TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The Lewis' acid and/or its complex contains a halogenated metal or boron trihalide (preferably boron trifluoride).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The curable composition further contains Lewis' acid guanidine complex (D). Preferred Compound: The guanidine compound is of formula (2). The guanidine compound is chosen from 1-phenyl guanidine, 1-(o-tolyl)guanidine, 1-phenyl biguanide and 1-(o-tolyl)biguanide.R2=same as R1;anda=1-4.    TECHNOLOGY FOCUS - POLYMERS - Preferred Polymer: The organic polymer is chosen from polyoxyalkylene-type polymer, saturated hydrocarbon-type polymer and (meth)acrylic ester-type polymer. The polyoxyalkylene-type polymer is polyoxypropylene-type polymer. EXAMPLE - Sodium methoxide aqueous solution was added to polypropylene oxide having number average molecular weight of 19000 and mixed, and methanol was distilled off. Then, allyl chloride was added, such that terminal hydroxyl group of the polypropylene oxide was converted into allyl group. Subsequently, n-hexane (in weight parts) (300) and water (300) were added and centrifuged. Then, devolatilization was performed under reduced pressure, to obtain polypropylene oxide having allyl group in its terminal. Methyl dimethoxysilane (1.35) and platinum vinyl siloxane complex isopropanol solution (150 ppm) containing 3 weight% of platinum were added and reacted at 90 degrees C for 5 hours to obtain polypropylene oxide (I) having methyldimethoxy silyl group in its terminal. Boron trifluoride diethylether complex (105) was dripped into a liquid mixture containing 1-phenyl guanidine (100) and diethyl ether (300) and reacted for 2 hours. The reaction mixture was concentrated under reduced pressure, and diethyl ether was removed. Then methanol (150) was added and mixed to obtain solution of boron trifluoride 1-phenyl guanidine complex. The obtained polypropylene oxide (I) (100) and solution (4) of boron trifluoride 1-phenyl guanidine complex were mixed to obtain curable composition. A cured product formed using the obtained curable composition, had excellent hardenability and tensile strength.