Living Radical Polymerization General Mechanisms

Scheme 1. General mechanisms of controlled/" living radical polymerizations. (Reproduced with permission from American Chemical Society).

RAFT polymerization is another powerful controlled/living radical polymerization technique for surface modification.The generally accepted mechanism of the RAFT polymerization is depicted in Scheme 5.4. The... 

There are three general classifications of living radical polymerization based on differences in the reversible activation reaction step described in the previous section. These three mechanisms are termed dissociation-combination, atom transfer and degenerative chain transfer, respectively [17, 18]. 

When the end groups of the polymers obtained by radical polymerization using certain iniferters still have an iniferter function, such radical polymerization is expected to proceed via a living radical mechanism even in a homogeneous system, i.e.,both the yield and the molecular weight of the polymers produced increase with reaction time. The generalized model is shown in Eq. (18) [16] ... 

Living free radical polymerizations were also carried out in miniemulsion systems via the reversible addition-fragmentation chain transfer mechanism [66]. The colloidal stability of miniemulsions is the key issue, and nonionic surfactants result in the best results. The polydispersity index of molecular weight distribution for the resultant miniemulsion polymer is generally smaller than 1.2. 

Since the number of monomers, and thus the resulting polymer structures, are limited by any of the specific living polymerization techniques, appropriate combination of different polymerization mechanisms can lead to a variety of new and useful polymeric materials. Therefore combinations of controlled radical polymerizations and other polymerizations applied to synthesize block copolymers have been developed. Generally, polymers with active sites, such as carbon-halogen or nitroxide or dithioester terminal groups, are synthesized by other living polymerizations, and the product is further used to initiate the controlled radical polymerization. In many cases, this method is essentially a variant of the macroinitiator method discussed above. However, in some cases, these kinds of macromolecules do not act as initiators, and may act as transfer agents. For example, an AB-type amphiphilic block copolymer, CLB-2 was prepared by RAFT polymerization of 2-(N-dimethylamino)ethyl methacrylate... 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

The specificity of the reaction mechanism to the chemistry of the initiator, co-initiator and monomer as well as to the termination mechanism means that a totally general kinetic scheme as has been possible for free-radical addition polymerization is inappropriate. However, the general principles of the steady-state approximation to the reactive intermediate may still be applied (with some limitations) to obtain the rate of polymerization and the kinetic chain length for this living polymerization. Using a simplified set of reactions (Allcock and Lampe, 1981) for a system consisting of the initiator, I, and co-initiator, RX, added to the monomer, M, the following elementary reactions and their rates may be... 

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