Hydroboration of residual carbon-carbon double bonds in preformed polyethylene and poly(vinyl chloride) films has been shown to be an effective and mild way to initiate grafting chemistry. This chemistry presumably produces carbon-boron intermediates that can, in turn, be used with vinyl monomers and oxygen to carry out radical graft polymerizations. Suitable vinyl monomers examined include methyl methacrylate, acrylamide, NN-dimethylacrylamide, acrylic acid, hydroxyethyl methacrylate, isobutyl methacrylate, tert-butyl acrylate, and methacrylonitrile. The products of these grafting reactions have been studied gravimetrically, by transmission and ATR-IR spectroscopy and by XPS spectroscopy. These spectroscopic studies as well as physical and mechanical studies show that this procedure produces a covalently modified substrate polymer in which bulk grafting and significant modification of the substrate polymer’s bulk and surface properties have occurred.