Redox initiating methods

Grafting reactions are conventionally carried out by using the ceric ion induced redox initiation method [51, 52]. The details of the synthesis and the reaction conditions are as follows ... 

An alternative procedure consisted of trichloroacetylation of the end groups and redox initiation with sulfur dioxide and pyridine309. In these two methods the block efficiency was not high. An improved method was proposed by Furukawa in which a macromolecular initiator is prepared from polyetherglycol and azobiscyanovaleroyl dichloride310 311. ... 

Redox initiation is often an efficient method for graft polymerization. Hydroxyl-containing polymers such as cellulose and poly(vinyl alcohol) undergo redox reaction with ceric ion or other oxidizing agents to form polymer radicals capable of initiating polymerization... 

This method involves graft copolymerisation using redox initiators [70] or free radical initiators [71, 72, 73] usually in the solution phase, occasionally under the influence of temperature, predominantly in the latter case. Redox systems have extensively been used to generate active sites especially on the natural polymers [74] (like cellulose). Transition metals viz. Cr+6, V+5, Ce+4,... 

Another successful example is the separation of a series of steroids listed in Fig. 6.11 using a monolithic capillary column prepared by redox initiated polymerization of a solution of acrylamide 4, methylene bisacrylamide 5, vinylsulfonic acid 12, and dodecyl acrylate 18 in N-methylformamide/TRIS-boric acid buffer (pH 8.2) to which polyethylene glycol) (MW 10,000) was added (overall composition 5% T, 60% C, 10% vinylsulfonic acid, 15% lauryl acrylate, 3% polyethylene glycol)). The capillary tube was first vinylized and its part beyond the detection window was coated with linear polyacrylamide to avoid band broadening. Since laser induced fluorescence was used to decrease the detection limit of the method to about 100 attomoles for neutral steroids, all of the analytes were first tagged with dansylhydrazine. Fig. 6.12 shows an... 

Thermoplasticization of Wood by Heterogeneous Graft Copolymerization (Redox Initiation). A redox grafting method using ferrous sulfate-hydrogene peroxide as the initiator was conducted to copolymerize MMA to wood meal. The results of grafting are shown in Table III. 

Note that the initiation step dominates the overall temperature dependence of the rate of polymerization. When the method of initiation varies, Er will also change. For redox initiation, for example, d is of the order of 40-60 kJ/mol and R for redox polymerizations is about 40 kJ/mol. For photochemical or radiation-induced polymerizations, d is practically zero and the rate of polymerization in such cases does not change much with the reaction temperature. 

The Me-p-CD/monomer complexes 3a and 4a were homopolymerized in water at room temperature using the redox initiator system K SiOs/NaiSiOs (Figs. 6 and 7). To compare our technique with traditional polymerization methods, all polymerizations were also carried out starting from the free monomers 3, 4, and 5 in organic solvents (toluene or benzene) at 80°C using AIBN as initiator (Table 2). 

In free-radical polymerization carried out in aqueous medium, the decomposition of peroxide or persulfate is greatly accelerated by the presence of a reducing system. This method of free-radical initiation is referred to as redox initiation. The initiation resulting from the thermal decomposition of oiganic compounds discussed above is appropriate only for polymerizations carried out at room temperature or higher. The enhanced rate of free-radical formation in redox reactions permits polymerization at relatively lower temperatures. Typical redox reactions for emulsion polymerization are shown in Equations 1.5-11. 

An alternative method is to use a semi-coitinuous process in which inverse emulsion and initiator solution are progressively added in the course of the reaction. When redox initiators operating at room temperature are used, a common practice is first to inject the oxidant part in the water-in-oil emulsion and to add the reducing portion progressively [11]. This initiation mode is an additional way to control the polymerization reaction since the half-life of a redox initiator is shorter than that of an azo initiator 10 h at 64 °C for AIBN (2,2 -azobis(isobutyrtHiitrile)) compared to 8 min at 45 °C for /-butyl hydroperoxide/cobalt stearate pair [16]. 

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