Ketonization of a nitrile-butadiene rubber by nitrous oxide: Comparison with the ketonization of other type diene rubbers

Noncatalytic ketonization of a nitrile-butadiene rubber (21 mol% acrylonitrile units) by nitrous oxide was shown to yield polymeric products functionalized with carbonyl (mainly ketone) groups. The reaction was conducted in a benzene solvent at 180–230° and pressure of 3–6 MPa. An assumed ketonization mechanism includes a 1,3-dipolar cycloaddition of N2O to Cdouble bond; length as m-dashC bonds in butadiene units. According to the NMR and GPC data, the main route of the reaction (ca. 85%) proceeds without cleavage of the Cdouble bond; length as m-dashC bonds and yields ketone groups in the polymer backbone. The minor route (ca. 15%) includes the cleavage of Cdouble bond; length as m-dashC bonds resulting in fragmentation of the macromolecules that leads to a decrease in their molecular weight. The nitrile (–CN) groups remain untouched. The resulting product is a bifunctional low-molecular rubber containing, in addition to originally present nitrile groups, a regulated amount of new ketone groups randomly distributed along the polymer backbone. The results for nitrile-butadiene rubber are compared with the earlier studied ketonization of butadiene and isoprene rubbers. The molecular structure of monomeric units was shown to be an important characteristic of a parent rubber affecting the reaction rate, degree of fragmentation, and consistency (rubber-like, plastic, or liquid) of the resulting material.