Tetrahydrofuran polymerizations

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Solution polymerization of these compounds can be brought about by nucleophilic initiators including n-butyllithium, triethylamine, and sodium cyanide. In the absence of such initiators, solution polymerization proceeds very slowly. As an example, l-(p-chlorothiobenzoyl)aziridine at a concentration of 0.5 mole percent in tetrahydrofuran polymerizes at room temperature when initiated with n-butyllithium to give a 94% yield of polymer. Melting point of the polymer is 90-100° C and its reduced viscosity in N-methylpyrrolidone (1% concentration at 30° C)i is 0.15. 

Fig. 5. Equilibrium constant of tetrahydrofuran polymerization to polyether as function of temperature (medium pure tetrahydrofuran). Reproduced, with permission, from Dreyfuss and Dreyfuss J. Polymer...

Summaries of the work in tetrahydrofuran polymerizations (7, 2, 3) have appeared as late as 1963. However, in the last four to five years the number of publications has been so numerous and the advances in the understanding of tetrahydrofuran polymerizations have been so rapid that it is worth reviewing again at this time. New catalysts have been reported, significant studies with old catalysts have been made, and a number of papers on the physical properties of polytetrahydrofuran have appeared. We will emphasize this new work and attempt to point out some areas where new investigations or a reinvestigation of earlier studies would be helpful. 

The initiation of tetrahydrofuran polymerization by direct addition of oxonium salts is of interest because it reveals a good deal about the mechanism, but for practical purposes the salts may be formed in the reaction mixture. The obvious method is, of course, to add a little epichlorohydrin to the mixture of monomer and Friedel Crafts reagent for only antimony pentachloride is sufficiently active to start the reaction with monomer alone, but other reactions which accomplish the same purpose are ... 

Initiation with Tropylium Ion. When cycloheptatrienyl hexachlor-antimonate is used as initiator for tetrahydrofuran polymerization, the reactions are somewhat cleaner, and strong colors do not develop as readily as when the corresponding trityl salts are used (17). Rates of initiation are much lower, and the reaction is hardly noticeable at room temperature. However, at 50 °C. and above initiation is significant, and the polymerizations proceed almost to the expected theoretical conversion of monomer to polymer even when hexachlorantimonate is the anion (Table III). Therefore, the apparent low equilibrium conversion obtained with the rapidly initiating trityl salts is minimized in this case by the comparatively low rate of consumption of initiator. Once again GLC demonstrates clearly that the initiation reaction involves primarily hydride abstraction from the ether. 

In tetrahydrofuran polymerization, chain propagation occurs at the ion pairs... 

The studies of initiation of tetrahydrofuran polymerization with differently substituted carbenium salts shed some light on the reasons of low... 

Manufacture Styrene-diene copolymers are produced by anionic solution polymerization, typically using i-butyl lithium in cyclohexane at 60-120°C. The system may also be promoted by small amounts of an amine such as tetramethylethylene diamine or an ether, such as tetrahydrofuran. Polymerization solids are in the range of 20-25%. 

Tetrahydrofuran polymerization is initiated by a nucleophilic attack of the free electrons on the oxygen atom in THF by tertiary oxonium salts, carbon-onium salts, and superacid esters. The polymer is formed in the propagation step by a SN2 mechanism. The termination step uses common nucleophiles such as water, alcohols, amines, and carboxylic acids. 

Other initiators for tetrahydrofuran polymerizations also include Lewis acids in combinations with promoters. These are complexes of Lewis acids, like BF3, SnCU, or C2H5AICI2 with epirane compounds like epichlorohydrin. The small-ring compounds are more reactive toward many Lewis acids, or protonic acids, than tetrahydrofuran and act as promoters of the initiation reactions. The initiations in the presence of small quantities of oxirane compounds can be illustrated as follows ... 

The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, CH2 , that are more active. All evidence to date, however, shows that in tetrahydrofuran polymerizations the presence of carbon cations is negligible in the propagation process. Also, the rate constant for propagation of free macroions with the counterions is equal, within experimental error, to the rate constant for macroion-counterion pairs. This does not appear to depend upon the structure of the anion studied." Such information, however, was obtained on large anions. > th smaller anions, differences in the rates of propagation of macrocations and those of macroion-counterion pairs has not been ruled out. 

Terminations in tetrahydrofuran polymerizations can depend upon the choice of the counterion, particularly if the reaction is conducted at room temperature. In many reactions the chain continues to grow without any considerable chain termination or transfer. This produced the term living polytetrahydrofuran. Thus, in polymerizations of tetrahydrofuran withPFe or SbFe counterions, the molecular weights of the products can be calculated directly from the ratios of the initiators to the monomers. The molecular weight distributions of the polymers from such polymerization reactions with PFe and SbF6 , however, start out as narrow, but then broaden. This is believed ... 

Discuss, including chemical equations, the initiation reactions in tetrahydrofuran polymerization, including the mechanism and various initiators. 

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