Dead-ending depletion

The synthesis of telechelics by what Tobo]sky,9> termed dead-end polymerization is described in several review s.191,191 In dead-end polymerization very high initiator concentrations and (usually) high reaction temperatures are used. Conversion ceases before complete utilization of the monomer because of depletion of the initiator. Target molecular weights are low (1000-5000) and termination may be mainly by primary radical termination.. The first use of this methodology to prepare lelechelic polystyrene was reported by Guth and Heitz.177... 

Increasing the temperature of polymerization does not always lead to higher rates of polymerzation. Higher temperature leads to faster dissociation of the initiator and complete depletion of the initiator resulting in a "dead end" polymerization (Bohme and Tobol-sky (1966)). Dead-end polymerization refers to one in which initiator concentration decreases to such a low value that the polymerization stops short of completion and a limiting conversion of monomer to polymer is observed (Odian (1970)). 

A closer look at the nonisothermal and isothermal policy results reveals some additional interesting features with regard to optimization. As mentioned earlier, isothermal policies were determined by two factors. One was the M, value and the other was the dead end polymerization caused by depletion of initiator. It was also observed that the minimum time from a nonisothermal policy was considerably less than the minimum time due to the isothermal policy whenever H>, was the controlling factor in the isothermal policy when the isothermal policy was controlled by initiator depletion, a nonisothermal policy did not show significant improvement in minimum time relative to the isothermal one. 

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

If the initiator concentration used in a free-radical polymerization system is low and insufficient, leading to a large depletion or complete consumption of the initiator before maximum conversion of monomer to polymer is accomplished, it is quite likely to observe a limiting conversion poo which is less than the maximum possible conversion pc, as shown in Fig. 6.2. This is known as the dead-end effect and it occurs when the initiator concentration decreases to such a low value that the half-life of the kinetic chains approximates that of the initiator. However, if there is autoacceleration effect or gel effect (described later) leading to a sharp rise in rate of polymerization, viscosity of medium, and degree of polymerization, pure dead-end effect cannot be observed. 

Fig. 1.3.3 Stages of conventional free-radical chain polymerization 1 -induction period, 2 - growth period, 3 - period of dead ending, monomer depletion, or end of radical production...

Zinc metal is oxidized to zinc ions, and copper ions are reduced to copper metal. The copper cathode becomes depleted of electrons because these are taken up by the copper ions in solution. At the same time the zinc anode has an excess of electrons because the neutral zinc atoms are becoming ionic and liberating electrons in the process. The excess electrons from the anode flow to the cathode. The flow of electrons is the source of external current the buildup of electrons at the anode and the depletion at the cathode constitute a potential difference that persists until the reaction ceases. The reaction comes to an end when either all the copper ions are exhausted from the system or all the zinc metal is dissolved, or an equilibrium situation is reached when both half-cell potentials are equal. If the process is used as a battery, such as a flashlight battery, the battery becomes dead when the reaction ceases. 

In Fig. 11, the radial distribution of the end segments in an annealing PE star, with different number p of arms, is shown for a range of pH values. A dead zone , i.e., a region close to the center of the star where free ends are depleted, is visible. The end-point distribution is clearly bimodal for pH > pK, when the star corona... 

Run 1 was conducted using waterflood alone. No gelation or any other chemical agent was used in this run. As expected, the water breakthrough took place very quickly. At the end of the displacement test, only 7% of the olMn-place was recovered. For the sake of comparison, oil recoveries at 5% oil cut were used as standard values. Note that in this experlmentation,dead oil was used. Consequently, this 7% recovery does not include any production through primary depletion which is likely to happen in a pressurized reservoir. Results of this run show why displacement-type recovery has not been used in naturally fractured reservoirs. 

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