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Element blocks, which are polymerizable functional units composed of diverse elements, have increasingly attracted attention to construct advanced organic–inorganic hybrid polymer materials. Polyhedral oligomeric silsesquioxanes (POSSs) are, in this context, promising element blocks because their inorganic core is covered with organic substituents. However, well established main-chain type POSS polymers are difficult to be synthesized due to the highly symmetrical structure of POSS; equivalent reactivity of the eight corners makes it difficult to obtain bi-functional POSS monomers. In this work, a one-pot synthesis of POSS monomers has been attained by utilizing incompletely condensed POSS (IC-POSS). A sterically hindered triphenylsilyl group was introduced to cap one of the three silanol groups in the open structure, and the remaining two silanol groups were converted to polymerizable functional groups. Notably, material properties of the resulting polymers were controlled not only by the comonomers but also by the substituents on the IC-POSS. This facile and versatile molecular design of POSS monomers will open a new avenue to sophisticated element block polymer materials.

Polyhedral oligomeric silsesquioxanes (POSSs) are promising building blocks for organic–inorganic hybrid materials. However, polymers containing POSS units in the main chains have been hardly studied because the synthesis of bifunctional POSS monomers is very difficult in general. In this work, a novel molecular design of a POSS monomer has been proposed; a completely condensed POSS (CC-POSS) was linked to one reaction site of an incompletely condensed POSS (IC-POSS), and the remaining two reaction sites of the IC-POSS were used for polymerization. Notably, substituents on the IC- and CC-POSSs can be widely varied based on the present molecular design. It has been demonstrated that the resulting polymers are suitable for optically transparent materials because of their high thermal stability and transparency and that the properties were controlled by the substituents of the POSSs and comonomers.