Termination kinetics

The role of water is two-fold. It is a co-catalyst, and its consumption during the reaction is, or is associated with, a kinetic termination. Moreover, there is evidence that the reaction in which water is consumed is of a lower order with respect to monomer than the propagation reaction, which therefore is most probably of first order with respect to monomer. 

In the present context the word termination is applied not to the breaking-off of a physical chain, i.e., the cessation of growth of a particular molecule, but to the complete destruction of a kinetic unit, which means the irreversible annihilation of one ion pair. This kinetic termination, which is a well-understood feature of radical polymerizations, is a comparatively rare event in cationic polymerizations it may occur in several different ways and in some systems not at all. 

The same phenomenon was found in the polymerization (to dimers and trimers) of trans-stilbene by TiCl4-CCl3COOH in benzene [8]. In this case chain initiation is presumably by protons, and there appears to be no kinetic termination at all. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

A (amphiphilic core-shell) R =C H - Tenninaling agent N,M-di-n-octadecylamine polymeiizalion time 72h B (eK sriu kinetics) Terminating agent piperidine polymeiization time 2. S. 24. 79h... 

The iGLE also presents a novel approach for studying the reaction dynamics of polymers in which the chemistry is driven by a macroscopic force that is representative of the macroscopic polymerization process itself The model relies on a redefined potential of mean force depending on a coordinate R which corresponds locally to the reaction-path coordinate between an n-mer and an (n -t 1 )-mer for R = nl. The reaction is quenched not by a kinetic termination step, but through an (R(t))-dependent friction kernel which effects a turnover from energy-diffusion-limited to spatial-diffusion-Iimited dynamics. The iGLE model for polymerization has been shown to exhibit the anticipated qualitative dynamical behavior It is an activated process, it is autocatalytic, and it quenches... 

The BF3/l,3j5-trioxane system is one of the few so far discovered in which there is a possibility that monomeric units add at the cationic end of a macrozwitterion. Fortunately, the cation seems to be stable in the presence of its counter anion. As a simple model system with which to study cationic propagation through zwitterion intermediates it is marred by its equilibrium nature and the insolubility of the polymer. Whilst kinetic termination seems to be absent, the authors report transfer to the solvent methylene chloride. Such a reaction would introduce non-zwitterionic chains. 

Shortly after Plesch and Westermann s work was published, Jaacks et al. [132] also reported a study of the polymerization of 1,3-dioxolane by HCIO4 in CH2CI2 at 20°C. Again the need for the most rigorous absence of water is emphasized [133]. They found a gradual but quantitative initiation reaction with no kinetic termination reaction. Eventually from further studies [134] they concluded that the slow initiation involves two reactions... 

It is doubtful, however, that a true living system without termination or transfer exists in these polymerizations instead we believe that the narrow distributions may result from a combination of essentially instantaneous initiation, relatively long kinetic lifetime, and kinetic termination without transfer. H. Morawetz has derived an equation relating the molecular weight distribution to the relative rates of propagation and termination and the concentration of active species for such a case ( ). The equation accounts for the possible occurrence of narrow distributions, and we are presently experimentally investigating these calculations and predictions for the polymerization of the p-isopropyl monomer. 

In the present study, kinetic termination in the presence of Et3Al or Et2AlCl is reflected by the formation of C12H (by hydridation) and Ct4 (ethylation). With Me3Al and Me2AlCl only one product, C13, due to methylation can form. Elimination/termination ratios are shown in Table 6. 

No general rate equation is known to exist for the processes of ionic polymerization, but if we restrict ourselves to the case relatively typical of ionic processes, when reactions of kinetic termination are absent and initiation proceeds rapidly, the polymerization rate may be... 

Such a process is thus equivalent to kinetic termination. In the homogenous cationic polymerization of cyclic acetals (e.g. DXL), chain transfer to polymer proceeds efficiently, because polymer is more basic than monomer. In this case, however, the polymeric oxonium ions remain active and may further propagate ... 

Intermolecular transfer reactions occur relatively frequently in cationic polymerizations. The cation from one growing chain is transferred to another in these transfer reactions. Thus, one individual growing chain is killed and another is started. Consequently, such reactions are of significance in the chemical sense for an individual chain, for which they represent a termination reaction, but these reactions are insignificant in the kinetic sense. Kinetically, termination reactions are defined as when a chain (or a pair of chains, see below) is killed and no new chain is formed. Genuine (kinetic) termination reactions are rare in cationic polymerization. 

A model for linear polymerizations proposed by Russell el al. [11, 12] suggests the following equations for predicting the kinetic termination constant when reaction diffusion is the dominant termination mechanism ... 

True (kinetic) termination may be brought about by the addition of an active hydrogen compound such as an alcohol ... 

Chain growth may be terminated without loss of active centres by various types of transfer, such as transfer to counter ion and to monomer, as illustrated above. In some instances kinetic termination may be brought about by combination. This mode of termination has been suggested for systems in which water is the co-catalyst hydroxy end-groups are then formed ... 

Subject headings polymerization / radical reactions / reaction kinetics / termination chain-length dependence... 

Chain transfer reactions are considered here irrespective of the exact chemical mechanisms of transfer that are discussed in detail, for example, in the comprehensive review of Glasse [9]. It should be stressed that all calculations were performed bearing in mind anionic polymerization, so the main concern in all theoretical papers reviewed is the absence of kinetic termination rather than the anionic mechanism of chain propagation. Thus, the conclusions drawn are valid for all processes that satisfy this condition. 

In true (kinetic) termination there is irreversible loss of propagating ability. This may be brought about in several ways, the most important of which is... 

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