Tetrahydrofuran anionic polymerization solvent

On each of the curves, the points at lowest X represent swelling in cyclohexane, the next in tetrahydrofuran and the last in benzene. In all cases, the samples were swollen in the pure solvent. The curves are reproduced from Figure 13 of Reference 19. The networks were made from anionically polymerized polyr-styrene using a bifunctional initiator crosslinked subsequently by divinyl benzene. The curves correspond to different ratios of divinyl benzene (DVB) per polystyrene living end (LE),... 

Hiller and Funke obtained easily dissolvable linear macromolecules of PVS by anionic polymerization of 1,4-DVB up to conversions of 80-90% [230,231]. In these experiments very low concentrations of n-butyl lithium (n-BuLi) were used and tetrahydrofuran (THF) as solvent. The reactions were carried out at -78 °C and for 7 min. The contents of pendant vinyl groups in the polymer were determined by infrared spectroscopy, mercury-II-acetate addition and catalytic... 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

The need for solvation in anionic polymerization manifests itself in some instances by other deviations from the normal reaction rate expressions. Thus the butyllithium polymerization of methyl methacrylate in toluene at — 60°C shows a second-order dependence of Rp on monomer concentration [L Abbe and Smets, 1967]. In the nonpolar toulene, monomer is involved in solvating the propagating species [Busson and Van Beylen, 1978]. When polymerization is carried out in the mixed solvent dioxane-toluene (a more polar solvent than toluene), the normal first-order dependence of Rp on [M] is observed. The lithium diethylamide, LiN(C2H5)2, polymerization of styrene at 25°C in THF-benzene similarly shows an increased order of dependence of Rp on [M] as the amount of tetrahydrofuran is decreased [Hurley and Tait, 1976]. 

Remarkably few systematic studies have been made of the kinetics of anionic polymerization in non-polar solvents containing small amounts of ethers in contrast, studies of bulk ether systems abound. Several studies have appeared 156 158) in which the propagation reactions involving styryllithium were measured in mixtures of benzene or toluene with ethers. The kinetic orders, in some cases, of the reactions were identical to those observed in the absence of the ether. Thus, in part, the conclusion was reached 157,1581 that the ethers did not disrupt the dimeric degree of aggregation of poly(styryl)lithium. The ethers used were tetrahydrofuran 156), anisole 157), diphenyl ether 158), and the ortho and para isomers of ethylanisole157). 

Johnston and Pepper35 have found similar behaviour in another anionic polymerization. When dichloromethane was substituted for tetrahydrofuran the rate of amine initiated cyanoacrylate polymerizations fell markedly. The dielectric constants of these two solvents are similar. However, THF is a good donor, weak charge acceptor solvent, methylene chloride the reverse. 

Chlorinated aliphatic solvents are useful only for cationic reactions, because they would be attacked by the strong bases used to initiate anionic polymerizations. Conversely, tetrahydrofuran is a useful solvent in some anionic processes but is polymerized by appropriate cationic initiators. 

The data on isoprene suggest a similar interpretation [140, 141]. There is little difference in the fep values in Table 5 between diethylether and tetrahydrofuran, and the difference would be even smaller if the results were corrected to the same temperature. It would seem that with this monomer solvent-separated ion-pairs are not easily formed even in the favourable case of Li counter-ions. The cheirge delocalization is restricted to three carbon atoms in the anion giving a less diffuse ion. These results must suggest the possibility that the apparent importance of solvent-separated ion-pairs in anionic polymerization is to some extent caused by the fact that most detailed studies have been made largely on styrene and its derivatives. 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

Chlorinated aliphatic solvents are useful for cationic polymerization but cannot be used in anionic polymerization. Conversely, tetrahydrofuran which is a useful solvent in anionic polymerization cannot be used in cationic polymerization. Why ... 

