One-Pot Synthesis of Strong Anionic/Charge-Neutral Amphiphilic Block Copolymers

Despite the ever more versatile polymerization techniques that are becoming available, the synthesis of macromolecules with tailored functionalities can remain a lengthy endeavor. This becomes more conspicuous when the implementation of incompatible chemistries (i.e., strong polyelectrolytes) within sequence-controlled polymers is desired, often requiring (i) polymerization, (ii) chain extension, and (iii) postpolymerization modification. Herein, we explore the production of strong anionic/charge-neutral block copolymers (BCPs) in a one-pot fashion. This straightforward three-step process includes the synthesis of a macroinitiator and chain extension via rapid and efficient photomediated atom transfer radical polymerization, followed by in situ deprotection to expose the polyanionic domains. The resulting BCPs, which are strong amphiphiles by nature, are capable of self-assembly in aqueous media, as evidenced by dynamic light scattering, small-angle X-ray scattering, ζ-potential measurements, and transmission electron microscopy. We further demonstrate the versatility of our methodology by producing several BCPs through sampling of a single reaction mixture, enabling the straightforward production of strong polymer amphiphiles.

Commercially-available monomers were passed through a short AlOx column to remove inhibitors prior to polymerizations. All other chemicals were used as received.
Me6-TREN/CuBr2 stock solutions were prepared freshly prior to polymerization by introducing 1 eq CuBr2, 6 eq Me6-TREN and DMSO into a glass vial and thorough mixing. A calculated volume of the stock solution was introduced into the reaction mixtures to obtain 0.02 eq CuBr2 and 0.12 eq Me6-TREN, with respect to the initiator. An example of a stock solution is as follows: CuBr2 (1 eq, 4.52 mg, 20.3 μmol), Me6-TREN (6 eq, 28.1 mg, 122 μmol) and DMSO (5.07 mL). A volume of 128 μL of this stock solution is required for a reaction using 5.00 mg EBiB.
The light reactor was built using a 2 m LED strip (SimpleColor Blue, purchased from Waveform) emitting at 365 nm with an output of ~ 29 W (original length of 5 m for 72 W) winded inside a Ø14 cm crystallization dish, which was covered on the exterior with aluminum foil with its reflective side inward (see Figure S1). Note that no active cooling was required, as the temperature inside the reactor never exceeded 35 °C during the course of the reactions.
Samples were dissolved in an appropriate solvent or solvent mixture (≈ 5 g L -1 ) and analyzed with a pulse width of 45 μs, spectral width of 12/-2 ppm, recycle delay of 1 s and either 32 or 256 scans (conversion or purified samples respectively). Spectra were analyzed with Mestrenova software version 14.1.
Size-exclusion chromatography (SEC) was performed on a GPCMax system from Viscotek equipped with a 302 TDA detector array and two columns in series (PolarGel L and M, both 8 μm 30 cm) from Agilent Technologies. The columns and detectors were maintained at a temperature of 50 °C. DMF containing 0.01 M LiBr was used as the eluent at a flow rate of 1 mL min -1 . Near monodisperse poly(methyl methacrylate) standards from Polymer Standard Services were used for the construction of a calibration curve. Samples were dissolved in the eluent at a concentration of ≈ 3 g L -1 and passed through a 0.45 μm PTFE filter prior to injection. Data acquisition and calculations were performed using Viscotek Omnisec software version 5.0.

