THF polymerization

THF polymerizes only by a cationic mechanism. Therefore, all of the catalysts are of the strong acid or Lewis acid type or of salts derived from them. 

Polymerization of THF was first observed by Meerwein and his coworkers (8) and was studied extensively by this group in the 1930 s and 1940 s. However, this work did not become generally known until after World War II and even then it was available only in the form of microfilmed reports (9). It was not until 1960, with the publication of Meerwein s review (3), that the scope of this excellent work became generally available. The catalysts used by this group are basically of the trialkyl oxonium ion type, either preformed or generated in situ. Meerwein classified combinations which generate oxonium ions into three broad groups  

1) Combinations of metal halides (e. g. FeCl3, A1C13) with compounds containing an active halogen atom (e. g. a-chloro-dimethyl ether, benzyl chloride, 2,3-dichloro THF). 

2) Unsaturated tertiary oxonium salts, [RC(OR )2]+X, where R=H, CHS, C2Hs R =CH3, CjH6 and X = BF, SbCl3. These salts can be preformed or formed in situ from the corresponding orthoester and the metal halide. 

It must be emphasized that the above catalysts differ considerably in their effectiveness. Some give solid polymers but most of the combinations give only liquid polymers. In addition the conversions realized vary widely. 

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Chain Transfer. A number of materials act as tme transfer agents in THF polymerization notable examples are dialkyl ethers and orthoformates. In low concentrations, water behaves as a transfer agent, and hydroxyl end groups are produced. The oxygen of dialkyl ethers are rather poor nucleophiles compared to THF and are therefore not very effective as transfer agents. On the other hand, orthoformates are effective transfer agents and can be used to produce alkoxy-ended PTHFs of any desired molecular weight (169). 

PTHF does not behave ideally in solution and the equiHbrium monomer concentration varies with both solvent and temperature. Kinetics of THF polymerizations fit equation 2, provided that the equiHbrium monomer concentration is deterrnined for the conditions used. 

Other THF polymerization processes that have been disclosed in papers and patents, but which do not appear to be in commercial use in the 1990s, include catalysis by boron trifluoride complexes in combination with other cocatalysts (241—245), modified montmorrillonite clay (246—248) or modified metal oxide composites (249), rare-earth catalysts (250), triflate salts (164), and sulfuric acid or Aiming sulfuric acid with cocatalysts (237,251—255). 

We observe the a-methylene carbons of the methyl tetrahydro-furanium ion, the a-carbons of the two types of propagating chain heads, the macroion and the macroester (17). The observation of the a-methylene carbon resonances of the acyclic tertiary oxonium ion provides a direct proof of chain transfer reaction in THF polymerization. 

We first confirmed the formation of these macrocycles in the polymerization of THF by using coupled gas chromatography/mass spectrometry ( 2). Macrocyclic ethers containing up to 8 THF units could be separated and identified by this method (23). The two predominant macrocyclic species found in THF polymerization mixtures are a cyclic tetramer and a cyclic pentamer. In analogy to the "crown ether" nomenclature, we proposed the name 20-crown-4 for the cyclic tetramer and 25-crown-5 for the cyclic pentamer (22). 

The thermodynamic constants of THF polymerization have been investigated by a number of authors. A variety of experimental techniques have been utilized including determinations of conversion to polymer, combustion, heat capacities eind vapor pressure. Comparison of our results with some previously published data shows that our results are within the range of the values reported (Table 3). 

Figure 8. Determination of thermodynamic constants of THF polymerization (plot of Equation 1) ( ), C-13 NMR (decoupled and fast pulsing) (O), C-13 NMR (gated and delayed pulsing) ( Z ), NMR. = —2.2 kcal mol ASp = —10.4 cal deg moV. ...

THF polymerization in CH3NO2 at 0 C. was found to be 1.5 x 10 3i.mol-lsec l. This is in good agreement with the propagation constant of a simileir polymerization mixture at this temperature calculated from data (18). 

D. J. Worsfold, NRC, Ont. I was glad to see that you were able to identify and measure the amount of dormant polymer present in the THF polymerization. The amount of the dormant chain end... 

