The ring-opening polymerization (ROP) of cyclic butylene terephthalate oligomers (cBT) using an aluminum salen catalyst was investigated. The kinetic analysis of the ROP of cBT in tetrachloroethane (TCE) revealed a first order dependence of the rate constant in catalyst as well as monomer concentration. Comparison with the ROP in other solvents showed that TCE significantly retards the reaction by short-term reversible catalyst deactivation and by long-term permanent deactivation. The apparent activation energy for the ROP of cBT and epsilon-caprolactone (CL) were determined independently, and the difference in reactivity was used to investigate the possibility to synthesize blocky copolymers. Sequence distribution analysis of the copolymers obtained over a period of time from a single feed copolymerization revealed that over the full reaction range the degree of randomness was high. A sequential feed approach, in which CL was added after the polymerization of cBT was completed, showed that the propagation reaction was not fast enough to avoid transesterification, yielding copolymers with an increasing degree of randomness over time. Additionally, the copolymerization of cBT with omega-pentadecalactone (PDL) was performed in bulk at temperatures above the crystallization temperature of poly(butylene terephthalate) (PBT), yielding random copolymers over the full composition range between polypentadecalactone (PPDL) and PBT. Copolymerizations using aluminum salen as catalyst were rapid and high molecular weights could be achieved. The melting temperature of both PPDL and PBT decreased upon the introduction of either counit, reaching a eutectic point between 50 and 70 mol % PDL. Furthermore, a combination of thermal analysis and X-ray diffraction shows that PDL units are fully excluded from the PBT crystal lattice, while butylene terephthalate units are partially incorporated into the PPDL crystal lattice.