This study investigates the role of polymer chain mobility, through the addition of a plasticizer, N-butylbenzene sulfonamide (BBSA), on the overall blending dynamics and interfacial copolymer chain formation in the reactive blending of brominated poly(isobutylene-co-p-methylstyrene) (BIMSM) and polyamide (PA). Compared to a non-reactive poly(isobutylene-co-p-methylstyrene) (IMSM)/PA blend, the reactive BIMSM/PA blend demonstrates a rapid, order of magnitude increase in viscosity, and attains a plateau viscosity at higher temperatures. The addition of a BBSA plasticizer (5-10 wt%) surprisingly increases the viscosity even further, suggesting an enhancement in interfacial reaction due to increased chain mobility. It is also found to increase the level of grafting from about 46 to 57% and has a significant influence on the morphology of the blend system. It is shown that the reactive grafting has an onset at lower temperatures for the blend system with 10 and 43% plasticizer. The morphologies of the blends with 5-10% plasticizer have a more uniform distribution (d(v)/d(n)) with a reduced d(v) for the dispersed BIMSM particles. At 43 wt% plasticizer the blends show an increased phase size resulting from the significantly enhanced phase coalescence. These results indicate that enhanced chain mobility through plasticizer usage can impact interfacial compatibilization and phase coalescence and that a threshold level of 5-10 wt% plasticizer provides an optimum level of chain mobility for this reactive blending system. (C) 2014 Elsevier Ltd. All rights reserved.