Alkoxides, polymeric ethylene oxide polymerization

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

Figure I indicates the approach used to synthesize poly(oxyethylene)-b-poly(pivalolactone) telechelomers. An acetal capped anionic initiator, X (13) polymerizes ethylene oxide (EO) to give 2> a potassium alkoxide of a masked polyether, and this "new" initiator is to be used to polymerize pivalolactone (PVL). Since potassium alkoxides are strong nucleophiles, they can randomly attack at both the carbonyl carbon and the 3-methylene carbon in lactones, (Figure 2) such a random attack would result in a pivalolactone segment containing irregularities. Lenz (15), and Hall (16), and Beaman (17) have investigated PVL polymerization and have shown that the less nucleophilic carboxylate anion is preferable in polymerizing PVL smoothly. The weaker carboxylate anion will attack only at the methylene...

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

An existing polymer with an appropriate end group can be reacted with an alkoxyamine for instance, alkoxide polymerization of ethylene oxide yields a hydroxyl-terminated polymer that undergoes substitution (in the presence of sodium hydride) with a halogen-containing alkoxyamine. 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

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... 

The aluminium alkyls and alkoxides were found to be effective also in the polymerization of ethylene oxide and phenyl glycidyl ether the latter gave considerable amounts of low molecular weight, crystalline polymer. Aluminum triethyl was examined by Kambaka and Hatano (38) in the polymerization of several cyclic ethers and found to be fairly effective for propylene oxide and 2-methyl-oxacyclobutane but not for oxacyclobutane and tetrahydrofuran. The combination of zinc diethyl and alumina gave a high rate of polymerization with ethylene or propylene oxide (39). 

The ring-opening polymerization of cyclic monomers can be performed by ionic chain polymerization, as is the case of epoxy monomers. Anionic polymerization of ethylene oxide propylene oxide, and caprolactone can be initiated by alkoxides ... 

Introduced as a chain-end group, the BBN group was converted to an alkoxide group, that is, -0 K+, an efficient initiator for anionic polymerization for ethylene oxide. The ring opening living anionic polymerizations of ethylene oxide were carried out with potassium alkoxide polyolefins to prepare polycthylcnc-block-poly(cthylcnc oxide) (PE-fc-PEO), poly(ethylene-co-styrene)-foZock-poly(ethylene oxide), and poly(ethylene-co-octene)-Mock-poly(ethylene oxide) (EOR-fo-PEO) [37]. 

Polymerization of ethylene oxide with an acetal-protected alkoxide afforded a-aldehyde-co-methacryloyl PEO macromonomer, 17, after termination with methacrylic anhydride followed by acid hydrolysis [24]. 

Polyethylene oxide)macromonomers were also prepared by reaction of methacryloyl chloride with living monofunctional poly(ethylene oxide) I6) the polymerization of which can be efficiently initiated by diphenylmethyl potassium25 or by potassium alkoxides 26 >. In the latter case care must be taken to avoid any excess of alcohol which would play the role of a transfer agent and impede the macromonomer synthesis. The obtained macromonomers... 

The polycondensation of BHET to PET proceeds in the melt at temperatures of 270-305 °C, under vacuum (< 1 mbar absolute pressure) and in the presence of Lewis acid metal compounds, such as titanium alkoxides, dialkyltin oxide, gallium oxide, germanium oxide, thallium oxide, lanthanide salts, and most commonly, antimony oxide [1,2, 22-26]. Under polymerization reaction conditions, these catalysts are generally converted to their alkoxides with ethylene glycol. Typical of such alkoxides is antimony(III) glycolate, the active catalyst for the majority of the world s PET production [27] (cf. Structure 1). 

Absence of Termination Processes. The possibility of having carbanionic species that show a negligible rate of termination is now realized. In other words, just as the growing chain end in the alkoxide polymerization of ethylene oxide represents a"stable" salt of an alkali metal and an alcohol, the styryl sodium chain end, in the polymerization of styrene by sodium naphthalene, represents a "stable" salt of sodium and a hydrocarbon. This relationship was first noted in the particular case of the sodium naphthalene systems in which the organometallic species is stabilized by a high degree of solvation by an ether, such as tetrahydrofuran, so that no observable side reactions exist, that is, termination of chains, at least not within the time scale of the polymerization reaction. 

The living nature of ethylene oxide polymerization was recognized by Flory21, who conceived the ramifications of such a situation. However, the earlier studies of this reaction involved chain-transfer. The investigators added alcohol to the polymerizing solutions to solubilize the otherwise insoluble alkoxides and thus, the reaction... 

The anionic polymerization of epoxides can be initiated by hydroxides, alkoxides, metal oxides, and some organometallic derivatives. For simpUcity the initiator will be generally represented as M A", e.g., K (Bu O) for potassium butoxide. Thus, the polymerization of ethylene oxide can be represented (Young and Lovell, 1990 Odian, 1991) by the initiation step ... 

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