The nature of the solvent influences greatly the polymerization yield Y, eg Y is around 10% maximum for SiW „Si (styryl) in DMF and 100% in PC or ACN. Table I summarizes the best results obtained for each monomer with an optimized solvent-initiator system. SiWuSi (vinyl) is quite unreactive toward radical polymerization11 and polymerization of SiW,, Si (styryl) is extensive. The re-activity of the different organically modified POM follows the order SiW,, Si (vinyl)«SiWuSi (allyl)< SiWnSi (methacryl)< SiWnSi (styryl). These results are quite com-parable with those obtained with (OX) 3SiR and X3SiR monomers(X being the polymerizable radical). 12 Note that best polymerization yields do notalways correspond to polymers with highest hydrodynamic radius. For solid-state characteristic purposes, the mixed organic inorganic compounds were precipitated with water, washed, and dried. The infrared spectrum of the SiWnSi (methacrylate) polymer below 1100 cm'1 exhibits the set vibrations characteristic of the oxometalatenetwork (i> Sio» 1050 cm'1; msk), 1020 cm'1; mw= o> 900 cm" 1; i< wow> 860, 760, 710 cm'1; vSlC, 615 cm'1), indicating that the POM backbone remains unchanged after the polymerization reaction (no degradation of the polyanion occurs in the reaction bath). A decrease of the intensity of the mc_c vibration (1590 cm" 1) is observed in agreement with the occurrence of the polymerizationreaction. High-resolution^ Si NMR spectrahave been recorded in both the liquid and solid state (cross-polarization magic angle spinning (CP-MAS)). The MSi NMR resonance corresponding to the modified silica clusters RSiO „(6=-65/-70 ppm)) was not observed. Such experiments confirm that polymerization of both compounds occurs without degradation of the polyanionic structure. The decoupled [• Hf-^ Si spectra (liquid), recorded with a se-quence [*•/2-acquisi t ion-delay]„(acquisition+ delay=