Polymers having polyhedral oligomeric silsesquioxane (POSS) in the main chains are an important class of organic-inorganic hybrid materials. Despite the increasing attention to the POSS polymers, variation of the monomers is still limited. Herein, we have proposed side-opening POSS (SO-POSS) monomers. Platinum-catalyzed hydrosilylation polymerization proceeded to produce polysiloxanes having SO-POSS in the main chains. The obtained polysiloxanes showed good solubility, high thermal stability, high transparency, and tunable reflective index. In addition, cyclic compounds were obtained during the investigation of the polymerization, and were synthesized with high selectivity under the slightly diluted conditions. The obtained cyclic compounds showed high thermal stability due to the silsesquioxane backbone, and the high dispersibility as a filler in poly(methyl methacrylate) was demonstrated. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019