ng 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms.Specific examples of the compounds represented by the general formula (XI) may include the following reaction products:(a) Reaction products obtained by reacting trimethylaluminum with methylboric acid at a molar ratio of 2:1;(b) Reaction products obtained by reacting triisobutylaluminum with methylboric acid at a molar ratio of 2:1;(c) React