Chain termination activation volumes

The over-all rates for free radical polymerizations increase with increasing pressure. This means simply that the pressure-induced retardation of the initiator decomposition rate is more than offset by the increase in the rate of chain propagation and the decrease in the rate of chain termination. This is formally stated in terms of activation volumes in Equation 4 (15)... 

A theoretical derivation [72] was based on the idea, that linear chain termination is the act of self-burial of macroradical s active center and manifests itself as the act of chain propagation, leading into a trap. Taking into account the fractal properties of pol5mier chain and assuming that a set of traps in its conformational volume is a fractal as well, we obtain the expression similar to Eq. (54) ... 

Thus, the probability of monomolecular chain termination first of all depends on the concentration of traps which, in turn is determined by the conformation of a propagating polymeric chain. From this point of view any additive that does not contain a functional group able to induce polymerization, displacing the monomer from the conformation volume of propagating polymeric chain, increases the traps concentration. In other words, the probability that the active center of a radical will be blocked from the functional groups of a monomer is increased. The stronger this effect, the longer the chain, namely its conformation volume. 

NMR Tj are in qualitative agreement with those obtained from electrical conductivity measurements at intermediate and low water contents, suggesting that motion of the sulfonic acid terminated pendant chains contributes to the conduction mechanism at low water contents. McLin et al. also determined the activation volume from the NMR spin—lattice relaxation time measurements at about 8.6 cm mol, in consistent with high pressure electrical conductivity measurement [113]. 

While the chain length dependence of termination was discussed earlier, the reality is that termination is much more strongly dependent on pressure [47,48] than on chain length. The large negative activation volumes typical for termination describe this effect. Because increased pressure not only decreases the rate of termination but also increases k,... 

For this example the procedures for the hybrid model design and the sub-model design, as described above, are followed. As a consequence of the requirements, for the hybrid model, all known balances involved are incorporated density balance, energy balance and component balances for monomer, hydrogen, non-activated catalyst, and activated catalyst. The volume of the reactor is constant. The reaction rates for the chain propagation, chain termination, catalyst activation and catalyst deactivation, determine the shifts between the component balances. For these rates, fuzzy relationships are birilt. 

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... 

At a low pressure in the reactor, active centers rapidly reaeh the reactor wall, are adsorbed on it, and reeombine chains terminate on the wall. If the probability of recombination on the wall is low (e 1), then chain termination occurs after multiple collisions of the aetive eenter with the surface and is not limited by their diffusion to the surface. In this ease, the eoneentration of active eenters is the same in the bulk of the whole volume, and the rate of ehain termination is the following ... 

The overall activation volume is composed of the activation volumes of the different polymerization steps, initiation, or initiator decomposition, chain propagation and chain termination ... 

In the chain propagation reaction, the decrease in the distance between the radical and the monomer molecule is greater than the increase in length of the double bond of the monomer. Hence the activation volumes An listed in Section D are always negative. The data for An are measured at high pressures by the method of the rotating sector together with An, the activation volume of ehain termination. 

The activation volumes An are also negative as shown in Section E. Mostly the termination reactions are diffusion-controlled. The reactant molecules undergo translational diffusion at first to form a collision pair. It follows the movement of the reactive sites on these molecules into a position favorable to chemical reaction. This process has been considered to be the rate-determining step. The activation volume involved in this so-ealled segment diffusion is observed. When chain termination is governed by translational diffusion, the activation volume, Au j for viscous flow, is the key parameter for the evaluation of the influence of pressure. 

The difference in the activation volumes of the various component reactions results in a different influenee of the pressure on the rate eonstants and leads to modified polymers. Beeause of its larger negative aetivation volume the chain propagation reaction is more favored by high pressures than the termination. The ehain length and consequently the tensile strength as well as the tensile... 

The interfacial activity is determined by the sterical properties of the molecule. At the interface the spatial demand A0 of the hydrophobic part of the molecule is higher because of the second chain of the internal sulfonate compared with the terminal sulfonate. Thus, the surface concentration of the surfactant molecules is lower. That means that the hydrocarbon chains are laterally oriented and therefore cover the interface between the solution surface and air more completely. Because the ratio of the spatial demand of the head group to the volume of the alkyl chain governs the radius of the micellar surface, it... 

Little effort has been devoted to ascertain how the coenzyme is re-formed (the termination step). In the radical initiation step (ii, Figure 3), deoxyadenosine is formed. To re-form the Co-C bond to the 5 position of the nucleoside, a methyl group in the 5 -deoxy adenosine must be activated. Such activation requires either H atom abstraction, perhaps by the amino acid side chain radical or by a rearranged radical intermediate derived from the substrate (iv, Figure 3). This step is still not well understood. In the first edition of this volume [75], I expressed the hope that the steps that regenerate the coenzyme would become obvious once the rearrangement process was understood. Recent studies [67] to be described... 

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. 

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