Surface grafting, redox initiators

In the early 1970s, surface modification of most polymers was achieved using redox initiators. Ce+4-induced initiation was employed to achieve surface grafting of acrylamide onto LDPE film [117]. The film was first oxidised by chromic acid and then reduced with diborane to form a hydroxyl-rich surface which was then used to initiate graft polymerisation of acrylamide using Ce+4/HN03. The mechanism of chromic-acid-facilitated surface oxidation of LDPE surface is shown in Scheme 6a and that of free-radical generation is represented in Scheme 6b. 

Surface grafting of barium sulfate is interesting Ifom the point of view of the kinetics of such reactions. Barium sulfate like calcium carbonate, is an inert filler. So it is necessary to modify its surface. First, barium chloride is reacted with sodium sulfate in the presence of a small amount of sodium 12-hydroxystearate. This introduces a controlled number of hydroxyl stearate sites onto the barium sulfate surface. The reaction is followed by a redox graft polymerization of acrylamide initiated by the hydroxyl stearate groups and ceric ion as a catalyst. Figures 6.9 to 6.11 show the effect of reaction substrates concentrations on polymerization rate. 

In conventional grafting procedures redox initiators are commonly used to initiate the grafting reactions. These initiators produce ions in the reaction medium which may be adsorbed at the copolymer surface to some extent. To avoid this, there has been recent interest in the use of microwave induced grafting reactions. Using microwave irradiation, methylmethacrylate (MMA) has been efficiently grafted onto Cassia marginata seed gum in the absence of... 

Chemical oxidation is achieved by introducing oxygen-containing gronps into the membrane surface, using oxidants such as potassium permanganate, nitric acid, and chromic acid, or redox initiators such as ferric chloride. They can oxidize the membrane surface to create active sites where surface graft polymerization is conducted. 

Based on the membrane surface properties and the HA properties, various researchers have attempted to change the membrane surface characteristics by surface modification. Different techniques have been performed, such as ion beam irradiation, plasma treatment, redox-initiated graft polymerization, photochemical grafting, and interfacial polymerization (IP). In this chapter, two surface modification techniques, IP and photochemical grafting, are discussed by means of experimental examples. The surface characteristics of the unmodified membrane and the modified membranes are studied and their relationships with irreversible fouling and NF performance are reported. 

Redox-controlled reversible loading of the -CD polymer on the gold substrates with surface-grafted and ferrocene-functionalized polymer brushes. (Reprinted with permission from Xu et al. 2010. One-Pot Preparation of "Ferrocene-Functionalized Polymer Brushes on Gold Substrates by Combined Surface-Initiated Atom Transfer Radical Polymerization and "Click Chemistry." Langmuir 26 (19) 15376-15382, copyright (2010) American Chemical Society.)... 

Hritcu et al. [149] synthesized first cationic PS latexes covered with a shell-containing poly(styrene-co-2-hydroxyethyl acrylate) by a seed copolymerization procedure using an azo initiator. In the second step, grafted chains anchored to the surface were produced by polymerization of fV-(2-methoxyethyl) acrylamide (MEA), in the presence of Ce(lV) as a redox initiator. However, the percentage of MEA found covalently attached to the surface was very low because bulk MEA polymerization forming soluble polymers occurred to a much greater extent than the actual grafting reaction on the particles. 

Graft polymerization of methacrylic acid monomer could increase the hydro-philicity and impart negative charges on the membrane surface. It has been used to remove endocrine disrupting chemicals and pharmaceuticals active compounds [159]. In addition, surface grafting using redox initiation has been developed, which offers simplicity to the process. The reaction can be performed in an aqueous media at room temperature without an external activation [161]. However, redox initiation has relatively slow kinetics that requires a high concentration of monomer [164]. 

In contrast to UV-initiated graft polymerization, the redox-initiated grafting allows the modification of the polymer membranes in situ, even inside commercial wound membrane elements [71]. Redox system most often used consists of potassium persulfate (K2S2O8) and potassium metabisulfite (K2S2O3) [71,72]. Redox reaction that occurs generates radicals on the membrane surface, upon which monomers can be attached ... 

For chromatographic sorbents it is necessary that the polymeric cover be uniformly distributed over the silica surface and chemically coupled to it. The appropriate introduction of the initiator is one of the decisive steps of this task. The first method (binding to the surface) increases the yield of grafted polymer. However in this case a large amount of homopolymer is formed. This disadvantage could be prevented by the application of hydroperoxide initiators in combination with the proper redox-agents [78-81],... 

This paper summarizes chemical grafting techniques explored in this laboratory that have potential biomedical application. These reactions, initiated by ceric ions, persulfate-bisulfite redox systems, or the presence of comonomers forming donor-acceptor complexes, were carried out in an aqueous environment under conditions which, with suitable modifications, might be tolerated in vivo. Grafting onto tissue surfaces by means of ionizing radiation will not be discussed since techniques for avoiding undesirable side reactions have not yet been developed. 

Mazurowski, M., Gallei, M., Li, J., Didzoleit, H., Stiihn, B., Rehahn, M. (2012). Redox-responsive polymer brushes grafted from polystyrene nanoparticles by means of surface initiated atom Transfer radical polymerization. Macromolecules, 45, 8970-8981. 

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