Aqueous RDRP in the Presence of Cu⁰: The Exceptional Activity of Cu^I Confirms the SARA ATRP Mechanism

Abstract

Polymerizations and mechanistic studies have been performed to understand the kinetic pathways for the\ud polymerization of the monomer oligo(ethylene oxide)\ud monomethyl ether acrylate (OEOA) in aqueous media.\ud Typically, the medium consisted of 18 wt % OEOA in\ud water, in the presence of Cu catalysts coordinated by tris[2(dimethylamino)ethyl]amine (Me6TREN). Well-controlled\ud polymerization of OEOA can be achieved in the presence of\ud halide anions and Cu wire with≲600 ppm of soluble CuII\ud species, rather than previously reported ca. 10 000 ppm of CuII and Cu0 particles formed by predisproportionation of CuI prior to monomer and initiator addition. The mechanistic studies conclude that even though disproportionation is thermodynamically favored in aqueous media, the SARA ATRP, not SET-LRP,\ud mechanism holds in these reactions. This is because alkyl halides are much more rapidly activated by CuI than by Cu0\ud (contribution of Cu0 to activation is <1%). Because of the high activity of CuI species toward alkyl halide activation,\ud [CuI/Me6TREN] in solution is very low (<5μM) and classical ATRP equilibrium between CuI and CuII species is maintained.\ud Although in aqueous media disproportionation of CuI/Me6TREN is thermodynamically favored over comproportionation, unexpectedly, in the presence of alkyl halides, i.e., during polymerization, disproportionation is kinetically minimized.\ud Disproportionation is slow because its rate is proportional to [CuI/Me6TREN]2 and [CuI/Me6TREN] is very small. Thus, during polymerization, comproportionation is 104 times faster than disproportionation, and the final thermodynamic equilibrium between disproportionation and comproportionation could be reached only after polymerization is completed. Activation of alkyl\ud halides by CuI/Me6TREN in aqueous media occurs 8 orders of magnitude faster than disproportionation

PSt-b-PU-b-PSt copolymers using tetraphenylethane-urethane macroinitiator through SARA ATRP

“…These results suggest the involvement of Cu 0 in the rate-determining step of the reaction, see Equation (1): (1) where R a is defined as an activation rate of macroinitiator [13]. Moreover, molecular weights increased linearly with conversion (Figure 4b), and were almost identical to theoretical ones.…”

“…The DP of the resulting copolymers increased linearly with monomer conversion and semilogarithmic kinetic plots were linear, which confirmed the living nature of the SARA ATRP of styrene. The results from 1 H and 13 C NMR spectral studies support the formation of PSt-b-PUb-PSt tri-block copolymers. Therefore SARA ATRP -method without pre-disproportionation of Cu I species provide a powerful tool for the synthesis of a broad range of copolymers with controlled chain length and narrow molecular weight distributions.…”

“…The apparent molecular weights and dispersity (M w /M n ) were determined with a calibration based on PS standards using TRISEC software from Viscotec Corporation. 1 H and 13 C NMR spectra of the macroinitiator and copolymers were Chmielarz and Król -eXPRESS Polymer Letters Vol.10, No.4 (2016) 302-310 Figure 1. Mechanism of SARA ATRP.…”

“…Lowering the concentration of the catalysts not only results in 'greener' polymerization processes but also allows for the synthesis of polymers with high MW and high chainend functionality [12]. Moreover, SARA ATRP offers simple removal and reuse of unreacted solid Cu 0 , control of the polymerization rate by the amount of ligand and the surface area of Cu 0 [13,14]. SARA ATRP, has Cu I /Cu II reactions as the major activation/deactivation ones, with Cu 0 acting as a supplemental activator of alkyl halides and a reducing agent for excess Cu II , and limiting disproportionation of Cu I ( Figure 1) [15,16].…”

“…Taking into account the Matyjaszewski's researches [17,23,24] it can be stated that the SARA ATRP mechanism is correct. SARA ATRP/SET-LRP was successfully applied to relatively nonpolar monomers (e.g., styrene [25], butyl acrylate [26], methyl acrylate [14,24,26,27], 2-(diisopropylamino) ethyl methacrylate [26,28], methyl methacrylate [19,26,29]), as well as polar monomers (e.g., oligo(ethylene oxide) monomethyl ether acrylate [13], N-isopropylacrylamide [30]) for the preparation of a variety of polymeric materials with different structures and architectures, including homopolymers, and well-defined block copolymers. However, tri-block copolymers have sparked much interest and their potential has been realized in many areas.…”