On the effect of the nature of the side chain over the crystalline structure in aliphatic polyketones

The crystalline structure of a number of random polymers of perfectly alternating l-olefins/carbon monoxide aliphatic polyketones has been studied by wide angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and Raman spectroscopy. From previous studies, WAXS, Raman and DSC have shown to be suitable techniques for the characterisation of the two crystalline polymorphs, α (denser) and β, detected in ethene/carbon monoxide (ECO) and in ethene/propene/carbon monoxide (EPCO) polymers. In this paper for the first time, polyketones with butene and hexene as the second olefin are reported. It was found that the ethene/propene/carbon monoxide polymers and ethene/butene/carbon monoxide (EBCO) polymers, predominately contain the β-rich crystalline phase. The crystalline density of this phase drops with increasing second olefin content, albeit at a faster pace for propene polymers. From the latter results, and from the behaviour of the melting point, crystallinity, and crystal thickness across composition, inclusion of methyl and ethyl side chains into the crystals as defects was inferred. Ethene/hexene/carbon monoxide (EHCO) polymers do seem to behave differently: they show lower crystallinity, the presence of a larger quantity of the denser α crystals and a relatively high and constant crystalline density for the β phase throughout composition; observations that unambiguously support the exclusion argument for the butyl branches. The above behaviour is surprising since for instance in polyethylene copolymers it is considered that only methyl branches can enter the crystal lattice. The relative presence of α crystals was found to decrease with increasing the concentration of branches and in the order EHCO>EBCO>EPCO.