Oxirane polymerization active sites

Many group 13 compounds have been prepared with porphyrins. The majority of these compounds were created to serve as models for the active sites of enzymes, such as cytochrome c oxidase. Additionally, the gallium compounds are more robust than their iron counterparts. To a much lesser extent the Por-AlX compounds are used as oxirane polymerization catalysts [8]. 

With monomers of the first class, methyl oxirane for example, the active sites are formed in an irreversible way after the reaction (or very strong complexation) of the initial monomer with the initiator. As a proof, one finds that the stereoelectivity (r), i.e. the enahtiomorphic distribution of sites, is not modified by a change of the temperature of polymerization, but (r) is strongly depending on the enantiomeric composition of the initial monomer. 

The number and the nature of the active sites remain constant during the polymerization for monomers such as methyl oxirane and methyl thiirane and this determines a first order kinetic law in monomer. 

Penczek and Kubisa developed a new cationic polymerization technique for cyclic monomers in which the chain propagation involves the reaction of a protonated (activated) monomer molecule with a nucleophilic site in the neutral growing macromolecule. This so-called activated monomer (AM) polymerization is depicted mechanistically in Scheme 58. According to this mechanism, when the polymerization of an oxirane (a cyclic ether) is carried out in the presence of... 

In a first step the monomer reacts with the initiator to form a full spectrum of sites having different R and S character. Some of these formed species have a complete selectivity and produce crystalline isotactic polymers. The proportion of such selective species for a given initiator is depending on the nature of the monomer. We have seen that for monomers with bulky substituents like t-butyl thiirane almost all the sites are purely selective, while for other monomers like methyl oxirane only 20 % of the active species are selective. If the initiator is optically active there is an unbalanced amount of R type and S type species and therefore stereoelection will occur when polymerizing a racemic monomer mixture. 

Although polymerization of oxiranes by the AM mechanism offers several advantages, there are also some limitations. Instantaneous [M]/[HO] ratio should be kept low thus monomer should be fed slowly into reaction mixture. The reaction is, therefore, slow and to achieve higher molecular weights, long reaction times are required. Moreover, the proton from a catalyst is exchanged quickly between all basic sites and only its fraction participates in the activation of the monomer. At the... 

With certain cyclic ethers (dioxolane, oxiranes, etc.), the use of particular conditions results in a limitation of the back-biting reactions and a certain control of the polymerization. For instance, in polymerizations carried out in the presence of an alcohol and with a very low instantaneous monomer concentration, obtained by a slow addition of the monomer solution, almost the totality of electrophilic entities carried by the initiator reacts with the monomer to give protonic species ( activated monomer) the latter then react with the nucleophilic sites that are the hydroxy groups of alcohols. Propagation occurs through nucleophilic attacks of hydroxyls onto activated monomer molecules ... 

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