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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 teaching of polymer chemistry, when teaching a ring-opening polymerization initiator to a student, the ring tension of a monomer is a very important thermodynamic factor that determines whether or not it can undergo ring-opening polymerization, but is not always a fundamental factor. Reference is made in some texts to cis- and trans-8-oxabicyclo [4.3.0] nonane (I), cis- and trans-7,9-dioxabicyclo [4.3.0] Alkane (II) two pairs of cis and trans isomers, they are considered to be ring-opening polymerization due to the large ring tension of the trans-isomer. In this paper, we calculated the Gibbs free energy of their ring-opening polymerization through Density Functional Theory (DFT) method. We found that the energy of two isomers of the former I monomer is very close and there is no difference in ring tension, The trans-isomer ring-opening products are more stable in energy and conducive to the ring-opening process. The latter II monomer does, indeed, because the ring-tension of the trans-cyclic monomer is higher than that of the cis-isomer, favoring ring-opening polymerization. This article hopes to clarify these two different mechanisms through the DFT calculations of these two examples and introduce students to quantum chemistry calculations.