The chemistry of free radical graft copolymerisation, initiated with t-butoxy radicals, has been investigated using 3-methylpentane and 2,4-dimethylpentane as models for LLDPE and PP, respectively. The tertiary C-H reaction site of 3-methylpentane is twice as reactive as the secondary position. However, the site of grafting from the polymer is determined by the concentration of tertiary and secondary reaction sites and the relative reactivity at these positions and hence grafting occurs most frequently from the secondary C-H reaction sites. Abstraction from 2,3-dimethylpentane is solvent dependent and occurs most readily at the tertiary position. Therefore, grafting will be predominantly concentrated at the tertiary C-H site. The models also indicate that PP is less reactive than LLDPE. Competition reactions between 3-methylpentane and the monomers methyl methacrylate, styrene or 4-vinylpyridine suggest that the nucleophilicity of the monomer influences the competing abstraction-monomer initiation reactions. Evidence for grafting methyl methacrylate from 3-methylpentane is also presented. This study also provides evidence of the incorporation of peroxide linkages into the graft even when standard deoxygenating techniques are used. (C) 2000 Elsevier Science Ltd. All rights reserved.