Oxirane polymerization active groups

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]. 

Early work identified Group II organometallic compounds such as diethylzinc and ethylmagnesium bromide as coordinate initiators of oxirane polymerization. Under carefully controlled conditions with anhydrous, high-purity monomer, the initiator activity was usually found to be weak. Bailey (80) found that with propylene oxide, distilled and maintained anhydrous with molecular sieves, the rate of polymerization of propylene oxide with dibutylzinc actually decreased as the concentration of dibutylzinc increased. This observation strongly suggested that a trace cocatalyst was needed to effect polymerization. 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

End-labelling of the active centres with diphenyl chlorophosphate, combined with31P NMR spectroscopy, was shown to be an effective method for the determination of structure and concentration of the active species in the polymerization of several oxirane compounds.96 98 The chemical shifts of31P signals from the diphenylphosphoryl end-groups are sensitive to the structure of polymer chain end to which it is attached. Thus the 31P NMR spectrum provides information whether the propagating anion is secondary or primary. A similar... 

The other mode of activation of the oxirane moieties is through cationic polymerization, which will generate a network for polyfunctional epoxidized glycerides and thermoplastic materials with fatty acids bearing a single epoxy group [43],... 

This mechanism is similar to that which seems to operate in the cationic oligomerization of oxiranes in the presence of compounds containing hydroxyl groups ( activated monomer polymerization ), discussed in detail in Chapter 4. 

Cyclization by backbiting cannot be avoided if active species of propagation are tertiary oxonium ions located at the chain end. Several years ago, we proposed that if acid-catalyzed polymerization of oxiranes is conducted in the presence of hydroxyl-group-containing compounds, the other mechanism competes with the ACE mechanism. This mechanism is based on the established mechanism of acid-catalyzed hydrolysis of oxiranes.In analogy to the hydrolysis mechanism, if an excess of oxirane (e.g., EO) is present, successive reactions of proto-nated monomer molecules with hydroxyl groups lead to extension of the chain as shown in Scheme 17... 

With methyl oxirane, propylene oxide, ring opening is by cleavage of the less-substituted carbon-to-oxygen bond. Since the terminal oxyalkanol groups are still active, the polymerizations, as represented, are "living" polymerization reactions. However, there are termination side reactions that become particularly important at elevated reaction temperatures. 

The activator polyether, in this case dioxane, opens-up the cyanide-zinc lattice in a manner that permits coordination with oxirane, which displaces the activator polyether. This is followed by hydroxyl attack, in this case from water, and initiation of polymerization. Polymerization proceeds by rearward attack of alkoxide on oxirane, so that the polyether produced is terminated, in the case of propylene oxide, by secondary hydroxyl groups. 

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 ... 

Even if the AM mechanism operates in a cationic polymerization of oxiranes in the presence of hydroxyl groups, it does not eliminate the possible contribution of a conventional active chain end (ACE) mechanism (active center oxonium ion located at the macromolecular chain end). In order for an AM-type propagation to prevail, the instantaneous concentration of monomer should be kept as low as possible (e.g. via continuous slow monomer addition). 

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