Hyper-branched polystyrenes (HBPS) were synthesized. The bulk glass transition temperature (T-g) measured by differential scanning calorimetry (DSC) for two kinds of HBPS with an equivalent M-w, which were fractionated from different lots, were different, being respectively higher and lower than that of the corresponding linear polystyrene (PS). Infrared spectroscopy revealed that the T-g of HBPS increased with an increasing extent of intramolecular cross-linking, or cyclization, in the molecule. The segmental dynamics of HBPS was examined by dynamic mechanical analysis. The relaxation temperature for the segmental motion in HBPS was consistent with the DSC results. The fragility index was always lower for HBPS than for the linear PS, regardless of its primary structure and chain end chemistry. This would indicate that the segmental motion for HBPS is less cooperative than that of the linear PS, probably due to a lack of intermolecular chain entanglements in HBPS. (C) 2009 Elsevier Ltd. All rights reserved.