「Spotlight Research」新型O-Phenylene Bisurea (OPBU)催化剂

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作者：石油醚

本期热点研究，我们邀请到了本文第一作者，来自加斯坦福大学的张嘉博士为我们分享。

2024年8月5日，J. Am. Chem. Soc.在线发表了来自斯坦福大学的Robert M. Waymouth教授团队题为「Highly SelectiveO-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket」的研究论文。在本文中，作者开发了一种新型的O-Phenylene Bisurea (OPBU)催化剂，用于环状开链聚合（ROP）。这种催化剂不仅反应迅速、可调性强，而且选择性优异，比传统催化剂（如硫脲、尿素和TBD）高出8到120倍。OPBU催化剂能在几秒到几分钟内实现超过95%的高转化率，产生分子量精准且分散度极低的聚合物。

“Highly SelectiveO-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

Jia Zhang, Kai Hin Lui, Rachele Zunino, Yuan Jia, Romain Morodo, Niklas Warlin, James L. Hedrick, Giovanni Talarico, Robert M. Waymouth*

J. Am. Chem. Soc.2024,ASAP, doi:10.1021/jacs.4c04740”

Q1.请对“Highly SelectiveO-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket”作一个简单介绍。

我们开发了一种新型的O-Phenylene Bisurea (OPBU)催化剂，用于环状开链聚合（ROP）。这种催化剂不仅反应迅速、可调性强，而且选择性优异，比传统催化剂（如硫脲、尿素和TBD）高出8到120倍。OPBU催化剂能在几秒到几分钟内实现超过95%的高转化率，产生分子量精准且分散度极低的聚合物。密度泛函理论（DFT）计算显示，催化剂通过类似于酶催化中的“氧负离子孔洞”的氢键口袋稳定氧负离子过渡态。这些新型催化剂在有机催化领域提供了新的选择，其优越的性能和可调性为控制聚合物合成带来了新的可能性。

In this study, we introduced a new type of O-Phenylene Bisurea (OPBU) catalyst for ring-opening polymerization (ROP). This catalyst is not only fast and easily tunable but also exhibits exceptional selectivity, outperforming traditional catalysts (such as thiourea, urea, and TBD) by 8 to 120 times. OPBU catalysts enable ROP of various monomers, achieving over 95% conversion in seconds to minutes, producing polymers with precise molecular weights and very low dispersity (Đ ≈ 1.01). Density functional theory (DFT) calculations reveal that the catalysts stabilize the oxyanion transition state via a hydrogen bond pocket similar to the “oxyanion hole” in enzymatic catalysis. These new insights provide a new class of organic catalysts with excellent performance and tunability, opening up new opportunities for controlled polymer synthesis.

Q2.有关本次研究的时候遇到过怎样的困难呢？又是怎样克服的呢

在这项研究中，我们遇到了两个主要困难。首先是如何对选择性进行定量测量。以往文献中报告的新型ROP催化剂通常仅依赖GPC结果，由于GPC的精度有限，难以直接比较不同催化剂的选择性。为了解决这个问题，我们设计了一个实验，以更准确地估算链增长与链转移的选择比，从而实现了对许多之前催化剂的准确比较。

其次，关于DFT计算，我们面临了复杂的挑战。反应物复合物可以形成多种不同的构象，使得计算变得非常复杂。尽管这方面我不是专家，但我们的合作伙伴Rachele Zunino（共同一作）非常出色地解决了这个问题，成功地展示了合理的反应机制。

We encountered two main difficulties in this research. First was developing a quantitative measurement of the selectivity parameter. Previously, new selective ROP catalysts reported in the literature usually relied solely on GPC results, which are not precise enough for good comparison between different catalysts. To address this, we designed kinetic experiments to provide a more accurate estimation of the selectivity ratio of chain growth versus chain transfer, allowing us to make fair comparisons among many previous catalysts.

Second, regarding DFT calculations, we faced challenges due to the many different conformations that reactant complexes can access, making the computations very complex. Although I am not an expert in this area, our collaborator Rachele Zunino brilliantly tackled this issue and was able to demonstrate a reasonable reaction mechanism.

Q3.本次研究主体，有没有什么让您感觉特别辛苦和烧脑呢？

论文最大的卖点肯定是催化剂的结构设计，但是研究过程中最耗时且挑战性最大的部分其实是收集可靠的动力学数据和pKa测量。我为了这个课题前后进行了超过一百次动力学实验，打了近千次核磁谱。这些实验对建立关系并深入理解反应机理至关重要。此外，pKa测量也非常困难，因为这些测量需要在极低浓度下进行，并且水分含量必须控制在极低水平，否则会导致测量不准确。这些繁琐的工作确实让人感到非常吃力。

The most time-consuming and overwhelming aspects were gathering reliable kinetic data and pKa measurements. To ensure accurate data, I probably conducted over a hundred kinetic experiments and nearly a thousand NMR runs. These experiments were crucial for establishing relationships and understanding the reaction mechanisms. Additionally, pKa measurements for a library of compounds were challenging due to the need for measurements at very low concentrations and maintaining an exceptionally low level of water/oxygen content to avoid inaccuracies. These tasks were indeed quite laborious and mentally demanding.

Q4.将来想继续研究化学的哪个方向呢？

我最近刚刚毕业，并且进入了一家涂料公司工作。未来，我的研究将主要集中在开发高性能且环保的涂料上。我希望通过研究和开发新的涂料配方来提高产品的性能，同时减少对环境的影响。

I recently graduated and have started a position at a coatings company. In the future, my research will focus on developing high-performance and eco-friendly coatings. I aim to improve product performance while reducing environmental impact through the development of new coating formulations.

Q5.最后，有什么想对各位读者说的吗？

我希望我们的研究能够激发更多的科研人员在催化剂设计和功能化学方面的探索。科学研究是一项团队合作的工作，我要特别感谢我的团队成员,尤其是Kai Hin Lui和Rachele Zunino的鼎力相助。希望大家能继续关注和支持我们的研究成果。也祝愿大家科研顺利，多发论文。

I hope our research inspires more scientists to explore catalyst design and functional chemistry. Scientific research is a collaborative effort, and I am especially grateful to my team members and funding agencies for their support. I look forward to continued attention and support for our research results.