Anionic polymerization crown ethers

Slomkowski, S., and Penczek, S., Influence of dibenzo-18-crown-6 ether on the kinetics of anionic polymerization of p-propiolactone, Macromolecules, 9, 367-369, 1976. 

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. 

Block copolymers of (R,S)-(3-butyrolactone and eCL have been synthesized by combining the anionic ROP of the first monomer with the coordinative ROP of the second one (Scheme 15) [71]. The first step consisted of the synthesis of hydroxy-terminated atactic P(3BL by anionic polymerization initiated by the alkali-metal salt of a hydroxycarboxylic acid complexed with a crown ether. The hydroxyl end group of P(3BL could then be reacted with AlEt3 to form a macroinitiator for the eCL ROP. 

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]. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

Crown ethers and cryptands have also found widespread use as modifiers in the polymerization process (79MI52104.81MI52110). Anionic polymerization is strongly affected by the addition of crown ethers and cryptands which cause increases in the polymerization rate and in the yield and molecular weight of the final polymer. 

Anionic polymerization of phenylacetylene to a trans-cisoid polymer in the presence of crown ether phase-transfer catalysts initiated by sodium amide has been reported.425 In contrast, the zwitterionic rhodium complex Rh+(COD)BPhJ yields a ds-transoid product in the presence of Et3SiH.426... 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

In addition to post-functionalizing polymers by bonding the macrocycle to the preformed polymer backbone, macrocycles can be incorporated into polymer matrices by direct polymerization of the macrocycle, either by a step-growth mechanism or a chain-growth mechanism. [46] Polymeric crown ether stationary phases were pioneered by Blasius et al. [34, 59-62] These resins were used to separate both cations (including protonated amines) with a common anion, and anions with a common cation in high... 

By the addition of crown ethers or cryptands, the rate of the anionic polymerization of methylthiiran, /Mactones, and other monomers may be enhanced by up to two orders of magnitude [218-220]. 

A method for preparing homopolymers or copolymers of oxiranes by anionic polymerization using s-butyl lithium and triisobutylaluminum but without crown ethers or cryptands during the polymerization process is described. 

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