4-Urazoylbenzoic acid groups are attached to the chain ends of polyisobutylene. The cooperative assembling process of these polar groups is studied by DSC and dielectric and dynamic mechanical spectroscopy. The melting of the ordered clusters occurs in the temperature range 380-390 K. Distortions within the U4A clusters (Sigma process) are monitored below the "melting" temperature T-m with dielectric spectroscopy. On a larger length scale, these distortions also lead to stress relaxation which can be probed by dynamic mechanical measurements. Near T-m, the relaxation of U4A multiplets (a* relaxation) is detected with dielectric spectroscopy. In this temperature range, dynamic mechanical measurements show a transition from elastic behavior to viscous flow. The results are rationalized with molecular modeling calculations.