The crosslinking reaction of liquid carboxylated poly(acrylonitrile-co-butadiene) [or nitrile rubber (NBR); acrylonitrile = 10 wt %] with dicumyl peroxide (DCPO) was studied in dioxane by means of Fourier transform near-infrared spectroscopy (FT-NIR) and electron spin resonance spectroscopy (ESR). Among the three butadiene units (1,2, cis-1,4, and trans-1,4 units) of NBR, only the pendant vinyl group of the 1,2 unit showed an absorption at 6110 cm(-1) from the FT-NIR examination of dioxane solutions of NBR, 1-octene, 3,3-dimethyl-1-butene, trans-2-octene, cis-5-octen-1-ol, poly-cis-1,4-butadiene, and poly-1,2-butadiene. The crosslinking reaction was followed in situ in dioxane by the monitoring of the disappearance of the pendant vinyl double bond with FT-NIR. The initial disappearance rate (R-0) of the vinyl group was expressed by R-0 = k[DCPO](0.9)[NBR](-0.2) (120degreesC). The overall activation energy of the reaction was calculated to be 20.7 kcal/mol. This unusual rate equation suggests unimolecular termination due to degradative chain transfer and depressed reactivity of the vinyl group caused by crosslinking. ESR study of the reaction mixture revealed that an allyl-type polymer radical was formed in the reaction, and its concentration increased with time and was then saturated. (C) 2003 Wiley Periodicals, Inc.