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An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes
Macromolecules ( IF 5.5 ) Pub Date : 2017-11-09 00:00:00 , DOI: 10.1021/acs.macromol.7b01721
Philipp Jöckle 1, 2, 3 , Caroline Schweigert 3 , Iris Lamparth 4 , Norbert Moszner 4 , Andreas-Neil Unterreiner 3 , Christopher Barner-Kowollik 1, 2, 5
Affiliation  

Five para-substituted monoacyltrimethylgermane derivatives, i.e., p-fluorobenzoyltrimethylgermane (pFBG, λmax = 405 nm), p-methoxybenzoyltrimethylgermane (pMBG, λmax = 397 nm), benzoyltrimethylgermane (pHBG, λmax = 409 nm), p-cyanobenzoyltrimethylgermane (pCBG, λmax = 425 nm), and p-nitrobenzoyltrimethylgermane (pNBG, λmax = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The para-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of ∼112 J (∼28 J for typical PLP experiments with pMBG, pFBG, and pHBG at ∼320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethylgermane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2–4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (∼320 J/pulse with 90 000 pulses resulting in ∼28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and p-cyanobenzoyldimethylgermyl radicals at a factor 4 higher total energy of ∼112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG.

中文翻译:

深入研究单酰基锗的自由基引发行为

五种对位取代的单酰基三甲基锗烷衍生物,即氟苯甲酰基三甲基锗烷(pFBG,λmax = 405 nm),甲氧基苯甲酰基三甲基锗烷(pMBG,λmax = 397 nm),苯甲酰基三甲基锗烷(pHBG,λmax = 409 nm),氰基苯甲酰基三甲基锗烷pCBG,λmax = 425 nm)和硝基苯甲酰基三甲基锗烷(pNBG,λmax通过脉冲激光聚合与随后的电喷雾电离和质谱分析(PLP-ESI-MS)以及飞秒瞬态吸收光谱法的组合,研究了429 nm(= 429 nm)。使用PLP-ESI-MS确定pFBG,pMBG和pHBG的起始苯甲酰基自由基片段的相对起始效率。该与未取代的pHBG相比,具有给电子基团的p取代的衍生物pFBG和pMBG分别显示出1.5和1.3倍的总引发效率。相比之下,带有吸电子基团的衍生物pCBG和pNBG仅在约112 J的4倍高总能量下显示出较弱的起始行为(对于典型的pMBG,pFBG和pHBG在约320 J和90的PLP实验,约为28J。 000个脉冲)。引发效率的差异代表了两类单酰基三甲基锗烷引发剂,即有效引发剂和弱引发剂,每种引发剂在其特定的自由基裂解机理上各不相同。有效的启动器pMBG,pFBG,和pHBG在脉冲辐照后的2-4 ps内显示出超快的系统间交叉,并随后形成了苯甲酰基和三甲基锗烷基残基。相比之下,弱激发剂pCBG和pNBG分别在13 ps和2 ps内通过主要的超快内部转换(IC)在光激发后弛豫到基态,在典型的PLP条件下(约320 J /脉冲,具有90 000脉冲)不允许有效的引发导致每个样本的总能量约为28 J)。pCBG具有弱启动行为,另外会形成甲基和 不允许在典型的PLP条件下有效启动(约320 J /脉冲,带有90 000个脉冲,每个样品的总能量约为28 J)。pCBG具有弱启动行为,另外会形成甲基和 不允许在典型的PLP条件下有效启动(约320 J /脉冲,带有90 000个脉冲,每个样品的总能量约为28 J)。pCBG具有弱启动行为,另外会形成甲基和氰基苯甲酰基二甲基锗基自由基的总能量约为112 J,高4倍。与IC弛豫快得多相一致,pNBG的单体转化率比pCBG弱10倍。
更新日期:2017-11-09
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