Selective functionalization of the ethylene rich phase of a heterophasic polypropylene

The purpose of the present work was to investigate the effect of the solubility parameters of various peroxides on the grafting level of maleic anhydride (MA) on the different phases of a heterophasic polypropylene (PP). In order to characterize the different phases, the material was separated by solvent extraction. The molecular structure of the different fractions was investigated by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). A qualitative comparison of the grafting levels was obtained by FTIR and quantified by titration. The degradation of the different phases was investigated by gel permeation chromatography (GPC). The investigation showed that both the degradation and the functionalization mainly occurred in the ethylene rich phase. A preferential attack of the free radicals at single tertiary hydrogens between ethylene units, or at the ends of a PP block, adjacent to one or multiple ethylene units, results in a selective functionalization of the ethylene rich copolymers, regardless of the solubility parameter or decomposition rate of the peroxides. These tertiary hydrogen atoms are not sterically protected by adjacent methyl groups, and are therefore more accessible to the generated free radicals and the bulky MA.