Polymerization of rac-lactide using schiff base aluminum catalysts:: Structure, activity, and stereoselectivity
Polymerization of rac-lactide using schiff base aluminum catalysts:: Structure, activity, and stereoselectivity
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DOI：
10.1021/ma062194u
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发表时间：
2007-03-20
期刊：
影响因子：
5.5
通讯作者：
Jing, Xiabin
中科院分区：
文献类型：
--
作者：
Du, Hongzhi;Pang, Xuan;Jing, Xiabin
合成了一系列以N，N，O，O-四齿席夫碱为骨架的铝乙基和异丙醇盐。X-射线衍射分析和核磁共振氢谱证实了这些席夫碱铝乙基和异丙氧基铝都是固体结构中具有五配位中心铝的单体物种。与在溶液中均保持单体结构的乙基铝相比，异丙醇铝在溶液中具有较少的空间位阻取代基，也观察到了二核现象。研究了这些配合物对RAC-丙交酯开环聚合的活性和立体选择性。聚合实验表明，(Sb-2d)(AlOPr)-Pr-I[(Sb-2d)=2，2-dimethyl-1，3-propylenebis(3，5-di-tert-butylsalicylideneiminato)]]具有最高的立体选择性，(Sb-3b)(AlOPr)-Pr-I[(Sb-3b)=2，2-dimethyl-1，3-propylenebis(3，5-dichlorinesalicylideneiminato)]]具有最高的活性。两个氮原子之间的取代基和桥联方式都对聚合反应的进程有重要影响，影响孤立聚合物的规整性或聚合速度。对以(Sb-3b)(AlOPr)-Pr-I为催化剂的聚合动力学进行了详细的研究，实验结果表明，(Sb-3b)(AlOPr)-Pr-I的聚合反应速率对[LA]为一级反应，对(Sb-3b)(AlOPr)-Pr-I的反应为1.81级，表明在无空间位阻取代基保护的情况下，体系中的增长物种分布不均匀。此外，以(Sb-3b)(AlOPr)-Pr-I为引发剂的聚合反应可以在较低温度(0℃)下进行，不改变立体选择性。
A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and H-1 NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)(AlOPr)-Pr-i [(SB-2d) = 2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)(AlOPr)-Pr-i [(SB-3b) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization. The polymerization kinetics using (SB-3b)(AlOPr)-Pr-i as a catalyst were studied in details, and the experimental results revealed that the rate of polymerization was first-order in [LA] and 1.81th-order with respect to (SB-3b)(AlOPr)-Pr-i, which indicated that the propagating species was not uniform in the system without the protection of steric hindered substituents. Furthermore, the polymerization by initiating with (SB-3b)(AlOPr)-Pr-i could be progressed at low temperatures (0 degrees C) without the change of stereoselectivity.