Low polydispersity high molecular weight poly(acrylic acid) (PAA) is formed rapidly by the aqueous polymerization of acrylic acid (AA) initiated by an azo radical source (V-70) in the presence of cobalt complexes of tetra(3,5-disulfonatomesityl) porphyrin. Water-soluble cobalt(II) porphyrins in conjunction with an azo radical source and acrylic acid react to produce organocobalt complexes that mediate the radical polymerization of AA in water. Linear increase in number-average molecular weight with monomer conversion indicates the living character of the polymerization. Kinetic−mechanistic studies demonstrate that organocobalt complexes mediate the living radical polymerization by both a degenerative transfer radical interchange polymerization (RIP) mechanism and cobalt(II) stable free radical reversible termination.