Insight into the composition distribution created by free-radical mediated addition of vinyltrimethoxysilane (VTMS) to high-density polyethylene (HDPE) has been gained using physical methods of polymer characterization as well as detailed structural studies of a model hydrocarbon system. Analysis of poly(ethylene-graft-vinyltrimethoxysilane), or HDPE-g-VTMS, by moisture curing and by differential scanning calorimetry-successive self-nucleation and annealing (DSC-SSA) indicated that the distribution of pendant alkoxysilane grafts amongst polymer chains was non-uniform. Fractionation and characterization of a graft-modified model compound, tetradecane-g-VTMS, showed that composition distributions were influenced strongly by intramolecular hydrogen atom abstraction, yielding multiple grafts per chain in the form of single pendant units as opposed to oligomeric grafts. Chain transfer to the methoxy substituent of bound VTMS was found to contribute significantly to the product distribution. (C) 2003 Elsevier Science Ltd. All rights reserved.