摘要/Abstract

合成了一系列非对称β-二亚氨基镁硅氨基络合物，通过¹H NMR、¹³C NMR和元素分析进行了表征，典型络合物经X-射线单晶衍射确定了其晶体结构.利用上述镁络合物在四氢呋喃中催化外消旋丙交酯开环聚合主要得到较低至中等杂规度的聚合物（P_r=0.64~0.80）.镁络合物在室温、甲苯中对ε-己内酯开环聚合表现出中等的催化活性，得到较宽分子量分布的聚合物（M_w/M_n=1.37~1.67）.利用典型镁络合物2进行催化L-丙交酯和ε-己内酯共聚的研究，采用分步加料法（两种不同加料顺序）以及一锅法均得到了两嵌段结构为主的共聚物.所得共聚物通过核磁氢谱、碳谱等测试，进一步确定了其微观链结构.
关键词: β-二亚氨基配体, 镁络合物, 内酯, 开环聚合, 共聚
Magnesium silylamido complexes 1~7 bearing non-symmetric β-diketiminate ligands were synthesized via the reactions of corresponding proligands HL^1~7 with one equivalent of Mg[N(SiMe₃)₂]₂ at 80℃ in toluene. All complexes were characterized by ¹H NMR, ¹³C NMR and elemental analysis. The monomeric nature of complexes 1 and 5 in the solid state was further confirmed by X-ray diffraction studies. In both complexes, the metal center is tri-coordinated by the β-diketiminate ligand and one silylamido group. The very close bond lengths of two Mg-N bonds as well as close C-N distances of the chelate ring indicate significant delocalization. The apparent deviation of the magnesium center from the ligand backbone plane suggests a certain ηⁿ-coordination nature of the ligand to the metal center. These magnesium silylamido complexes showed good catalytic activities for the ring-opening polymerization of rac-lactide under ambient conditions, and could polymerize 300 equivalents of rac-lactide to high molecular weight polymers within short time in THF. The solvent effect played a critical role during the polymerization process. All complexes showed higher catalytic activity in THF than in toluene. Taking complex 2 as an example, a monomer conversion of 96% could be achieved within 10 min in THF, whereas a conversion of 80% could only be achieved within extended polymerization time of 210 min in toluene (Table 1, Entries 6 and 9,[LA]₀/[Mg]₀=100). Upon the addition of isopropanol, the activities of magnesium silylamido complexes 1~7 increased significantly. For instance, when the polymerization runs were carried in THF in the presence of isopropanol, the reaction time could be reduced to 10~20 min even with a high molar ratio of[LA]₀/[Mg]₀=300. Moreover, the type and location of the substituent(s) on the N-aryl group of the β-diketiminate ligand exerts a significant influence on the catalytic activity of the corresponding complex toward the polymerization of rac-lactide. In the absence of excess isopropanol, the introduction of sterically demanding substituent to the ortho-position of N-aryl ring leads to a decrease of the polymerization activity; but the influence of electron-withdrawing group is different in different solvents (toluene or THF). These magnesium complexes could produce heterotactic polymers in THF (P_r=0.64~0.80) and atactic polymers in toluene (P_r=0.45~0.58). Magnesium complexes 1~7 also displayed high catalytic activities for the ring-opening polymerization (ROP) of ε-caprolactone, among them complexes bearing β-diketiminate ligand with bulky ortho-substituted N-aryl rings showed higher activities. Generally, the ROPs of ε-caprolactone initiated by these magnesium silylamido complexes were not well-controlled, giving moderately distributed polymers (M_w/M_n=1.37~1.67). Additionally, diblock copolymers of L-lactide and ε-caprolactone were obtained by using 2 as the initiator via both sequential feeding of two monomers (in either order) and the one-pot method. The formation of diblock copolymers were verified by ¹H NMR, ¹³C NMR, DSC and GPC analysis.
Key words: β-diketiminate ligand, magnesium complex, cyclic ester, ring-opening polymerization, copolymerization


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