No abstract

We prepared a series of dendritic−linear block copolymers (DLBCPs) bearing a semirigid Percec-type dendron with ionophilic poly(ethylene glycol) (PEG) tails and a polystyrene (PS) linear polymer by nitroxide-mediated living radical polymerization (NMRP). As the DLBCPs are connected by an ester linkage, through hydrolysis the molecular weights of the DLBCPs were precisely characterized by gel permeation chromatography and MALDI-TOF MS. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to investigate the phase behaviors of the DLBCPs. Results show that the PEG tails of the semirigid dendron display a cold crystallization peak and a melting peak during the second heating process, while for the neat DLBCPs, the crystallization of the PEG tails is completely inhibited, and only the glass transition temperature (T g ) of the PS block is observed. However, T g of the dendron block can be observed by complexing the DLBCPs with LiCF 3 SO 3 , suggesting that microphase separation occurs in the doped DLBCPs. Comparing the phase behaviors of the DLBCPs having the same dendron weight fraction (w D = 0.14) with varying dendron generation and salt concentration, we found that the G 1 or G 2 DLBCP undergoes a phase transition from a hexagonally packed cylinder structure to a lamellar structure with increasing content of LiCF 3 SO 3 . However, the G 3 DLBCP only displays a lamellar phase, and the lamellar thickness increases with increasing salt concentration. The difference can be attributed to the different degree of chain branching, which leads to different interface curvature.

Mg–Al oxides were prepared by facile solvent-free grinding of Mg–Al layered double hydroxide and were used as catalysts for continuous-flow transesterification of dimethyl carbonate and ethanol. The surface and structure properties of the prepared Mg–Al oxides were characterized by X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectra, and N2 adsorption–desorption. Moreover, the acid–base properties of the prepared catalysts were characterized using NH3-TPD, CO2-TPD, and pyridine-IR. The Mg–Al oxides prepared using the solvent-free method with a molar ratio of 2.0 (CHT-2-SF) exhibit good catalytic performance, which has medium basic sites, weak acidic sites, and high Brunauer–Emmett–Teller surface areas. A correlation between basicity–acidity and catalytic activity demonstrates that the conversion of ethanol increases with increasing medium basicity and weak acidity of the catalyst. The activity of CHT-2-SF remained almost unchanged after 1800 h on stream.

Amphiphilic rod−coil dendritic−linear block copolymers PEG(G m )-b-PMPCS (where m is the number of dendron generation, and m = 1, 2, 3) composed of a semirigid Percec-type dendron with hydrophilic poly(ethylene glycol) (PEG) tails and a rodlike mesogen-jacketed liquid crystalline polymer, poly{2,5-bis[(4′methoxy-phenyl)oxycarbonyl]styrene} (PMPCS), were successfully prepared. The self-assembled structures undergo a transition from vesicles through large compound vesicles (LCVs) to short cylindrical micelles with increasing dendron generation. PEG(G 2 )-b-PMPCS forms stable LCVs with porous surfaces of a narrow size distribution in a mixed solvent of tetrahydrofuran and water. The formation mechanism of the supramolecular structure with nano-and microsized scales is studied through changing the rate of water addition. It is composed of two steps: morphological transformation and vesicles fusion or differentiation. Vesicles are precursors for LCVs regardless of what the initial morphology is. However, the final LCV structures are different. Slow addition of water produces spherical LCVs, while those formed during fast water addition are irregular (like deformed spherical) LCVs.