To investigate the effects of reactive end-cappers on the performance of PI precursors and the resulting polyimide (PI) films, poly(amic acid)s (PAAs) as a base polymer of the positive-working photosensitive polyimides (PSPIs) were synthesized via ring-opening polymerization of 4,4'-oxydiphthalic anhydride and 4,4'-oxydianiline with four different reactive end-cappers [maleic anhydride, citraconic anhydride, 2,3-dimethylmaleic anhydride (DMA), and 5-norbornene-2,3-dicarboxylic anhydride (NDA)]. During imidization of these end-capped PAAs to form the corresponding PI films, chain extension and crosslinking reactions of the respective end groups occurred, resulting in an improvement in the mechanical and thermal properties despite the low molecular weight of the precursors. However, the UV transmittance at similar to 365 nm, an important property of PSPIs for thick-film applications, such as stress buffer layers, was strongly influenced by the type of end-capper used. These behaviors were understood in terms of the optimized geometries and the simulated UV-vis spectra of modeled end groups determined from density functional theory calculations. (c) 2006 Wiley Periodicals, Inc.