POLYMER COMPOSITION CONTAINING MODIFIED POLYMER HAVING BORON-CONTAINING FUNCTIONAL GROUP, AND METHOD FOR PRODUCING SAME

A production method is provided that is capable of giving, in a high yield, polymers having a boron-containing functional group which are suited for modification of various materials such as modification of polar materials and dispersing of inorganic materials. A polymer composition is also provided that includes modified polymers with a boron-containing functional group, and exhibits high reactivity with respect to various polar functional groups and attains excellent storage stability at the same time. A method for producing a polymer composition including modified polymers having a boron-containing functional group includes a step (1) of anionically polymerizing at least one monomer selected from the group consisting of conjugated dienes and aromatic vinyl compounds in the presence of an active metal or an active metal compound capable of catalyzing anionic polymerization to produce a terminal-active polymer (Z), and a step (2) of reacting the terminal-active polymer (Z) obtained in the step (1) with a boric acid compound having at least two or more partial structures represented by the formula (I) below and having a Taft's steric parameter Es of -0.30 or less with respect to at least one of Rs bonded to the oxygen atom in the partial structures, B-O-R (I) (in the formula (I), B denotes a boron atom, ∘ denotes an oxygen atom, and R denotes an organic group).