Fig. 7-4 Temperature dependence of the equilibrium monomer concentration in THF polymerization by (JjNj PFg. After Dreyfuss and Dreyfuss [1966] (by permission of Wiley-Interscience, New York).

The chemistry of polymerization of the oxetanes is much the same as for THF polymerization. The ring-opening polymerization of oxetanes is primarily accomplished by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, insertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

Certain related complex ions have been successfully applied in recent studies of THF polymerizations. These include Ph3C+SbCle (17, 18), Tropylium+SbClg (19), Ph3C+PF (20), and 4-ClC6H4NjPF (21). These catalysts also give high molecular weight PTHF. 

Sims (37) made a similar interpretation of the results of his study of ECH promoted PF5 catalyzed THF polymerizations. The scheme he... 

Note added in proof A recent patent (Neth Appl. 6,509,888, Feb. 1966 Chem. Abstr. 65, 828D [1966]) diseloses the use of a variety of SbFJ salts for THF polymerization. 

Chain transfer reactions in THF polymerizations have not been considered until rather recently. Compounds known to be effective chain transfer agents include dialkyl ethers, orthoesters, and water. In addition, chain transfer to polymer and with gegenion is possible. 

With acyclic ethers. In a study of THF polymerization using PFjT gegenions Dreyfuss (25) show that in the presence of dialkyl ethers chain transfer occurs and continues to occur after equilibrium is reached. The ultimate conversion to polymer is not affected but the intrinsic viscosity of the polymer decreases with time (Fig. 6). The reaction involved is essentially the reverse of the initiation reaction with trialkyl oxonium salts (equation 5). In the case of transfer the dialkyl ether reacts with the propagating oxonium ion to give a trialkyl oxonium ion which has one long chain alkyl and two short alkyls derived from the ether. 

Sims (40) has studied the effects of water on PF5 initiation of THF polymerization (Section IIIB2b). Possibly, in addition to cocatalysis and destruction of catalyst, his results are complicated by transfer. 

Lyudvig, Rozenberg et al. (41) made a study of the kinetics of THF polymerization initiated by triethyloxonium hexachloroantimonate. 

Imai, Saegusa, Furukawa et al. (48,66) carried out kinetic studies of THF polymerization in bulk and in cyclohexane solution at 0° C. They used a ternary catalyst system consisting of AlEt3-H20(2 l)-epichloro-hydrin (ECH). They obtained high molecular weight polymer and noticed no evidence for either termination or transfer. Their polymerizations were preceded by an induction period as shown in Fig. 13 but after that their data could be fitted to an equation of the same form as equation 42. This time [f0] was defined as the concentration of propagating species ([P ]) determined from the amount and the molecular weight of product polymer. 

From these studies it appears that the kinetics of polymerization of THF are closely approximated by equation 42. The equation does not always apply from the beginning of the polymerization and frequently cannot be applied before a steady-state of active centers is achieved. The initiator term, / , in this equation is often a function of several components. Only in the case of preformed trialkyloxo nium ions of the form R30+X is the initiation simple. These results suggest that in order to theoretically study the kinetics of polymerization of THF or to compare the kinetics of THF polymerization in the presence of different gegenions, it is desirable to use preformed trialkyl oxonium salts. Ideally... 

Fig. 25. Influence of the molar ratio of H2O/H3PW12O40 on THF polymerization (333 K). (A) Solid-liquid phase, (B) two-liquid phase, (C) homogeneous liquid phase. (From Ref. 164.)...

The use of an unsaturated nucleophile to introduce into the chain end of the macromolecule a double bond has also proved successful for the synthesis of poly-THF macromonomers. The oxolane polymerization is started with any efficient initiator. When the growing chains have reached the desired length, the unsaturated deactivator is added. The reaction between the oxonium sites and the nucleophile should be fast and free of side reactions. Various unsaturated nucleophiles have been employed, e.g. p-vinylphenoxide used by Asami50). The THF polymerization was initiated with triethyloxonium tetrafluoroborate and carried out atO °C. Addition of the nucleophile (obtained by reaction of the phenol with NaH) yields the corresponding macromonomer the structure of which was characterized by various techniques ... 

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