CO2/顺式-2,3-环氧丁烷交替共聚物的立体寡聚物合成及微结构分析

Synthesis and microstructure analysis of stereoregular oligomers of CO2/cis-2,3-butene oxide alternating copolymer

聚合物主链的立体结构与其热力学性能密切相关.精确控制聚合物立体化学是配位聚合催化最重要的研究目标之一,而合成确定分子量的各种立构规整性寡聚物和它们的核磁碳谱分析是精确表征聚合物微结构的关键.本研究以手性2,3-丁二醇为起始原料,通过迭代合成方法,采用三光气为羰化试剂,制备了全同和间同结构的CO2/顺式-2,3-环氧丁烷交替共聚物(聚碳酸-2,3-丁烯酯)的四聚体和八聚体,并从它们的碳谱信息精确分析聚碳酸-2,3-丁烯酯的微结构.发现全同聚合物的羰基、次甲基和甲基的核磁碳谱分别对应于154.12、75.23和15.71 ppm化学位移处,而间同结构聚碳酸酯的羰基、次甲基和甲基的核磁碳谱分别对应于154.09、75.07和15.51 ppm.

The utilization of carbon dioxide(CO2)as a nontoxic,abundant,economical and renewable C1 building block for the production of valuable organic chemicals can contribute to a more sustainable chemical industry.In the chemical process using CO2 as a raw material,the synthesis of degradable aliphatic polycarbonates by alternating copolymerization with epoxides has attracted widespread attention.Notably,aliphatic polycarbonates with biocompatibility and biodegradability have gradually attracted great attention in the field of biomedicine in recent decades,since Inoue and co-workers first reported the copolymerization of CO2 and epoxides to produce biodegradable CO2-based polycarbonates by using heterogeneous catalyst in 1969.This method used non-toxic and inexpensive CO2 to replace the highly toxic phosgene in the traditional polycondensation as carbonylation reagent.Since then,many heterogeneous and homogeneous catalysts systems have been developed for this copolymerization.However,the commercial applications of aliphatic polycarbonates have very limited due to their poor thermal stability and the mechanical properties in comparison with polyolefins.Recently,the binary or bifunctional catalyst based on metal-salen complexes was found to be very efficient for this transformation in immortal polymerization mode,affording various polycarbonates with complete alternating nature,and excellent region-and stereo-selectivity.Along with the discovery of these highly stereoregular catalyst systems that can generate copolymers with high stereoregularity,study on the regiochemistry of epoxides ring-opening during the copolymerization and the stereochemistry of the carbonate unit sequence in the polymer main chain was also reported.The stereochemistry of a polymer significantly affects its physical property.For biodegradable polymers,e.g.,polylactide,the degradation rate of isotactic polymers is much slower than that of amorphous polymers due to the orderly arrangement of building units in the former.Additionally,the relative stereochemistry in a polymeric chain also bears a memory of the reaction pathway leading to its formation.Therefore,the assignment of the stereochemistry information on a polymer chain is beneficial for understanding the polymerization mechanism and further designing more efficient catalyst systems.The microstructure information of a polymer can be obtained by analyzing its NMR spectrum.However,since the difference in chemical shifts of the same atom in different stereochemical environments is very small,it is quite difficult to assign the nuclear magnetic signals of polycarbonates.The signals of carbonyl and methine regions in the main chain in the 13C NMR spectra are significantly affected by stereochemistry of polycarbonates.As a consequence,the chemical shift and split of carbonyl and methane in the 13C NMR spectra are used to characterize the microstructure of polycarbonates.The accurate microstructure analysis of a polymer is performed by synthesizing the corresponding model oligomers and analysis of their 13C NMR spectra.In the present contribution,four kinds of model compounds of poly(2,3-butene carbonate):isotactic and syndiotactic tetramers and octamers,were synthesized by the use of chiral 2,3-butanediols as starting materials and triphosgene as carbonylation reagent,which not only avoided the degradation to cyclic carbonate during synthesis but also ensured a fast and reliable increase in the number of oligomer units.By comparing the signals in the 13C NMR spectra of steroregular oligomers,it could be concluded that the signals at 154.12,75.23 and 15.71 ppm were attributed to carbonyl,methine and methyl of isotactic polymer,respectivey,while the signals at 154.09,75.07 and 15.51 ppm were assigned to carbonyl,methine and methyl of syndiotactic polymer,respectively.