Amorphous to high crystalline PE made by mono and dinuclear Fe-based catalysts

Abstract

We designed a series of dinuclear structures (F1–F5) bearing different linkers (rigid to flexible) between the Fe centers to investigate that how they could control the catalyst behaviour and polymer properties. The dinculear structures were used for polymerization of ethylene in presence of MMAO and TiBA. Based on initial results, catalyst F2 containing methyl-substituted phenyl bridge owned the highest activity (4.9 × 106 g PE/mol Fe.h) through dinuclearity and optimum bulkiness among the structures studied. This performance was along with the greatest crystallinity (χc) of PE made by the catalyst. Polymerization at higher temperature and monomer pressure exhibited high thermal stability and performance of catalyst F2, respectively. For further structures, decreasing of effective electronic and steric features led to lower activity. In addition, not only catalyst F4 bearing the short ethylene bridge exhibited the lowest productivity, but also produced the PE containing high level of short chain branches (37.2 SCB/1000C) affording a branched microstructure of polyethylene in presence of TiBA. It could attribute to low steric and electronic effects and short distance between the centers (ethylene linker) that made it suitable and active for SCB formation. Regarding to it, the electrophilicity index (ω) of F4 also was greater that led to high capacity or propensity of the specie to accept the (macro) monomer. For this sample, virtually no χc observed in DSC and SSA thermograms. Moreover, MMAO acted as an effective cocatalyst in comparison to TiBA, according to the kinetic profiles of ethylene polymerization. The observations were in respect of strength and affinity of the cocatalysts in deactivation and reactivation of the centers at prolonged time.