Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment

Abstract

Aminopyridine iron(ii) chloride complexes coordinating a type of flexible ligand are reported as precursors for controlled polymerization of isoprenes.

Highly Active Iminopyridyl Iron-Based Catalysts for the Polymerization of Isoprene

“…In the past few months, the same research group disclosed the polymerization of isoprene using a series of iron-based complexes supported by aminopyridine ligands (Figure 2(D)) [28]. Upon activation with an excess of MAO, these pre-catalysts provided a wide range of polyisoprene microstructures with, in most cases, high content of 3,4 units.…”

Enhancing Thermal Stability in Aminopyridine Iron(II)-Catalyzed Polymerization of Conjugated Dienes

“…In addition, when the IP/Fe ratio was increased to 10000, although the conversion of monomer was reduced, the activity was improved to 2.34 × 10 6 g polymer mol –1 h –1 within 10 min (Table , entry 11), which indicated that the capacity of Fe- i Pr -catalyzed isoprene polymerization was superior to that of secondary aminopyridine iron­(II) complexes with a maximum value of 1.9 × 10 6 g polymer mol –1 h –1 previously reported . Next, the Al/Fe ratio was investigated (Table , entries 12 and 13).…”

“…Thus, the electrophilic aminopyridine iron center (LFe 2+ ) tends to coordinate with the incoming monomer through cis -η 4 mode rather than η 2 mode (Scheme ). , In addition, the 1,4-regularity is generated via the active iron center bound to the C1 position of anti -η 3 or syn -η 3 allyl intermediates, while 3,4-regularity is achieved though the active iron center bound to the C3 position . When the iron center is bound to the C1 position via syn -η 3 allyl intermediates, isomerized by the anti -η 3 -allyl intermediates, the trans -1,4-regularity would become visible.…”

“…It was a remarkable improvement compared to previous results. For example, the yield of polyisoprene catalyzed by the secondary aminopyridine iron­(II) chloride complex decreased from 100% to 71% as the temperature increased from 25 to 50 °C, and the yield of polyisoprene obtained from the terpyridine/FeCl 2 complex was observed to decrease sharply from 99% to 24% when the temperature was increased from 25 to 60 °C . These results may be ascribed to the unstable state of the active species at high temperatures .…”

“…Later, Ritter, Chen, and other groups designed a novel iminopyridine iron complex, where the hybridized nitrogen atoms (sp 2 , sp 2 ) are symmetric but ligand structures are unsymmetric, producing low to moderate 3,4-polyisoprene with high molecular weights . Recently, new types of iron catalysts bearing aminopyridine ligands, for which both the geometric configuration and the hybridization of nitrogen atoms (sp 2 , sp 3 ) are unsymmetric, were explored for isoprene polymerization by our group . However, the reported aminopyridine iron catalysts showed moderate thermostability and the polyisoprenes with low M n and wide Đ , which might be ascribed to the deprotonation of the catalyst by AlMe 3 in MAO.…”