Chain-End Stereostructure of Heterotactic Poly(ethyl methacrylate) Prepared with t-Butyllithium/Bis(2,6-di-t-butylphenoxy)methylaluminum as Studied by 13C NMR Spectroscopy
Chain-End Stereostructure of Heterotactic Poly(ethyl methacrylate) Prepared with t-Butyllithium/Bis(2,6-di-t-butylphenoxy)methylaluminum as Studied by 13C NMR Spectroscopy
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DOI：
10.1295/polymj.28.61
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发表时间：
1996-01
期刊：
影响因子：
2.8
通讯作者：
T. Kitayama;Tomohiro Hirano;K. Hatada
中科院分区：
文献类型：
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作者：
T. Kitayama;Tomohiro Hirano;K. Hatada
在-78 °C和-95 °C下，在甲苯中用叔丁基锂/双（2，6-二叔丁基苯氧基）甲基铝制备了异规聚甲基丙烯酸乙酯[poly（EMA）s]。在聚（EMA）的13 C NMR谱中的端基信号被分配在引发和终止链端附近的三单元构型。对引发剂链端的分析表明：（1）二聚体阴离子优先进行外消旋加成，形成具有外消旋（r）二联体的三聚体阴离子（t-Bu-M-M−→t-Bu-r-M−）;（2）具有r二联体的三聚体阴离子有利于内消旋（m）加成（t-Bu-r-M−→t-Bu-r-m-M−）;（3）降低聚合温度可显著提高m-选择性。对终止链端的分析表明：（1）在链端具有r个二联体的活性聚合物（rM−）比具有m个二联体的活性聚合物（rM −）存在更多，表明r-末端阴离子的稳定性更高;（2）活性阴离子在与醇的终止反应中有利于r-加成，表明增长物种固有的r-特异性。在此基础上，讨论了异规聚合中立体定向性的机理。
Heterotactic poly(ethyl methacrylate)s [poly(EMA)s] were prepared with t-butyllithium/bis(2,6-di-t-butyl-phenoxy)methylaluminum in toluene at −78°C and −95°C. The end-group signals in 13C NMR spectra of the poly(EMA) were assigned in terms of triad configurations near the initiating and terminating chain-ends. The analysis for the initiating chain-end revealed that (1) the dimer anion preferentially undergoes racemo addition to form a trimer anion with racemo (r) diad (t-Bu-M-M−→t-Bu-r-M−), (2) the trimer anion with r diad favors meso (m) addition (t-Bu-r-M−→t-Bu-r-m-M−), and (3) the m-selectivity is much enhanced by lowering the polymerization temperature. The analysis of the terminating chain-end showed that (1) living polymer having r diad at the chain end (〰rM−) exists more than that with m diad (〰rM−), suggesting the higher stability of the r-ended anion, and (2) the living anion favors r-addition in their termination reaction with alcohol, indicating the inherently r-specific nature of the propagating species. Based on these structural analyses the mechanism of stereospecificity in the heterotactic polymerization is discussed.