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.
Statistical copolymers of N-isopropyl acrylamide (NIPAM) and N,N-diethyl acrylamide (DEAAM) show a pronounced synergistic depression in their cloud points, though both homopolymers phase separate at significantly higher temperatures close to 30 °C (e.g., Polymer 2009, 50, 519). While phase separation occurs at 20 °C for the statistical copolymers, the influence of the monomeric sequential arrangement along the backbone was not addressed so far. Thus, we report on the thermosensitive properties of a diblock copolymer PDEAAM-b-PNIPAM and compare it to the homopolymers, mixtures thereof, and to the statistical copolymer of the same molecular weight. These polymers were prepared by controlled radical polymerization, namely Reversible Addition− Fragmentation Chain Transfer (RAFT). Their solution behavior was mainly studied by infrared spectroscopy (IR) of the amide I′ band and by turbidimetry. IR spectroscopy sees a decreasing hydration with heating even below the cloud point for all polymers. This results finally in phase separation, which induces further spectral changes. Rather unexpectedly, the diblock copolymer shows phase separation at temperatures close to the homopolymers, well above the cloud points of the homopolymer mixtures. In turn, the transition temperature of the homopolymer mixture is reduced compared to its homopolymers, which indicates intermolecular attraction between both partners. This behavior can be explained by taking the block length dependencies of the respective cloud points into account and assuming a rather independent phase behavior of each short block (within the copolymer). Then, the increased inherent cloud point of each "half-length" block (compared to the homopolymers) has a stronger effect than the aggregating tendency inherited by the connectivity of the comonomer units. As a result, IR spectroscopy reveals almost ideal behavior of the diblock copolymer, which can be comprehended as an ideal mixture of the homopolymers, each one contributing to the overall signal by its concentration. Finally, 1 H NMR suggests that intermediate aggregation (as seen by light scattering) is not induced by segregation of just one block, but rather by partial and weak complexation between the two components within the diblock copolymer.
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).
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