Differential scanning calorimetry (DSC) was used to evaluate the thermal behavior and isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers containing 2-methyl-1,3-propanediol as a comonomer unit. The addition of comonomer reduces the melting temperature and decreases the range between the glass transition and melting point. The rate of crystallization is also decreased with the addition of this comonomer. In this case it appears that the more flexible glycol group does not significantly increase crystallization rates by promoting chain folding during crystallization, as has been suggested for some other glycol-modified PET copolyesters. The melting behavior following isothermal crystallization was examined using a Hoffman-Weeks approach, showing very good linearity for all copolymers tested, and predicted an equilibrium melting temperature (T-m(0)) of 280.0 degrees C for PET homopolymer, in agreement with literature values. The remaining copolymers showed a marked decrease in T 0 with increasing copolymer composition. The results of this study support the claim that these comonomers are excluded from the polymer crystal during growth. (c) 2006 Wiley Periodicals, Inc.