AbstractThe adduct of Kaolin and 2,2′‐azobisisobutyramidine hydrochloride has been used to initiate the polymerization of methyl methacrylate, styrene, styrene‐butadiene, vinyl acetate, chloroprene, acrylamide, and acrylonitrile. Both extractable and nonextractable polymers are obtained. The adduct of bentonite and 2,2′‐azobisisobutyramidine hydrochloride will initiate the polymerization of methyl methacrylate, acrylamide, vinyl acetate, 4‐vinylpridine, and styrene. Both extractable and nonextractable polymers are formed. When the bentonite or the hectorite adducts are more than 99% decomposed prior to the addition of methyl methacrylate, essentially no extractable polymer is formed, but an increased quantity of nonextractable polymer is produced. This is probably due to an increased accessibility of the monomer to the clay's interior surface and a decrease in the rate of chain termination. As is the case of the kaolin adduct, the clay surface of bentonite appears to accelerate the chain growth c