Apolar solvents, anionic polymerization

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

The rate of polymerization of living anionic polymerizations depends not only on the molar concentrations of monomer and initiator, but also on the relative proportions of free ions, ion pairs, and ion associates. Very complicated kinetic expressions are obtained for the general case. Fortunately, almost all anionic polymerizations fall within two limiting cases. One of the limiting cases consists of the polymerization of apolar monomers in apolar solvents ion pairs and their associates only occur in this case with free ions being absent. The other limiting case consists of the polymerization of polar monomers in polar solvents. Ion associates need not be considered here, but the equilibrium between free ions and the various kinds of ion pairs must be considered. 

Anionic polymerizations of polar monomers in polar solvents are only poorly stereospecific, whereas, according to conditions, the anionic polymerization of apolar monomers in apolar solvents can be strongly stereo-controlled. All possible levels of stereocontrol lie within the two extremes. 

Anionic polymerizations require polar systems, since polyinsertions and not anionic polymerizations occur in strongly apolar systems. Consequently in many cases the polymerizations are carried out in polar solvents. Ethers and nitrogen bases are especially suitable for such solvents. Tetrahydrofuran, ethylene glycol dimethyl ether (glyme), diethylene glycol dimethyl ether (diglyme), pyridine, and ammonia are often used. 

Such triblock copolymers can be derived by anionic polymerization using dilithium initiators. For the central polydiene block to exhibit a high content in l,4-c units, it is essential that the polymerization be carried out in hydrocarbon solvents (apolar medium) and in absence of any polar additive. Such dilithium initiators are generated from the reaction of butyllithium with an adequate precursor as shown below ... 

The most effective initiators for anionic polymerization are alkali metals, alkali metal oxides (lithium, sodium and potassium alkoxides) and alkah metal naphtha-lenide complexes with crown ethers [2]. Anionic polymerizations are sometimes associated with termination and transfer reactions, while cycHc oHgomers are formed as a result of back-biting reactions. In CL polymerization initiated with potassium tertibutoxide, a huge amount of cyclic lactones was formed, yet in the presence of Hthium tertibutoxide in an apolar solvent the oligomer formation was significantly reduced. 

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