Propagating species, tetrahydrofuran polymerization

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. 

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. 

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 ... 

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). 

In lithium alkyl-initiated polymerizations only chain initiation and propagation steps need be considered in hydrocarbon solvents. Both reactions are strongly influenced by extensive association of all lithium compounds. The reactive species in chain propagation is the small amount of dissociated material which probably exists as an ion pair. Association phenomena disappear on adding small amounts of polar additives, and the aggregates are replaced by solvated ion pairs. In polar solvents of relatively high dielectric constant (e.g. tetrahydrofuran), some dissociation of the ion pairs to free ions occurs, and both species contribute to the propagation step. The polymerizations are often complicated in tetrahydrofuran by two side reactions, namely carbanion isomerization and reaction with the solvent. 

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. 

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].)...

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