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[摘要]：A computational study of the branching in polyacrylates is performed for both atom-transfer radical polymerization (ATRP) and free radical polymerization (FRP). In both cases the secondary radical formed can transfer to polymer to generate a tertiary radical, which can propagate with monomer to re-form the secondary species. The critical difference between these two processes is that the exchange between tertiary and secondary species is supplemented in ATRP by additional activation and deactivation reactions for both the secondary and tertiary species. This leads to a competition between the activation-deactivation and exchange processes in ATRP, while there is no such competition in FRP. This introduces the idea of competing processes or equilibria. These competing Processes can alter the fate of the tertiary radical in ATRP, by introducing a deactivation step, in addition to the propagation, or branch formation, available in FRP. Various simulations show that, in order to effectively decrease the branching fraction in ATRP, the tertiary radical must be deactivated relatively rapidly. Then, the rate of branch formation is slower than the rate of transfer, resulting in a decrease in the branching fraction. Kinetic simulations also find that concentrations of copper catalysts have minimal effect on the branching fractions and that higher initiator concentrations tend to decrease the branching levels in ATRP. Furthermore, Monte Carlo simulations found that chain length dependence and presence or absence of intermolecular transfer had minimal effect on the branching fraction. |
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