Synthesis and Operating Optimization of the PEG Conjugate via CuAAC in scCO2

Abstract

A new sustainable green protocol for obtaining polyethylene glycol (PEG) conjugates, with a prototype molecule, which in this work was coumarin, by means of click chemistry is presented. The organic solvents commonly used for this type of reaction were replaced by supercritical carbon dioxide (scCO₂). The synthesis and characterization of PEG-coumarin were successfully reported using FTIR, ¹H NMR, and MALDI TOF. Subsequently, a preliminary study was carried out using the response surface methodology to examine the variables that most affect the use of scCO₂ as a reaction medium. The main effects caused by these variables, individually and their binary interaction, have been estimated. The response surface methodology has been used in this work to screen variables using a factorial design 2³. The p-values of temperature and pressure were 0.006 and 0.0117, being therefore the most significant variables of the response surface methodology study. Subsequently, a more intensive study has been carried out on the variables that have shown the greatest significant effect on reaction performance where an 82.32% synthesis success was achieved, which broadens the scope of the use of scCO₂ as a reaction medium. The conjugated coumarin with mPEG-alkyne and coumarin were evaluated for their in vitro antioxidant activities by the DPPH radical scavenging assay and were found to exhibit substantial activities. The click product showed comparable or even better efficacy than the initial coumarin.

Scheme 1. Synthesis Reaction Scheme for mPEG-Coumarin in scCO₂

Figure 1

FTIR spectra: (a) mPEG-alkyne; (b) click product mPEG-coumarin in scCO₂; (c) 4-azidomethyl-7-methoxycoumarin.

Figure 2

MALDI TOF MS spectra: (a) mPEG-alkyne; (b) mPEG-coumarin. (square) Peaks of mPEG-alkyne. (star) Peaks of mPEG-coumarin.

Figure 3

Standardized Pareto chart for the yield.

Figure 4

MALDI-TOF MS spectra: (a) 0.5 C/A molar ratio (run 5); (b) 0.1 C/A molar ratio (run 4); (c) the absence of the catalyst. (square) Peaks of the polymer. (star) Peaks of the click product.

Figure 5

Effect of pressure and density on the yield of cycloaddition reaction. Experimental conditions: azide-to-alkyne molar ratio, 1:1; reaction time, 24 h; temperature, 35 °C; C/A molar ratio, 0.5. (square) Values of density of scCO₂ determined by the equation of Bender and (circle) values of the yield of reaction.

Figure 6

Increase in percentage antioxidant activity of different compounds with the increase in test concentrations.

Figure 7

Schematic diagram of the experimental setup for reaction in the supercritical medium. V-1, V-3: check valves; V-2: purge valve. P-1: pump; E-1: cooler; C-1: batch reactor; V-4: valve of depressurization; TIC: temperature digital controller; FI: flow indicator; PI: pressure indicator.