Chain transfer activation volumes

Winyl polymerization as a rule is sensitive to a number of reaction variables, notably temperature, initiator concentration, monomer concentration, and concentration of additives or impurities of high activity in chain transfer or inhibition. In detailed studies of a vinyl polymerization reaction, especially in the case of development of a practical process suitable for production, it is often desirable to isolate the several variables involved and ascertain the effect of each. This is difficult with the conventional batch polymerization technique, because the temperature variations due to the highly exothermic nature of vinyl polymerization frequently overshadow the effect of other variables. In a continuous polymerization process, on the other hand, the reaction can be carried out under very closely controlled conditions. The effect of an individual variable can be established accurately. In addition, compared to a batch process, a continuous process normally gives a much greater throughput per unit volume of reactor capacity and usually requires less labor. 

This equation was used to compute the quantity AVf — AV ), the standard state diSerence in activation volume for transfer and propagation, from chain-transfer data in homogeneous bulk polymeriaation at two widely separated pressures (75). Note that the standard states of the pure components are assumed to correspond to the pressure of... 

Chain-transfer reactions would be expected to increase in rate with increasing pressure since transfer is a bimolecular reaction with a negative volume of activation. The variation of chain-transfer constants with pressure, however, differ depending on the relative effects of pressure on the propagation and transfer rate constants. For the case where only transfer to chain-transfer agent S is important, Cs varies with pressure according to... 

Despite the complex interaction between the components of a catalyst recipe, for example consisting of catalyst, co-catalyst, electron donors (internal and external), monomers, chain-transfer agents such as hydrogen, and inert gases and the catalyst support, the local polymer production rate rate (polymerization rate) in a given volume, Rp (kg polymer hr"1), can often be described by a first-order kinetic equation with respect to the local monomer concentration near the active site, cm (kgm"3), and is first order to the mass of active sites ( catalyst ) in that volume, m (kg) ... 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

Inhibition of peroxidation of unsaturated lipid chains in biomembranes is of particular significance and interest, because uncontrolled oxidation disrupts the protective layer around cells provided by the membranes. Furthermore, radical chain transfer reactions can also initiate damage of associated proteins, enzymes and DNA. The volume of literature is immense and expanding in the field of antioxidants. We will select certain milestones of advances where micelles and lipid bilayers, as mimics of biomembranes, provided media for quantitative studies on the activities of phenolic antioxidants. One of us, L. R. C. Barclay, was fortunate to be able to spend a sabbatical in Dr. Keith Ingold s laboratory in 1979-1980 when we carried out the first controlled initiation of peroxidation in lipid bilayers of egg lecithin and its inhibition by the natural antioxidant a-Toc . A typical example of the early results is shown in Figure 4. The oxidizability of the bilayer membrane was determined in these studies, but we were not aware that phosphatidyl cholines aggregate into reverse micelles in non-protic solvents like chlorobenzene, so this determination was not correct in solution. This was later corrected by detailed kinetic and P NMR studies, which concluded that the oxidizability of a lipid chain in a bilayer is very similar to that in homogeneous solution . ... 

This relationship has been central to commercial catalyst manufacture for half a century. Notwithstanding its importance, however, the underlying science was not explained [500]. Both activity and MW are affected by pore volume, but the data of Figure 49, and also many other related data, show that it is not certain that the two relationships are necessarily identical. That is, activity depends on catalyst fragility, but it is not obvious how catalyst fragility could control chain transfer. Thus, the connection between MW and porosity may or may not involve fragility. The influence of pore volume is seen in all polyethylene processes— slurry, gas phase, and solution. No satisfying explanation has yet been proposed to account for this relationship. 

Chain transfer to the aluminium alkyl was also deserved. Using the method of moments the authors obtained an equation for the first three moments of active, temporarily deactivated, and dead chains. As a result of a computerized search for the values of constants, based on the model and on the experimental data obtained in a batch reactor (volume = 131), some of the values were found to differ considerably from those published in the literature. 

Four chapters in this volume are addressed to the uses of chelated alkali metal complexes in various polymerizations, telomerizations, and polymer grafting applications. They fully cover all of the published work in these areas. There are, however, several general features based on our unpublished results which warrant a general discussion. These include the effect of catalyst ion pair structure on polymerization activity and polybutadiene microstructure, the effect of steric hindrance on catalyst activity, and the mechanisms for chain transfer. 

As anticipated for a chemically controlled reaction, CO2 has only a minor influence on the rate coefficient for chain-transfer to DDM and to the MMA tri-mer in MMA and styrene homo- and copolymerizations. Going from bulk polymerization to solution polymerization with 40 wt%> CO2 present enhances Cx by about 10%, but leaves the associated activation volume, AV (Cx), unchanged [48]. As pointed out in the previous section, the observed lowering of kp,app upon increasing CO2 content is no true kinetic effect, and the propagation rate coefficient kp,kin most likely remains unaffected by the presence of CO2. Thus, ktr for DDM and for the MMA trimer should not be significantly varied by the presence of CO2. 

To develop their model, Wen and McCormick adopted a number of simplifying assumptions. These are (1) initiation produces two equally reactive radicals, (2) chain transfer reactions are neglected, (3) the rate constants for radicals of different sizes are assumed identical, (4) the propagation rate constant kp, termination rate constant kp and the rate constant for radical trapping kb are all simple functions of free volume as shown below, and (5) there is no excess free volume. The material balance equations for the initiator, the functional group, the active radical, and the trapped radical concentrations... 

Three cases of Type la activations illustrate a class of reactions expected to give positive results. The first one is provided by SrnI or ETC processes. Figure 1 shows the chain mechanism of the reaction of lithium nitronate with 4-nitro-benzyl bromide established by Komblum and Russell. This reaction was expected to display sonochemical switching, which was indeed foimd. The mechanism suggests that the sonochemical activation should find its origin either in creating species 1 or 2 (no direct entry to 3 seems plausible). The creation of 1 within a cavitation bubble could result either from high-pressure-promoted electron transfer (activation volumes for some electron transfer reactions may be found in Ref. 9) or local conditions at the interface between the cavitation bubbles and the bxilk solution (Qi. 1). The creation of radical 2 could result from a direct sonolysis of the benzylic C-Br bond (p. 86) but... 

Where kp is the rate constant of propagation, C is the active site concentration and M is the monomer concentration. Calculation of kp requires the knowledge of Rn, [C ], and [M]. The uncertainty in the determinations of [C ] by various techniques has been discussed by Tait in this volume. Depending upon the catalyst system, the active sites may all be present initially, or more may be produced as the catalyst agglomerates or crystals fracture during polymerization. If there is catalyst deactivation by either chain termination, chain transfer or poison, [C ] may decrease with time. At the initial stage of reaction [M] is the concentration of monomer dissoved in the diluent. If, during reaction, the catalyst is completely encapsulated by the polymer... 

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