Both full and semi‐interpenetrating polymer networks (IPNs) of polybutadiene and poly(methyl methacrylate) were synthesized by sequential polymerization. The effect of compositional variation and the cross‐linking agent of both elastomer and plastomer on the physical, mechanical, and morphological properties were investigated. Full‐IPNs exhibited improved tensile strength, modulus, tear strength, gel content, and density, whereas the corresponding semi‐IPNs exhibited better toughness and elongation at break. Phase morphology of full‐IPNs were characterized by compact, tight network structures compared to those of semi‐IPNs. © 1994 John Wiley & Sons, Inc. Copyright © 1994 John Wiley & Sons, Inc.