Polymerization oxirane

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

Many other cyclic ethers have been polymerized using cationic polymerization. Ethylene oxide (also called oxirane) polymerizes forming poly(ethylene oxide) (PEO) (structure 5.24) in the presence of acids such as sulfuric acid, producing a wide range of chain-sized polymers sold under various trade names including Carbowax and Poly ox. PEO is also used in cosmetics and pharmaceuticals (as water-soluble pill coatings and capsules). 

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]. 

In the first step the oxirane polymerization initiated by the isomerized EN dianion has been studied. The results reported in Table III and the low polydispersity observed (about 1.1 Figure 5) are conclusive for a living anionic system. Moreover the initiation rate of which is higher than the propagation one. 

From the kinetics of the oxirane polymerization initiated hy alcoholate (l6) and hy fluorenyl potassium (27) and as fluorenyl and dihydronaphthalene mono anion (28) have approximately the same basicity, the ka2 over kpr0p ratio may be estimated to 20. Therefore, the length of the two growing polyether chains must be largely independent on the nature of the initiating site. 

In the three-membered heterocycles, the oxiranes, polymerization has been induced with basic catalysts. Usually, these polymerizations are slow, and the polymers formed are of relatively low molecular weight. Certain, specially prepared carbonates and amides of calcium constitute a limited exception to this generalization, as they allow the formation of a solid polymer from ethylene oxide. 

A very recent paper by Saegusa, 7) describes a method of synthesizing polyoxyethylene macromonomers bearing a polymerizable heterocycle at the chain end. Here again the method involves initiation of the oxirane polymerization by means of an alcoholate (derived from 2-p-hydroxyphenyl)-oxazoline). As metalation agent butyllithium, was used since lithium alcoholates are not very reactive towards oxirane. Indeed, the macromonomers obtained exhibit very low degrees of polymerization. Deactivation was performed with methyl iodide ... 

Bis(chlorodimethylsilyl)benzene-AgPF6 system was shown to act as a bifunctional initiator of substituted oxirane polymerization [42], Tri-methylsilyl iodide and triflate were used also as initiators of the cationic polymerization of oxazolines [43]. In this system, however, in contrast to typical initiation mechanism of oxazoline polymerization, O-silylation leads to initiation, because of the unfavorable charge distribution in N-siiyiated species ... 

The polymerization of oxiranes, a reaction of importance for both industry and commerce, has been abundantly described in the literature. Several hundred articles are published in this field annually. The quantity and great variety of themes discussed mean that a survey of this immense literature material exceeds the scope of the present review. Accordingly, we shall merely mention some of the works attempting to clarify the situation regarding the mechanism of polymerization reactions. ° We shall also outline the fundamental types of oxirane polymerizations. These can be classified into three groups with anionic, cationic, " and coordination mechanisms. 

It was soluble in benzene, and the benzene solution exhibited a catalytic activity for the oxirane polymerization at 80 C, but no activity at room temperature. 

A soln. of 8-benzyloxy-2,6-dimethyl-2,3-epoxyoctane in ether added to /-Pr2NH and 4 eqs. H2O in the same solvent under a balloon of SiF4 at 0°, the mixture stirred for 1 h, then quenched with aq. KF -> 8-benzyloxy-2,6-dimethyl-2-fluorooctan-3-ol. Y 91%. The method is short and is undertaken with conventional glass apparatus olefins, ethers and 1,2-disubstd. oxiranes (aliphatic) were unaffected, while mono-subst. aliphatic oxiranes polymerized. Stereoselectivity is illustrated by the formation of 5y -fluorohydrins. F.e.s. M. Shimizu, H. Yoshioka, Tetrahedron Letters 29, 4101 (1988) l-fluoro-2-hydroxysilanes s. Tetrahedron Letters 30, 967-70 (1989). 

Tsuruta, T. Molecular level approach to the origin of the steric control in organozinc catalyst for oxirane polymerization. Makromol. Chem., Macromol. Symp. 1986,6, 23-31. 

Cyclic ethers polymerize by ionic mechanism. Three-membered cyclic ethers (oxiranes) polymerize by both cationic and anionic mechanisms. Four-membered and higher cyclic ethers polymerize by cationic mechanism only (although examples of anionic polymerization are occasionally mentioned in the literature). Thus, cationic mechanism is a general mechanism of cyclic ether polymerization. 

The main representatives of multinuclear catalysts for oxirane polymerization are Al and Zn compounds, which are characterized by moderate nucleophilicity and relatively high Lewis acidity. The appropriate Lewis acidity of the metal and the appropriate nucleophilicity of the metal substituent in these catalysts make monomer coordination preferable to the... 

It was later shown that a small amount of water added to the FeCVpropylene oxide initiator system increased both the polymerization rate and the amount of crystalline product formed (67, 70) (Figure 2). This effect of an added reactive species such as water, an alcohol, or acetylacetone to one of the stereospecific initiator structures having a strong influence on polymerization rate and stereospecificity has been found in a number of the important oxirane polymerization systems. 

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. 

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