Upper Critical Solution Temperature of Poly(N-acryloyl glycinamide) in Water: A Concealed Property

Abstract

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.

Polymers with Upper Critical Solution Temperature in Aqueous Solution

“…The heating/cooling rate was 1.0 °C/min and the inflection points were defined as cloud points. Reprinted with permission from reference [20]. Copyright (2012) American Chemical Society.…”

One-pot synthesis of responsive sulfobetaine nanoparticles by RAFT polymerisation: the effect of branching on the UCST cloud point

“…al. 4 The same group has reported several examples of the previously unnoticed UCST behaviour of well-known polymer systems such as poly(acrylamide-co-acrylonitrile) 5 and poly(N- 35 acryloyl glycinamide); 6 however few other examples exist. Polybetaines are electrically neutral polymers that contain both cationic and anionic groups within a single monomer unit.…”

Thermo-Responsive Hydrogels for Stimuli-Responsive Membranes

“…Since this first observation and study there have been very few works further investigating the characteristics and mechanisms of complexation of NAGA as either homopolymer or co-monomer [83]. A renewed interest in 2007 in patent literature [84,85] and again in 2010 in the solution properties of single polymer chains and synthesis methods for PNAGA [30,86,87] suggests that more investigations of the characteristics and properties of NAGA-based gels may be expected in the future. The only previous work discovered to observe the thermophilic behavior of NAGA-based gels was with IPNs by Sasase et al [69].…”

“…The VPTT was determined to be ~22.5 °C upon heating and ~12.3 °C upon cooling by a turbidity photometer at 670 nm at a heating rate of 1 °C min −1 . A later publication by Seuring et al investigated why thermophilic behavior for PNAGA had not been observed while it had been know to undergo thermo-reversible gelation for near half a century [87]. It was determined that any ionized groups in the polymer prevent phase separation, ionic groups can be 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".…”