Poly(ethylene terephthalate) copolymers containing 5‐tert‐butyl isophthalic units

Poly(ethylene terephthalate‐co‐5‐tert‐butyl isophthalate) copolymers, abbreviated as PET^tBI, with compositions ranging between 95/5 and 25/75, as well as the two parent homopolymers, PET and PE^tBI, were prepared from comonomer mixtures by a two‐step melt‐polycondensation. Polymer intrinsic viscosities varied from 0.4 to 0.7 dL g^?1 with weight‐average molecular weights ranging between 31,000 and 80,000. The copolymers were found to have a random microstructure with a composition according to that used in the corresponding feed. The melting temperature and crystallinity of PET^tBI decreased with the content in 5‐tert‐butyl isophthalic units, whereas the glass‐transition temperature increased from 82 °C for PET up to 99 °C for PE^tBI. Copolymerization slightly improved the thermal stability of PET. Preliminary X‐ray diffraction studies revealed that PET^tBI adopt the same crystal structure as PET with the alkylated isophthalic units probably excluded from the crystal lattice. The homopolymer PE^tBI appeared to be a highly crystalline polymer taking up a crystal structure clearly different from that of PET and PET^tBI copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 1994–2004, 2001