The central challenge that has limited the development of catalytic hydrogenation of diene-based polymer latex (i.e., latex hydrogenation) in large-scale production pertains to how to accomplish the optimal interplay of accelerating the hydrogenation rate, decreasing the required quantity of catalyst, and eliminating the need for an organic solvent. Here, we attempt to overcome this dilemma through decreasing the dimensions of the polymer substrate (such as below 20 nm) used in the hydrogenation process. Very small diene-based polymer nanoparticles were synthesized and then used as the substrates for the subsequent latex hydrogenation. The effects of particle size, temperature, and catalyst concentration on the hydrogenation rate were fully investigated. An apparent first-order kinetic model was proposed to describe the rate of hydrogen uptake with respect to the concentration of the olefinic substrate (C?C). Mass transfer of both the hydrogen and catalyst involved in this solid (polymer)liquid (water)gas (hydrogen) three-phase latex system is discussed. The competitive coordination of the catalyst between the C?C and acrylonitrile units within the copolymer was elucidated. It was found that (1) using very small diene-based polymer nanoparticles as the substrate, the hydrogenation rate of polymer latex can be increased vastly to achieve a high conversion of 95% while a quite low level of catalyst loading is required; (2) this latex hydrogenation process was completely free of organic solvent and no cross-linking was found; (3) the mass transfer of hydrogen is not a rate-determining step in the present hydrogenation reactions; (4) the catalyst was dispersed homogeneously within the polymer nanoparticles; (5) for the reaction that has reached about 95 mol % conversion, the kinetic study shows that the reaction is chemically controlled with an apparent activation energy of 100110 kJ/mol; (6) the strong coordination of C[tbond]N to the catalytically active species RhH2Cl(PPh3)2 imposed a negative effect on the hydrogenation activity. The present research provides a comprehensive study to appreciate the underlying chemistry of latex hydrogenation of diene-based polymer nanoparticles and more importantly shows great promise toward the commercialization of a green catalytic hydrogenation operation of a diene-based polymer latex in industry. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012