Semi-rigid polyesters from diphenyl dicarbonic acid and some branched propyl and butyl spacers have been investigated using Differential Scanning Calorimetry. From Δcp at Tg and the determination of the fragility index m, we studied the “strong-fragile” behaviour of these materials. All the samples appear thermodynamically “strong” and “kinetically” fragile. A comparison of these results with those obtained from glass-forming liquids leading to linear polymers - such as PET, PCT, PEN or poly-methyl (α-n-alkyl) acrylates - shows that a modification of the polymer rigidity leads to change the fragility index m and the Δcp at the glass transition.