Nga H. Nguyen

Visualization of the crucial step in SET-LRP

The crucial step in SET-LRP is the disproportionation of the CuX generated by activation with Cu(0) wire or powder, into nascent, extremely reactive Cu(0) nanoparticles, and CuX2. Nascent Cu(0) activates the initiator and dormant chains via a heterogeneous single-electron transfer (SET) mechanism. Here we report model reactions visualizing the disproportionation of CuBr and activation by nascent Cu(0) in protic, dipolar aprotic, and nonpolar solvents, and in protic, polar and nonpolar monomers. The nascent Cu(0) nanoparticles and the green/blue color of the solution of CuBr2/N-ligand were visible, demonstrating that disproportionation occurs under all SET-LRP and many ATRP conditions. Unexpectedly, nascent Cu(0) nanoparticles and insoluble CuBr2 were also formed via a surface disproportionation of CuBr/Me6-TREN in a range of nonpolar solvents and monomers. The effect of solvent polarity on the rate of SET activation was visualized by adding methyl 2-bromopropionate (MBP) initiator to the disproportionation mixture and monitoring the disappearance of the nascent Cu(0) nanoparticles. The consumption of nascent Cu(0) was extremely rapid in DMSO, fast in MeCN, and slower in toluene. This trend confirms the expected dependence of SET activation on solvent polarity. The enhanced stabilization of Cu(0) nanoparticles in DMSO compared to MeOH was visualized, and used to explain the synergistic solvent effect in DMSO–MeOH and other solvent–monomer mixtures. Visualization of the disproportionation and activation also explains the rate acceleration in CuX-catalyzed ATRP in polar media in which the active catalyst is most likely the extremely reactive nascent Cu(0) generated by disproportionation, and therefore, rapidly discriminates between SET-LRP and ATRP.

A comparative study of the SET-LRP of oligo(ethylene oxide) methyl ether acrylate in DMSO and in H2O

A comparative analysis of the SET-LRP of oligo(ethylene oxide) methyl ether acrylate (OEOMEA) in DMSO and in H2O at 25 °C is reported. Both the catalysis with activated Cu(0) wire/Me6-TREN and with mimics of “nascent” Cu(0) nanoparticles/Me6-TREN resulted in a higher rate of polymerization in water than in DMSO. This result is consistent with the acceleration expected for SET-LRP by a more polar reaction solvent, and with the difference between the equilibrium constants of disproportionation of CuBr in DMSO (Kd = 1.4–4.4) and in water (Kd = 106 to 107), both much higher in the presence of Me6-TREN. The inefficient access of the Cu(0) catalyst to the hydrophobic reactive centers of the monomer and initiator assembled in micellar structures explains the induction time observed in the SET-LRP of OEOMEA in water. This induction period is longer for Cu(0) wire. The use of “nascent” Cu(0) nanoparticles prepared by the disproportionation of CuBr in DMSO, in combination with 5 mol% CuBr2, led to an extremely efficient SET-LRP of OEOMEA in water. This SET-LRP in water is fast and follows first order kinetics to complete monomer conversion with linear dependence of experimental Mn on conversion, and narrow molecular weight distribution. Under the polymerization conditions investigated in both water and DMSO, no reduction in the absorbance of CuBr2/Me6-TREN was observed by online UV-vis spectroscopy. This excludes the formation of CuBr by reduction of CuBr2 by Cu(0) during the SET-LRP in DMSO and in water.

Interrupted SET-LRP of methyl acrylate demonstrates Cu(0) colloidal particles as activating species

To determine how “nascent” Cu(0) colloidal nanoparticles mediate the single electron transfer-living radical polymerization (SET-LRP), two different series of interruption experiments were performed on SET-LRP of methyl acrylate in DMSO with [MA]0/[MBP]0/[Me6-TREN]0 = 222/1/0.1 and hydrazine-activated Cu(0) wire. When the polymerization was interrupted by lifting the Cu(0) wire wrapped around the stirring bar out of the reaction medium with a magnet, leaving colloidal Cu(0) particles, soluble CuBr and CuBr2 formed during the reaction in solution, the polymerization still proceeded, but at a much slower rate. However, by carefully decanting the reaction mixture from one Schlenk tube containing the Cu(0) wire catalyst to a second one without the Cu(0) catalyst, the polymerization reached a complete stop. This decantation experiment demonstrates that soluble CuBr/L cannot be a major contributor to the activation process in SET-LRP. Therefore colloidal Cu(0) particles generated from the disproportionation of CuBr/L are the catalytic species. The presence of the Cu(0) colloidal particles in the reaction medium in the direct lifting setup is most likely responsible for the continued polymerization in the absence of the Cu(0) wire catalyst. Furthermore, SET-LRP was demonstrated to be “immortal” towards catalyst removal. Finally, it was demonstrated that colloidal Cu(0) nanoparticles formed via disproportionation agglomerate into highly visible particles that were immediately consumed upon addition of the methyl 2-bromopropionate initiator, leaving no observable Cu(0) aggregates.

