Shampa R. Samanta

Single electron transfer in radical ion and radical-mediated organic, materials and polymer synthesis.

The article discusses the mechanisms and the applications in organic synthesis, materials, supramolecular, and polymer synthesis of most organic reactions mediated by single electron transfer. Each reaction or class of reactions will be discussed by starting with the original discovery publication, followed by a summary of all or most review articles published in the field, and a discussion of the mechanism(s) and of the most important methodologic and synthetic developments since the most recent review was published. The mechanisms of most organic reactions are considered to proceed by two-electron transfer pathways, even though both biology and radical chemistry rely extensively on one-electron transfer processes. Radicals generated by homolytic cleavage at high temperature were traditionally employed in the industrial production of polymers and to a lesser extent in the synthesis of organic molecules.

Aqueous SET-LRP catalyzed with "in situ" generated Cu(0) demonstrates surface mediated activation and bimolecular termination

The aqueous SET-LRP catalyzed with “in situ” generated Cu(0) of the two amphiphilic monomers 2-hydroxyethyl acrylate (HEA) and oligo(ethylene oxide) methyl ether acrylate (OEOMEA) was investigated at temperatures from −22 to +25 °C. The kappp values of both monomers are higher at 0 °C (4.61 min−1 for OEOMEA and 2.60 min−1 for HEA) than at 25 °C (1.60 min−1 for OEOMEA and 1.12 min−1 for HEA). These unexpected and unprecedented results are explained by the lower Cu(0) particle size obtained by the disproportionation of CuBr at 0 °C in H2O. Poly(OEOMEA) obtained by aqueous SET-LRP at 0 °C with the unexpectedly high kappp = 4.61 min−1 exhibits 88% chain-end functionality at 100% monomer conversion, while the theoretical value would have to be ∼0%. This high experimental chain-end functionality was explained by the slow desorption of the hydrophobic backbone containing the propagating radicals of these amphiphilic polymers from the surface of the catalyst due to their strong hydrophobic effect. Polymer radicals adsorbed on the surface of Cu(0) undergo monomer addition and reversible deactivation but do not undergo the bimolecular termination that requires desorption. This amplified adsorption–desorption process that mediates both the activation and the bimolecular termination explains the unexpectedly high chain-end functionality of the polymers synthesized by SET-LRP.

Self-activation and activation of Cu(0) wire for SET-LRP mediated by fluorinated alcohols

Herein we report the self-activation and activation of Cu(0) wire used to form a catalyst in single-electron transfer living radical polymerization (SET-LRP) in two fluorinated alcohols employed as solvents, 2,2,2-trifluoroethanol (TFE) and 2,2,3,3-tetrafluoropropanol (TFP). Self-activation and activation of Cu(0) wire by TFP exhibit a stronger increase of the apparent rate constant of propagation, kappp, compared to TFE. SET-LRP of methyl acrylate (MA) in DMSO catalyzed with TFP-activated Cu(0) wire showed a comparable kappp value to that of the hydrazine-activated Cu(0) wire-catalyzed counterpart. SET-LRP catalyzed with Cu(0) wire pretreated with TFE or TFP, and the in situ activation of Cu(0) wire while using TFE or TFP as solvent, proceeded with no initial induction period and exhibits predictable molecular weight evolution with conversion and narrow molecular weight distribution.

SET-LRP of hydrophobic and hydrophilic acrylates in tetrafluoropropanol

Efficient disproportionation of CuBr–Me6-TREN in 2,2,2-trifluoroethanol (TFE) to produce “nascent” Cu(0) and CuBr2–Me6-TREN indicates that this semifluorinated alcohol has the potential to be an excellent solvent for Cu(0) mediated single electron transfer-living radical polymerization (SET-LRP). Herein, we report the SET-LRP of a range of hydrophobic and hydrophilic acrylates, including methyl acrylate (MA), n-butyl acrylate (nBA), tert-butyl acrylate (tBA), 2-ethylhexyl acrylate (EHA) and 2-hydroxyethyl acrylate (HEA) using 2-bromopropionate (MBP) and 2-ethylbromoisopropionate (EBiB) as initiators, and Me6-TREN as ligand in TFE. Analysis of the kinetics of polymerization and of the polymer chain ends by a combination of 1H NMR, GPC, MALDI-TOF and chain-end functionalization by “thio-bromo” click reaction demonstrated the synthesis of perfect or near-perfect chain-end functional polyacrylates by SET-LRP in TFE.

Synthesis of high molar mass poly( n -butyl acrylate) and poly(2-ethylhexyl acrylate) by SET-LRP in mixtures of fluorinated alcohols with DMSO

SET-LRP of n-butyl acrylate (nBA) and 2-ethylhexyl acrylate (EHA) initiated with bis(2-bromopropionyl)ethane (BPE) to synthesize high molar mass poly(nBA) and poly(EHA) was carried out in binary mixtures of 2,2,2-trifluoroethanol (TFE) or 2,2,3,3-tetrafluoropropanol (TFP) with DMSO at 50 °C. Using a solvent mixture of TFP containing 30% DMSO led for the first time to the synthesis of poly(nBA) with Mn = 527700, Mw/Mn = 1.21 in 12 h, and poly(EHA) with Mn = 913100, Mw/Mn = 1.20 in 15 h via SET-LRP. Although these two fluorinated alcohols provide complete solubilization of the hydrophobic monomers, nBA and EHA, and of the resulting polymers in addition to an efficient disproportionation of Cu(I)Br and subsequent stabilization of the Cu(II)Br2/L complex, SET-LRP targeting high molar mass poly(nBA) and poly(EHA) in these solvents alone resulted in a relatively slow polymerization with limited conversion. By contrast, DMSO, in spite of being the preferred solvent for SET-LRP with ability to disproportionate Cu(I)Br, stabilizes the “nascent” Cu(0) nanoparticle and provides an efficient SET process, generating a non-living polymerization for these hydrophobic monomers due to the insolubility of the resulting polymers. Remarkably, by a cooperative and synergistic effect, the binary mixtures of TFE or TFP with DMSO provide excellent reaction media for the synthesis of high molar mass poly(nBA) and poly(EHA) by SET-LRP.

