Polymer-silica hybrid precipitate could be observed in urea and formaldehyde condensation reaction in silica sol-gel. The amount of the precipitate increased with that of the organic component used in the reaction and achieved its maximum when the urea to formaldehyde molar ratio was above the value of 1.0. The IR absorption of the precipitate at ca. 3348, 1572, 1635 and 784 cm^－1 could be attributed to the characteristics of a linear structure —HNCONHCH₂—. XRD pattern of the precipitate was correlated with the semicrystallization of the linear structure via hydrogen bond interaction (C＝O…H—N). Decomposition of the crystal species occurred around 290 ℃ according to DTGA and XRD analyses. SEM results show that the hybrid particles have a three-dimensional network structure, which tended to crack even under the room temperature drying conditions. With increasing the reaction time, acidity or content of the inorganic component, the hybrid particles became more compact. BET results demonstrate that the calcined samples have a lamellar structure. A large pore size of 14.3 nm, a pore volume of 1.68 cm³/g, and a BET surface area of 536 m²/g were obtained under the highest acidity reaction conditions. Meanwhile the structural characteristics of the silica or hybrids could be controlled by the acidity and molar ratio of the reactants.

Key words: urea-formaldehyde resin, mesoporous silica, semicrystallization, template, hybridization process