Modulating the solubility of zwitterionic poly((3-methacrylamidopropyl)ammonioalkane sulfonate)s in water and aqueous salt solutions via the spacer group separating the cationic and the anionic moieties

Abstract

Modulating the solubility of zwitterionic poly((3-methacrylamidopropyl) ammonioalkane sulfonate)s in water and aqueous salt solutions via the spacer group separating the cationic and the anionic moieties Postprint Complementary to the well-established zwitterionic monomer 3-((3-methacrylamidopropyl)dimethylammonio)propane-1-sulfonate (SPP), the closely related monomers 2-hydroxy-3-((3-methacrylamidopropyl)dimethylammonio)propane-1-sulfonate (SHPP) and 4-((3-methacrylamidopropyl)dimethylammonio)-butane-1-sulfonate (SBP) were synthesised and polymerised by reversible addition-fragmentation chain transfer (RAFT) polymerisation, using a fluorophore labeled RAFT agent. The polyzwitterions of systematically varied molar masses were characterised with respect to their solubility in water and aqueous salt solutions. Both poly(sulfobetaine)s show thermoresponsive behaviour in water, exhibiting phase separation at low temperatures and upper critical solution temperatures (UCST). For both polySHPP and polySBP, cloud points depend notably on the molar mass, and are much higher in D 2 O than in H 2 O. Also, the cloud points are effectively modulated by the addition of salts. The individual effects can be in parts correlated to the Hofmeister series for the anions studied. Still, they depend in a complex way on the concentration and the nature of the added electrolytes, on the one hand, and on the detailed nature of the spacer group separating the anionic and the cationic charges of the betaine moiety, on the other hand. As anticipated, the cloud points of polySBP are much higher than the ones of the analogous polySPP of identical molar mass.Surprisingly, the cloud points of polySHPP are also somewhat higher than the ones of their polySPP analogues, despite the additional hydrophilic hydroxyl group present in the spacer separating the ammonium and the sulfonate moieties. These findings point to a complicated interplay of the various hydrophilic components in polyzwitterions with respect to their overall hydrophilicity. Thus, the spacer group in the betaine moiety proves to be an effective additional molecular design parameter, apparently small variations of which strongly influence the phase behaviour of the polyzwitterions in specific aqueous environments.

Effect of the zwitterion structure on the thermo-responsive behaviour of poly(sulfobetaine methacrylates)

“…The polymerisations in trifluoroethanol proceeded smoothly, in agreement with the literature on other sulfobetaine monomers. [28][29][30]41,46,66 The ratio of monomer to the fluorophore-functionalised CTA in the reaction mixtures was varied between 100 : 1 up to 600 : 1, in order to modulate the molar masses, while the ratio between CTA and the azoinitiator was always kept constant as 5 : 1 (Table 1). Under such conditions, the vast majority of the polymer chains were initiated by the "R"-residue of the RAFT agent and consequently labelled by the fluorophore.…”

“…28,30 This enabled not only a facile tracking of the polymers during the various experiments by virtue of their strong fluorescence, but also the ability to apply end-group analysis to support the molecular characterisation. 67 As shown previously, 28,30 the signals of both the aromatic moieties of the "R"-and the "Z" end groups could be resolved and quantified in the 1 H NMR spectra for moderate molar masses (<10 5 ), while the UV/vis absorbance band of the naphthalimide chromophore enabled quantification of the "R" end groups, even for high molar masses. This enabled the facile and quite reliable determination of the absolute number average molar mass (M n ) of the polyzwitterions as well as of the extent of the preservation of the RAFTactive end groups.…”

“…4), in analogy to the closely related poly(sulfobetaine methacrylamides). 28,30 The differences observed between the cloud points in H 2 O and in D 2 O were about 6°C in the cases of P-7 and P-8, and about 10°C in the cases of P-1, P-3, P-4 and P-5. The highest difference of about 25°C was found for P-6.…”

“…for producing biocompatible or low-fouling materials). Only recently has interest in the UCST behaviour of polysulfobetaines gained more impetus, mostly in order to † Electronic supplementary information (ESI) available: Synthesis of tertiary amine bearing methacrylates; a enlarge the scope of the "traditional" thermo-responsive polymers with LCST behaviour 15,[24][25][26][27][28][29][30] or to obtain stimuli-responsive materials of increasing complexity. [31][32][33][34][35][36][37][38][39][40] Chemically well-defined polysulfobetaines are most conveniently prepared via free radical polymerisation of the underlying monomers.…”

“…28,30 By varying the spacer groups between the cationic ammonium and the anionic sulfonate groups, we were able to study their aqueous solution behaviour and dependence on temperature and on the concentration of selected inorganic salts. While strong effects on the phase transition temperature were induced by apparently small structural modifications, the observed increase -or decrease -in the temperature did not follow a simple obvious logic.…”