Anionic polymerization of epoxides

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by hydroxides, alkoxides, oxides, and metal alkyls and aryls, including radical-anion species such as sodium naphthalene. Thus, the polymerization of ethylene oxide by e.g., K (Bu 0 ), involves 

This propagation reaction can be visualized as involving the formation of an incipient alkoxide anion on cleavage of the oxygen-metal bond in the propagating chain — hence the name anionic coordination. Many anionic coordination polymerizations proceed with stereochemical consequences. 

Polymer molecular weights are low for anionic polymerizations of propylene oxide ( 5000) since polymerization is severely limited by chain transfer to monomer. Chain transfer to monomer can take place by proton abstraction from the methyl group attached to the epoxide ring  

Chain transfer to monomer is much less prevalent for polymerizations with most of the anionic coordination initiators. Much higher molecular weights are thus possible in these polymerizations. For example, molecular weights of the order of 10 are reported for propylene polymerization by an initiator derived from diphenyltin sulfide and bis(3-dimethylaminopropyl)zinc. 

Most epoxide polymerizations have the characteristics of living polymerizations, that is, the ability to polymerize successive monomer charges forming block copolymers. The expressions for the rate and degree of polymerizations are essentially those used in living chain polymerizations (see Chapter 8). The polymerization rate is given by 

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Potassium carboxylate groups introduced onto the surface of carbon fibers initiated anionic polymerization of epoxides (e.g., styrene oxide, epichlorohydrin, and glycidyl phenyl ethers) and cyclic acid anhydrides (e.g., maleic anhydride, succinic anhydride, and phthalic anhydride) in the presence of 18-crown-6 [41]. 

As described in the previous sections, the living anionic polymerizations of epoxides and methacrylic esters initiated with aluminum porphyrins 1 [67] are dramatically accelerated upon addition of sterically hindered Lewis acids such as 3 [68,69], where the monomers are coordinated to and activated for nucleophilic attack by the Lewis acids. Successful extension of this method is the living anionic polymerization of oxetane [70]. 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

Alkoxide-Type Initiators. Using the guide that an appropriate initiator should have approximately the same structure and reactivity as the propagating anionic species (see Table 1), alkoxide, thioalkoxide, carboxylate, and silanolate salts would be expected to be useful initiators for the anionic polymerization of epoxides, thiiranes, lactones, and siloxanes, respectively (106—108). Thus low molecular weight poly(ethylene oxide) can be prepared... 

In the polyurethane industry, the polymeric glycols are prepared by anionic polymerization of epoxides such as ethylene oxide and propylene oxide. Poly(tetra-methylene glycol), which was prepared by polymerization of tetrahydrofuran, was subjected to chain extension by reaction with diisocyanate (polyurethane formation) and with dimethyl terephthalate (polyester by alcoholysis). 

Anionic polymerization of epoxides can be induced by Lewis bases (usually tertiary amines) or by metal hydroxides. The amine-type catalysts are by far the most Important type of catalyst for epoxide homopolymerization. The initiation of the polymerization of epoxides has been proposed by Narracott (15) and Newey (16) to result from the attack by the tertiary amine on the epoxide (Reaction 35), with the resulting alkoxlde amine being the propagating species (Reaction 36). 

Among these heterocyclic monomers, the anionic polymerizations of epoxides have been examined most extensively. 

Scheme 10.17 Mechanism of the initiation of the anionic polymerization of epoxides by a tertiary amine in conjunction with a polyol [56].

Heterocyclic Monomers. A variety ofheterocycHc monomers can be polymerized by anionic ring-opening polymerizations. The types of anionically polymerizable heterocyclic monomers include oxiranes (epoxides), thiacyclopropanes, thiacyclobutanes, lactones, lactides, lactams, anhydrides, carbonates, and silox-anes (92). Among these heterocyclic monomers, the anionic polymerizations of epoxides have been examined most extensively. 

In this chapter, the anionic and related nudeophiUc polymerizations of epoxides are reviewed. The elementary mecharrisms involved in the presence of different initiators and catalysts and the main sjmthetic strategies developed for the preparation of epoxide homopolymers and copolymers are described. In the second section, the anionic polymerization of epoxides involving alkali metal derivatives is described. The use of orgarric derivatives as counterions or catalysts is presented in the third section. The fourth section is devoted to epoxide-coordinated polymerization. Finally, in the last sertion, monomer-activated epoxide polymerization is described. The cationic polymerization of epoxides is described in another chapter. 

Alkah metal derivatives, including hydrides, alkyls and aryls, hydroxides, alkoxides, and amides, have been used as initiators for the anionic polymerization of epoxides. Sodium, potas-sirrm, and to a lesser extent cesium are the most often used alkali metals. Lithium derivatives are generally avoided since after the insertion of the first epoxide unit, they yield alkoxide species that are unable to properly propagate the polymerization, due to sUong association. Anionic polymerization of... 

Dr. Amdie Barr re is associate professor at the University of Paris 13, France. Previously, she graduated from the University of Bordeaux 1, France, and obtained a PhD in 2007 on the activated anionic polymerization of epoxides under the supervision of Dr. A. Deffieux. She joined the laboratory Bio-ingenierie des Polym es Cardiovasculaires, supervis l by Dr. D. Letoumeur where her research area is oriented toward the development of coatings of cardiovascular devices bas ... 

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