Tetrahydrofuran polymerization Initiation

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. 

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. 

This concept of polymerization initiation is applicable to cationic processes, too [71] see the example in Scheme 7 for copper or silver ions (Me+) and tetrahydrofuran (THF). A THF coordination is a prerequisite for the polymerization reaction. But, it is unclear, whether expulsion of Me(O) occurs immediately following excitation (path a) or follows the attack of the excited complex by the ground state of THF (path b). 

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

In the polymerization of tetrahydrofuran (THF) initiated with HO-SO2CF3 in CD3NO2 solvent at 35° C (molar ratio [THF]/[CD3N02]/ [CF3SO3H] = 20.8/24.6/1), the equilibrium monomer concentration (i.e., the ultimate conversion of monomer to polymer) is reached in 2 hr. At this stage of polymerization, concentration of cyclic oligomers is still very low [90]. 

The complex of n-butyllithium with diethylzinc has also been studied as a polymerization initiator [180] for methylmethacrylate. In pure toluene the results obtained with this system were complex and not readily interpretable, but in presence of 1—10% tetrahydrofuran a simpler behaviour was reported. Monomer disappears by a first order process, and the observed first order coefficients can be expressed in terms of initiator, monomer and tetrahydrofuran concentrations in the following way... 

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. 

This equation assumes each initiator and alcohol molecule to be a potential polymer chain. Alcohol or other protonic substances can thus be used to control polymer molecular weight. The molecular weight limitation due to exchange reactions, as represented by Eq. (10.11), does not, however, apply to polymerizations initiated by alkoxides and hydroxides in aptotic polar solvents, nor does it apply to polymerizations initiated by other initiators such as metal alkyls and aryls and the various coordination initiators, since the latter initiators are dissolved in aprotic solvents such as benzene or tetrahydrofuran (Odian, 1991). 

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 ... 

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. 

Precaution Extremely flamm. sol ns. (> 20%) will ignite spontaneously in air ignites on contact with water or CO2 may cause potentially explosive polymerization of styrene NFPA Health 3, Flammability 4, Reactivity 2 Uses Solution polymerization initiator for polyolefin elastomers polymerization catalyst for food-contact polybutadiene for repeated use lithiation reagent Grignard-type reagent intermediate in prep, of lithium hydroxide org. synthesis reagent for metallation of org. compds. rocket fuel components to generate tetrahydrofurans from (tributylstannyl) methyl ethers... 

Anionic polymerization initiator sodium naphthalene, tetrahydrofuran solvent <1.1 (17)... 

Conventional sodium-, potassium-, or cesium-based initiators in an ether solvent, such as tetrahydrofuran (THF), or in polar media, such as dimethyl sulfoxide (DMSO) or hexamethylphosphortriamide (HMPTA), afford a controlled polymerization allowing the synthesis of end-functional PEO. The chemical mechanism of EO polymerization is relatively simple in view of the well-known stability of alkoxide growing chains toward termination and transfer reactions. The standard method for producing low-MW PEO (PEG) is based on controlled addition of EO to water or alcohols in the presence of alkaline catalysts. The kinetics of EO polymerization, initiated by sodium methoxide in the presence of a small excess of methanol in dioxane involves a contact ion pair of the initiator. The reaction rate in the case of alkaU/alkoxide initiation alone is slow, but increases with the concentration of excess alkanol. Within the variation of excess alkanol concentration, the MW distribution (MWD) of the polymers produced remains narrow. Formation of an alkanol-alkoxide complex as an effective initiator is envisaged. The complex formation loosens the bonding of the tight alkoxide ion pair. 

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