Organic free radical initiators

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Free radical initiators are compounds containing covalent bonds that readily undergo hemolytic cleavage producing free radicals. The most widely used organic free radical initiators are peroxides and azo compounds. Here, we will briefly describe the synthesis of the more widely employed free radical initiators. 

Dixon, K.W. (1999) Decomposition rates of organic free radical initiators. In J. Brandrup, E.H. Immergut and E.A. Grulke (Eds) Polymer Handbook (4th edn). Wiley, New York, p. 1-76. 

However, further investigation of this reaction found identical KIEs (within statistical differences) for both the AIBN (free-radical) and Cu(l)-initiated processes.DFT calculations reasonably reproduced the isotope effects and indicated that the a-carbon KIE was associated with the weakening of the two vibrational modes related to the Cq=C stretch in going from the alkene (1720 and 1395cm ) to the transition state (1616 and 1322cm ). The authors concluded that the previous report was statistically anomalous and that ATRP reactions exhibit identical chain-propagation steps as organic free-radical-initiated polymerizations. 

The decomposition of most organic free radical initiators follows first order kinetics. With certain peroxides, however, higher order deeompositions are observed. Generally, the higher order reaetion is caused by a reaction of radicals with the initiator (indueed decomposition). The value of the rate for unimoleeular decomposition m be determined either by extrapolation of the rate back to zero initiator concentration or by use of a monomer or other radical trap . Some of the peroxides may also decompose by non-radical routes. Acids, bases, and polar solvents favor ionic intermediates. Koenig (296) presents an excellent discussion of dw and peroxide decomposition pathways. 

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