Five new block copoly(imide siloxane)s have been prepared by reacting two different diamines, 4.4 ''-bis(p-aminophenoxy)-3,3 ''-trifluoromethyl terphenyl (APTTFT) and aminopropyl terminated polydimethylsiloxane (APPS), separately with 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride); BPADA. The reactions were conducted by a two pot solution imidization technique. The diamine APTTFT and the dianhydride BPADA composed the hard block segment while APPS and BPADA composed the soft block segment. The soft and hard blocks of different block lengths were generated by different stoichiometric imbalance in two different flasks and the final polymers were obtained by reacting both the blocks together. Different block copoly(imide siloxane)s were prepared on increasing the hard block lengths (DP) from 7 to 12, 18, 23 and 28 and the soft block lengths (DP) from 4 to 6, 8, 10 and 12, respectively. The resulting polymers have been well characterized by NMR, DSC and DMA techniques. The properties of the block copolymers were compared with the analogous random copolymers and homopolyimide prepared without APPS. (C) 2009 Elsevier Ltd. All rights reserved.