Free radical initiators transfer constants

If an oil-soluble monomer is dispersed in a continuous aqueous phase without the use of surfactants, suspension polymerization results. The viscosity of the resulting suspension will remain essentially constant over the course of the polymerization. Oil-soluble free radical initiators are used to effect polymerization. The monomer is dispersed into beads by the action of an agitator. Since little or no surfactant is used, no emulsification takes place, and, if the agitation is stopped, the monomer will form a separate bulk phase, usually above the aqueous phase. The monomer is polymerized by the initiator within the droplets, forming polymer beads of approximately the same size as the monomer droplets (0.1-10 mm diameter). The product can be readily separated from the aqueous phase (via filtration or decantation) in the form of macroscopic particles or beads, which can be easily packaged and/or transported. Heat transfer is facihtated by the presence of the continuous aqueous phase. Blocking agents such as clays or talcs are used to prevent particle ag-... 

Vinyl chloride (VC) readily copolymerizes with MA (see table in the appendix to this chapter) maleates and maleimides, using free-radical initiators to give random copolymers.The effect of MA on the chain-transfer constant (C ) during polymerization of vinyl chloride has been studied at 40-70°C. At 60°C, the MA Cs was 7.7 x 10, compared to a styrene Cs of 72.2 X 10. Melville and Burnett determined the copolymerization rate constant for the VC-MA pair to be 2.1 x 10" liter moF s Several studies have shown that equimolar copolymers may also be obtained for the monomer pair (see Chapter 10). It has been observed that the presence of MA enhanced the polymerization rate of vinyl chloride. As shown in the table in this chapter s appendix, vinylidene chloride also undergoes free-radical copolymerization with MA. ... 

Nuclear magnetic resonance determinations showed the various (27A-27C)-MA charge-transfer complexes exhibited fairly high equilibrium constants, i eq = 0.18-1.27 liter/mole. Considering this, the equimolar compositions of the copolymers, and all other results, a mechanism for the free-radical-initiated copolymerization of 27A-27D with MA was conceived " [Eq. (4)] ... 

The thiol ( -dodecyl mercaptan) used ia this recipe played a prominent role ia the quaUty control of the product. Such thiols are known as chain-transfer agents and help control the molecular weight of the SBR by means of the foUowiag reaction where M = monomer, eg, butadiene or styrene R(M) = growing free-radical chain k = propagation-rate constant = transfer-rate constant and k = initiation-rate constant. 

Equation (l) shows the rate of polymerization is controlled by the radical concentration and as described by Equation (2) the rate of generation of free radicals is controlled by the initiation rate. In addition. Equation (3) shows this rate of generation is controlled by the initiator and initiator concentration. Further, the rate of initiation controls the rate of propagation which controls the rate of generation of heat. This combined with the heat transfer controls the reaction temperature and the value of the various reaction rate constants of the kinetic mechanism. Through these events it becomes obvious that the initiator is a prime control variable in the tubular polymerization reaction system. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

Vinyl acetate was polymerized in a free-radical reaction. The initial monomer concentration was 1 mol/liter and its concentration after I h was 0.85 mol/liter. Chloroform was present as a chain transfer agent, with concentrations 0.01 mol/liter at time zero and 0.007 mol/liter after I h. What is the chain transfer constant C in this case ... 

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