Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota , 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States.
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University , Ithaca, New York 14853, United States.
J Am Chem Soc. 2017 Oct 25;139(42):15222-15231. doi: 10.1021/jacs.7b09079. Epub 2017 Oct 6.
Mechanistic studies involving synergistic experiment and theory were performed on the perfectly alternating copolymerization of 1-butene oxide and carbic anhydride using a (salph)AlCl/[PPN]Cl catalytic pair. These studies showed a first-order dependence of the polymerization rate on the epoxide, a zero-order dependence on the cyclic anhydride, and a first-order dependence on the catalyst only if the two members of the catalytic pair are treated as a single unit. Studies of model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide. In addition, ring-opening of epoxide by an intermediate comprising multiple carboxylates was found to be rate-determining. On the basis of the experimental results and analysis by DFT calculations, a mechanism involving two catalytic cycles is proposed wherein the alternating copolymerization proceeds via intermediates that have carboxylate ligation in common, and a secondary cycle involving a bis-alkoxide species is avoided, thus explaining the lack of side reactions until the polymerization is complete.
采用（salph）AlCl/[PPN]Cl 催化对，对 1-丁烯氧化物与碳酸酐的完全交替共聚进行了协同实验和理论的机理研究。这些研究表明，聚合速率对环氧化物呈一级依赖，对环状酸酐呈零级依赖，且仅当将催化对的两个成员视为单个单元时，对催化剂呈一级依赖。模型配合物的研究表明，混合烷氧基/羧酸盐铝中间体优先打开环状酸酐而不是环氧化物。此外，发现由多个羧酸盐组成的中间体开环环氧化物是速率决定步骤。基于实验结果和 DFT 计算分析，提出了一种涉及两个催化循环的机理，其中交替共聚通过具有共同羧酸盐连接的中间体进行，避免了涉及双烷氧基物种的二次循环，从而解释了聚合完成之前缺乏副反应的原因。
