Redistribution of chain lengths

Not all transfers to polymer give rise to branched polymers. Many lead to a redistribution of chain lengths. 

The way in which redistribution of load over individual chains occurs was described earlier for two individual chains. In the model under consideration, this can be best described by Fig. 22, which shows the initial distribution of load for the model / = 48, m = 2, p = 0.5 and n = 104, and the distribution as a function of the time-strain variable Yde. If all chains are assumed to be equally strong, and if catastrophic failure is further assumed to proceed from the failure of one chain, the model vul-canizate will become stronger under instantaneous loading after relaxation in accordance with Fig. 22. However, due to the increasing lengths of the mobile chains, the fixed chains bear an increasing proportion of the load until they become the principal elements in the system this is the point of maximum strength, and is a discontinuity on the curve. 

Alkoxydialkylboranes1 and dialkoxyalkylboranes are formed rapidly in the >BH-catalyzed redistribution reaction of trialkoxyboranes with trialkylboranes. If the difference in the chain length is sufficiently large, distillative separations are often possible. 

The on rate constant, /c+, is diffusion-controlled and depends little on surfactant and micelle size (cf. Table 19.5). The off rate constant, on the other hand, is strongly dependent on alkyl chain length, micelle size, etc. Because of the co-operativity in micelle formation there is a very deep minimum in the size distribution curve. This leads to a two-step approach to equilibrium after a perturbation. In a fast step, quasiequilibrium is reached under the constraint of a constant total number of micelles. The redistribution of unimers between abundant micelles is a fast process. In order to reach a true equilibrium, the number of micelles must change. Because of the stepwise process, this also involves the very rare intermediate micelles. Therefore, this process is slow. 

In common examples, essentially all of the polymer chains in each of the immiscible polymers are capable of participating in the copolymer-forming reaction by redistribution. This is in contrast to many other processes for in situ copolymer formation where only those few chains bearing reactive functionality participate. Unless the redistribution process is carefully controlled, it is difficult to stop the process to make stable, compatibUized polymer blends. If the reaction is thermally initiated, the blend processing temperatures and residence times must be strictly and reproducibly controlled within narrow Emits to achieve reproducible properties. For prolonged reaction times at a temperature above that necessary to initiate the reaction, one may obtain a broad distribution of block lengths and... 

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