Basic aspects of step polymerization via reactive intermediate were elucidated for the first time by using a typical example. The polycondensations of 2,2-dichloro-1,3-benzodioxole (1) and 4,4＇-isopropylidenediphenol (2) with a variety of feed ratios were carried out in refluxing chlorobenzene (2.5 mol/L) for 3 h with continuous removal of hydrogen chloride to give polyorthocarbonate 3. The highest molecular weight of 3 was obtained when ca. 0.7 equivalent excess of 1 was used and was much higher than that calculated according to the conventional step polymerization theory. The excess I remained aff er the polymerization was completed. In the model reaction of 1 and phenol at -40 degrees C, the rate of the first nucleophilic displacement reaction (k(1)＇) was 27 times lower than that of the second reaction (k(2)＇) of monochloride formed by the first condensation. The attribution of the anomeric effect was assumed from the results of semiempirical molecular orbital calculation. Under the condition that the first condensation is 27 times faster than the second one, analyses of the kinetic equations of the polycondensation showed that the number-averaged molecular weight (M-n) of the polymer is higher with more stoichiometric imbalance (S) for a shorter polymerization time. As time elapses, a greater M-n is observed for smaller S value. Infinite M-n will be obtained for S = 1 at infinite time, which is impossible to reach. For step polymerization in which the first reaction gives rise to acceleration of rate of the second reaction, in general, stoichiometric imbalance practically enhances molecular weight of the polymer.