Metalloporphyrins polymerization initiators

In contrast to the above polymerizations via anionic and/or coordination anionic mechanisms, radical polymerization initiated with metalloporphyrins remains to be studied. The only example of controlled radical polymerization by metalloporphyrins has been reported by Wayland et al. where the living radical polymerization of acrylic esters initiated with cobalt porphyrins was demonstrated. In this section the radical polymerization of MMA initiated with tin porphyrin is discussed. 

Metalloporphyrins as initiators of vinyl polymerization 89YGK1017. Metalloporphyrins, aspects of organometallic chemistry of 88-CRV1121. 

A key to achieving living polymerization is to develop well-behaved initiators, because the initiator affects both the relative rate of initiation to propagation and the potential for side reactions. - In this chapter, we focus on the precision synthesis of macromolecules with metalloporphyrins as initiators. Extensive initial studies by Inoue and Aida, - and more recent studies by Wayland et al.have shown that the metalloporphyrins which are applicable to controlled macromolecular synthesis include compounds of aluminum (1-4), zinc (5), manganese (6), cobalt (7) and rhodium (8). The first two nontransition metal metalloporphyrins are effective for anionic (nucleophilic) polymerization while... 

Some monomers give the polymers with narrow molecular weight distribution even in the presence of a protic compound as a chain transfer agent. This is different from living polymerization, and a new concept of immortal polymerization has been presented . Living and immortal polymerizations initiated with metalloporphyrins can provide various types of block copolymers and end reactive polymers . ... 

The activity of metalloporphyrins as initiator is dependent on (1) nucleophilicity of metal-axial ligand bond. (2) Lewis acidity of metal. (3) electronic and steric effect of porphyrin group, and (4) photo-excitation of porphyrin group. In this article will be described some of the recent advances in our studies on polymerizations with metalloporphyrins. 

The metalloporphyrin-initiated polymerizations are accelerated by the presence of steri-cally hindered Lewis acids [Inoue, 2000 Sugimoto and Inoue, 1999]. The Lewis acid coordinates with the oxygen of monomer to weaken the C— O bond and facilitate nucleophilic attack. The Lewis acid must be sterically hindered to prevent its reaction with the propagating center attached to the prophyrin structure. Thus, aluminm ortho-substituted phenolates such as methylaluminum bis(2,6-di-/-butyl-4-methylphenolate) accelerate the polymerization by factors of 102-103 or higher. Less sterically hindered Lewis acids, including the aluminum phenolates without ortho substituents, are much less effective. 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

Compared to polymerization with metalloporphyrins of nontransition metals, those with transition-metal complexes have not been well explored to date. Nevertheless, some metalloporphyrins of transition metals have been found to serve as initiators for controlled polymerization. Examples include manganese porphyrins for controlled ring-opening polymerization and organo-cobalt and -rhodium porphyrins for controlled addition polymerization. " -... 

When compared to the abovementioned highly associated multinuclear coordination catalysts of very low initiation efficiency, metalloporphyrins with mononuclear Mt-X species have been reported to be excellent catatysts for the living polymerization of epoxides. The molar mass distribution of... 

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