Porphyrins alkaline earth metal complexes

Cationic polymerization of alkylene oxides generally produces low molecular weight polymers, although some work [26] seems to indicate that this difficulty can be overcome by the presence of an alcohol (Fig. 1.3). Higher molecular weight polyethylene oxides can be prepared by a coordinated nucleophilic mechanism that employs such catalysts as alkoxides, oxides, carbonates, and carboxylates, or chelates of alkaline earth metals (Fig. 1.4). An aluminum-porphyrin complex is claimed to generate immortal polymers from alkylene oxides that are totally free from termination reaction [27]. 

As noted in Section 3.1.2, the introduction of the crown ethers in the late 1960s gave legitimacy to the concept of stable coordination complexes of the alkali metals. Their presence, and that of many other macrocyclic counterparts (e.g., porphyrins) and three-dimensional chelators (e.g., cryptands, calixar-enes) is now pervasive in both alkali and alkaline-earth coordination chemistry, and the literature on these complexes is vast. Early work in this area was summarized in CCC (1987), and examined in a more focused manner in Comprehensive Supramolecular Chemistry. It is not the intent of this section to repeat such material, but rather to highlight new developments since the mid-1990s. In some cases, specialist reviews are available on these subjects they will be noted where relevant. 

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