Polymers with a carbon?carbon (C?C) backbone are animportant class of polymers, which can be regarded as the derivatives ofpolyethylene (PE). To investigate the effect of side chains on the singlechainenthalpic elasticity (SCEE) of polymers with a C?C backbone,several polymers with pendants or side chains of different lengths andshapes have been studied by single-molecule AFM. We find that bothlength and shape of the side chains count: only the side chains that areboth long and bulky (i.e., bulky dendrons of second or higher generationas side chains) affect the SCEE. Thus, only rare polymers have specialSCEE. For the vast majority of polymers, the SCEE is identical to that ofPE, which means that the SCEE is determined by the nature of the C?Cbackbone. It is expected that this conclusion can also be popularized to allpolymers with various backbones. This study is an important update tothe understanding of polymers at the single-chain level.