Structure and melting of perfectly alternating ethylene‐carbon monoxide copolymers

The perfectly alternating ethylene carbon monoxide copolymer (polyketone; POK) has been studied by means of 1H nuclear magnetic resonance and thermal analysis, and the crystal structure, determined by wide angle x‐ray scattering methods, is presented. The crystal structure of this polymer in well‐oriented fibers (POK‐α) is as follows: Space group Pbnm, α = 6.91 (2) Aå, b = 5.12 (2) Aå, c = 7.60 (3) Aå (fiber axis), pc = 1383 kg/m3. This differs from the structure reported earlier by Chatani et al. (POK‐β). The very dense packing in the POK‐α structure is a result of the arrangement of the dipoles in the crystal lattice, giving rise to strong lateral forces between the polymer chains. Owing to the all‐trans conformation of the polymer chain in the crystal lattice, high moduli can be achieved for well‐oriented fibers. A first approximation results in a value of 360 GPa for the theoretical modulus. From the melting data for a series of low molecular weight polyketone homologs, a first, estimate is derived for the crystalline heat of fusion (215–330 J/g) for infinite chain length. As a result of the strong lateral forces, this polymer shows a (very) high crystalline heat of fusion, whereas creep and compressive strength of oriented fibers are expected to be superior to those of high‐modulus polyethylene fibers.