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 of ΔG. 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 of ΔG. The above two different mechanisms are to be explained in class. DFT calculation are to be introduced as well.