Polymerization of THF

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

Block or graft copolymem can be obtained by cationic polymerization of THF with macromolecular initiators. The recommended groups for the initiation are the dioxolenium cation, the acyl cation and the super acid ester, each of which can be introduced into the backbone polymer by reaction with silver salts of strong acids. Introduction of the dioxolenium group into polystyrene was carried out by the following route32 ... 

The bifunctional initiator 4-hydroxy-bulyl-2-bromoisobulyralc, HBBIB, promoted the ATRP of styrene as well as the cationic ring opening polymerization of THF [134], In the presence of Cu/CuBr2/PMDETA styrene was polymerized through the bromoisobutyrate function of HBBIB, to give PS chains end-functionalized with hydroxyl groups, PS-OH. The in situ... 

Polymerization Equilibria. As mentioned earlier, esters of strong acids, e.g. trifluoromethane sulfonic acid ("triflates"), are excellent initiators for the polymerization of THF. With such initiators, however, a complication arises. In addition to the normal propagation i depropagation equilibria of oxonium ions, Smith and Hubin postulated that the macroion ( ) may also convert into a corresponding nonpolar macroester ( ) by attack of the anion (14). ... 

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). 

As an example of a cyclic ether copolymerization, we will briefly discuss the polymerization of THF with OXP initiated with methyltriflate. The homopolymerizations of both cyclic monomers follow a similar mechanism, and both were found to proceed via macrooxonium ion and/or the macroester mechanism depending on the polarity of the polymerization medium. There should then be 8 possible end-groups, i.e. two types of methoxy tails having a penultimate THF or OXP unit, respectively, two covalent macroesters, and four different oxonium ion propagating chain heads two from a THF oxonium center attached to penultimate THF or OXP units, and two from an OXP oxonium center attached to THF and OXP penultimate units (Scheme III). ... 

Figure 9. Determination of kinetic constants (plot of Equation 6). Polymerization of THF in CH3NO2 at 0°C kp = 1.5 X 10 k, mol sec. ...

The polymerization of THF is an equilibrium polymerization. It fits the equation that relates the enthalpy of polymerization, AH, and entropy of polymerization at 1 Af, to the equilibrium monomer concentration, [AT], as a function of the absolute temperature, T, where R is the gas constant... 

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 ... 

It is now generally accepted (7, 25, 29) that the propagating species in the cationic polymerization of THF is the tertiary oxonium ion ... 

In the use of the BFJorthoester combination as a catalyst for the polymerization of THF, Meerwein (3) considers that this reaction proceeds only part way ... 

In the only study to date of the mechanism of PFS initiation, Sims (40) could conclude only that polymerization of THF with PF5 as catalyst is a very complex system. He found that there appears to be a slow initiation reaction which does not reach completion until at least half of the polymerization is over. Sims also studied the effect of water on the polymerization, but could not fully explain the complex results. He... 

It is interesting to consider the BF8 case. BF3 in the absence of promotor molecules does not initiate polymerization of THF unless very high concentrations of BF3 are employed [Burrows 42)). Perhaps the equilibrium constant of the reaction ... 

There have been numerous claims (16, 18, 21, 26, 31, 48, 49) that the polymerization of THF in the presence of certain catalysts proceeds without termination. A number of authors hasten to add that termination and transfer are not important under the conditions of their experiments but that slow termination or transfer reactions may exist. Several kinds... 

Some insight into the unusually high efficiency of TMOF was obtained by allowing the polymerization of THF in the presence of a large amount of TMOF (40%) to proceed for 14 days. Under these conditions degrada-tive chain transfer proceeded to the ultimate limit and only oligomers were isolated (Table 8). In this way it was clearly demonstrated that the end groups were both methoxy and that methyl formate was formed as a byproduct. 

With water. Rozenberg et al. (31) have shown that in the BF3/ ECH catalyzed polymerization of THF water can act as a chain transfer agent. They found that lower molecular weight polymers were produced when the amount of water added was less than the catalyst concentration. At the same time the polymerization rate remained the same. The mechanism they proposed for the reaction is as follows ... 

Bawn, Bell, Fitzsimmons, and Ledwith (20) have suggested that transfer reaction in the bulk polymerization of THF must involve either hydride ion abstraction from the alpha methylene of THF or of tetra-methyleneoxy units in the polymer, or degradative oxonium ion formation with the ethereal oxygen atoms of polymers of the type discussed in Section IIID4. 

Rozenberg et al. (24) have also studied the kinetics of polymerization of THF initiated by triethyl oxonium tetrafluoroborate. They generated their catalyst in situ from epichlorohydrin and the boron trifluoride-ether cofnplex and carried out their polymerizations in bulk and in... 

Sims 37,40) has studied the polymerization of THF in ether solution initiated either by PFS-ECH or PF6. 

Lyudvig, Rozenberg et al. (41) have also considered the kinetics of polymerization of THF initiated by antimony pentachloride. Their rate curves are clearly S-shaped (Fig. 17). This is similar to the results obtained in the triethyl aluminum system (Fig. 13) and in the PF5 case (Fig. 16). 

Fig. 18. Dependence of maximal rate on monomer concentration in polymerization of THF in the presence ol...

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