Noncatalytic transformation of cis-1,4-polyisoprene rubber (M-n = 320,000) into functionalized liquid rubbers containing various amounts of carbonyl groups was studied. The process is performed via selective carboxidation of the polymer C=C bonds by nitrous oxide (N2O) in the temperature range of 180-230 degrees C and under 3-6 MPa pressure. The carboxidation proceeds by the non-radical type mechanism involving the 1,3-dipolar cycloaddition of N2O to the C=C bond. The main route of the reaction (ca. 65%) proceeds without cleavage of the internal C=C bonds and leads to the formation of ketone groups in the polymer backbone. The second route (ca. 35%) includes the cleavage of C=C bonds, yielding the molecules of a smaller size. This route results in a manifold decrease of the molecular weight, which, depending on the carboxidation degree, may be more than two orders of magnitude less than that of the parent rubber. A series of functionalized liquid rubbers having M,, value from 1000 to 19,000, and the oxygen content from 0.3 to 3.9 wt % was obtained in the form of the liquid unsaturated polyketones. Similar polyketones can also be prepared by carboxidation of the natural rubber. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1241-1249, 2009