A study on the optimization of conditions for polycondensation of 3,3.3',3'-tetramethyl-1,1'-spirobisindane-5,5',6,6'-tetrol. TTSBI, with 1,4-dicyanotetrafluorobenzene (DCTB) in dimethylformamide (DMF) at 50-70 degrees C is reported. A linear PIM-I polymer (PIM = polymer with intrinsic microporosity) with high molecular weight was obtained by optimized reaction conditions. Of the solvents tested, DMF was the preferred solvent for this reaction. The optimized molar ratio TTSBI/DCTB/K2CO3 was 1/1/2.04. The mechanism of PIM-I crosslinking during polymerization was studied, and gel-free, high molecular weight PIM-I polymer was Prepared. The physical properties of anhydrous K2CO3 significantly affected the reaction rate. Flexible tough polymer films were cast from PIM-I solution in chloroform, and pure permeability data are reported for several gases.