Department of Chemistry and Center for Sustainable Polymers, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota, 55455-0431, USA.
J Am Chem Soc. 2010 Aug 25;132(33):11649-57. doi: 10.1021/ja103841h.
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 (InCl(3)), benzyl alcohol (BnOH), and triethylamine (NEt(3)). 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 InCl(3) and zero-order in both BnOH and NEt(3). The rate of D,L-LA conversion is also dependent on the indium(III) halide (i.e., t(1/2)(InCl(3)) approximately = 43 min versus t(1/2)(InBr(3)) approximately = 7.5 h, 21 degrees C, CD(2)Cl(2), D,L-LA/BnOH approximately = 100, D,L-LA = 0.84 M, InX(3)/BnOH = 1) and lactide stereoisomer (i.e., k(obs)(D,L-LA) approximately = 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 NEt(3). The product of the reaction of deapH with InCl(3) was identified as InCl(3)(deapH)(H(2)O) 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 CD(2)Cl(2) 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 InCl(3)/BnOH/NEt(3) system is similar to InCl(3)(deapH)(H(2)O) and of the type InCl((3-n))(OBn)(n). 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 InCl(3)/BnOH/NEt(3) system, we found this system to be effective for the polymerization of other cyclic esters, including epsilon-caprolactone and several substituted derivatives.
我们报告了使用三氯化铟（InCl(3)）、苄醇（BnOH）和三乙胺（NEt(3)）原位制备的引发剂对消旋丙交酯（D,L-LA）进行的立体选择性和可控聚合的综合研究结果。数均分子量（M(n)）与单体与醇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，该过程在InCl(3)中为一级，在BnOH和NEt(3)中为零级。D,L-LA 的转化率也取决于三价卤化铟（即 t(1/2)(InCl(3))约= 43 分钟，t(1/2)(InBr(3))约= 7.5 小时，21°C，CD(2)Cl(2)，D,L-LA/BnOH约= 100，D,L-LA=0.84 M，InX(3)/BnOH=1）和丙交酯立体异构体（即 k(obs)(D,L-LA)约= k(obs)(meso-LA)＞k(obs)(L-LA)）。使用 3-二乙氨基-1-丙醇（deapH）代替 BnOH 和 NEt(3)，开发了一种聚合 D,L-LA 的具有相同高立体选择性的模型体系。通过元素分析、X 射线晶体学、NMR 和傅里叶变换红外光谱鉴定了 deapH 与 InCl(3)反应的产物为InCl(3)(deapH)(H(2)O)。当小心避免外来水时，还分离出该配合物的无水形式，并通过脉冲梯度自旋回波（PGSE）NMR 实验表明，在与 D,L-LA 立体选择性聚合相同的条件下，该配合物在 CD(2)Cl(2)溶液中采用双核结构。综合数据表明，InCl(3)/BnOH/NEt(3)体系的引发物种类似于InCl(3)(deapH)(H(2)O)，其类型为InCl((3-n))(OBn)(n)。有了这些信息，我们提出了一个可以解释 D,L-LA 聚合中观察到的立体控制的机制。最后，在探索 InCl(3)/BnOH/NEt(3)体系的范围时，我们发现该体系对其他环状酯的聚合有效，包括己内酯和几种取代衍生物。
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