Poly(N,N-bis(2-methoxyethyl)acrylamide), a thermoresponsive non-ionic polymer combining the amide and the ethyleneglycolether motifs

Abstract

Poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMOEAm) featuring two classical chemical motifs from non-ionic water-soluble polymers, namely, the amide and ethyleneglycolether moieties, was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. This tertiary polyacrylamide is thermoresponsive exhibiting a lower critical solution temperature (LCST)–type phase transition. A series of homo- and block copolymers with varying molar masses but low dispersities and different end groups were prepared. Their thermoresponsive behavior in aqueous solution was analyzed via turbidimetry and dynamic light scattering (DLS). The cloud points (CP) increased with increasing molar masses, converging to 46 °C for 1 wt% solutions. This rise is attributed to the polymers’ hydrophobic end groups incorporated via the RAFT agents. When a surfactant-like strongly hydrophobic end group was attached using a functional RAFT agent, CP was lowered to 42 °C, i.e., closer to human body temperature. Also, the effect of added salts, in particular, the role of the Hofmeister series, on the phase transition of PbMOEAm was investigated, exemplified for the kosmotropic fluoride, intermediate chloride, and chaotropic thiocyanate anions. A pronounced shift of the cloud point of about 10 °C to lower or higher temperatures was observed for 0.2 M fluoride and thiocyanate, respectively. When PbMOEAm was attached to a long hydrophilic block of poly(N,N-dimethylacrylamide) (PDMAm), the cloud points of these block copolymers were strongly shifted towards higher temperatures. While no phase transition was observed for PDMAm-b-pbMOEAm with short thermoresponsive blocks, block copolymers with about equally sized PbMOEAm and PDMAm blocks underwent the coil-to-globule transition around 60 °C.

Aggregation Behavior of Nonsymmetrically End-Capped Thermoresponsive Block Copolymers in Aqueous Solutions: Between Polymer Coils and Micellar States

“…Compared to the values typically reported for the homopolymers pNiPAm and pDEAm, the phase transition temperature of the corresponding block copolymers was strongly increased up to a temperature of 60 °C. This increase is only to a small part a consequence of the short lengths of the thermoresponsive blocks but mainly attributed to the attached long hydrophilic pDMAm block and the resulting overall higher hydrophilicity of the copolymers, as observed before. ,,,, …”

“…This is a distinct advantage over most of the reported hydrophobized RAFT agents, where the hydrophobic group is part of the Z-group, which is quite sensitive to hydrolysis. − CTA 6 bears a trimethylsilyl (TMS) moiety in the Z-group that is a powerful 1 H NMR label (see Figure S3), as its signal appears close to 0 ppm in the spectra, a region free of signals for most polymers and solvents. The intense singlet signal of the TMS group disposing of nine protons facilitates the determination of the number average molar mass by end group analysis. , Moreover, it also enables the estimation of the end group fidelity by comparing the integrals of Z- and R-group signals, whose aromatic protons appearing around 8 ppm are also well resolved …”

“…The synthesis of monomer N- acryloylpyrrolidine ( 5 , NAP) was adapted from reported procedures, , as described in detail in the Supporting Information. The synthesis of the chain-transfer agent (CTA) 4-(( n -dodecylcarbamoyl)­benzyl-3-(trimethylsilyl)­propyl)­carbonotrithioate ( 6 , CTA1) is described elsewhere . Deionized water was further purified by a Millipore Milli-Q Plus water purification system (Merck Millipore, Darmstadt, Germany) with a resistivity of 18.2 MΩ cm.…”

“…In contrast, the responsive end block is varied by employing four different acrylamide monomers, namely, N - n -propylacrylamide ( 2 , NPAm), N -isopropylacrylamide ( 3 , NiPAm), N , N -diethylacrylamide ( 4 , DEAm), and N -acryloylpyrrolidine ( 5 , NAP) whose homopolymers are known for increasing LCST values in aqueous solution between 20 and 60 °C (see Figure ). , Moreover, this study follows a preliminary one on HMTR polymers of this particular design, which used poly­( N,N -bis­(2-methoxyethyl)­acrylamide) as a thermoresponsive block, resulting, however, in transition temperatures that were unpractically high …”

“…10,11 Moreover, this study follows a preliminary one on HMTR polymers of this particular design, which used poly(N,N-bis(2-methoxyethyl)acrylamide) as a thermoresponsive block, resulting, however, in transition temperatures that were unpractically high. 19 ■ EXPERIMENTAL METHODS Chemicals. The reagents and solvents used are specified in the Supporting Information (SI).…”