Temporary Reversible Termination

The growing macrocation can combine with an anion or with a fragment thereof and form a covalent bond. This reaction can either be reversible or irreversible and we shall confine ourselves in this section exclusively to the reversible ones. 

In the following a few examples of reversible terminations through recombination of ion pairs are given  

The equaibrium according to Eq. (92) involving oxazolinium cations was apparently the first observation of the equilibrium between ionic and covalent species in the cationic polymerization of heterocyclic monomers A similar collapse, being however irreversible, has earlier been postulated for the cationic polymerization of oxetane propagating with C(N02)3 anions and of styrene propagating with CIO anions  

In the polymerization of THE, the kinetics and thermodynamics of the equilibrium between covalent macromolecules (macroesters) and macroion pairs have recently been elucidated e.g. for CF3SOJ anion  

This equilibrium was postulated by Smith several years ago in the same year it was strongly suf rted by kinetic evidence and, although the macroester was not detected by at the bemnning, this equilibrium was later amply documented by H-, F-, and C-NMR spectra  

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The borderline between transfer und termination is not very sharp in the Uter-ature and we shall use the following kinetic distinctions for trand er it has no direct kinetic effect, growing species are fully restored, every act of transfer forms one dead macromolecule. We drall also disoiss separately the special case of temporary termination being a reversible termination in which an active center becomes temporarily converted into its inactive (dormant) or much less active, isomeric counterpart. Termination forms one dead maaomolecule and annihilates one active species. 

The rate expression for Fiseher-Tropseh (FT) synthesis has been obtained using a 25 wt.% C0/AI2O3 eatalyst in a 1 liter continuously stirred tank reactor (CSTR) operated at 493K, 1.99 MPa (19.7 atm), H2/CO feed ratios of 1.0-2.4 with varying space velocities to produce 14-63% CO eonversion. Adjusting the ratios of inert gas and added water permitted the impact of added water to be made at the same total flow rate and H2 and CO partial pressures. The addition of water at low levels during FT sjmthesis did not impact CO conversion but at higher levels it decreased CO conversion relative to the same conditions without water addition. The catalytic activity recovered after water addition was terminated. The temporary reversible decline in CO conversion when water was added may be due to the kinetic effect of water by inhibition of CO and/or H2 adsorption. The data of this study are fitted fairly well by a simple power law expression of the form ... 

Remarkably, the same key concept applies in living radical polymerizations. It has been suggested that the term temporary deactivation is more appropriate than reversible termination, to emphasize control (20). 

Temporary Termination Reversible Recombination with Noncoraplex Anions. Temporary termination was evidenced for the first time in the polymerization of cyclic imino ethers (51). 

Living polymerization is defined as chain polymerization in which chain termination and irreversible chain transfer are absent. The rate of chain initiation is usually larger than the rate of chain propagation with the result that the number of kinetic-chain carriers is essentially constant throughout the reaction. Reversible (temporary) deactivation of active centers can take place in a living polymerization, and all the macromolecules formed possess the potential for further growth. The term controlled polymerization, on the other hand, indicates control of a certain kinetic feature of a polymerization or structural aspect of the polymer. ... 

The polymerization is characterized by a rapid (temporary) termination reaction occurring between the active spedes and a sulfide function in the polymer chain. This termination appears to be reversible so that these terminated groups are dormant rather than dead species. The reinitiation reaction can result in the same starting structures however, as shown in the next scheme, it is equally possible that a neighboring sulfide of another polymer chain produces the thiiranium ion. In that case, polymer is cormected to and the active spedes is... 

The kinetic scheme of living, reversible ROP can also encompass-apart from initiation (Equation 1.27a) and propagation (Equation 1.27b)-additional reversible reactions, such as segmental exchange (Equation 1.27c) or temporary termination (Equation 1.27d). 

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