Stereoselective Ring-Opening Polymerization of Racemic Lactide

由于具有良好的生物相容性和生物降解性，聚乳酸被广泛地应用于组织工程，药物控制释放和环境材料工程等领域。由于聚乳酸的物理、机械和降解等性质在很大程度上决定于其链序列结构，所以丙交醋的立构选择性聚合就成为了一个研究热点。本文合成了一系列无手性席夫碱一铝配合物，并将其用于外消旋丙交醋的立构选择性聚合，此外还对聚合所得的不同立构规整度的聚外消旋丙交酯进行了初步表征。具体的实验结果如下：1．合成了无手性席夫碱一乙基铝配合物（2）和席夫碱一异丙氧基铝配合物（3）。在等摩尔量异丙醇的存在条件下，配合物（2）对rac-LA的开环聚合具有良好的控制性和立体选择性。所得到的聚乳酸是结晶性的聚合物，可以形成一种PLA立体络合物。同核去偶~1HNMR和~(13)CNMR结果表明，它们都是立构嵌段型的聚合物，其平均嵌段长度为11个乳酸单元。端基分析发现：配合物（2）本身没有引发rac-LA的开环聚合，它是在原位先与异丙醇反应生成相应的烷氧基铝化合物，然后，后者再引发rac-LA的开环聚合。因此将席夫碱-异丙氧基铝配合物（3）直接用于引发rac-LA的开环聚合，也具有良好的控制性和立体选择性。2．制备了一系列的席夫碱一铝配合物（5-8）用考察席夫碱配体的性质对催化剂的催化活性和立构选择性的影响。实验结果表明，在苯环上引入较大的取代基（如叔丁基）有助于提高立构选择性，但是增加二元胺桥刚性则会使立构选择性下降。此外，还考察了聚合温度对于配合物（8）／异丙醇催化rac-LA开环聚合的影响，发现降低聚合温度有利于提高全同键接的含量。3．通过控制聚合温度得到了具有不同立构规整度的立构嵌段型聚外消旋乳酸（Pm0.77～0.88）。与PLLA和无规立构聚乳酸不同的是，这些聚外消旋乳酸都可以形成PLA立体络合物，其熔融温度也相对较高；并且随着聚外消旋乳酸的Pm值的增大，其结晶性能也相应增加。Because of their biocompatibility and biodegradation, polylactides were widely used in tissue engineering, drug controlled delivery and environmentally benign materials. The physical, mechanical as well as degradation properties of polylactides were intimately dependent on the chain stereochemistry. Therefore, the stereoselective ring-opening polymerization of lactides is attracting a lot of attention. In this thesis, a series of achiral Schiff-base aluminum complexes were synthesized and used for stereoselective ring-opening polymerizaition of rac-lactide. In addition, the obtained poly(rac-lactide)s with different degree of isotacticity were characterized. The main results were presented as follows: 1. Achiral schiff-base ethyl aluminum complex (2) and schiff-base isopropoxide aluminum complex (3) was synthesized. In the presence of equimolar isopropanol, complex (2) showed good controllability and stereoselectivity for the ROP of rac-lactide. The obtained poly(rac-lactide)s were crystalline polymers which could form PLA stereocomplex. Homonuclear decoupled !H NMR and I3C NMR spectra manifested that the obtained poly(rac-lactide)s were isotactic stereoblock polymers containing blocks with an average of 11 units of enantiomerically pure lactic acid. End group analysis indicated that the Al-Et bond itself was not active for the ROP of lactide. Firstly, complex (2) reacted with isopropanol in situ to produce Al-isopropoxy bond, and then, the latter initiated the ROP of lactide. Therefore, complex (3) was used to initiate the ROP of rac-lactide. And it also carried out the ROP of rac-lactide with high stereoselectivity and excellent molecular-weight control. 2. A series of Schiff-base aluminum complexes (5-8) were prepared in order to investigate the influence of the nature of schiff-base ligands on the activity and stereoselectivity of the catalysts. It was indicated that the introduction of bulk substituents such as tert-butyl into the aromatic rings increased the stereoselectivity significantly, and that increase in the rigidity of the diamino 'bridge' brought about decrease in stereoselectivity. Furthermore, the effects of polymerization temperature on the ROP of rac-lactide catalyzed by complex (8) and isopropanol was investigated. And it was revealed that lowering the polymerization temperature would increase the percentage of isotactic linkages in the resulting polymer chains. 3. Stereoblock poly(rac-lactide)s samples with different degree of isotacticity (Pm = 0.77-0.88) were prepared by ROP of rac-lactide under different polymerization temperature. By contract with PLLA and atactic poly(rac-lactide) samples, these stereoblock poly(rac-lactide)s could form PLA stereocomplex, which exhibited relatively higher melting temperature. And their crystallization abilities increased with increasing the degree of isotacitcity Pm