Oxirane polymerization Initiators alkoxides

Initiation of the ionic polymerization by means of an unsaturated compound may also give access to macromonomers, provided the double bond cannot get involved in the polymerization. This method chiefly applies to heterocyclic monomers such as oxolane or oxirane, the initiator being respectively methacryloyl hexafluoroantimonate or the potassium alkoxide derived from p-isopropenylbenzyl alcohol (Scheme 29). 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

Ethylene Oxide The anionic polymerization of ethylene oxide is complicated by the association phenomenon and the participation of ion-pair and free ion intermediates in the propagation reactions [129, 130]. Simple lithium alkoxides are strongly associated into hexamers and tetramers even in polar media such as THE and pyridine [130]. As a consequence, lithium alkoxides are unreactive as initiators for the anionic polymerization of oxiranes. Association effects can be minimized by effecting polymerizations in alcohol media or in dipolar aprotic solvents. 

Various basic (nucleophilic) initiators can be used to initiate anionic polymerization, including covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines, and organometallic compounds such as butyllithium. The polymerization is initiated by addition of the initiating species to the monomer, typically a vinyl compound with electron-withdrawing moieties. In anionic ROPs, cyclic monomers with electron-deficient carbon atoms such as amides (lactams) and esters (lactones), ethers (oxiranes), or Leuchs anhydrides are used as monomers (Scheme 13). 

The mechanism of the ferric chloride-propylene oxide complex-initiated polymerization has been considered to be a coordinate anionic polymerization in which monomer, propylene oxide, first coordinates with iron, followed by nucleophilic attack of the alkoxide group on one of the oxirane carbons. 

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. 

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