Accelerated Reaction Rates within Self-Assembled Polymer Nanoreactors with Tunable Hydrophobic Microenvironments

Performing reactions in the presence of self-assembled hierarchical structures of amphiphilic macromolecules can accelerate reactions while using water as the bulk solvent due to the hydrophobic effect. We leveraged non-covalent interactions to self-assemble filled-polymer micelle nanoreactors (NR) incorporating gold nanoparticle catalysts into various amphiphilic polymer nanostructures with comparable hydrodynamic nanoreactor size and gold concentration in the nanoreactor dispersion. We systematically studied the effect of the hydrophobic co-precipitant on self-assembly and catalytic performance. We observed that co-precipitants that interact with gold are beneficial for improving incorporation efficiency of the gold nanoparticles into the nanocomposite nanoreactor during self-assembly but decrease catalytic performance. Hierarchical assemblies with co-precipitants that leverage noncovalent interactions could enhance catalytic performance. For the co-precipitants that do not interact strongly with gold, the catalytic performance was strongly affected by the hydrophobic microenvironment of the co-precipitant. Specifically, the apparent reaction rate per surface area using castor oil (CO) was over 8-fold greater than polystyrene (750 g/mol, PS 750); the turnover frequency was higher than previously reported self-assembled polymer systems. The increase in apparent catalytic performance could be attributed to differences in reactant solubility rather than differences in mass transfer or intrinsic kinetics; higher reactant solubility enhances apparent reaction rates. Full conversion of 4-nitrophenol was achieved within three minutes for at least 10 sequential reactions demonstrating that the nanoreactors could be used for multiple reactions.


S1.1.2: Induction Time Determination
Initially, the spectra of the polymer nanoreactor dispersion was recorded as a reference to be subtracted from subsequent spectra as background. The 4-nitrophenol was added resulting in an increase in absorbance at 425 nm. The increase in absorbance due to the addition of 4-nitrophenol occurs within 30 seconds. After 1 minute, the sodium borohydride is added, which results in a further increase in absorbance at 425 nm due to formation of the 4-nitrophenolate ion. The initiation of reaction conditions corresponding to the start of the induction period was defined as the time at which the absorbance at 425 nm increased to at least 10% of the absorbance at 45 seconds. The induction time itself is characterized by a slow decrease (0.002 Abs/s) in absorbance that is followed by a sharp (> 0.01 Abs/s) decline in absorbance indicating beginning of the reduction reaction, which signifies the end of the induction period. Changes in the slope of the absorbance vs. time was used to quantitatively determine the induction time. Specifically, the absorbance vs. time during the induction time was presumed to be adequately described by a line calculated from fitting the initial region of the induction period (no less than 10% of the total approximate induction time domain). For example, if the induction period lasted roughly 300 seconds based upon visual inspection, the linear fit of the induction time was based on the first 60 seconds of the induction time. Next, the maximum value within the first five or more, consecutive data points which were at least 1% less than the fitted line was used to normalize the data set. Importantly, these consecutive data points must occur after the start of the induction time period. The time point designating the end of the induction time was then defined as the last occurrence of the normalized data point with a value of 1.

S1.1.3: Apparent Reaction Rate Constant
For analysis of the reaction rate, the natural log of the normalized absorbance was plotted with respect to time and regions of two distinct slopes were observed. The first region has been attributed to formation of an intermediate [2]. In order to avoid analysis of the intermediate reaction, the apparent reaction rate was calculated from the second region corresponding to when the normalized absorbance fell below 0.67. Data corresponding to a 15% conversion were analyzed when determining apparent reaction rate. If the 15% conversion domain contained less than 4 data points, one data point above the determined region and below the region was included, such that no analysis included less than three data points. The induction time and apparent reaction rate are reported as the average ± standard deviation of three experimental trials.
With a sufficiently large excess of sodium borohydride compared to 4-nitrophenol, the reaction kinetics can be described by pseudo-first-order kinetics. For heterogeneous catalysts, the apparent rate constant is assumed to be proportional to the surface of the catalyst described by [2]: where c is the concentration of 4-nitrophenol at time (t), kapp is the apparent rate constant, k1 is the rate constant normalized to surface area of gold nanoparticles per unit volume of the reaction. Experimentally, k1 is determined by the change in 4-nitrophenol concentration after the induction period and the mass of gold measured by ICP (surface area calculated assuming 5 nm spherical particles). Pseudo-first-order rate kinetics are characteristically described by a linear fit when the natural log of the reagent concentration is plotted with time.

S1.1.4: NMR Measurements
To evaluate effective transport of the 4-nitrophenol, 1 H-NMR spectroscopy and pulsed field gradient (PFG) NMR, combined with saturated transfer difference (STD) spectroscopy, using a Bruker 800 MHz cryoprobe (Billerica, MA, USA) was performed in accordance with the methods described previously (our paper). Briefly, 4-nitrophenol molecules in close proximity to the nanoreactor core were analyzed based on spin diffusion of selectively saturated core material (polystyrene or castor oil), in conjunction with an applied magnetic field gradient. Relevant peak intensities were analyzed as a function of gradient strength to determine the diffusion coefficient of 4-nitrophenol molecules of interest. Since nanoreactors diffuse in free solution at least 3 orders of magnitude slower than molecules, the measured diffusion coefficient was considered the effective diffusion coefficient of the solute within the nanoreactor [3,4]. A detailed method explanation has been published previously [26]. Figure S1: Results of saturation-transfer-differentiated PFG-NMR of 4-nitrophenol in a solution of castor oil nanoreactors. The blue line corresponds to the curve fit. The difference spectra corresponds to signal from the 4-nitrophenol in closest proximity to the hydrophobic phase (within the nanoreactors) which we interpret as an effective diffusion coefficient of 4-nitrophenol within the nanoreactors of 1.7 x 10 -8 m 2 /s.    The HSP distance or RA 2 , is a measure of how alike two compounds are. The smaller the RA 2 value, the more likely they are to be compatible with each other. The HSP distance is calculated as follows: 2 = 4( 1 − 2 ) 2 + ( 1 − ) 2 + ( ℎ 1 − ℎ 2 ) 2 (S2)

S2.7: Diffusion Coefficient of 4-Nitrophenol in CO NR
Of the hydrophobic core materials chosen, castor oil presents the lowest RA 2 value and would likely interact with 4-nitrophenol the most favorably.