Polymerization halide chain ends dependence

Reactive impurities are substances that can react with the monomers, the growing chain-ends, or the acid acceptor to terminate the polymerization prematurely. They can be introduced with the solvent or with the intermediates. The acid chloride may contain impurities originating in its synthesis or storage such as hydrogen chloride, thionyl chloride, phosphorus halides, or monoacid halides. The diamine may contain monoamines, water, or carbonates. It may degrade oxidatively in air or absorb moisture and carbon dioxide. The degree of interference caused by these impurities depends on both the quantity of the impurities as well the relative reaction rates of the desired polymerization vs. those of the impurities. 

The control of the polymerization reaction afforded by ATRP is the result of the formation of dormant alkyl (pseudo)halides. This reduces the instantaneous concentration of the active radicals and thereby suppresses bimolecular termination reactions. The reversible deactivation and activation leads to a slow, but steady growth of the polymer chain with a well defined end group (Scheme 27). Control and properties of the synthesized polymers depend on the stationary concentration of active radicals and the relative rates of propagation and deactivation. When one or less than one monomer unit is incorporated into the polymer chain during one activation step, the polymerization is well controlled. The ATRP equilibrium can be approached from both directions in Scheme 27. Beginning with an alkyl halide and the lower valent metal complex, the process is called direct ATRP. If a conventional thermal initiator like AIBN and the higher valent metal complex are the starting materials, the polymerization process is named reverse ATRP [287]. 

Thus, direct determination by EPR of coppa Il) species was reported by Matyjaszewski and coworkers [139] in the case of styrene ATRP. The polymerization proceeds by monomer addition to free radicals reversibly generated by an atom transfer process from dormant polymer chains with halide end-groups. In these reactions, a small amount of copper(II) species was used as a deactivator which moderates rates and keeps low polydispersity. An example of time-dependent EPR signals of copper species in the ATRP of styrene in toluene, initiated by 1-phenylethyl bromide (styrene/l-phenylethyl bromide/CuBr/ dNbipy=l(X)/l/l/2) at 110°C is shown in Figure 10.7 [139]. 

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