Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)
Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)
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DOI：
10.1021/jacs.9b02158
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发表时间：
2019-05-08
影响因子：
15
通讯作者：
Liu, Peng
中科院分区：
文献类型：
--
作者：
Fang, Cheng;Fantin, Marco;Liu, Peng
铜催化原子转移自由基聚合（Cu-ATRP）是应用最广泛的可控自由基聚合技术之一。尽管在文献中进行了广泛的机理研究，但尚未通过计算研究生长的聚合物链的活化/失活的过渡态，这是Cu-ATRP中的关键平衡。因此，对配体和引发剂对活化/失活速率的影响的起源的理解仍然有限。在这里，我们提出了第一个计算分析的Cu-ATRP激活过渡态，以揭示影响激活和失活率的因素。聚合物链和Cu催化剂之间的Br原子转移通过不寻常的弯曲几何形状发生，该几何形状涉及聚合物链末端和Cu催化剂上的辅助配体之间的显著相互作用。因此，活化/失活的速率由Cu催化剂的电子性质和配体引发剂的空间排斥决定。此外，我们的计算揭示了配体骨架的灵活性对激活的重要作用。这些理论分析导致了三个化学上有意义的描述符，即HOMO能量的催化剂（E-HOMO），百分比埋体积（V-bur%），和扭曲能量的催化剂（Δ E-dist），描述的电子，空间，和灵活性的反应性的影响，分别。一个强大的和简单的预测模型配体反应性的影响，从而建立相关的实验活化速率常数，使用多元线性回归这三个描述符。验证使用一组结构不同的配体显示的平均误差小于+/-2千卡/摩尔相比，实验推导的活化能。同样的方法也适用于开发不同的烷基卤引发剂的反应性的预测模型，使用R-X键离解能（BDE）和Cu-X亲卤性作为描述符。
Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst (E-HOMO), percent buried volume (V-bur%), and distortion energy of the catalyst (Delta E-dist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than +/- 2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.