Polymers showing an upper critical solution temperature (UCST) in water are rare. Recently, the nonionic homopolymer poly(N-acryloyl glycinamide) (poly(NAGA)) has been shown to exhibit a sharp upper ...critical solution temperature in pure water as well as in electrolyte solution. Although poly(NAGA) is known for decades the UCST behavior had not been reported. The first controlled radical polymerization of poly(NAGA) by the RAFT (reversible addition–fragmentation transfer) process was also achieved recently, but no UCST was observed. The present study shows that traces of ionic groups in the polymer prevent phase separation. Failure to notice the UCST in the past was because ionic groups have been introduced unintentionally by either acrylate impurities in the monomer, hydrolysis of the polymer side chains, and/or usage of ionic initiators or chain transfer agents. A synthetic procedure for high purity NAGA monomer free of ionic impurities is reported. It is also shown how to obtain stable aqueous solutions of nonionic poly(NAGA) so that the UCST behavior can be exploited in pure water as well as in a physiological milieu. Further, ultrasensitive differential scanning calorimetry and light scattering were used to get insights into the phase separation mechanism. We believe that this knowledge is transferable to other systems and will greatly accelerate research in the field of macromolecules that feature thermally reversible hydrogen bonding.
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This review focuses on polymers with upper critical solution temperature (UCST) in water or electrolyte solution and provides a detailed survey of the yet few existing examples. A guide for synthetic ...chemists for the design of novel UCST polymers is presented and possible handles to tune the phase transition temperature, sharpness of transition, hysteresis, and effectiveness of phase separation are discussed. This review tries to answer the question why polymers with UCST remained largely underrepresented in academic as well as applied research and what requirements have to be fulfilled to make these polymers suitable for the development of smart materials with a positive thermoresponse.
Why are polymers with UCST in water underrepresented in academic and applied research? This review tries to answer this question. Furthermore, a guide for the synthesis of novel UCST polymers is presented and possible handles to tune the phase transition temperature, sharpness of transition, hysteresis, and effectiveness of phase separation are discussed.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Polymers showing an upper critical solution temperature (UCST) in aqueous solution were not rare, but the UCST was rarely observed under practically relevant conditions. Recently, much progress has ...been made in the synthesis of polymer systems that display UCST behavior under mild and physiologic conditions. Current developments focus on polymers that rely on hydrogen bonding. This viewpoint explains the historical context, presents the major properties, and concludes with a discussion of the most recent examples.
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A well‐studied example of a thermoresponsive polymer is PNiPAAm, which exhibits a sharp coil‐to‐globule transition in water at its LCST. A study relating to the missing counterpart of LCST polymers ...is presented: N‐acryloylglycinamide (NAGA) homopolymer and copolymers from NAGA and N‐acetylacrylamide (NAcAAm) that show a sharp UCST (where the sharpness depends on the composition). The polymers were synthesized by free‐radical copolymerization and the copolymerization parameters were determined by the method of Kelen‐Tüdös. The UCST was investigated by turbidimetry, regarding the influence of the copolymer composition, the polymer concentration and the addition of electrolytes.
Detailed studies regarding the correlation between the UCST and the copolymer composition, polymer concentration and the presence of electrolytes for aqueous solutions of the homopolymer of N‐acryloylglycinamide (NAGA), bearing H‐donor and H‐acceptor sites, and a UCST copolymer system based on H‐donor NAGA and H‐acceptor N‐acetylacrylamide (NAcAAm) monomers are reported.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We present a powerful universal and versatile approach for the synthesis of polymers that show a UCST in water. Up to now only a few polymers were known that show an upper critical solution ...temperature (UCST) in water. This study establishes general requirements to obtain polymers with a UCST in water as well as electrolyte solution. It is demonstrated that old homo- and copolymer systems like poly(methacrylamide) and poly(acrylamide-co-acrylonitrile) can exhibit a UCST in water and how polymers with a tunable UCST can be synthesized by copolymerization of acrylamide and acrylonitrile, monomers that are industrially produced on large scales. Controlled increase of the UCST by copolymerization of acryamide with varying amounts of acrylonitrile was shown, and it could be varied between 6 and 60 °C. The hysteresis between the cloud point upon cooling and heating was very small with only 1–2 °C in most cases. The cloud points in pure water were similar to the cloud points measured in phosphate buffered saline. Also, it is possible to prepare highly concentrated thermoresponsive polymer solutions without gel formation.
