Xiaoshuang Feng

Dendrimer-like polymers: a new class of structurally precise dendrimers with macromolecular generations

A new class of highly branched polymers referred to as dendrimer-like polymers has been developed in the last decade, the first synthetic example dating back to 1995. Dendrimer-like polymers exhibit molecular features similar to those of regular dendrimers, such as the presence of a central core, a precise number of branching points and terminal functions, but comprise of generations of macromolecular size between their branching junctions. In this highlight article synthetic strategies to dendrimer-like polymers are reviewed as well as some of their characteristic properties. A special emphasis is placed on synthetic methodologies designed in our group to generate dendrimer-like homopolymers and block copolymers by iterative divergent approaches based on anionic ring-opening polymerization of ethylene oxide and atom transfer radical polymerization. Are also thoroughly described the methods used for selectively branching polymeric chain-ends and introducing ω-geminal functionalities from which further macromolecular generations could be grown.

Using UCST Ionic Liquid as a Draw Solute in Forward Osmosis to Treat High-Salinity Water

The concept of using a thermoresponsive ionic liquid (IL) with an upper critical solution temperature (UCST) as a draw solute in forward osmosis (FO) was successfully demonstrated here experimentally. A 3.2 M solution of protonated betaine bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) was obtained by heating and maintaining the temperature above 56 °C. This solution successfully drew water from high-salinity water up to 3.0 M through FO. When the IL solution cooled to room temperature, it spontaneously separated into a water-rich phase and an IL-rich phase: the water-rich phase was the produced water that contained a low IL concentration, and the IL-rich phase could be used directly as the draw solution in the next cycle of the FO process. The thermal stability, thermal-responsive solubility, and UV-vis absorption spectra of the IL were also studied in detail.

Metal-Free Alternating Copolymerization of CO2 with Epoxides: Fulfilling “Green” Synthesis and Activity

Polycarbonates were successfully synthesized for the first time through the anionic copolymerization of epoxides with CO2, under metal-free conditions. Using an approach based on the activation of epoxides by Lewis acids and of CO2 by appropriate cations, well-defined alternating copolymers made of CO2 and propylene oxide (PO) or cyclohexene oxide (CHO) were indeed obtained. Triethyl borane was the Lewis acid chosen to activate the epoxides, and onium halides or onium alkoxides involving either ammonium, phosphonium, or phosphazenium cations were selected to initiate the copolymerization. In the case of PO, the carbonate content of the poly(propylene carbonate) formed was in the range of 92-99% and turnover numbers (TON) were close to 500; in the case of CHO perfectly alternating poly(cyclohexene carbonate) were obtained and TON values were close to 4000. The advantages of such a copolymerization system are manifold: (i) no need for multistep catalyst/ligand synthesis as in previous works; (ii) no transition metal involved in the copolymer synthesis and therefore no coloration of the samples isolated; and (iii) no necessity for postsynthesis purification.

Dendritic Carrier Based on PEG: Design and Degradation of Acid-sensitive Dendrimer-like Poly(ethylene oxide)s

Degradable dendrimer-like PEOs were designed using an original ABC-type branching agent featuring a cleavable ketal group, following an iterative divergent approach based on the anionic ring opening polymerization (AROP) of ethylene oxide and arborization of PEO chain ends. A seventh generation dendrimer-like PEO carrying 192 peripheral hydroxyls and exhibiting a molar mass of 446 kg · mol(-1) was obtained in this way. The chemical degradation of these dendritic scaffolds was next successfully accomplished under acidic conditions, forming linear PEO chains of low molar mass (≈2 kg · mol(-1)), as monitored by (1)H NMR, SEC, and MALDI-TOF mass spectrometry as well as by AFM.

Cs2CO3-promoted polycondensation of CO2 with diols and dihalides for the synthesis of miscellaneous polycarbonates

A one-pot protocol for the direct synthesis of polycarbonates through polycondensation of diols, dihalides and CO2 in the presence of Cs2CO3 is described. The conditions were optimized by studying the polycondensation of CO2 with 1,4-phenylenedimethanol and 1,4-dibromobutane as model monomers. Then, diols and dihalides with different spacers between the reactive groups including aliphatic, aromatic and poly(ethylene glycol) were tested under optimal conditions. Miscellaneous polycarbonates exhibiting molar masses in the range of 43 000 g mol−1 (GPC) and conversion higher than 96% could be obtained. The proposed mechanism rules out the possibility of ether linkage formation during polycondensation and accounts for the creation of carbonate linkages in two different ways. The thermal properties of the synthesized polycarbonates were unveiled by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA).

Synthesis of polyglycocarbonates through polycondensation of glucopyranosides with CO2

Starting from α-methyl D-glucopyranoside (MDG), three strategies of synthesis of polyglycocarbonates through direct polycondensation with CO2 were tried. Using unprotected MDG for reaction with CO2, water soluble oligoglycocarbonates could be obtained; α-methyl-2,3-di-O-methyl D-glucopyranoside (MDMG) which had its hydroxyls in the C2 and C3 positions protected was also subjected to polycondensation with CO2, affording polyglycocarbonates of limited molar mass due to an equilibrium that prevented the progress of the condensation reaction as in the previous case. Lastly, the polycondensation of MDMG with CO2 and aliphatic or aromatic dihalides was carried out in the presence of Cs2CO3; this resulted in the formation of polyglycocarbonates of rather high molar mass containing either aliphatic or aromatic linkers. The structures of the synthesized monomers and polyglycocarbonates were thoroughly characterized. The thermal properties of the obtained polyglycocarbonates were further investigated by TGA and DSC.

Osmotic Heat Engine Using Thermally Responsive Ionic Liquids

The osmotic heat engine (OHE) is a promising technology for converting low grade heat to electricity. Most of the existing studies have focused on thermolytic salt systems. Herein, for the first time, we proposed to use thermally responsive ionic liquids (TRIL) that have either an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) type of phase behavior as novel thermolytic osmotic agents. Closed-loop TRIL-OHEs were designed based on these unique phase behaviors to convert low grade heat to work or electricity. Experimental studies using two UCST-type TRILs, protonated betaine bis(trifluoromethyl sulfonyl)imide ([Hbet][Tf2N]) and choline bis(trifluoromethylsulfonyl)imide ([choline][Tf2N]) showed that (1) the specific energy of the TRIL-OHE system could reach as high as 4.0 times that of the seawater and river water system, (2) the power density measured from a commercial FO membrane reached up to 2.3 W/m2, and (3) the overall energy efficiency reached up to 2.6% or 18% of the Carnot efficiency at no heat recovery and up to 10.5% or 71% of the Carnet efficiency at 70% heat recovery. All of these results clearly demonstrated the great potential of using TRILs as novel osmotic agents to design high efficient OHEs for recovery of low grade thermal energy to work or electricity.