Well-defined poly(methyl methacrylate) (PMMA)-grafted natural rubbers (NRs) were prepared to Study the structure-property relationships. Syntheses were achieved by the photopolymerization of methyl methacrylate initiated by N,N-diethyldithiocarbamate groups created beforehand in side positions on the NR chains. With this procedure, good control of the graft density and PMMA content could be obtained. Thermal, morphological, and mechanical properties of NR-g-PMMA copolymers were studied as a function of the NR/PMMA composition and graft density. NR-g-PMMAs containing 15-80% grafted PMMA showed characteristics of heterogeneous materials (characterized by two glass-transition temperatures, those of PMMA and NR, in differential scanning calorimetry). Under these conditions, they developed the morphology of thermoplastic elastomers with PMMA nodules dispersed in the rubber matrix when the PMMA content was near 20%; conversely, they developed the morphology of softened thermoplastics with rubber nodules dispersed in PMMA when the PMMA content was near 80%. Graft copolymers containing about 20% PMMA remained essentially rubbery, but they were already different from pure NR. On the other hand, the thermal stability of NR wash improved after the introduction of PMMA grafts onto NR chains. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 788-799, 2009