Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes
Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes
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DOI：
10.1002/chem.200601308
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发表时间：
2007-01-01
影响因子：
4.3
通讯作者：
Kondo, Tadao
中科院分区：
文献类型：
--
作者：
Nomura, Nobuyoshi;Ishii, Ryohei;Kondo, Tadao
通过外消旋丙交酯与非手性salen-和homosalen-铝络合物（salenH（2）= NN '-双（亚水杨基）乙二胺; homosalenH（2）= NN'-双（亚水杨基）三亚甲基-1，3-二胺）的开环聚合（ROP）合成了高度全同立构的聚丙交酯或聚（乳酸）。配体的系统探索表明了席夫碱部分对全同立构规整度和高活性骨架的空间影响的重要性。原位制备的配合物纯度足够高，无需纯化即可用于聚合反应。通过X-射线衍射确定了配合物的晶体结构，证实了配合物的手性性质。然而，H-1和C-13 NMR谱的分析明确地表明，它们的构象是如此灵活，以至于络合物的手性环境不能在25 ℃下保持在溶液中，并且络合物在聚合条件下是非手性的。还记录了传播步骤中骨干的灵活性。因此，聚合物的全同立构规整度由于链端控制机制而发生。在本体系中，在骨架中具有2，2-二甲基取代基的homosalen配体（ArCH= NCH 2CMe 2CH 2N =CHAr）获得最高的反应性，而在亚水杨基部分的3-位上的tBuMe（2）Si取代基导致最高的选择性（P-meso = 0.9（g）; T-m=210 ℃）。发现外消旋丙交酯和L-丙交酯的ROP中的速率常数的比率与本系统中的立体选择性相关。该配合物可用于工业上最有吸引力的本体聚合，尽管在高温下立体选择性有所损失，并且所得到的聚合物显示出比纯手性聚（L-丙交酯）（T-m = 162-180 ℃）更高的熔融温度（P-meso =0-9（2），T-m，高达189 ℃）。即使在高转化率（97-98%）和延长的聚合时间（1-2小时）下，在130 ℃下本体聚合的“活性”也得以保持。
Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymerization (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH(2)=NN'-bis(salicylidene)ethylene-1,2-diamine; homosalenH(2) = NN'bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepared in situ are pure enough to apply to the polymerizations without purification. The crystal structures of the key complexes are elucidated by X-ray diffraction, which confirms that they are chiral. However, analysis of the H-1 and C-13 NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in solution at 25 degrees C and that the complexes are achiral under the polymerization conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-dimethyl substituents in the backbone (ArCH=NCH2CMe2CH2N=CHAr), whereas tBuMe(2)Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (P-meso = 0.9(g); T-m=210 degrees C). The ratio of the rate constants in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymerization, which is the most attractive in industry, although with some loss of stereoselectivity at high temperature, and the afforded polymer shows a higher melting temperature (P-meso =0-9(2), T-m, up to 189 degrees C) than that of homochiral poly(L-lactide) (T-m = 162-180 degrees C). The "livingness" of the bulk polymerization at 130 degrees C is maintained even at a high conversion (97-98%) and for an extended polymerization time (1-2 h).