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

Poly(ethylene terephthalate‐co‐5‐tert‐butyl isophthalate) copolymers, abbreviated as PETtBI, with compositions ranging between 95/5 and 25/75, as well as the two parent homopolymers, PET and PEtBI, 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 PETtBI 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 PEtBI. Copolymerization slightly improved the thermal stability of PET. Preliminary X‐ray diffraction studies revealed that PETtBI adopt the same crystal structure as PET with the alkylated isophthalic units probably excluded from the crystal lattice. The homopolymer PEtBI appeared to be a highly crystalline polymer taking up a crystal structure clearly different from that of PET and PETtBI copolymers.