Efficiency Boosting of Surfactants with Poly(ethylene oxide)-Poly(alkyl glycidyl ether)s: A New Class of Amphiphilic Polymers.

Twenty years ago, it was found that adding small amounts of amphiphilic block copolymers like poly(ethylene propylene)-co-poly(ethylene oxide) (PEP-b-PEO) to microemulsion systems strongly increases the efficiency of medium-chain surfactants to solubilize water and oil. Although being predestined to serve as a milestone in microemulsion research, the effect has only scarcely found its way into applications. In this work, we propose new types of efficiency boosters, namely, poly(ethylene oxide)-poly(alkyl glycidyl ether carbonate)s (PEO-b-PAlkGE) and their "carbonated" poly(ethylene oxide)-poly(carbonate alkyl glycidyl ether) analogs. Their synthesis via anionic ring-opening polymerization (AROP) from commercially available long-chain alkyl glycidyl ethers (AlkGE) and monomethoxypoly(ethylene glycol)s as macroinitiators can be performed at low cost and on a large scale. We demonstrate that these new PEO-b-PAlkGE copolymers with dodecyl and hexadecyl side chains in the nonpolar block strongly increase the efficiency of both pure and technical-grade n-alkyl polyglycol ether surfactants to form microemulsions containing pure n-alkanes or even technical-grade waxes, a result that could be of interest for industrial applications where reduced surfactant needs would have significant economic and ecological implications. For n-decane microemulsions, the boosting effect of PEO-b-PAlkGE and PEP-b-PEO polymers can be scaled on top of each other, when plotting the efficiency semilogarithmically versus the polymeric coverage of the amphiphilic film. Interestingly, a somewhat different scaling behavior was observed for n-octacosane microemulsions at elevated temperatures, suggesting that the polymers show less self-avoidance and rather behave as almost ideal chains. A similar trend was found for the increase of the bending rigidity κ upon polymeric coverage of the amphiphilic film, which was obtained from the analysis of small-angle neutron scattering (SANS) measurements.

"Dumb" pH-Independent and Biocompatible Hydrogels Formed by Copolymers of Long-Chain Alkyl Glycidyl Ethers and Ethylene Oxide.

The formation and rheological properties of hydrogels based on amphiphilic ABA triblock polyether copolymers are described, relying solely on the hydrophobic interaction of long-chain alkyl glycidyl ether (AlkGE)- based A-blocks that are combined with a hydrophilic poly(ethylene glycol) (PEG) midblock. Via anionic ring-opening copolymerization (AROP), ethylene oxide (EO) and long-chain alkyl glycidyl ethers (AlkGEs) were copolymerized, using deprotonated poly(ethylene glycol) (PEG) macroinitiators (Mn of 10, 20 kg mol-1). The polymerization afforded amphiphilic ABA triblock copolymers with molar masses in the range of 21-32 kg mol-1 and dispersities (D) of D = 1.07-1.17. Kinetic studies revealed random copolymerization of EO and AlkGE, indicating random spacing of the hydrophobic AlkGE units by polar EO units. Following this approach, the hydrophobicity of the apolar blocks of amphiphilic ABA triblock polyethers can be tailored. Detailed rheological measurements confirmed the successful formation of hydrogels at different pH values as a consequence of nonpolar interactions and alkyl chain crystallization. Hydrogel formation was also observed at different ionic strengths (i.e., varied salt concentration), based on the hydrophobic aggregates. This behavior is in contrast to other often-used supramolecular cross-linking strategies, such as Coulomb interactions, complexation, or hydrogen bonding. Micro-differential scanning calorimetry (µ-DSC) measurements of the hydrogels revealed crystalline hydrophobic domains with melting temperatures in the physiological temperature range. In 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide (MTT) assays, diblock copolymers possessing structural analogy to the triblock copolymers were studied to assess the general cytotoxicity of amphiphilic polyethers bearing long alkyl chains at the polyether backbone, using splenic immune cells. At intermediate polymer concentrations, no cytotoxic effects were observed. This indicates that long-chain alkyl glycidyl ethers are promising for the introduction of highly hydrophobic as well as crystalline motifs at the polyether backbone in hydrogels for biomedical purposes.

Long-Chain Alkyl Epoxides and Glycidyl Ethers: An Underrated Class of Monomers.

Long-chain epoxides and specifically alkyl glycidyl ethers represent a class of highly hydrophobic monomers for anionic ring-opening polymerization (AROP), resulting in apolar aliphatic polyethers. In contrast, poly(ethylene glycol) is known for its high solubility in water. The combination of hydrophobic and hydrophilic monomers in block and statistical copolymerization reactions enables the synthesis of amphiphilic polyethers for a wide range of purposes, utilizing micellar interactions in aqueous solutions, e.g., viscosity enhancement of aqueous solutions, formation of supramolecular hydrogels, or for polymeric surfactants. Controlled polymerization of these highly hydrophobic long-chain epoxide monomers via different synthesis strategies, AROP, monomer-activated anionic ring-opening polymerization, catalytic polymerization, or via postmodification, enables precise control of the hydrophilic/lipophilic balance. This renders amphiphilic polymers highly interesting candidates for specialized applications, e.g., as co-surfactants in microemulsion systems. Amphiphilic polyethers based on propylene oxide and ethylene oxide, such as poloxamers are already utilized in many established applications due to the high biocompatibility of the polyether backbone. Long alkyl chain epoxides add an interesting perspective to this area and permit structural tailoring. This review gives an overview of the recent developments regarding the synthesis of amphiphilic polyethers bearing long alkyl chains and their applications.

Crystalline CO2 -Based Aliphatic Polycarbonates with Long Alkyl Chains.

Carbon dioxide (CO2 ) is an easily available, renewable carbon source and can be utilized as a comonomer in the catalytic ring-opening polymerization of epoxides to generate aliphatic polycarbonates. Dodecyl glycidyl ether (DDGE) is copolymerized with CO2 and propylene oxide (PO) to obtain aliphatic poly(dodecyl glycidyl ether carbonate) and poly(propylene carbonate-co-dodecyl glycidyl ether carbonate) copolymers, respectively. The polymerization proceeds at 30 °C and high CO2 pressure utilizing the established binary catalytic system (R,R)-Co(salen)Cl/[PPN]Cl. The copolymers with varying DDGE:PO ratios are characterized via NMR, FT-IR spectroscopy, and SEC, exhibiting high molecular weights between 11 400 and 37 900 g mol-1 with dispersities (Р= M w /M n ) in the range of 1.37-1.61. Copolymers with T g s of -11 °C or T m s from 5 to 15 °C and thermal decomposition >200 °C depending on the comonomer ratio, are obtained as determined by differential scanning calorimetry/TGA.