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首页> 外文期刊>Chemistry: A European journal >Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes
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Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes

机译：外消旋丙交酯和高salen-铝络合物的外消旋丙交酯的立体选择性开环聚合

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摘要

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).
机译：高度全同立构的聚乳酸或聚乳酸是在外消旋丙交酯与非手性salen-和均salen-铝络合物（salenH（2）= NN'-双（水杨基）乙烯-1,2）的开环聚合（ROP）中合成的-二胺; homosalenH（2）= NN'双（水杨基）三亚甲基-1,3-二胺）。对配体的系统研究表明，席夫碱部分对等规度和高活性骨架的立体影响很重要。原位制备的配合物足够纯，无需纯化即可用于聚合反应。通过X射线衍射阐明了关键配合物的晶体结构，证实了它们是手性的。但是，对H-1和C-13 NMR谱的分析明确表明它们的构象是如此灵活，以至于在25℃下溶液中不能保持络合物的手性环境，并且该络合物在聚合条件下是非手性的。还记录了骨干在传播步骤中的灵活性。因此，聚合物的全同立构规整度由于链端控制机理而发生。在本系统中，使用在骨架上具有2,2-二甲基取代基的高均配体（ArCH = NCH2CMe2CH2N = CHAr）获得最高的反应活性，而在水杨基部分的3位上的tBuMe（2）Si取代基导致最高选择性（P-介孔= 0.9（g）; Tm = 210摄氏度）。发现外消旋丙交酯和L-丙交酯的ROP中的速率常数之比与本系统中的立体选择性相关。该配合物可用于本体聚合，这是工业上最有吸引力的方法，尽管在高温下会损失一些立体选择性，并且所提供的聚合物显示出更高的熔融温度（P-meso = 0-9（2），Tm，最高可达189摄氏度），而不是同级手性聚（L-丙交酯）（Tm = 162-180摄氏度）。即使在高转化率（97-98％）下并在延长的聚合时间（1-2小时）下，仍可保持本体聚合在130℃下的“活性”。

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