Poly(ethylene terephthalate) (PET) ionomers containing terminal units derived from 3-sulfobenzoic acid, sodium salt (SSBA), were synthesized using metal-catalyzed melt polymerization techniques. SSBA (1.0-5.0 mol %) in the presence of sodium acetate (10 mol % compared to SSBA) served as an ideal chain-end functionalization reagent, and molecular weights (eta(inh) = 0.28-0.80) were reproducibly obtained at various SSBA levels. H-1 NMR spectroscopy quantified the presence of the sodiosulfonate end groups and the formation of diethylene glycol. For comparative purposes, polyesters containing telechelic alkyl ester end groups derived from dodecanol were also synthesized at nearly equivalent molecular weights in order to fundamentally understand the role of ionic end groups on rheological and thermal performance. Differential scanning calorimetry indicated that the presence of ionic end groups significantly increased the crystallization half-time. A comparison of the crystallization half-time for telechelic ionomers vs non-ionomers demonstrated that ionic aggregation exerted a more profound effect in the diffusion-controlled regime (lower than 150 degreesC). Melt rheology confirmed that the ionic end groups increased the melt viscosity compared to non-ionomers at equivalent molecular weights. Moreover, ionomers and non-ionomers with nearly equivalent molecular weights exhibited similar slopes in log(melt viscosity) vs temperature curves, which is consistent with crystallization data. Melt viscosity vs temperature studies for telechelic ionomers also suggests that ionic aggregation was more stable below 150 degreesC compared to high molecular weight non-ionomer analogues.