The effects of incorporating 5-tert-butylisophthalic acid ((BI)-B-t) and 1-3/1-4 cyclohexanedimethanol (CHDM) in the polymer chain of poly(ethylene terephthalate) (PET) on the crystallization behavior and thermal, optical and tensile properties of this polyester (PETGB) were evaluated. These random copolyesters that contained between 0 and 20 mol % of CHDM and between 0 and 10 mol % of (BI)-B-t units were prepared by esterification followed by melt copolycondensation. The compositions and molecular weights of the copolyesters were determined by H-1-NMR spectroscopy and viscometry, respectively. The composition of the polyester was consistent with the composition of the feed. The intrinsic viscosities of the copolymers ranged between 0.62 and 0.74 dL/g. The thermal behaviors were investigated over the entire range of copolymer compositions, using DSC under the heating and cooling rate of 20 degrees C/min and TGA. The copolyesters with (BI)-B-t and CHDM. of < 20 mol % were crystallizable, whereas the copolyesters with (BI)-B-t and CHDM of >= 20 mol % were amorphous. They appeared to be stable up to 395 degrees C. The optical transmissions of the amorphous polyesters were more than 88% in the visible region. The mechanical behavior was investigated by performing a tensile test. (c) 2007 Wiley Periodicals, Inc.