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