Living chain ends, propagation

Enolates, which are the actual propagating species, exist in the E (11) and Z (12) configurations. The E/Z ratio of the living chain-ends can be indirectly determined by reacting the propagating enolates 11 and 12 with chlorotrimethylsilane and converting them into the corresponding ketene silylacetals 13 and 14, which are characterized by NMR spectroscopy (equations 23 and 24). ... 

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. 

In contrast to the free-radical copolymerization, the lack of termination in living ionic copol5mierization enables the direct determination of the rate coefficients of the cross-propagation step. The reaction rate of monomer B with the living chain end a is directly accessible via the concentration decrease of a , which may be traced via a suitable spectroscopic method. Alternatively, the concentration of B can be determined as a function of time with the concentration of a being held constant. The latter experimental technique requires extrapolation of time toward zero, because the cross-propagation step is immediately followed by homopropagation of B. 

Living polymerizations exhibit a linear first-order plot of monomer consumption (provided that initiation is fast) which indicates that the number of propagating chain ends is constant. Such a plot is obtained by considering the relationship between time (x-axis) and logarithmic monomer consumption. If the chain ends propagate at a constant rate in the absence of detectable amounts of chain termination, then a linear relationship results. Deviations from ideal linear behavior are observed as a result of termination events that decrease the slope, or slow initiation processes that increase the slope. However, this method is not sensitive to chain transfer. 

When the initial monomer supply is exhausted, the anionic chain ends retain their activity. Thus, these anionic chains have been termed living polymers. If more monomer is added, they resume propagation. If it is a second monomer, the result is a block copolymer. 

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

The polymerization kinetics have been intensively discussed for the living radical polymerization of St with the nitroxides,but some confusion on the interpretation and understanding of the reaction mechanism and the rate analysis were present [223,225-229]. Recently, Fukuda et al. [230-232] provided a clear answer to the questions of kinetic analysis during the polymerization of St with the poly(St)-TEMPO adduct (Mn=2.5X 103,MW/Mn=1.13) at 125 °C. They determined the TEMPO concentration during the polymerization and estimated the equilibrium constant of the dissociation of the dormant chain end to the radicals. The adduct P-N is in equilibrium to the propagating radical P and the nitroxyl radical N (Eqs. 60 and 61), and their concentrations are represented by Eqs. (62) and (63) in the derivative form. With the steady-state equations with regard to P and N , Eqs. (64) and (65) are introduced, respectively ... 

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