Segmented copolyurethanes comprising 40-60% by weight of polyurethane hard segments (HS) and polyether soft-segment (SS) with different functionalities (SS-f(n)), have been formed by reaction injection moulding (RIM). The HS were formed from 4,4' diphenylmethane diisocyanate (MDI) reacted with ethane diol (ED). The three SS-prepolymers used were all hydroxyl-functionalised poly(oxypropylene-b-oxyethylene)s with different nominal functionalities (f(n)) of 2, 3 and 4 but with a constant molar mass per functional group of similar to2000 g mol(-1). RIM materials were characterised using differential scanning calorimetry, dynamic mechanical thermal analysis, tensile stress-strain and single-edge notch fracture studies. Predictions using a statistical model of the RIM-copolymerisation showed that increasing SS-f(n) lead to more rapid development of copolymer molar mass with isocyanate conversion. Experimentally, the RIM-PU exhibited a wide range of mechanical behaviour resulting from differences in molecular and morphological structures. Increasing SS-fn produced materials with improved mould release behaviour and fracture resistance. However, increasing SS-f(n) also reduced the degree of phase separation developed in the copolyurethanes, resulting in increased modulus-temperature dependence and poorer tensile properties. (C) 2003 Elsevier Science Ltd. All rights reserved.