Semi-interpenetrating polymer networks (IPNs) composed of poly(methyl methacrylate) (PMMA) and aromatic/aliphatic siloxanes have been made via sequential and simultaneous polymerizations. As the percentage of aliphatic siloxane increases, flexibility and, in general, toughness of the IPNs increases and clarity is reduced. This loss in clarity is due to the mismatch of refractive indices (1.49 form PMMA vs. 1.43 for aliphatic siloxane). PMMA is quite transparent. On the other hand, in making aromatic siloxane/PMMA IPNs clarity is retained as aromatic siloxane is increased due to better matching refractive index -1.49 for PMMA and -1.49 for poly(diphenyl siloxane). Gel permeation chromatography (GPC) indicates slightly crosslinked IPNs with the THF soluble portions having number-average molecular weight, (M) over bar(n), of 10(5)-10(6). NMRs of IPNs essentially show peaks for the components, PMMA and the siloxane, which make up the respective IPNs. Si-29-NMRs indicate crosslinking and grafting. Mechanical properties show increased toughness of IPNs versus PMMA as percentage of siloxane and crosslinker increases, but with a corresponding loss in tensile strength.