The article reports on the noncatalytic transformation of a polybutadiene (number-average molecular weight = 128,000) into a functionalized liquid rubber via the carboxidation of the polymer C=C bonds by nitrous oxide. The reaction was conducted in a benzene solution at 160-230 degrees C and a pressure of 3-6 MPa. The carboxidation mechanism was determined. The main route (95%) of the reaction proceeded without cleavage of C=C bonds and led to the formation of ketone groups in the polymer backbone. A minor route (5%) of the reaction proceeded with the cleavage of C=C bonds, yielding two smaller fragments containing aldehyde and vinyl end groups. The availability of the cleavage route could lead to a dramatic decrease in the molecular weight, which, depending on the carboxidation degree, could be 1-2 orders of magnitude less than that of the initial material. Thus, the carboxidation of more than 15% of the polybutadiene C=C bonds transformed it into a C=O-functionalized liquid rubber with a narrow molecular weight distribution. (c) 2006 Wiley Periodicals, Inc.