Propagating species, tetrahydrofuran

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

Jaacks and co-workers [17], however, maintain that in these polymerisations the propagating species is a tertiary oxonium ion (VII), and they consider the propagation to be essentially the same as in the polymerisation of tetrahydrofuran, as is shown in Reaction (C). 

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]. 

The propagating species in the cationic polymerization can be examined from the copolymerization behavior (21). Cyclic ethers such as tetrahydrofuran (THF) or 3,3-bischloromethyloxetane (BCMO), and cyclic esters such as 0-propiolactone (/3-PL) or -caprolactone (c-CL) are classified as oxonium ion type monomers. Copolymerizations between these monomers are observed easily as in the case of BCMO-THF (12, 13), BCMO-/3-PL (14, 15), BCMO-c-CL (16), and THF- -CL (21). 

These initiators produce anionic propagating species by attack on the C=C double bond of vinyl and diene monomers. A common example of this type is n-butyl lithium. The C—Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. The unaggregated form is more active for initiation. Butyl lithium is usually available as a solution in hexane. Addition of tetrahydrofuran to this solvent increases the concentration of the unaggregated initiator by forming a 1 1 complex with this compound. This accelerates the rate of initiation of styrene ... 

A novel transformation reaction of living poly(tetrahydrofuran) from cationic into anionic propagation species has been published. This species was formed by end-capping of living poly(THF) with potassium iodide followed by the reduction with bis(pentamethylcyclopentadienyl)samarium (Cp 2Sm). The formed terminal anionic carbanion is... 

Figure 10.1 Tennination by chain transfer to polymer in cationic polymerization of tetrahydrofuran. Nucleophilic attack by an ether oxygen (2) in a polymer chain on an oxonium ion propagating center (1) forms a tertiary oxonium ion (3), which then undergoes nucleophilic attack by monomer leading to a dead polymer (4) and a regenerated propagating species . (After Odian, 1991.)...

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

In contrast to these initial reports on the living CROP of tetrahydrofuran which were performed without additional solvents, Penczek and coworkers demonstrated that the solvent plays an important role in the cationic ROP of tetrahydrofuran since it controls the proximity and stability of the ion pair at the living chain end [100, 101]. The polymerization rate increases in more polar solvents because of stabilization of the ion pair, whereby it was demonstrated that the methyl-triflate-initiated CROP of tetrahydrofuran involves an equilibrium between the cationic propagating oxonium species and the covalent triflic acid adduct, which can be shifted by the solvent choice as depicted in Scheme 8.19. Nonetheless, as a result of the much higher reactivity of the cationic propagating species, the polymerization rate is almost exclusively determined by the concentration of oxonium ions. 

Scheme 8.19 Schematic representation of the methyl-triflate-initiated CROP of tetrahydrofuran comprising equilibration between cationic and covalent propagating species. The ratio of the cationic versus covalent propagating in different solvents is also indicated.

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

Figure 6.1 Top Equilibrium between cationic and covalent propagating species for the living cationic ring-opening polymerization of tetrahydrofuran (THE) initiated with methyltriflate. Eower panels H NMR spectra obtained during the methyl triflate-initiated polymerization of THE in different solvents, demonstrating the different equilibria between cationic and covalent species. (Reprinted with permission from Ref [33].)...

In the present communication the strontium salt of one-ended living polystyrene (SrS2) was studied In tetrahydrofuran (THF) and tetrahydropyran (THP), In order to check the validity of the triple Ion mechanism. The Ionic dissociation of SrS2 In THP was expected to be smaller than In THF and therefore It was thought that perhaps a contribution to the propagation from species, other than the free S anions, would be detectable. 

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