Processes for increasing the molecular weight of bis-maleimides are provided. The bis-maleimides are reacted with certain cyclic dienes to form reversible adducts. These adducts, upon elimination of the cyclic diene, yield an extension of the bis-maleimides to higher molecular weight polymers, ultimately to thermally stable, infusible polymers. The process can be interrupted to yield tractable polymers which are also convertible to cross-linked, infusible products. Modifications of the process include preparing the adducts in the presence of aprotic solvents as well as Bronsted acid salt catalysts, and the isolation of intermediate solid products.