Stable free radical polymerization concept

Three basic concepts concerning stable free-radical polymerization (SFRP) also called nitroxide-mediated radical polymerization (NMP), atom-transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT) have been developed during the last decade. 

A similar concept was used in the development of artificial chymotrypsin mimics [54]. The esterase-site was modeled by using the phosphonate template 75 as a stable transition state analogue (Scheme 13.19). The catalytic triad of the active site of chymotrypsin - that is, serine, histidine and aspartic acid (carboxy-late anion) - was mimicked by imidazole, phenolic hydroxy and carboxyl groups, respectively. The catalytically active MIP catalyst 76 was prepared using free radical polymerization, in the presence of the phosphonate template 75, methacrylic acid, ethylene glycol dimethacrylate and AIBN. The template removal conditions had a decisive influence on the efficiency of the polymer-mediated catalysis, and best results were obtained with aqueous Na2CC>3. 

Atom transfer free-radical polymerization (ATRP) proceeds by a transient/ stable radical mechanism analagous to nitroxide-mediated free-radical polymerizations (see Section 3). This controlled polymerization concept was first described independently by two research groups in 1995, and exhibits a high degree of control over the molecular weight of the desired polymer and more remarkably, the ability to realize very narrow molecular weight distributions < 1.05). ATRP methodologies involve (see... 

It should be noted that, whereas the preceding discussion has been cast in terms of free-radical polymerizations, the thermodynamic argument is independent of the nature of the active species. Consequently, the analysis is equally valid for ionic polymerizations. A further point to note is that for the concept to apply, an active species capable of propagation and depropagation must be present. Thus, inactive polymer can be stable above the ceiling temperamre for that monomer, but the polymer will degrade rapidly by a depolymerization reaction if main chain scission is stimulated above T.. 

The concept of reversible termination by using a stable free radical has recently been shown to control growing free-radical chains [104,105]. The stable nitroxide radicals such as 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) are known to act as strong polymerization inhibitors [106,107]. In the mean time, nitroxides react at nearly diffusion-controlled rates with carbon-centered radicals, and the reaction rates are influenced by solvent... 

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