The specially designed reactive copolymers with pendant peroxide groups (RPC) and poly(ethylene glycols) of various molecular weights (PEG) were used for obtaining the cross-linked reactive polymer matrices useful for in-situ synthesis of semiconductor nanocrystals. The networks in the polymer matrices were formed by annealing the mixtures of RPC and PEG owing to both a sequence of radical reactions and acylation reactions yielding ester linkages between macromolecules of RPC and PEG. The size of NCs and the nanocomposite optical properties depended on several factors, among which the most important were the nature of RPC, MW of PEG, PEG/RPC ratio, concentration of precursor (metal salts). The notable benefit of the method developed is feasibility of obtaining the nanocomposites with a high concentration of NCs (up to 20 wt% and more). It is very important that, in spite of high NC concentrations, the nanocomposites were transparent, not opaque evidently because of formation of NCs inside of the swelled polymer gel matrix that avoids their aggregation.