A polymerized product is obtained from a mixture containing (i) a free radical polymerizable monomer, oligomer or polymer; (ii) an organoborane amine complex, (iii) a poor or non-solvent for the polymerized product of (i); and (iv) an amine reactive compound. The mixture may include (v) an active ingredient to be encapsulated by the polymerized product, and (vi) an optional component(s). Polymer particles are obtained by (A) forming a composition containing components (i)-(iv); (B) agitating components (i)-(iv) in the presence of oxygen to initiate polymerization, at a rate and for a time sufficient to provide a uniform dispersion and reaction of the components; (C) allowing the reaction to proceed to completion at room temperature; and (D) recovering the polymer particles by removing component (iii). The composition may include (v) the active ingredient to be encapsulated by the polymer particles.