pH-Responsive Nanogels Generated by Polymerization-Induced Self-Assembly of a Succinate-Functional Monomer

Colloidal nanogels formed from a pH-responsive poly(succinate)-functional core and a poly(sulfonate)-functional corona were prepared via a previously unreported reversible addition–fragmentation chain-transfer (RAFT)-mediated aqueous emulsion polymerization-induced self-assembly (PISA) route. Specifically, a poly(potassium 3-sulfopropyl methacrylate) (PKSPMA50) macromolecular chain-transfer agent (macro-CTA) was synthesized via RAFT solution polymerization followed by chain-extension with a hydrophobic, carboxylic acid-functional, 2-(methacryloyloxy) ethyl succinate (MES) monomer at pH 2. Colloidal nanoparticles with tunable diameters between 66 to 150 nm, depending on the core composition, and narrow particle size distributions were obtained at 20% w/w solids. Well-defined pH-responsive nanogels that swell on increasing the pH could be prepared even without the addition of a cross-linking comonomer, and introducing an additional cross-linker to the core led to smaller nanogels with lower swelling ratios. These nanogels could reversibly change in size on cycling the pH between acidic and basic conditions and remain colloidally stable over a wide pH range and at 70 °C.


Synthesis of PKSPMA50 via RAFT solution polymerization:
The preparation of PKSPMA by RAFT solution polymerization has been reported before. 1 The protocol used for the preparation of PKSPMA50 is as follows.KSPMA monomer (19.0 g, 77.1 mmol), PETTC RAFT agent (523.7 mg, 1.5 mmol, dissolved in dioxane), ACVA (86.5 mg, 0.3 mmol, PETTC/ACVA molar ratio = 5), and pH 5.5 acetate buffer (82.9 g, final buffer/dioxane ratio = 3) were weighed into a 250 ml round-bottomed flask, which was then sealed and purged with N2 for 30 min.The sealed flask was placed in a preheated water bath at 70 °C for 90 min.The reaction was quenched by immersion in an ice bath and opening the flask to air.The resulting PKSPMA was purified by dialysis against 10:1 water/methanol overnight.The purified solution was dried under vacuum at 30 °C to evaporate volatiles and then freeze-dried from aqueous solution overnight to yield a yellow product (61.2% yield).The mean degree of polymerization (DP) of PKSPMA was calculated to be 50, as determined by 1 H-NMR using D2O.
Deionized water and 0.25 M HCl were added to adjust the mixture to pH 2 to prevent ionization of the -COOH groups on MES during the polymerization and to produce a 20% w/w aqueous mixture.The solution was then purged with N2 for 30 min prior to immersion in a water bath set at 70 °C.The heated reaction was stirred for 10 h before the polymerization was quenched by cooling in an ice bath and exposure to air.Monomer conversions were determined via 1 H NMR using a methanol-d4/D2O mixture (80/20 % w/w).
Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded on a Bruker Advance III 400 MHz spectrometer with 128 scans averaged per spectrum at 25 °C.
Aqueous gel permeation chromatography (GPC) measurements were conducted using a phosphate buffer eluent (pH 9) containing 30 % v/v methanol at a flow rate of 1.0 mL min -1 at ambient temperature.The instrument was equipped with two PL aquagel-OH MIXED-H 8 μm columns and a refractive index detector (Shodex RI-101) was used to assess molar mass distributions.The system was calibrated with a series of nearmonodisperse poly(ethylene oxide) standards.Samples were prepared in the phosphate buffer eluent at 2 mg mL -1 .
Dynamic light scattering (DLS) studies were conducted using a Malvern Zetasizer Ultra instrument to measure both hydrodynamic diameter (Dh) and zeta potential.The instrument is equipped with a He-Ne solidstate laser operating at 633 nm, detecting back-scattered light at a scattering angle of 173°.All samples were diluted to 0.1 % w/w using 1.0 mM KCl or 1.0 mM NaCl as background electrolyte and data were averaged over three consecutive runs.Disposable folded capillary cells (Malvern DTS1070) were used for measuring both Dh and zeta potential.The dispersion pH was adjusted manually from pH 2 to pH 10 using KOH or NaOH (0.25 M/0.025 M) and HCl (0.25 M/0.025 M).The dispersion temperature was automatically adjusted from 15 °C to 70 °C by the Malvern Zetasizer Ultra instrument.All DLS samples were equilibrated for 240 seconds before measurements.
Transmission electron microscope (TEM) observations were carried out on a FEI Tecnai G2 F20 instrument operating at an accelerating voltage of 200 kV and connected to a Gatan 1k CCD camera.Copper/palladium TEM grids (Agar Scientific, UK) were surface coated with a thin film of amorphous carbon, then subjected to a plasma glow discharge for 30 seconds to produce a hydrophilic surface.The obtained dispersions were diluted from 20% w/w to 0.1% w/w solids at pH 2. A hydrophilic grid was placed onto an aqueous droplet (40 μL) of a 0.1 w/w dispersion for 1 min and then blotted with filter paper to remove excess solution.This grid was then negatively stained via placing onto a uranyl acetate solution (0.5 wt %) droplet (40 μL) for 30 seconds.
Excess stain was removed by blotting and the grid was carefully dried with a vacuum hose.

