Solvent effects, atom transfer radical polymerization

The synthesis of mixed peroxides formed from /-butyl hydroperoxide and carbon-centred radicals has been studied. The reactions were strongly effected by solvents as well as catalytic amounts of Cun/Fem. The kinetic data suggest that the conditions for the Ingold-Fischer persistent radical effect are fulfilled in these cases.191 The use of Cu /Cu" redox couples in mediating living radical polymerization continues to be of interest. The kinetics of atom-transfer radical polymerization (ATRP) of styrene with CuBr and bipyridine have been investigated. The polymer reactions were found to be first order with respect to monomer, initiator and CuBr concentration, with the optimum CuBr Bipy ratio found to be 2 1.192 In related work using CuBr-A-pentyl-2-... 

Matyjaszewski, K., Nakagawa, Y., and Jasieczek, C. B. (1998). Polymerization of n-butyl acrylate by atom transfer radical polymerization. Remarkable effect of ethylene carbonate and other solvents. Macromolecules, 31(5) 1535-1541. 

Braunecker WA, Tsarevsky NV, Gennaro A, Matyjaszewski K. Thermodynamic components of the atom transfer radical polymerization equilibrium quantifying solvent effects. Macromolecules 2009 42 6348-6360. 

Feng W, Chen R, Brash JL, Zhu S (2005) Surface-initiated atom transfer radical polymerization of oligo(ethylene glycol) methacrylate effect of solvent on graft density. Macromol Rapid Commun 26(17) 1383-1388... 

Clearly the benzylic H-atoms attached to the polystyrene backbone are not as labile as in cumene. This is likely due to the steric effect of the coil configuration of the polymer chain which blocks access of the /er/-butoxy radicals. Nonetheless, some backbone H-atom abstraction from the polystyrene backbone does occur during radical polymerization of styrene. The extent of abstraction is proportional to the concentration of peroxide initiator added to the process. Typically, in commercial continuous bulk polymerization processes the concentration of peroxide initiator is kept below 500 ppm. Also a few percent of a solvent having some chain transfer activity (ethylbenzene) is added to the styrene feed. This is done so that the extent of branching is small. If the concentration of initiator is increased to >500 ppm and/or the chain transfer solvent falls below a certain level, the extent of branching can increase to a level where gels began to appear in the product. The mechanism of... 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

Transfer to solvent. A significant decrease in polymer chain length is often found when polymerizations are carried out in solution rather than in the undiluted state, and this variation is a function of both the extent of dilution and the type of solvrait used. The effectiveness of a solvent in a transfer reaction depends largely on the amount present, the strength of the bond involved in the abstraction step, and the stability of the solvent radical formed. With the exception of llnorine, halogen atoms are easily transferred, and the reaction of styrene in CCI4 is a good example of this chain transfer. 

In many polymerization systems the polymer molecular weight is observed to he lower than predicted on the basis of the experimentally observed extents of temtination by coupling and disproportionation. This effect is due to the premature temtination of a growing polymer by the transfer of a hydrogen or other atom or species to it from some compound present in the system—the monomer, initiator, or solvent, as the case may be. These radical displacement reactions are termed chain-transfer reactions and may be depicted as... 

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