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Atom transfer radical polymerization (ATRP) of
N
-acryloylglycinamide has been used as a tool to make poly(NAGA) showing UCST-type phase transitions in water and electrolytes independent of molar ...mass and end-groups. We hypothesized that similarity in the structure of polymer chain ends with the repeat unit of poly(NAGA) could help in eliminating the effect of molar mass dependence of the cloud point especially in the low molar mass region. The monomer-like end-groups were introduced by choosing an appropriate initiator for ATRP like chloropropionamide (CPA). The catalyst system CuCl/CuCl
2
and tris2-(dimethylamino)ethyl-amine (Me
6
TREN) as a ligand provided controlled polymerization of NAGA in DMSO at 45 °C with UCST-type transitions retained in water and electrolytes without being influenced by chain ends/molar mass and high concentration of salts like NaCl and Na
2
SO
4
.
ATRP has been used as a tool to provide poly(
N
-acryloylglycinamide) with UCST-type transitions in water and electrolytes independent of molar mass and chain ends.
The first example of a chemically crosslinked synthetic non‐ionic hydrogel showing reversible positive swelling behavior in pure water as well as electrolyte solution is highlighted. Homopolymeric ...hydrogels are synthesized from N‐acryloylglycinamide with N,N′‐methylenbis(acrylamide) as a chemical crosslinker via free radical polymerization in dimethyl sulfoxide (DMSO). The swelling‐ratio dependency of the hydrogels upon temperature as well as on the amounts of crosslinker from 1 to 4.8 mol% is studied. With 1 mol% crosslinker, the hydrogel is able to take up over three times water of its weight at 37 °C in pure water and phosphate‐buffered saline. All the samples show almost 100% of reversibility for at least 6 d up to 37 °C irrespective of the amount of the crosslinker, making them promising candidates for biomedical applications. The sample with a higher amount of crosslinker, i.e., 4.8 mol% is even stable for over 6 d at 70 °C.
The first example of a chemically crosslinked synthetic non‐ionic thermophilic hydrogel is reported, showing positive swelling behavior in pure water as well as electrolyte solution. The swelling ratio dependency upon temperature as well as on the amount of crosslinker is studied. All the samples show almost 100% swelling reversibility with temperature between 4 and 37 °C for at least 6 d.
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A power conversion efficiency of 11.8% is demonstrated for planar perovskite solar cell based on acid water‐free poly(3,4‐ethylenedioxythiophene) (PEDOT) as the hole‐transporting layer. This ...efficiency is further improved to 14.2% using a pH neutralized PEDOT, which reduces the sub‐gap defects at the surface of perovskite. All the active layers reported are solution‐processed at temperatures below 140 °C making it compatible with roll‐to‐roll production.
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A new polymerizable surfactant, 12-methacryloyl dodecylphosphocholine (MDPC), has been synthesized using a three-step procedure in moderate yield. Phase transitions were characterized by DSC and ...phase behavior in water was determined by surface tension and polarizing microscopy. MDPC showed typical surfactant behavior and self-aggregated to micelles above a distinct concentration. The critical micelle concentration (CMC) of MDPC was determined to be 5
×
10
−4
mol/L. MDPC showed mesomorphic properties between 75 and 86
°C as studied by differential scanning calorimetry (DSC). The formation of black lipid membranes was further investigated. The methacrylate functionalized MDPC could form a bilayer membrane (BLM) although it was very unstable (collapsed after 10–30
s). However, it was possible to form stable BLMs in mixture with non-polymerizable two chain phospholipids, i.e. asolectin and diphytanoyl phosphatidylcholine (DPhPC). Stable bilayers could be obtained up to a MDPC content of 50
mol%. Gramicidin A was incorporated into MDPC/DPhPC membranes and exhibited ion-channel activity shown by single channel conductivity measurements.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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