SET-LRP of methacrylates in fluorinated alcohols

Activated Cu(0) wire-mediated single-electron transfer living radical polymerization (SET-LRP) of methyl methacrylate (MMA), ethyl methacrylate (EMA) and n-butyl methacrylate (BMA) was performed in two fluorinated alcohols, 2,2,2-trifluoroethanol (TFE) and 2,2,3,3-tetrafluoropropanol (TFP) using tosyl chloride (TsCl) as initiator, hydrazine-activated Cu(0) wire as catalyst, and Me6-TREN as ligand at 50 °C. The ability of these two solvents to solubilize these monomers, the resulting polymers, and mediate an efficient disproportionation of Cu(I)Br in the presence of Me6-TREN ligand to produce the “nascent” Cu(0) activator, Cu(II)Br2 deactivator and to stabilize Cu(II)Br2/Me6-TREN complex make them excellent solvents for SET-LRP of methacrylates. To the best of our knowledge, this is the first report of SET-LRP of methacrylates in fluorinated alcohols and also the first efficient SET-LRP of EMA and BMA in any solvent. The kinetic experiments and molecular weight analyses demonstrate that in all experiments a living polymerization takes place with excellent evolution of molecular weight and with narrow molecular weight distribution until almost complete monomer conversion. It was also demonstrated that the rate of polymerization conducted in TFE and TFP could be significantly enhanced by a synergistic solvent effect upon addition of a small amount of water, which mediates an even faster disproportionation of Cu(I)Br into Cu(0) activator and Cu(II)Br2 deactivator.

SET-LRP of semifluorinated acrylates and methacrylates

Single electron transfer-living radical polymerization (SET-LRP) of 1H,1H,2H,2H-perfluorooctyl acrylate, 2,2,3,3,4,4,4-heptafluorobutyl acrylate, and 1H,1H,5H-octafluoropentyl methacrylate up to [M]0/[I]0 = 30 and 1H,1H,5H-octafluoropentyl acrylate up to 93% monomer conversion targeting [M]0/[I]0 = 260 with excellent control of molecular weight and molecular weight distribution was accomplished in 2,2,2-trifluoroethanol (TFE) at 25 °C for the acrylates and 50 °C for the methacrylate. These experiments demonstrated that TFE serves as a very good solvent for SET-LRP of semifluorinated monomers using hydrazine activated Cu(0) wire as a catalyst, bis(2-bromopropionyl)ethane and p-toluene sulfonyl chloride as initiators, and Me6-TREN as a ligand. Analysis of the kinetics of polymerization and of the polymer chain ends by a combination of 1H-NMR, GPC, MALDI-TOF and chain-end functionalization by “thio-bromo” click reaction demonstrated the synthesis of perfect or near-perfect chain-end functional semifluorinated homopolymers.

A rational approach to activated polyacrylates and polymethacrylates by using a combination of model reactions and SET-LRP of hexafluoroisopropyl acrylate and methacrylate

The transesterification of hexafluoroisopropyl esters mediated by two mild bases, 1,8-diazabicycloundec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was investigated as model reaction for the transesterification of poly(1,1,1,3,3,3-hexafluoroisopropyl acrylate) [poly(HFIPA)] and poly(1,1,1,3,3,3-hexafluoropropyl methacrylate) [poly(HFIPM)]. Unexpectedly, the rate of transesterification of the hexafluoroisopropyl esters was higher than that of the more reactive pentafluorophenyl esters although the rate of uncatalyzed aminolysis followed the reverse trend. Subsequently SET-LRP of HFIPA up to [M]0/[I]0 = 92 at 25 °C and of HFIPM up to [M]0/[I]0 = 86 at 50 °C with activated Cu(0) wire as catalyst, Me6-TREN as ligand, and trifluoroethanol as solvent in the presence of acetic acid to generate the corresponding polymers with well-defined molecular weight, narrow molecular weight distribution and near-quantitative chain-end functionality was elaborated. Using reaction conditions established with model compounds a highly efficient transesterification of poly(HFIPA) and of poly(HFIPM) using DBU as catalyst at 25 °C has been developed. Therefore, this combination of experiments demonstrated that hexafluoroisopropyl esters provide a new class of activated carboxylic acids with applications in functional group transformation of interest both to organic and macromolecular synthesis.

Release of guests from encapsulated masked hydrophobic precursors by a phototrigger.

Examples of release of organic acids from encapsulated p-methoxyphenacyl esters provided here demonstrate the value of a phototrigger strategy to release chemicals of interest in water from hydrophobic precursors. In this study, a photochemical β-cleavage process centered on the p-methoxyphenacyl group is exploited to release the acid of interest from a water-soluble capsule made up of octa acid.

Photodimerization of hydrophobic guests within a water-soluble nanocapsule

Conducting reactions in environmentally benign conditions is one of the major objectives of “green chemistry.” In this context, developing ways to conduct reactions in water seems obvious. In this report, we present our results on photodimerization of select guest molecules placed within the rigid reaction cavity of a water-soluble cavitand, octa acid. The results presented herein highlight the value of a supramolecular approach in achieving selectivity in photoreactions and opening reaction pathways that are latent in solution chemistry.