In order to determine the effect of the molecular structure of short side branches in lowdensity polyethylene upon the physical properties of the resin, a study was carried out in which small amounts of various comonomers were added to an otherwise relatively unbranched polymer. It was found that linear short side branches have about the same effect in decreasing stiffness and increasing toughness as the natural short-chain branches of polyethylene have. However, branches containing a tert-butyl group increased resin toughness more than linear branches while decreasing stiffness by about the same amount. Thus, by adding a small amount of branched, short side branches, it is possible to obtain an optimum balance of physical properties not obtainable from low-density ethylene homopolymer.