Anionic epoxide polymerization

Anionic Epoxide Polymerization Initiated by Alkali Metal Derivatives... 

Propylene oxide and other epoxides polymerize by ring opening to form polyether stmctures. Either the methine, CH—O, or the methylene, CH2—O, bonds ate broken in this reaction. If the epoxide is unsymmetrical (as is PO) then three regioisomers are possible head-to-tad (H—T), head-to-head (H—H), and tad-to-tad (T—T) dyads, ie, two monomer units shown as a sequence. The anionic and... 

A number of organometallic compounds acting as initiators can cause epoxide polymerization to proceed through anionic coordination mechanism. Propagation in such systems involves a concerted or coordinated process in which the epoxide monomer is inserted into an oxygen-metal (0-M) bond (Odian, 1991) ... 

Hydrocarbon solubility is important for diene polymerization, where a high 1,4 polymer microstructure is often desired. Polar solvents have a tendency to decrease the 1,4 content and elevate the 1,2 addition product. Hydrocarbon solubility is less of an issue for styrene or epoxide polymerization. Thus, the hydrocarbon soluble t-butyldimethylsilyl (TBDMS) protected initiator (5) (Table I) is recommended for high 1,4 polydienes. Most of the other conpounds in Table I can be used with other anionic monomers or where high 1,4 microstructure is not needed. 

Aluminum tetraphenylporphyrin with axial bound chloride, (TPP)AICI, Ic is an effective initiator for the anionic ringopening polymerization of epoxides (11). - - This complex can be easily synthesized by reacting free-base tetraphenylporphyrin, (TPP)H2, with chlorodiethylaluminum in CH2CI2 under nitrogen, where the reaction proceeds rapidly and quantitatively at room temperature with evolution of two equivalents of ethane with respect to (TPP)H2-According to Scheme 1, a variety of aluminum porphyrins can be synthesized using some organoaluminum compounds and they are easily identified by H NMR (Table 2). 

Effect of Temperature and Photoinitiator Anion. The polymerization temperature has a significant effect on both the rate of polymerization and the final limiting conversion. For example. Figure 9 contains a series of plots of reaction rate as a function of time for photopolymerization of an epoxide monomer. This figure illustrates that as the temperature is increased, the peak reaction rate increases and the reaction time decreases. The increase in reaction rate with increasing temperature ultimately arises from the effect of temperature on the propagation rate constant (which increases with increasing temperature as described by the Arrhenius equation for the rate constants). The rate of photoinitiation is... 

FUK 87] Fukuchi Y., Takahashi T., Noguchi H. et al, Photoinitiated anionic coordination polymerization of epoxides, a novel polymerization process . Macromolecules, o. 20, pp. 2316-2317, 1987. 

Epoxide polymerization can be described in terms of three different mechanisms (1) anionic (base-catalyzed), (2) cationic (acid-catalyzed), and (3) coordinate. The third actually combines features of the first two extremes, since it involves coordination of the monomer oxygen at a Lewis acid catalyst site (L), followed by attack on the thus activated monomer by an alkoxide already bound to the site. 

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. 

Detailed studies on the mechanisms of initiation and propagation of epoxide polymerization have been conducted using well-defined and stable initiators. Ethylene oxide (EO) was generally chosen as the reference monomer, owing to the living character of its polymerization, whereas the anionic polymerization of most other epoxides, including propylene oxide (POx), is subject to side reactions. 

One major interest in these organic bases is the activation of living polymer chains with lithium counterions toward epoxide polymerization. This is of particular interest for the synthesis of poly(EO)-containing block copolymers by sequential anionic... 

Anionic and anionic coordination polymerizations of epoxides are often slow processes that require long reaction times to achieve high monomer conversions. Moreover, as reported in previous sections, a majority of these polymerizations suffer from side reactions, illustrated by the chain transfer reaction to monomer in alkali metal anionic polymerization and by a very low initiation efficiency and the formation of several polyether populations in coordination polymerizations. 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

An alternate way to make block copolymers involving PDMS blocks 124,125) is to have these chains fitted with epoxide functions at chain end, and to react them with a vinylic or dienic polymer carrying terminal COOH functions. Sequential addition of monomers has also been used, the ring opening polymerization of the cyclic trimer (D3) being initiated by the anionic site of a living polymer126). 

Equation 1 expresses a state of equilibrium between an alcohol A. on a molecule whose degree of polymerization is j, the catalyst C and the alkoxide anion A.C. In Relation 2 this activated intermediate reacts with monomeric anhydride A, forming an acid adduct A.AC, which dissociates, forming an unassociable carboxylic acid A.A. Reactions 3-5 depict the union of a carboxylic intermediate with a monomeric epoxide E, or with pendant oxiranes on macromole- ... 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

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