Polymerization of rac-Lactide Using Schiff Base Aluminum Catalysts: Structure, Activity, and Stereoselectivity

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• 作者：Hongzhi Du ; Xuan Pang ; Haiyang Yu ; Xiuli Zhuang ; Xuesi Chen ; Dongmei Cui ; Xianhong Wang ; Xiabin Jing
• 刊名：Macromolecules
• 出版年：2007
• 出版时间：March 20, 2007
• 年：2007
• 卷：40
• 期：6
• 页码：1904 - 1913
• 全文大小：229K
• 年卷期：v.40,no.6(March 20, 2007)
• ISSN：1520-5835

文摘

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligandframework have been prepared. X-ray diffraction analysis and ¹H NMR confirmed that these Schiff base aluminumethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solidstructures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, thedinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution havealso been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerizationof rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)AlOⁱPr [(SB-2d) = 2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)AlOⁱPr [(SB-3b) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highestactivity among these aluminum isopropoxides. The substituents and the mode of the bridging part between thetwo nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencingeither the tacticity of isolated polymers or the rate of polymerization. The polymerization kinetics using (SB-3b)AlOⁱPr as a catalyst were studied in details, and the experimental results revealed that the rate of polymerizationwas first-order in [LA] and 1.81th-order with respect to (SB-3b)AlOⁱPr, which indicated that the propagatingspecies was not uniform in the system without the protection of steric hindered substituents. Furthermore, thepolymerization by initiating with (SB-3b)AlOⁱPr could be progressed at low temperatures (0 C) without thechange of stereoselectivity.