN-fibers had a tenacity of 195 ± 40 MPa and a Young's modulus of 5.2 ± 0.7 GPa. The flame-retardant properties have been determined by Limiting Oxygen Index (LOI) flame tests. The LOI value of the melt-spinnable PAN was 25.1; it therefore meets the flame retardancy criteria for many applications. In short, the reported method shows that the disadvantage of high comonomer content necessary for flame retardation can be turned into an advantage by enabling melt spinning.
RESUMO
The thermoresponsive and pH-sensitive behavior of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA), and poly(N,N-diisopropylaminoethyl methacrylate) (PDiPAEMA) is compared by use of different techniques. We employed temperature- and pH-dependent turbidimetry, fluorescence spectroscopy (of the polarity indicator 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, 4HP, which is sometimes also abbreviated as DCM), and IR spectroscopy (of the carbonyl band). Within specific pH windows, all polymers showed phase separation at elevated temperatures (showing a lower critical solution temperature behavior, an LCST behavior). By increasing the hydrophobicity of the dialkylaminoethyl substituent, the phase separation is shifted to lower pH (at constant temperatures; pH(PDMAEMA) > pH(PDEAEMA) > pH(PDiPAEMA)) or to lower temperatures (at constant pH; T(PDMAEMA) > T(PDEAEMA) > T(PDiPAEMA)). While PDMAEMA does not exhibit pronounced changes in polarity upon phase separation (as seen by fluorescence spectroscopy), PDEAEMA and PDiPAEMA provide a nonpolar surrounding for the 4HP uptake above their collapse. In addition, PDiPAEMA causes the sharpest transition (as seen by the 4HP probe), although the carbonyl hydration experiences a more gradual (sigmoidal) transition for all polymers (as seen by IR). These observations allow a distinction of the phase separation mechanisms. While the LCST properties of PDMAEMA are mainly caused by backbone/carbonyl interactions, its rather polar dimethylaminoethyl group does not inflict pronounced hydrophobicity, but promotes a higher water content within the phase-separated polymer. In contrast, the phase separation of PDEAEMA and PDiPAEMA is mainly influenced by the less polar dialkylaminoethyl groups, leading to drastic changes in the hydrophobicity around the cloud points. Further, the IR data suggest that the diisopropylaminoethyl groups of PDiPAEMA tend to backfold to the carbonyl groups/backbone to minimize water-polymer contact already in its soluble state. Finally, this study might lead to advanced lasing applications of the laser dye 4HP.
RESUMO
A novel robust pyridine-bridged bis-benzimidazolylidene nickel pincer complex 3 accessible from inexpensive, commercially available precursors efficiently catalyzes the first practical Suzuki-Miyaura cross-coupling reactions with various less-reactive electrophiles ArX (X = Br, Cl, OTs and OMs) and even tolerates electron-rich, sterically demanding and heterocyclic arenes in the presence of catalytic amounts of PPh(3).
