Segmented copolyureas and copoly(urethane-urea)s comprising 50 % by weight of polyurea hard segments (HS) and polyether soft segments (SS) with different functionalities, have been formed by reaction injection moulding (RIM). The HS were formed from 4,4’-diphenylmethane diisocyanate reacted with mixed isomers of 3,5-diethyltoluene diamine. The nominal functionality of the SS prepolymers used (either amino- or hydroxyl-functionalized polyoxypropylenes with a constant molar mass per functional group of similar to 2000 g mol(-1)) was systematically increased from 2 to 4. RIM materials were characterized using a simple demould toughness test, and d.s.c. and d.m.t.a. were used to obtain SS and HS glass transition temperatures, T-g(S) and T-g(H), and the degree of phase separation. Variations in the development of copolymer molar mass and HS sequence length, resulting from reactivity differences between the monomers and increasing functionality of the SS prepolymer, have been modelled using a statistical analysis of the RIM copolymerization. Schematic phase diagrams are presented, to aid interpretation of the complex effects of SS structure on the kinetic competition between polymerization and phase separation processes during the formation of RIM copolymers.