The methods of x-ray analysis, mechanical testing, and density measurements were used in an investigation of the connection between the quantity and geometrical dimensions of the side groups (statistically introduced into the linear polyethylene chain), on the one hand, and physicomechanical characteristics of the polymer system, on the other. It was shown that an increase in the number of “defects” of a relatively small size brings about “loosening” of the crystal lattice and the loss of the ability to crystallize of an increasingly large number of fragments of macromolecules, up to the complete disappearance of the crystalline phase, starting from a certain critical composition. The effects are the more pronounced, the larger the dimensions of the side groups. It appears that comparatively long pendants which are incorporated in the lattice virtually without distorting the latter do not cause any considerable changes in the degree of crystallinity of the polymer. The authors come to a conclusion that the “optimal” copolymer (i.e., showing a perceptible distortion of the crystal lattice with a relatively small loss of crystallinity) is that which contains 1 mol% of defective links, with the length of the branchings being 2–6 carbon atoms in the side chain.