Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini,ACS Macro Letters

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Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini
ACS Macro Letters ( IF 5.1 ) Pub Date : 2017-08-16 00:00:00 , DOI: 10.1021/acsmacrolett.7b00499
Andrea Lauer _{1,

2,

3} , David E Fast ₄ , Jan Steinkoenig _{1,

2,

3} , Anne-Marie Kelterer ₄ , Georg Gescheidt ₄ , Christopher Barner-Kowollik _{1,

2,

3}

Affiliation

Herein, we report the unique—and first time—wavelength-dependent investigation with strictly monochromatic light of 305–405 nm wavelength into the stability of photoinitiator-derived chain termini of poly(methyl methacrylate) using a tunable laser system fused with pulsed-laser irradiation and size exclusion chromatography hyphenated to high-resolution electrospray mass spectrometry (PLI-SEC-ESI-MS). We assess several substitution patterns of methyl groups on the common benzoyl-type radical fragment. Critically, methyl substitution in the 2- and 6-positions of the benzoyl moiety, i.e., in both ortho-positions, resulted in stable chain ends up to approximately 350 nm. The stability can be attributed to a blue-shift of the n−π* transitions (relevant for the end group reactivity) as predicted by earlier density functional theory (DFT) calculations on model species. In sharp contrast, our experiments show a far reduced stability of the end groups commencing from 400 nm onwards, when the dual ortho-methyl substitution in the benzoyl fragment is missing. Thus, we demonstrate that the substitution pattern on the phenyl ring of the benzoyl group dictates the chain end stability as a function of wavelength in excellent agreement with the quantum chemical predictions. Our study thus provides critical insights into selecting suitable photoinitiation systems for specific wavelength regimes.

中文翻译：

光引发剂衍生的大分子链末端的波长依赖性光化学稳定性

在这里，我们报告了使用与脉冲激光融合的可调谐激光系统对聚（甲基丙烯酸甲酯）的光引发剂衍生链末端的稳定性进行了独特且首次的波长相关研究，该研究使用 305-405 nm 波长的严格单色光进行辐照和尺寸排阻色谱与高分辨率电喷雾质谱 (PLI-SEC-ESI-MS) 联用。我们评估了常见苯甲酰基型自由基片段上甲基的几种取代模式。重要的是，苯甲酰基部分的 2 位和 6 位上的甲基取代，即在两个邻位-位置，导致稳定的链末端高达约 350 nm。正如早期对模型物种的密度泛函理论 (DFT) 计算所预测的，稳定性可归因于 n-π* 跃迁（与端基反应性相关）的蓝移。形成鲜明对比的是，当苯甲酰基片段中的双邻位-甲基取代缺失时，我们的实验表明，从 400 nm 开始，端基的稳定性大大降低。因此，我们证明了苯甲酰基苯环上的取代模式决定了链端稳定性作为波长的函数，这与量子化学预测非常一致。因此，我们的研究为为特定波长范围选择合适的光引发系统提供了重要的见解。

更新日期：2017-08-16

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