环张力驱动环醚单体开环聚合的DFT验证
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摘要
高分子化学教学中,在向学生讲授环状单体开环聚合驱动力时,单体的环张力是决定其能否进行开环聚合的一个非常重要的热力学因素,但有时并非是根本因素。在部分教材上提及顺式-和反式-8-氧杂双环[4.3.0]壬烷(I)、顺式-和反式-7,9-二氧杂双环[4.3.0]壬烷(II)两对顺反异构体时,都认为它们是由于反式异构体环张力大而易于开环聚合。本文通过密度泛函理论(Density Functional Theory,DFT)方法分别计算了它们发生开环聚合的Gibbs自由能变化,发现前者I单体两个异构体能量非常接近,不存在环张力大小的区别,而反式异构体开环产物在能量上更加稳定,利于开环过程。后者II单体则确实是因为反式环状单体的环张力比顺式异构体高,从而有利于开环聚合。本文希望通过这两个实例的DFT计算,阐明这两种不同的机理,并且向学生们介绍量子化学计算方法。
In the ring-opening polymerization chapter of the course Polymer Chemisty,students are told that ring strain is a driving force and an important thermodynamic parameter to determine whether a cyclic monomer can be polymerized or not.However,it is not always the truth.Some textbooks use two cis-and trans-isomers as examples,which are cis-and trans-8-oxabicyclo[4.3.0]-nonane(I)and cis-and trans-7,9-dioxabicyclo[4.3.0]-nonane(II),and consider ring-strain as the only reason why trans-isomer is easier than the corresponding cis-one in both cases.In this work,we calculate the changes of Gibbs free energy of two ring-opening reactions with density functional theory(DFT).The Gibbs energies of cis-and trans-isomers I are very close to each other without difference in ring strain.Instead,the product of trans-isomer after ring opening reaction is more stable than that of cis-isomer by 1.81kcal/mol ofΔG.In contrary,the ring of trans-II is easier to be opened than that of cis-II because of higher ring strain by 3.00kcal/mol ofΔG.The above two different mechanisms are to be explained in class.DFT calculation are to be introduced as well.
In the ring-opening polymerization chapter of the course Polymer Chemisty,students are told that ring strain is a driving force and an important thermodynamic parameter to determine whether a cyclic monomer can be polymerized or not.However,it is not always the truth.Some textbooks use two cis-and trans-isomers as examples,which are cis-and trans-8-oxabicyclo[4.3.0]-nonane(I)and cis-and trans-7,9-dioxabicyclo[4.3.0]-nonane(II),and consider ring-strain as the only reason why trans-isomer is easier than the corresponding cis-one in both cases.In this work,we calculate the changes of Gibbs free energy of two ring-opening reactions with density functional theory(DFT).The Gibbs energies of cis-and trans-isomers I are very close to each other without difference in ring strain.Instead,the product of trans-isomer after ring opening reaction is more stable than that of cis-isomer by 1.81kcal/mol ofΔG.In contrary,the ring of trans-II is easier to be opened than that of cis-II because of higher ring strain by 3.00kcal/mol ofΔG.The above two different mechanisms are to be explained in class.DFT calculation are to be introduced as well.
引文
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[12]Guillaume S M,Annunziata L,del Rosal I,Iftner C,Maron L,et al.Polym Chem-Uk,2013,4(10):3077~3087.
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[2]Odian G G.Principles of polymerization.Wiley.com,2007.
[3]潘才元.高分子化学(第2版).合肥:中国科学技术出版社,2012.
[4]Kohn W,Sham L J.Phys Rev,1965,140(4A):A1133~A1138.
[5]Ling J,Shen J G,Hogen-Esch T E.Polymer,2009,50(15):3575~3581.
[6]Liu J Z,Ling J,Li X,Shen Z Q.J Mol Catal A:Chem,2009,300(1):59~64.
[7]Barros N,Mountford P,Guillaume S M,Maron L.Chem Eur J,2008,14(18):5507~5518.
[8]Lin C Y,Coote M L,Gennaro A,Matyjaszewski K.J Am Chem Soc,2008,130(38):12762~12774.
[9]Ling J,Liu J Z,Shen Z Q,Hogen-Esch T E.J Polym Sci Pol Chem,2011,49(9):2081~2089.
[10]Shi F T,Ling J,Lu J M,Han B Y,Liu L,Hogen-Esch T E.Polymer,2012,53(1):94~105.
[11]Darensbourg D J,Yeung A D.Macromolecules,2013,46(1):83~95.
[12]Guillaume S M,Annunziata L,del Rosal I,Iftner C,Maron L,et al.Polym Chem-Uk,2013,4(10):3077~3087.
[13]Penczek S,Cypryk M,Duda A,Kubisa P,Somkowski S.Prog Polym Sci,2007,32(2):247~282.
[14]Friscsh M J,Trucks G W,Schlegel H B,et al.Gaussian 03,Gaussian Inc,Wallingford 2004.
[15]Kops J,Spanggaard H.Makromol Chem,1975,176(2):299~313.