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Urea–formaldehyde resin is widely used as shell materials for microencapsulation of pesticide byin situpolymerization. A series of acetochlor microcapsules were prepared with different formaldehyde to urea molar ratios in this research. Fourier transform infrared spectroscopic analysis suggests that there hardly exists any chemical interactions between shell materials and the acetochlor core materials, and the structure of urea–formaldehyde pre-polymers and shell materials are significantly different when the ratios of formaldehyde to urea increases. The pre-polymer becomes more hydroxymethylated with higher molar ratios, and hydrogen bonds in shell materials interact stronger as well, thus the ratios of formaldehyde to urea affect encapsulation efficiency by hydrogen bonding interactions, which determines the cross-link degree of the shell materials of the prepared microcapsules. [ABSTRACT FROM PUBLISHER]