Zhong Zhiyuan, Dijkstra Pieter J, Feijen Jan
Department of Polymer Chemistry and Biomaterials and Institute for Biomedical Technology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
J Am Chem Soc. 2003 Sep 17;125(37):11291-8. doi: 10.1021/ja0347585.
Chiral aluminum isopropoxides based on enantiopure or racemic cyclohexylsalen ligand (Jacobsen ligand) have been prepared and employed for stereoelective/stereoselective ring-opening polymerization of lactide in toluene at 70 degrees C. The kinetics, selectivity of the catalysts, and microstructure of the obtained polylactides, using different combinations of lactide enantiomers and catalysts, were determined. In all cases, polylactides of controlled molecular weight, low polydispersity, and defined end groups were obtained. The polymerizations are first-order in both monomer(s) and catalyst. (R,R)-CyclohexylsalenAlO(i)()Pr [(R,R)-1] polymerizes l-lactide significantly faster than d-lactide with a rate constant ratio k(l)/k(d) of approximately 14. The polymerization of rac-lactide using (R,R)-1 yields crystalline polymers, for which a selectivity factor of approximately 5.5 could be calculated up to 50% conversion based on the optical purity of the isolated polymers. The polymerization of a l-lactide/d-lactide (molar ratio: 80/20) mixture by (R,R)-1 furnishes an isotactic-atactic block copolylactide, which is highly crystalline with a T(m) = approximately 155 degrees C. Polymerization of rac-lactide applying rac-cyclohexylsalenAlO(i)()Pr [rac-1] yields isotactic stereoblock polylactides with a high T(m) = approximately 185 degrees C and a high degree of crystallinity.
基于对映体纯或外消旋环己基水杨醛配体（雅各布森配体）的手性异丙醇铝已被制备，并用于在70℃下于甲苯中进行丙交酯的立体电子选择性/立体选择性开环聚合。使用丙交酯对映体和催化剂的不同组合，测定了所得聚丙交酯的动力学、催化剂的选择性以及微观结构。在所有情况下，均获得了分子量可控、低多分散性且端基明确的聚丙交酯。聚合反应对单体和催化剂均为一级反应。(R,R)-环己基水杨醛铝异丙醇[(R,R)-1]聚合L-丙交酯的速度明显快于D-丙交酯，速率常数比k(l)/k(d)约为14。使用(R,R)-1进行外消旋丙交酯的聚合得到结晶聚合物，基于分离出的聚合物的光学纯度，在转化率高达50%时可计算出选择性因子约为5.5。(R,R)-1引发L-丙交酯/D-丙交酯（摩尔比：80/20）混合物的聚合得到全同立构-无规立构嵌段共聚丙交酯，其结晶度高，熔点T(m)约为155℃。使用外消旋环己基水杨醛铝异丙醇[外消旋-1]进行外消旋丙交酯的聚合得到全同立构规整嵌段聚丙交酯，其熔点T(m)约为185℃，结晶度高。