Blue solutions of potassium in dimethoxyethane or tetrahydrofuran initiate polymerization of styrene77. With the former solvent the reaction was quantitative, both at 0° and - 70 °C, yielding living polystyrene. Termination was observed in tetrahydrofuran presumably it was caused by some slowly reacting impurities. The authors attributed this initiation to diamagnetic (e )2 however, the diamagnetic blue species are the K anions which act as the electron-transfer agents. 

A Side Reaction in the Anionic Polymerization of Methyl Methacrylate (MMA). As was reported in many papers, the anionic polymerization of MMA is more complicated than that of styrene because different side reactions are involved in the process. To separate these reactions, G. Lohr in our laboratory has tried to determine the polymerization conditions (e.g., solvent, counter ion, temperature) under which the reaction would proceed in the same, relatively simple way as does styrene. These are low temperature in tetrahydrofuran with Cs+ as counter ion (16). Under these conditions, one finds a relatively narrow distribution curve of Gaussian type. However, at increasing temperatures a tail towards lower DPs develops which results in complete disappearance of the usual distribution at —5°C (see Figure 11). 

Anionic initiator, ec-Butyllithium 1.3 M solution in cyclohexane (Aldrich) was used as obtained. Free-radical initiator, 1,1 -Azobiso(cyclohexane carbonitrile)(ACHN, Aldrich) was purified by recrystallization in methanol. l,r-Diphenylethylene (DPE, Aldrich) was purified by two consecutive distillations over CaH2 and n-butyl-lithium. The solvent, tetrahydrofuran (THF, Aldrich) used in anionic polymerization was dried by distillation over sodium wire in the presence of benzoquinone until a blue color developed and remained, and by consecutive vacuum distillations over LiAlH4 and n-butyllithium. Toluene used for fi e-radical polymerization was purified by distillation over CaH2 in a vacuum apparatus. All other chemicals used in monomer synthesis were used as purchased. 

Anionic polymerization active 1.1-1.2 center sodium thiolates, tetrahydrofuran solvent Anionic polymerization <1.1... 

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. 

Anionic polymerizations are initiated in polar systems by bases and Lewis bases. For example, alkali metals, alcoholates, metal ketyls, metal alkyls, amines, phosphines, and Grignard compounds act as initiators. However, the polymerization mechanism does not depend on the nature of the initiator alone. For example, tertiary amines and phosphines do not only initiate anionic polymerizations under certain conditions, they can also initiate zwitterion polymerizations. In addition, polyinsertions can proceed in less polar systems. Thus, anionic polymerizations are often carried out in polar solvents. Ethers and nitrogen compounds, such as tetrahydrofuran, ethylene glycol dimethyl ether (glyme), diethylene glycol dimethyl ether (diglyme), pyridine, and ammonia are most commonly used. 

The cationic polymerization of tetrahydrofuran is used commercially to produce a,CD-dihydroxypoly(tetramethylene oxide) (PTMO glycol). Although this polymer is not used by itself as an elastomer, it is used as one of the elastomeric block components for preparation of segmented thermoplastic polyurethane [133] and thermoplastic polyester [134] elastomers. The cationic polymerization of tetrahydrofuran (THF) is a living polymerization under proper experimental conditions [135-139], i.e., it does not exhibit any termination step, very much like the analogous anionic polymerizations which are discussed in Section VIII. However, these polymerizations are complicated by the fact that the ceiling temperature, where the free energy of polymerization is equal to zero, is estimated to be approximately 83 2°C in bulk monomer solution [140] therefore, the polymerization is reversible and incomplete conversion is often observed, especially in the presence of added solvent. For... 

Electrolytic polymerizations were described in the section dealing with cationic polymerizations. Anionic polymerizations can also be initiated in an electric field. When LiAlHj or NaAl(C2H5)4 are used as electrol3Tes in tetrahydrofuran solvent living polymers can be formed from a-methylstyrene [190]. The deep red color of carbanions develops first at the cathode compartments of divided cells. 

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