Synthesis and characterization of well-controlled isotactic polypropylene ionomers containing ammonium ion groups

This paper discusses the synthesis of a new family of well-controlled isotactic polypropylene ionomers (iPP-NH₃⁺Cl^-) containing up to 5 mol % of NH₃⁺Cl^- ionic groups, with high molecular weight and narrow molecular weight and composition distributions, as well as good processability in melt and solution. A systematic study was conducted using various isospecific Ziegler-Natta and metallocene catalysts in the copolymerization of propylene and a high α-olefin comonomer containing a silane-protected amino group and the subsequent work-up procedures that can prevent undesirable side reactions in forming iPP-NH ₃⁺Cl^- ionomers in a one-pot process. The resulting copolymers were carefully monitored by polymer solubility and a combination of NMR, GPC-triple detectors, DSC, and mechanical property measurements. Evidently, the most suitable reaction process requires a combination of the rac-Me₂Si[2-Me-4-Ph(Ind)]₂ZrCl ₂ metallocene catalyst system with a purified d-MAO (TMA-free), 6-bis(trimethylsilyl)amino-1-hexene comonomer during the copolymerization reaction and a work-up procedure to directly interconvert the silane-protected amino groups (-N(SiMe₃)₂) into -NH₃ ⁺Cl^- ionic groups before exposing to air. The attempt of isolating both iPP-N(SiMe₃)₂ and iPP-NH₂ intermediates resulted in the insoluble (cross-linked) products. On the other hand, the resulting iPP-NH₃⁺Cl^- ionomers were melt processed in air and showed a systematic increase of mechanical properties and high-temperature stability with the increase of NH₃ ⁺Cl^- content.