Where is Cu(0) generated by disproportionation during SET-LRP?

The nucleation and growth of nascent Cu(0) nanoparticles formed by disproportionation during SET-LRP on the surface of Cu(0) wire was demonstrated by Scanning Electron Microscopy analysis of the surface of Cu(0) wire before and after disproportionation and polymerization. The dynamics of nascent Cu(0) formation, activation of dormant species and their nucleation and growth on the existing Cu(0) surface determines the Cu(0) consumption during polymerization.

SET-LRP of N-(2-hydroxypropyl)methacrylamide in H2O

Cu(0) wire-catalyzed SET-LRP of N-(2-hydroxypropyl)methacrylamide (HPMA) initiated with methyl 2-chloropropionate (MCP) in H2O at 50 °C exhibited linear kinetics up to 90% conversion using 0.5 equivalents of Me6-TREN with respect to initiator concentration.

SET-LRP of 2-hydroxyethyl acrylate in protic and dipolar aprotic solvents

Cu(0) wire-mediated single-electron transfer living radical polymerization (SET-LRP) of 2-hydroxyethyl acrylate (HEA) was performed in protic solvents, MeOH, binary mixtures of MeOH and EtOH with H2O, and H2O, and in the dipolar aprotic solvent, DMSO. The tertiary alkyl halide initiator, ethyl 2-bromoisobutyrate (EBiB), and the tris[2-(dimethylamino)ethyl]amine (Me6-TREN) ligand mediated rapid SET-LRP of HEA providing poly(HEA) (PHEA) with narrow Mw/Mn. When SET-LRP of HEA was performed at high H2O content in MeOH, and in H2O, gel formation was observed exclusively on the Cu(0) wire surface. This demonstrated the heterogeneous nature of the Cu(0)-mediated SET activation that promotes a strong adsorption of PHEA and slow diffusion of PHEA radicals generated from activation on the Cu(0) wire surface by the hydrophobic effect. High molecular weight PHEA was obtained at [M]0/[I]0 = 400 and 800 with Mw/Mn < 1.45 in MeOH + 40% H2O in 50 min. This suggests significantly less termination and a much higher rate of SET-LRP of HEA at 25 °C than in the previously reported CuX-catalyzed polymerization of HEA in bulk (90% conversion, Mn = 30 000 after 14 h at 90 °C) or solution (87% conversion, Mn = 14 700 after 12 h at 90 °C) at elevated temperatures. When targeting higher [M]0/[I]0, Mw/Mn increases with conversion but first order kinetics was observed. This was attributed to the increased hydrophobic effect of PHEA at higher Mn that is accompanied by slow desorption of the polymer from the Cu(0) wire and a reduced exchange rate between dormant and active species.

Synthesis of Ultrahigh Molar Mass Poly(2-Hydroxyethyl Methacrylate) by Single-Electron Transfer Living Radical Polymerization

Cu(0)/Me6-TREN mediated SET-LRP of 2-hydroxyethyl methacrylate (HEMA) initiated with methyl α-bromophenylacetate (MBrPA) was performed in DMSO at 25 °C targeting [M]0/[I]0 = 100 to 10 000. At [M]0/[I]0 = 100, SET-LRP of HEMA is a living process, and provided PHEMA with Mn = 21 500 g mol−1 and Mw/Mn = 1.20 in 7 h. Using similar conditions, PHEMA with Mn = 35 000 to 152 200 g mol−1 and Mw/Mn = 1.28 to 1.39 was prepared within 9 h. When targeting higher [M]0/[I]0 (2000 to 10 000), Me6-TREN concentration was changed to 0.15 equivalent with respect to initiator concentration, for at higher ligand concentration the polymerization did not proceed beyond 30% conversion even after a long reaction time. PHEMA with Mn = 333 500 to 1 017 900 g mol−1 and Mw/Mn lower than 1.50 was synthesized for the first time by direct polymerization of HEMA without protecting the hydroxyl group.