Mechanism of Organoscandium-Catalyzed Ethylene Copolymerization with Amino-Olefins: A Quantum Chemical Analysis
Mechanism of Organoscandium-Catalyzed Ethylene Copolymerization with Amino-Olefins: A Quantum Chemical Analysis
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DOI：
10.1021/acscatal.9b02317
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发表时间：
2019-09-01
期刊：
影响因子：
12.9
通讯作者：
Marks, Tobin J.
中科院分区：
文献类型：
--
作者：
Chen, Jiazhen;Motta, Alessandro;Marks, Tobin J.
乙烯与极性单体的直接、高效共聚代表了极性聚乙烯合成的“圣杯”；然而，开发这种共聚反应的有效催化剂仍然是一个悬而未决的挑战。最近，有机钪催化剂被证明对乙烯与极性单体[H2C=CH(CH2)(n)CH(2)FG, FG =极性官能团]的共聚反应非常活跃。有趣的是，共聚单体的链结选择性随着连接体长度(n)的增加而降低，而总体聚合活性在很大程度上不受影响，其有趣的机制起源尚不清楚。本研究以（C5Me4SiMe3）Sc(CH2CH2CH3)B+(C6F5)(4)(-) （Sc-1）为模型活性物质，N-（1-丁烯基）“Pr-2”和N-（1-辛烯基）“Pr-2”为模型共聚单体，采用密度泛函数理论（DFT）方法研究了有机钪催化乙烯与氨基烯烃（AO）共聚的机理。在单体配位、活化和插入等可能的情况下，发现共聚活性在很大程度上受分子间氨基烯烃n配位的控制。氨基烯烃n依赖的链结模式源于链长对氨基烯烃螯合“自辅助”链结途径的能量势垒的调节。短链N-（1-丁烯基）“Pr-2”链通过自我辅助插入途径（6.0 kcal/mol能量势垒），而长链N-（1-辛烯基）“Pr-2”链通过外源胺配位的非辅助1,2插入途径（7.2 kcal/mol能量势垒）。这些发现解释了实验结果，展示了Sc催化剂在极性单体共聚中的特征反应性，并突出了极性单体共聚催化剂开发的潜力和挑战。
The direct, efficient copolymerization of ethylene with polar monomers represents a "holy grail" for the synthesis of polar polyethylenes; however, developing effective catalysts for such copolymerizations remains a largely unsolved challenge. Very recently, organoscandium catalysts were shown to be very active for ethylene + polar monomer [H2C=CH(CH2)(n)CH(2)FG, FG = polar functional group] copolymerization. Interestingly, comonomer enchainment selectivity decreases with increasing linker length (n), while overall polymerization activity is largely unaffected, and the intriguing mechanistic origins are not yet understood. In this study, density functional theory (DFT) methods are employed to investigate the mechanism of organoscandium-catalyzed ethylene + amino olefin (AO) copolymerization, using (C5Me4SiMe3)Sc(CH2CH2CH3)B+(C6F5)(4)(-) (Sc-1) as the model active species and N-(1-butenyl)"Pr-2 and N-(1-octenyl)"Pr-2 as model comonomers. Among conceivable scenarios in monomer coordination, activation, and insertion, it is found that copolymerization activity is largely governed by intermolecular amino olefin N-coordination. Amino olefin n-dependent enchainment patterns arise from chain-length regulation of the energy barrier for an amino olefin chelating "self-assisted" enchainment pathway. Short-chain N-(1-butenyl)"Pr-2 enchains via a self-assisted insertion pathway (6.0 kcal/mol energy barrier), while long-chain N-(1-octenyl)"Pr-2 enchains via unassisted 1,2-insertion with exogenous amine coordination (7.2 kcal/mol energy barrier). These findings explain the experimental results, showcase the characteristic reactivity of Sc catalysts in polar monomer copolymerization, and highlight the potential and challenges in developing catalysts for polar monomer copolymerization.