The grafting mechanism of poly(vinyl acetate) macroradicals prepared by cobalt-mediated radical polymerization onto C-60 is investigated. The experimental conditions directly impact the nature and stability of the PVAc/C-60 adducts. in the presence of residual initiating radicals that can compete with PVAc degrees macroradicals for addition onto C-60, mixtures of PVAc/C-60 adducts having between one and eight polymer chains per C-60 are formed. PVAc/C-60 adducts prepared with low [PVAc]:[C-60] ratios may contain weak C-60-C-60 bonds that further dissociate and account for the instability of the products. The formation of such dimers can be lessened by increasing the temperature from 30 degrees C to 100 degrees C. The temperature increase also allows a complete dissociation of the PVAc-Co dormant species into PVAc macroradicals and an almost quantitative grafting of eight PVAc chains onto C-60, leading to well-defined C-60(PVAc)(8) octa-adducts. These results might shed new light on the grafting onto C-60 of macroradicals prepared by other CRP techniques. (C) 2012 Elsevier Ltd. All rights reserved.