The solid-state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) has been studied and compared with that of poly(ethylene terephthalate) (PET). Because PTT and PET share the same SSP mechanism, the modified second-order kinetic model, which has successfully been used to describe the SSP behaviors of PET, also fits the SSP data of PTT prepolymers with intrinsic viscosities (IVs) ranging from 0.445 to 0.660 dL/g. According to this model, the overall SSP rate is -dC/dt = 2k(a)(C - C-ai)(2), where C is the total end group concentration, t is the SSP time, k(a) is the apparent reaction rate constant, and C, is the apparent inactive end group concentration. With this equation, the effects of all factors that influence the SSP rate are implicitly and conveniently incorporated into two parameters, k(a), and C-ai. k(a) increases, whereas C-ai decreases, with increasing SSP temperature, increasing prepolymer IV, and decreasing pellet size, just as for the SSP of PET. Therefore, the SSP rate increases with increasing prepolymer IV and increasing SSP temperature. The apparent activation energy is about 26 kcal/mol, and the average SSP rate about doubles with each 10degreesC increase in temperature within the temperature range of 200-225degreesC. The SSP rate increases by about 30% when the pellet size is decreased from 0.025 to 0.015 g/pellet. Compared with PET, PTT has a much lower sticking tendency and a much higher SSP rate (more than twice as high). Therefore, the SSP process for PTT can be made much simpler and more efficient than that for PET. (C) 2003 Wiley Periodicals, Inc.