Direct access to dithiobenzoate RAFT agent fragmentation rate coefficients by ESR spin-trapping.
Direct access to dithiobenzoate RAFT agent fragmentation rate coefficients by ESR spin-trapping.
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DOI：
10.1002/marc.201400518
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发表时间：
2014-12
影响因子：
4.6
通讯作者：
Kayte Ranieri;Guillaume Delaittre;C. Barner‐Kowollik;Thomas Junkers
中科院分区：
文献类型：
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作者：
Kayte Ranieri;Guillaume Delaittre;C. Barner‐Kowollik;Thomas Junkers
采用电子自旋共振（ESR）捕获方法，研究了叔丁基自由基与二硫代苯甲酸叔丁酯（一种可逆加成-断裂链转移（RAFT）试剂）加成反应的β断裂速率系数随温度的变化关系.新引入的ESR捕获方法进行了严格的评估，发现是可靠的。在20 °C时，观察到的碎裂速率系数接近0.042 s（-1），而首次测定的碎裂反应活化参数为EA = 82 ± 13.3 kJ mol（-1）和A =（1.4 ± 0.25）× 10（13）s（-1）。因此，ESR自旋捕获方法有效地探测RAFT加合物自由基在与RAFT过程的预平衡相关的条件下的稳定性。它特别表明，稳定的RAFT加合物自由基确实形成在RAFT聚合的早期阶段，至少当二硫代苯甲酸酯作为控制剂，如所谓的缓慢断裂理论所规定的。通过方法的设计，获得的碎裂速率系数代表上限。ESR自旋捕获方法，因此被视为一个合适的工具，用于评估的碎片速率系数的RAFT加合物自由基的范围很广。
The β-scission rate coefficient of tert-butyl radicals fragmenting off the intermediate resulting from their addition to tert-butyl dithiobenzoate-a reversible addition-fragmentation chain transfer (RAFT) agent-is estimated via the recently introduced electron spin resonance (ESR)-trapping methodology as a function of temperature. The newly introduced ESR-trapping methodology is critically evaluated and found to be reliable. At 20 °C, a fragmentation rate coefficient of close to 0.042 s(-1) is observed, whereas the activation parameters for the fragmentation reaction-determined for the first time-read EA = 82 ± 13.3 kJ mol(-1) and A = (1.4 ± 0.25) × 10(13) s(-1) . The ESR spin-trapping methodology thus efficiently probes the stability of the RAFT adduct radical under conditions relevant for the pre-equilibrium of the RAFT process. It particularly indicates that stable RAFT adduct radicals are indeed formed in early stages of the RAFT poly-merization, at least when dithiobenzoates are employed as controlling agents as stipulated by the so-called slow fragmentation theory. By design of the methodology, the obtained fragmentation rate coefficients represent an upper limit. The ESR spin-trapping methodology is thus seen as a suitable tool for evaluating the fragmentation rate coefficients of a wide range of RAFT adduct radicals.