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded on a
Bruker VERTEX 70 spectrometer equipped with an ATR diamond single reflection module. The spectra were collected in the range of 4000-500 cm -1 with a spectral resolution of 2 cm -1 and using 64 scans for each sample. Atmospheric compensation and baseline correction were applied to the collected spectra using Bruker's OPUS spectroscopy software version 7.0. Differential scanning calorimetry (DSC) measurements were recorded on a TA Instruments DSCQ1000 analyzer. The samples (~ 5 mg) were subjected to the following method: (i) equilibration at -60 °C, (ii) 5 min isotherm, (iii) ramp to 130 °C at 10 °C min -1 , (iv) 5 min isotherm, (v) ramp to -60 °C at 10 °C min -1 , (vi) 5 min isotherm and (vii) ramp to 130 °C at 10 °C min -1 . Data analysis was performed on the heat second cycle using TA Instruments TRIOS software.
Thermogravimetric analysis (TGA) measurements were recorded on a TA Instruments TGA5500 analyzer. The samples (~ 5 mg) were heated from 30 °C to 700 °C at a rate of 10 °C min -1 under a continuous nitrogen flow. The data acquisition and analysis was done using TA Instruments TRIOS software.
Dynamic light scattering (DLS) measurements were performed on a Malvern Panalytical Zetasizer Ultra system, equipped with a helium-neon laser (λ = 633 nm) and an Avalanche photodiode detector. The nanoparticles solutions were measured at 25 °C in back scattering mode after a 120 s equilibration time and using 30 cumulative recordings. Samples were recorded in triplicate. The results were analyzed with ZS Xplorer software. ζ-potential measurements were performed on a Malvern Panalytical Zetasizer Ultra system, equipped with a helium-neon laser (λ = 633 nm) and an Avalanche photodiode detector. The measurements were taken at 25 °C while the acquisition times were determined automatically.
Samples were recorded in triplicate and the results were analyzed with ZS Xplorer software.
Transmission electron microscopy (TEM) imaging was performed on a Philips CM120 transmission electron microscope using a tungsten filament operated at an accelerating voltage of 120 kV. Images were recorded using a Gatan 4k CCD camera. TEM grids (carbon, 400 mesh with carbon support film) were glow-discharged (15 s at 50 mA and 300 V) prior to sample preparation.
Specimens were prepared by deposition of 5 μL of the nanoparticle dispersion (c ~ 1 g L -1 ) onto the grid and adsorption for 1 min before blotting. Before the specimen was fully dried, 5 μL of 2 wt.% uranyl acetate staining solution was deposited onto the grid, immediately blotted and a new 5 μL drop of staining solution was deposited and left to adsorb for 1 min before blotting. TEM images were analyzed using Image J software, employing brightness and contrast correction tools to enhance the general quality of the snapshots, and software-imbedded measurement tool to determine the dimensions of the nanoparticles.
Small angle X-ray scattering (SAXS) experiments were performed at the MINA diffractometer of the University of Groningen. The instrument was equipped with a Cu rotating anode (X-ray wavelength λ = 1.5413 Å and energy of 8 keV) and a Dectris Pilatus 300K detector placed at a distance of 3.1 m. SAXS measurements of the block copolymers in bulk were conducted after annealing at 120 °C overnight (protected samples) or for a week (deprotected samples) and using a copper sample holder sealed with Kapton TM tape. The solution scattering experiments were carried out in glass capillaries, using 5 g L -1 solutions. The data were fitted with SASfit software. [2] The scattering length densities η (SLD) were computed using the nominal composition of the polymers according to the synthesis procedure. The SLD for the solvent at room temperature was used as ηm = 0.94 x 10 11 cm -2 . The contrast between the core and the solvent (ηcore-ηm) was calculated as 0.16 x 10 11 cm -2 and kept constant during the fitting procedure. The SAXS profiles for the polymer micelles were fitted using a dilute core-shell model for spherical particles. In this case, the scattering intensity is given by the following equation:

Deprotection of poly(3-isobutoxysulfopropyl acrylate).
The deprotection of poly(3-isobutoxysulfopropyl acrylate) was performed as reported before.  Table S1 below for the characterization of the blocks. In-situ chain extension of poly(3-isobutoxysulfopropyl acrylate).
This section details the conditions tested for the production of PMAx-b-PSPAy amphiphilic block copolymers in one-pot fashion. For the methodology, please refer to the following section.   Da, Đ = 1.16.

One-pot synthesis of poly(methyl acrylate)-block-poly(3-isobutoxysulfopropyl acrylate) amphiphilic block copolymers.
A typical example of one-pot polymerization can be found here. Detailed reaction mixture compositions of further reactions can be found in Table S5.  Table S6 for the characterization of the blocks.   The three large aliquots were then diluted with 6 mL of DMSO and NaI (770 mg, 5.13 mmol) was introduced. These solutions were deprotected at 70 °C for 20 hours, before precipitation in cold 1:2 n-hexane:ethanol, washing once with 1:2 n-hexane:ethanol and once with pure n-hexane before drying under high vacuum. See Table S7 for the characterization of the blocks.                              The fitted curves are reported in Figure S13 below and the fitted values for the core-shell particles are summarized in Table S11. The core size is in good agreement with the TEM analysis and the shell thickness scales according to the degree of polymerization of the SPA block. The scattering length density of the core was first derived from the fitting of the PMA193-b-PSPA-Na94 sample (xSPA-Na = 0.33, sample showing the most well-defined SAXS profile) and was kept constant during fitting for the other samples. The SLD for the solvent at room temperature was used as ηm = 0.94 x 10 11 cm -2 . The contrast of between the core and the solvent was calculated as ηcore-ηm = 0.16 x 10 11 cm -2 (based on the composition of the PMA) and kept constant during the fitting procedure. Conversely the shell scattering length density is much lower than the one expected for bulk SPA and is much closer to the solvent (i.e., 10 mM KNO3, ≈ water) value, meaning that the shell is highly swollen, as expected for charged systems. The background exponent was found to oscillate between 1.3 and 2.3. For PMA47-b-PSPA-Na254, the total particle size seems to be outside of the measured range and only scattering from the outer PSPA-Na corona is measured.  The core size polydispersity was described by a Schultz-Zimm distribution function. ηSPA-Na-ηm: scattering length difference between the SPA-Na phase and the surrounding media. Rcore: core radius. Rshell: outer shell thickness. σc : core polydispersity. background: Background function. S14: Schematic representation of the 'sampling' method. Figure S14: Schematic representation of the synthesis of several amphiphilic block copolymers using a 'sampling' method, including (i) the production of the first PMA block, followed by (ii) chain extension with BSPA monomer and (iii) deprotection of each block copolymer after sampling. S15: 1 H NMR of the block copolymers achieved through sampling.