Incorporation of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane (Dianol 220 (R)) into poly(butylene terephthalate) (PBT) via solid-state copolymerization (SSP) showed that Dianol, besides being the reactant, also acts as a swelling agent for rigid amorphous PBT chain segments. Being swollen, these amorphous chain segments become sufficiently mobile to contribute to the SSP process. The thermal behavior of the resulting copolyesters is comparable with melt copolymerized copolymers, although having a different chemical microstructure. The main reason is a full miscibility in the melt of unmodified PBT chain segments and modified chain segments, which eliminates the advantages of a blocky microstructure for the SSP copolyesters. However, incorporation of 2,2'-biphenyldimethanol (BDM) into PBT resulted in a higher crystallization temperature compared with PBT-Dianol copolymers of equal composition. Preordering of polymer chains in the melt by incorporating rigid, phase separating BDM-moieties, preferably via SSP to obtain a non-random distribution, may be the origin of the enhanced crystallization temperature. (C) 2008 Wiley Periodicals, Inc.