Anionic polymerization mechanism solvent effect

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. 

Effect of Solvents and Reaction Conditions Synthesis Capabilities Block Copolymers Functional End-Group Polymers Initiation Processes in Anionic Polymerization Initiation by Electron Transfer Initiation by Nucleophilic Attack Mechanism and Kinetics of Homogeneous Anionic Polymerization Polar Media Nonpolar Media... 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Glusker, D. L. Galluccio, R. A. Evans, R. A. The mechanism of the anionic polymerization of methyl methacrylate, in. Effects of solvents upon stereoregularity and rates in fluorenyUithium-initiated polymerizations. 7. Am. Chem. Soc. 1964, 86, 187-196. 

The presence of small amounts of wrater completely inhibited the electropolymerization reaction. However, the addition of small quantities of a free radical inhibitor had little effect on polymerization. Hence, it was ccmduded that the polymerization mechanism is anionic. Further, the reaction rate was rather slow, presumably because of the resonance stabilization of the anions at the ends of growing polymer chains. Such resonance stabilization may ako be a limiting factor in the maximum molecular wei t achievable by electropol3rmerization. The intrinsic viscosity of the polymer, measured at 35 in concentrated sulfuric add solution was found to be 0.07 dl/g. The polymer was partially soluble in se ral organic solvents induding... 

The only isomer that has been studied is 4-(26). The polymerization was effected in either ammonia solvent or in a solvent-free system.57 An anionic mechanism was proposed, and molecular weights were in the range of 10,000 to 260,000. As expected, higher current densities led to lower average molecular weights. 

Finally, the solvent isotope effect observed in the crystallization of zeolite A provides further insight into the mechanism. The deuteroxide anion (0D") is a stronger base than OH and should promote the depolymerization equilibria.(32) Also, D20 is more structured than H20 and should promote nucleation.(33) The isotope effect leading to a slower rate of reaction will occur in the condensation polymerization reaction leading to crystal growth, which involves elimination of both H20 and OH". (2) It is a combination of all these factors that lead to the observed nucleation rate. 

The mechanisms of these reactions are not completely established, but it has been suggested from the kinetic relationships, the effect of zinc and aluminium alkyls on molecular weight and polymer structure, that a coordinated anionic mechanism is probable in hydrocarbon solvents [215]. Similar conclusions have been drawn for the Cr(acac)3/AlEt3 system. For acetonitrile solution, however, the evidence is less certain and simultaneous coordinate anionic and free radical polymerization may occur. 

Isocyanates polymerize through the carbon-to-nitrogen double bonds by anionic mechanism. Reactions can be catalyzed by sodium or potassium cyanide at-58 °C. N,N -dimethylformamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of various carboxylic acids to sodium-naphthalene, are also effective. In addition, polymerizations can be carried out by cationic, thermal, and radiation-induced methods. 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

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