(b) (a)
Supporting Tables Table S1 a Measured using 1mM KCl as the dispersant and the pH was adjusted using HCl.
b Measured using 1mM KCl as the dispersant and the pH was adjusted using KOH.
c Measured using 80/20 % w/w methanol/water mixture as the dispersant.Instrument parameters (viscosity, refractive index, and dielectric constant values for methanol/water) used during measurements were as previous reported.
Figure S1.Assigned 1 H NMR spectra of (a) an unpurified PKSPMA reaction product and (b) a purified and freeze-dried PKSPMA50 macro-CTA.Both samples were dissolved in D2O prior to analysis.The conversion for this reaction was calculated using unpurified PKSPMA reaction product spectra by comparing the integrated proton signals corresponding to the methacrylic polymer backbone at 2.83-3.26ppm with that corresponding to the vinyl protons of the KSPMA monomer at 5.6 ppm-6.25 ppm.The degree of polymerization (DP) for the purified polymer was calculated by comparing the integrated proton signals corresponding to the methacrylic polymer backbone at 2.83-3.26ppm and 3.97-4.44ppm with that corresponding to the aromatic protons of the chain end at 7.2-7.4ppm.

Figure S2 .
Figure S2.Aqueous gel permeation chromatography chromatogram obtained for PKSPMA50 macro-CTA.A relatively narrow molecular weight distribution was achieved, suggesting successful RAFT polymerization.

Figure S4 .Figure S5 .
Figure S4.Kinetic studies for the RAFT emulsion polymerization of MES (target DP 300) using PKSPMA50 as a macro-CTA in pH 2 water at 70 °C.(a) Assigned 1 H NMR spectra at different reaction time (samples were dissolved in 80/20 % w/w methanol-d4/D2O for measurements).(b) Chemical structures of MES monomer and the resulting diblock copolymer products.

Figure S6 .Figure S7 .
Figure S6.(a) Hydrodynamic diameter on cycling between 25 °C and 70 °C and (b) zeta potential vs.temperature for PKSPMA50-PMES300 (red triangles) and PKSPMA50-PMES500 (blue triangles) nanoparticles.All measurements were performed on 0.1% w/w copolymer dispersions prepared in the presence of 1.0 mM KCl background salt and the pH was adjusted using HCl or KOH.

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Particle diameter and volume swelling ratio for PKSPMA50-P(MESm-EGDMA1-m)y nanoparticles under various conditions.Particle diameters were measured by DLS and polydispersity index values are indicated in brackets. 1

Table S2 .
Particle diameter and volume swelling ratio of non-crosslinked nanoparticles at 15 °C and 70 °C.Particle diameters were measured by DLS and polydispersity index values are indicated in brackets.