Termination by disproportion

This occurs when the radical of a propagating chain, abstracts an H atom from another propagating polymer chain, to achieve the mutual extinction of the radicals. 

In this case the two polymer chains do not combine, but remain as separate entities, one chain containing an unsaturated end unit, and the other a saturated end unit. 

A special case of intra-molecular abstraction can occur when the H atom is removed from the same chain as the abstracting radical. This process (called backbiting ) may occur where long chain polymers take up configurations which bring labile H atoms on the polymer chain, into close proximity with the radical at the chain end. This process will result in the termination of the growing chain and give rise to a C=C bond at the position on the polymer chain where the abstraction occurred. 

Whether a polymer terminates by a disproportionation or by a combination reaction depends upon the configuration of the monomers involved. If there are no labile H atoms available, then termination will be by a combination reaction, e.g. isobutyl styrene homopolymers. If labile H atoms are available, then termination may occur by both combination and disproportionation reactions, e.g. acrylic monomers. 

When disproportionation occurs, the product will contain polymer chains with unsaturated chain ends or with unsaturated sites along the polymer backbone, as a result of the abstraction of H atoms. 

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Termination by combination Termination by disproportion Transfer to monomer... 

Chain transfer to monomer, a familiar reaction in other monomer systems, is strikingly small with ethylene. A value of less than 0.00003 at 130 C was calculated from the data on vinyl concentration of Wood-brey and Ehrlich (70), assuming all vinyl end groups to be the result of chain transfer. Even at 250° C. the C, value for ethylene is only 0.00007 0.00002, if the same assumption is made. Since termination by disproportion and / -scission, as discussed later in this review, can also give rise to vinyl groups, the actual extent of chain transfer to ethylene is probably even less than this. 

In curve B, the first and second peaks correspond to the unsaturated chain end and to the saturated chain end of the polymer respectively. In curve C, one observes that the peak corresponding to polymer chains terminated through disproportion have disappeared this points out the fact that the cellulosic macromolecules could be crosslinked by PMMA chains. 

Termination takes place either in a bimolecular mode with the coupling of two primary radicals or by disproportion of the primary macroradicals. 

Indeed, both reactions in Schemes 13.2 and 13.3 shows that in the balances of the species involved in the solution and molten state processes, three polypropylene radicals remain active for every two grafted SA groups. These become involved in the termination of the radical step that could proceed, as it is well accepted by disproportion and recombination, as the two major processes involved in the termination step of the classical three-step radical processes. 

The polymerization of unsaturated monomer, in this case styrene, by chain polymerization is first discussed. The mechanism consisting of initiation, linear propagation and termination by combination and/or disproportion, as presented in many textbooks (Odian 1991), is adopted in this study. Thus based on the defined mechanism, the rate of decomposition of initiator can be presented as ... 

Processes in which macromolecules are produced are termination by coupling and/or disproportion, and transfer to other molecule, i.e. monomer. In this case it is assumed that there is no transfer to other molecules. Since it is not known which termination mechanism (coupling or disproportion) is in the majority, and since only one termination rate constant can be calculated, two extreme cases are considered. The calculated termination rate constant will be assumed to be equal to the termination rate constant by coupling and the termination rate constant by disproportion, as presented below. 

The proposed model was then validated with data obtained from a laboratory batch polymerization reactor (Boodhoo 1999). The polymerization system consisted of styrene as monomer in an initial concentration of 7.28 mol/dm, benzoyl peroxide as initiator in an initial concentration of 5.1 10 mol/dm and toluene as solvent in an initial concentration of 1.567 mol/dm. Batch temperature was set at 90°C and agitation speed was 500 rpm. The model results in the case of termination only by coupling, and termination only by disproportion, are compared with experimental results. The results are presented in Figs. 1,2,3 and 4. 

As can be seen from Fig. 1, conversion is well predicted by the proposed model. The model results are in agreement with the experimental results within a confidence interval of 5%. Fig. 2 presents the results for number average molecular weight as a function of time. As can be seen, the experimental data set lies, for the whole of the polymerization process, between the two extreme cases, termination only by coupling and termination only by disproportion. At the beginning of the polymerization process, the experimental data lies exactly between the two extreme cases but after 60 minutes of polymerization, the experimental data fluctuates toward the termination only by coupling and reaches this extreme mechanism after 140 minutes from the beginning of the polymerization. 

Vinylidene terminal groups can be formed by disproportioning they cause degradation by a zipper mechanism that reaches its maximum degradation rate at 280 °C [620]. Small amounts of statistically incorporated methyl acrylate disrupt this mechanism and increase thermal stability. 

A rather special case of bimolecular termination was described recently in the literature by Chien (7). It concerns the polymerization of ethylene initiated by a soluble biscyclopentadienyl titanium dichloride-dimethyl aluminium chloride complex. Such a polymerization should be classified as a coordination polymerization and not as an ionic polymerization. Nevertheless, some similarity to anionic polymerization justifies its discussion at this place. It was shown that the termination is kineti-cally bimolecular, and it is postulated that it involves the reduction of two TiIV+ to TiIII+ complexes, proceeding simultaneously with the disproportion of 2 polymeric chains,... 

The mechanisms and resulting kinetic equations are shown in Figure 4. Other mechanisms are possible as well as modifications of these—e.g., disproportion termination of chain reactions, and condensation between unlike monomers. The left sides of the equations represent the reactor operator (note that all resulting differential equations are nonlinear because of the second-order propagation and termination reactions). To this is added the complexity of considering separate equations for the thousands of separate species frequently required to define completely commercially useful polymers. Solution by direct application of classical techniques is impractical or impossible in most cases even direct numerical solution is often difficult. Simplifying assumptions or special mathematical techniques must be used (described below in the calculations of MWD). 

Isomerization of perfluoroalkenes can be realized by use of SbFs catalysis [37]. The terminal carbon-carbon bonds of these alkenes are usually moved to the 2-position under the influence of this catalyst (Eq. 19). A further inward shift generally occurs only if H or Cl atoms are present at the 4-position of the alkenes. As a rule, isomerization leads to the predominant formation of trans isomers. Terminal fluorodienes also isomerize exothermally into dienes containing internal double bonds in the presence of SbFj. With a catalytic amount of SbFs, perfluoro-l,4-cyclohexadiene disproportion-ates to hexafluorobenzene and perfluorocyclohexene. SbFs promotes the rearrangement of perfluoroepoxides to carbonyl compounds (Eq. 20) [38]. 

In the light of these findings, it may be hypothesized that the shortest chains, that is, those coming from disproportion termination reactions plus others produced by chain scissions processes (if there are any) should contribute to the (soluble)oo fraction. The inversion between the amounts of the different sequences in fractions... 

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