Lewis Pair Radical Polymerization “On-Water”

Abstract

An unprecedented radical polymerization of various (meth)­acrylates, diethyl acrylamide, and 4-vinylpyridine initiated by Lewis pair (LP) catalysts, specifically PPh3/Cu­(OTf)2 or PPh3/Sn­(OTf)2, is reported herein. The reaction occurred at the interface between monomer and water phases, namely, “on-water”, at room temperature. The polymerization of (meth)­acrylates “on-water” was significantly more efficient than the reaction in organic solvents or in bulk. It was determined that the propagation occurs via a single electron transfer, which was evidenced by the inhibition of the reaction by the addition of trace amounts of phenols or phenothiazine. Additionally, MALDI-TOF-MS analysis revealed the occurrence of disproportionation termination. Initiation at the interface was supported by various polymerization studies. Specifically, a higher rate of polymerization of methacrylates was observed for hydrophilic monomers. Moreover, the polymerizations in a mixed solvent of water and MeOH indicated that the heterogeneous system is critical. The generation of a PPh3 radical cation following a single electron transfer from LP was not plausible. Instead, we propose a mechanism involving a Michael addition of PPh3 to monomers activated by Lewis acids “on-water” to generate Cu­(II) or Sn­(II) enolates and/or free radical as propagating species.

Anionic or Radical Polymerization Catalyzed by Metal Triflate-Based Lewis Pairs: A Comprehensive Mechanistic Study

“…The metal enolates in the monomer phase were not decomposed even in the presence of water, but they are reluctant to undergo anionic Michael addition, probably because of the protic nature of water and/or ligand exchange during polymerization. Activation of the monomer carbonyl groups on-water on the surface probably facilitates the LPRP, as previously proposed …”

“…The terminal structure and effects of various inhibitors were then analyzed for the polymerization of DEAA using various LAs with PPh 3 . The LAs, including Fe­(OTf) 3 , Cu­(OTf) 2 , In­(OTf) 3 , Ga­(OTf) 3 , Zn­(OTf) 2 , and Sn­(OTf) 2 , facilitated the LPRP on-water (see entries 1–4, Table ), whereas the LPAP in THF proceeded using Sc­(OTf) 3 , Y­(OTf) 3 , Zn­(OTf) 2 , and Sn­(OTf) 2 (entries 5, 6, 8, and 9 in Table ). Interestingly, the propagation mechanism depends on the solvent used, Zn­(OTf) 2 and Sn­(OTf) 2 .…”

“…The polymerizations proceed with pseudozero-order kinetics with respect to monomer concentration ([M]), and the Michael addition of propagating enolaluminates or enolborates to the LA-activated monomer is the rate-determining step . However, the LAs used are limited to compounds of main group elements, except for rare-earth aryloxides. − We previously reported the LP of PPh 3 /Cu­(OTf) 2 -catalyzed polymerization of (meth)­acrylates, diethyl acrylamide, and 4-vinylpyridine. , The polymerizations were initiated at the interface between water and the monomer (on-water) via the radical propagation mechanism, which is distinct from other LP polymerizations via anionic propagation. Metal triflates exhibit water tolerance, high Lewis acidity, and satisfactory solubility in organic solvents, thereby catalyzing various reactions , including polymerizations such as cationic, radical and ring opening, , and polycondensation .…”

“…41−43 We previously reported the LP of PPh 3 /Cu(OTf) 2 -catalyzed polymerization of (meth)acrylates, diethyl acrylamide, and 4vinylpyridine. 44,45 The polymerizations were initiated at the interface between water and the monomer (on-water) via the radical propagation mechanism, which is distinct from other LP polymerizations via anionic propagation. Metal triflates exhibit water tolerance, high Lewis acidity, and satisfactory solubility in organic solvents, thereby catalyzing various reactions 46,47 including polymerizations such as cationic, 48 radical 49 and ring opening, 50,51 and polycondensation.…”

Pushing Photochemistry into Water: Acceleration of the Di‐π‐Methane Rearrangement and the Paternó‐Büchi Reaction “On‐Water”

“…Specifically, it has been shown that organic reactions are significantly accelerated “on water”, that is, in heterogeneous medium with no solvation of the reaction components. [2] Hence, the observed reaction rates of Aldol reactions, [3] cycloaddition reactions, [4] multicomponent reactions, [5] polymerization reactions, [6] and even metal–organic reactions [1] are larger in heterogeneous aqueous medium as compared with the ones obtained in homogeneous solutions in organic solvents. The mechanistic origin of the apparently general phenomenon is still not fully understood and presently debated,[ 1 , 2a ] and it certainly depends on the respective specific interactions of the substrates with the surrounding medium.…”