Two constitutionally isomeric alicyclic polyimides (PIs), head-to-head (HH) and random types, were successfully synthesized on the basis of a siteselective ring opening reaction of an unsymmetric spiroalicyclic dianhydride, rel-[1R,5S,6R]-3-oxabicyclo[3,2,1]octane-2,4-dione-6-spiro-3'-(tetrahydrofuran-2',5'-dione), with using several kinds of diamines only by changing reaction procedures. A model reaction study revealed that the origin of the siteselectivity in the reaction of the dianhydride was due to both the enhanced reactivity of six-membered anhydride embedded in bicyclo[3.2.1] system and the reduced reactivity of five-membered anhydride in the spiro[4.5] system. The HH-PIs showed higher glass transition temperature and higher dielectric constant than those of random PIs. Specific gravity of the PI films showed that the differences in physical properties stem from different packing density of the two constitutionally isomeric PIs. This is the first example for constitutionally isomeric both amorphous polycondensates with different physical properties. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012