Methyl, free-radical transfer reactions with

The rate of polymerization of polar monomers, for example, maleic anhydride, acrylonitrile, or methyl methacrylate, can be enhanced by coraplexing them with a metal halide (zinc or vanadium chloride) or an organoaluminum halide (ethyl aluminum sesqui-chloride). These complexed monomers participate in a one-electron transfer reaction with either an uncomplexed monomer or another electron-donor monomer, for example, olefin, diene, or styrene, and thus form alternating copolymers (11) with free-radical initiators. An alternating styrene/acrylonitrile copolymer (12) has been prepared by free-radical initiation of equimolar mixtures of the monomers in the presence of nitrile-coraplexing agents such as aluminum alkyls. 

Although the previously described polymerization with organocobalt porphyrins (7) is the first example of controlled radical polymerization, the applicability of this system is only limited to acrylic esters (20). Use of 7 for free-radical polymerization of methacrylic esters (21) results in a chain-transfer reaction with respect to the a-methyl group, to give oligomers with terminal unsaturation. 

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

An alternative route for the synthesis of TV-methyl amino acids without racemization is shown in Scheme 8.[98 This method includes the use of TBPB in the presence of copper(I) octanoate. The proposed mechanism of this free radical reaction is given in Scheme 8. Electron transfer from copper(I) to TBPB affords the copper(II), benzoate, and tBuO radical 4, which undergoes (3-scission to acetone and methyl radical 5. In turn, electron transfer from the urethane to the copper(II) ion, followed by proton transfer, affords the corresponding urethane radical 6, which reacts with the methyl radical 5 to give the desired product in overall yields of 54% (Z derivative) or 57% (Boc derivative), respectively. 

The key problems in a polymerization CSTR are the determination and characterization of micro- and macromixing, and the possibility of multiple steady states due to the exothermic nature of the reactions. Recent studies of CSTRs for bulk or solution free-radical polymerization indicate the possibility of multiple steady states due to the large heat evolution and difficult heat transfer that are characteristic of the reactors. Furthermore, even in simple solution polymerization (for example, in methyl methacrylate polymerization in ethyl acetate solvent), autocatalytic kinetics can lead to runaway conditions even with perfect temperature control for certain combinations of solvent concentration and reactor residence time. In practice, the heat evolution can be an additional source of autocatalytic behavior. 

Chain polymerizations are less often performed in die bulk, because of problems with the control of the reaction. [An interesting exception is poly(methyl methacrylate), a polymer that is soluble in its own monomer (not all polymers are), and which is synthesized commercially by chain (free radical) polymerization very slowly in bulk (Figure 3-44). The resulting polymer has outstanding optical properties (clarity) because there are very few impurities.] In bulk polymerizations there is a tendency for the reaction mass to form a gel (i.e., have an extraordinarily high viscosity) and hot spots can develop. At the extreme, the reaction rate can accelerate to runaway proportions (for reasons we will discuss when we consider kinetics) with potentially disastrous (explosive) consequences. Viscosity and heat control can be achieved, if necessary, by carrying out the polymerizations to a relatively low conversion, with the unreacted monomer being separated and recycled. Another way to control the viscosity and heat transfer problems of chain polymerizations is to perform the polymerization in solution A major concern with this method is that chain transfer to sol-... 

Termination by disproportionation. In Example 10.3, coupling of two polymer radicals was assumed to be the only termination mechanism, as is indeed essentially true for polymerization of styrene [34], However, various other mechanisms may contribute to termination or even dominate it. The most common of these is disproportionation, mainly observed for tertiary and other sterically hindered free radicals [35]. An example is methyl methacrylate [34] (see reaction 10.26 below). In disproportionation, two polymer radicals react with one another, transferring a... 

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