Mechanistic Study of the Stereoselective Polymerization of D,L-Lactide Using Indium(III) Halides
Mechanistic Study of the Stereoselective Polymerization of D,L-Lactide Using Indium(III) Halides
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DOI：
10.1021/ja103841h
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发表时间：
2010-08-25
影响因子：
15
通讯作者：
Tolman, William B.
中科院分区：
文献类型：
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作者：
Pietrangelo, Agostino;Knight, Spencer C.;Tolman, William B.
我们报告的结果，最近发现的立体选择性和控制聚合的外消旋丙交酯（D，L-LA）使用原位制备的引发剂，从氯化铟（III）（InCl 3），苯甲醇（BnOH），和三乙胺（NEt 3）的全面调查。数均分子量（M-n）与单体与醇的浓度比和单体转化率之间的线性关系与良好控制的聚合一致。聚合动力学研究表明，[InCl_3]（0）的聚合反应为一级动力学过程，[BnOH]（0）和[NEt_3]（0）的聚合反应均为零级动力学过程。D，L-LA转化的速率还取决于卤化铟（III）（即，t（1/2）（InCl 3）约为43分钟，t（1/2）（InBr 3）约为7.5小时，21 ℃，CD 2Cl 2，[D，L-LA]（0）/[BnOH]（0）约为100，[D，L-LA]（0）= 0.84 M，[InX 3]（0）/[BnOH]（0）= 1）和丙交酯立体异构体（即，k（obs）（D，L-LA）近似于k（obs）（meso-LA）> k（obs）（L-LA））。使用3-二乙基氨基-1-丙醇（deapH）代替BnOH和NEt 3开发了以相同高度立体选择性聚合D，L-LA的模型系统。通过元素分析、X射线衍射、核磁共振和傅里叶变换红外光谱等方法确定了deapH与InCl_3反应的产物为[InCl_3（deapH）（H_2 O）]（2）。当小心避免偶然的水时，也分离出无水形式的复合物，并且通过脉冲梯度自旋回波（PGSE）NMR实验显示，在与D，L-LA的立体选择性聚合中使用的那些条件相同的条件下，在CD 2Cl 2溶液中采用双核结构。综合数据表明，InCl_3/BnOH/NEt_3体系的起始物种类似于[InCl_3（deapH）（H_2 O）]（2）和[InCl（3-n）（OBn）（n）]（m）类型。有了这些信息，我们提出了一种机制，合理化所观察到的立体控制D，L-LA聚合。最后，在InCl 3/BnOH/NEt 3系统的范围的探索中，我们发现该系统对于其他环酯的聚合是有效的，包括ε-己内酯和几种取代的衍生物。
We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (D,L-LA) using an initiator prepared in situ from indium(III) chloride (InCl3), benzyl alcohol (BnOH), and triethylamine (NEt3). Linear relationships between number-average molecular weight (M-n) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl3](0) and zero-order in both [BnOH](0) and [NEt3](0). The rate of D,L-LA conversion is also dependent on the indium(III) halide (i.e., t(1/2)(InCl3) approximate to 43 min versus t(1/2)(InBr3) approximate to 7.5 h, 21 degrees C, CD2Cl2, [D,L-LA](0)/[BnOH](0) approximate to 100, [D,L-LA](0) = 0.84 M, [InX3](0)/[BnOH](0) = 1) and lactide stereoisomer (i.e., k(obs)(D,L-LA) approximate to k(obs)(meso-LA) > k(obs)(L-LA)). A model system that polymerizes D,L-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lieu of BnOH and NEt3. The product of the reaction of deapH with InCl3 was identified as [InCl3(deapH)(H2O)](2) by elemental analysis, X-ray crystallography, and NMR and FTIR spectroscopies. An anhydrous version of the complex was also isolated when care was taken to avoid adventitious water, and was shown by pulsed gradient spin echo (PGSE) NMR experiments to adopt a dinuclear structure in CD2Cl2 solution under conditions identical to those used in its stereoselective polymerization of D,L-LA. The combined data suggest that the initiating species for the InCl3/BnOH/NEt3 system is similar to [InCl3(deapH)(H2O)](2) and of the type [InCl(3-n)(OBn)(n)](m). With this information we propose a mechanism that rationalizes the observed stereocontrol in D,L-LA polymerizations. Finally, in an exploration of the scope of the InCl3/BnOH/NEt3 system, we found this system to be effective for the polymerization of other cyclic esters, including epsilon-caprolactone and several substituted derivatives.