Carbon-13 spin-lattice relaxation times are measured for poly(octadecyl acrylate) above and below the melting point of the crystalline side chains. The chain backbone has long spin-lattice relaxation times below the melting point that shorten by more than an order of magnitude as the melting point range is traversed. Below the melting point, the backbone is nearly immobilized with spin-lattice relaxation changing very slowly with temperature. Above the melting point, the shorter spin-lattice relaxation times are typical of a rubber above the glass transition and decrease with increasing temperature. The methylene groups in the side chain are quite mobile well below the melting point, indicating fairly rapid anisotropic motion within the crystal. The methyl group at the end of the chain and the adjacent methylene group have longer spin-lattice relaxation times, indicating the greatest side-chain mobility at the end, which is in the middle of the crystal structure. The side-chain carbon adjacent to the carbonyl group is as mobile as the majority of the side-chain carbon, indicating sidechain mobility extends to all of the side-chain CH2 groups. The abrupt transition in the mobility of the backbone is not typical of the amorphous phase in a semicrystalline polymer where the backbone units can crystallize. The close proximity of every backbone segment to the crystalline domain locks backbone segmental motion below the melting point. Melting and crystallization of the side chains switch segmental motion of the backbone on and off.