Chemical irregularities such as side chain branches or comonomers in an otherwise regular polymer influence its crystallization. In most polymer systems, these chemical defects are randomly distributed along the chain, and it is difficult to understand in detail their effect on crystallization. We have examined three precision polymers prepared by acyclic diene metathesis (ADMET) polymerization. This synthesis ensures exact placement of the chain defects; here they are all separated by 20 CH2 units. The polymers are two polyphosphoesters with a phosphate or phosphonate group in the main chain and one polyethylene with butyl branches. Although the alkyl part is identical for all three polymers, their thermal and crystal properties differ noticeably. By means of differential scanning calorimetry, X-ray scattering, and transmission electron microscopy, we characterize the lamellar crystals and correlate our findings to the observed difference in thermal behavior.