以理論計算研究第十三、十四族有機金屬化合物催化環酯類開環聚合的反應機制

Theoretical Study on the Mechanisms of the Ring-Opening Polymerization of Cyclic Esters Catalyzed by the 13^(th) and 14^(th) Organometallic Compounds

林雅凡(Ya-Fan Lin)

《化學》 78卷4期 (2020/12) Pp. 277-288

https://doi.org/10.6623/chem.202012_78(4).010

摘要

本文旨在回顧以密度泛函方法計算第十三、十四族有機金屬化合物催化環酯類單體開環聚合之反應機制，反應遵循「配位－嵌入」路徑：環酯類先與有機金屬化合物產生配位加成物，起使劑如烷氧基團接著進行親核加成反應，致使環酯單體的C=O基團嵌入於金屬－烷氧基團中間，並在金屬中心的協助之下進行開環。在此機制下，反應速率決定步驟與金屬中心的路易士酸性、配位基的立體障礙與電子效應、起使劑的選擇與單體的立體障礙和環張力等因素有關。一般而言，配位加成物的熱力學穩定度如果較低，速率比較有可能決定於親核加成步驟；另一方面，立體阻礙大的單體或催化劑，開環步驟的過渡態比較不穩定，因此有些系統探討了鏈增長步驟和起始步驟的機制，發現兩者在加成步驟的活化能差別不大；在開環步驟中兩者的活化能卻差異甚大。文中並回顧理論計算評估立體規整性與位置選擇性的方法設計與結果。

關鍵字

密度泛函理論計算 ； 配位－嵌入機制 ； 開環聚合反應 ； 第13,14族有機金屬化合物

並列摘要

In this article, the works using density functional theory to study the mechanisms of the ring-opening polymerization of cyclic esters catalyzed by the 13^(th) and 14^(th) organometallic compounds have been reviewed. Considered the coordination-insertion pathway, the reaction starts from the coordination of the cyclic ester monomer to the organometallic compound, followed by the nucle-ophilic addition of the initiator such as an alkoxide. In this step, formation of the intermediate that the carbonyl group insertion into the metal-alkoxide fragment is proposed. After that, the occurrence of the ring opening is assisted by the metal center. The rate determining step will be correlated to the Lewis acidity of the metal center, the steric and electronic effects of the ligand, the kind of the initiator, and the steric hindrance and ring strain of the monomer. In general, if the monomer-catalyst coordination adduct is thermodynamically unfavorable, the rate is likely to be determined by the nucleophilic addition step. On the other hand, the bulkier the monomer or catalyst is, the less stable the transition state of the ring-opening step exhibits. Moreover, for the nucleophilic addition step, the activation energies of initial and propagation processes are similar. However, they can be largely different in the ring-opening step. The computational methods designed for investigating the tacticity and the regioselectivity and the corresponding results were also discussed in this review.

並列關鍵字

Density functional theory ； Coordination-insertion mechanism ； Ring-opening polymerization ； 13^(th) and 14^(th) organometallic compounds