Urea and Thiourea H-Bond Donating Catalysts for Ring-Opening Polymerization: Mechanistic Insights via (Non)linear Free Energy Relationships,Macromolecules

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Urea and Thiourea H-Bond Donating Catalysts for Ring-Opening Polymerization: Mechanistic Insights via (Non)linear Free Energy Relationships
Macromolecules ( IF 5.1 ) Pub Date : 2018-04-13 00:00:00 , DOI: 10.1021/acs.macromol.8b00321
Jinal U. Pothupitiya ₁ , Rukshika S. Hewawasam ₁ , Matthew K. Kiesewetter ₁

Affiliation

Hammett-style free energy studies of (thio)urea/MTBD mediated ring-opening polymerization (ROP) of δ-valerolactone reveal the complicated interplay of reagents that give rise to catalysis through one of two mechanisms. The operative mechanism depends most greatly on the solvent, where polar solvents favor a (thio)imidate mechanism and nonpolar solvents favor a classic H-bond mediated ROP. Data suggest that the transition state is only adequately modeled with ground state thiourea–monomer interactions in the H-bonding pathway, and elusive urea/reagent ground state binding interactions may be irrelevant and, hence, not worth pursuing. However, neither relationship is robust enough to be predictive in the absence of other data. Isotope effects suggest that the base/alcohol binding event is directly observable in the ROP kinetics. New opportunities for catalysis emerge, and a reason for the observed mechanism change is proposed.

中文翻译：

用于开环聚合的尿素和硫脲氢键给体催化剂：通过（非）线性自由能关系的机理分析

δ-戊内酯的（硫）脲/ MTBD介导的开环聚合（ROP）的哈米特式自由能研究揭示了通过两种机理之一引起反应的复杂的试剂相互作用。其作用机理在很大程度上取决于溶剂，其中极性溶剂有利于（硫代）亚氨酸酯的机理，而非极性溶剂有利于经典的H键介导的ROP。数据表明，过渡态仅能通过H键合途径中的基态硫脲与单体相互作用进行充分建模，而难以捉摸的尿素/试剂基态结合相互作用可能无关紧要，因此不值得研究。但是，在没有其他数据的情况下，两种关系都不具有足够的鲁棒性来进行预测。同位素效应表明，在ROP动力学中可以直接观察到碱/醇结合事件。

更新日期：2018-04-13

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