The flexibility of a polymer chain depends on the groups present in the polymer backbone. Groups such as the following are called stiffening groups because they decrease the flexibility of the polymer chain.
As an example, poly(ethylene terephalate) is a stiffer molecule than poly(ethylene adipate) because a benzene ring is not as flexible as a chain of CH2 groups.
David Whisnant (Wofford College). Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grants DUE #9950809 and DUE #9950296. Additional support was provided by the Camille and Henry Dreyfus Foundation.