Microdroplet-Mediated Radical Polymerization

Abstract

Micrometer-sized aqueous droplets serve as a unique reactor that drives various chemical reactions not seen in bulk solutions. However, their utilization has been limited to the synthesis of low molecular weight products at low reactant concentrations (nM to μM). Moreover, the nature of chemical reactions occurring outside the droplet remains unknown. This study demonstrated that oil-confined aqueous microdroplets continuously generated hydroxyl radicals near the interface and enabled the synthesis of polymers at high reactant concentrations (mM to M), thus successfully converting the interfacial energy into the synthesis of polymeric materials. The polymerized products maintained the properties of controlled radical polymerization, and a triblock copolymer with tapered interfaces was prepared by the sequential addition of different monomers into the aqueous microdroplets. Furthermore, a polymerization reaction in the continuous oil phase was effectively achieved by the transport of the hydroxyl radicals through the oil/water interface. This interfacial phenomenon is also successfully applied to the chain extension of a hydrophilic polymer with an oil-soluble monomer across the microdroplet interface. Our comprehensive study of radical polymerization using compartmentalization in microdroplets is expected to have important implications for the emerging field of microdroplet chemistry and polymerization in cellular biochemistry without any invasive chemical initiators.

Deciphering the Kinetics of Spontaneous Generation of H2O2 in Individual Water Microdroplets

“…Due to the asymmetric molecular interactions imposed by surrounding medium, the water molecules positioned at the air–water or oil–water interface exhibit distinct physicochemical properties relative to that of bulk water, such as faster reorientation dynamics of water and unique acid–base behavior. − If the bulk water is atomized into sub-10 μm-sized droplets, named microdroplets, such an interfacial effect becomes more prominent and might accelerate chemical reactions or even activate the thermodynamically unfavorable reactions under ambient conditions. − A latest example is given by Zare and co-workers, who reported a striking discovery of spontaneous formation of H 2 O 2 in water microdroplets with diameters below 10 μm at room temperature, without any catalysts, chemical precursors, and external voltage . In view of the widespread use of H 2 O 2 in various fields and the high energy-consuming production of H 2 O 2 nowadays in industry (i.e., anthraquinone oxidation methods), such a mild production of H 2 O 2 holds significant potential in the field of disinfection and green-route chemical synthesis. − Considering the similar sizes of these water microdroplets to the atmospheric aerosol particles, such a spontaneous process of H 2 O 2 formation suggests important implications in atmospheric chemistry to further understand the role of aerosol on the formation of acid rain, haze, and ozone depletion in the stratosphere . Regarding the origin of life on Earth, this process probably acts as another source of oxygen gas to eventually promote the evolution of life, i.e., decomposition of spontaneously generated H 2 O 2 .…”

Catalyst-free selective oxidation of C(sp3)-H bonds in toluene on-water

“…Moreover, intrinsic strong electric eld 5,6 , high density and alignment of molecules 7 near the aqueous interfaces could facilitate unusual chemistry in water. Those unique chemistry occurring at the waterhydrophobe interfaces, which cannot be observed in bulk water, include the marked acceleration of chemical reaction rates 8,9 , spontaneous redox reactions 10,11 , C-N bond formation 12,13 , amination 14 , carboxylation 11 , and even polymerization 15 .…”

Mechanically Controlled Enzymatic Polymerization and Remodeling

“…24 The sonochemical formation of H 2 O 2 was characterized by the I 3 − method. 25 A continuous production of H 2 O 2 was observed during 10 min of US (Figure S1).…”

“…The mechanical activation of enzymes for initiating polymerization involves two consecutive processes: (i) sonolysis of water to produce H 2 O 2 ; (ii) enzymatic oxidation of H 2 O 2 to provide free radicals for polymerization . The sonochemical formation of H 2 O 2 was characterized by the I 3 – method . A continuous production of H 2 O 2 was observed during 10 min of US (Figure S1).…”