Control and Termination of Polymerization Reactions

In chain growth polymerization reactions the average molecular weight, the molecular weight distribution and in some cases the type of terminal group of the polymer can be varied within certain limits by proper choice of reaction conditions and/or the addition of low-molecular-weight compounds (regula- 

In ionic polymerizations, the molecular weight can be regulated by temperature, type of catalyst and nature of solvent. In some cases also regulators can be used which, as in the case of cationic polymerization of trioxane, lead to the incorporation of special endgroups. 

In polymerizations with Ziegler-Natta catalysts, molecular hydrogen is the preferred regulation agent for controlling the molecular weight. 

Control of molecular weight is much simpler for those polymerizations where the macromolecules are built up stepwise condensation and stepwise addition). In such cases, the reaction can be stopped at any time, for example by cooling, so that every step of molecular weight cem be obtained. In principle, the molec- 

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