Influences of Fluorine Substituents on Iminopyridine Fe(II)- and Co(II)-Catalyzed Isoprene Polymerization

Abstract

A series of iminopyridine complexes of Fe(II) and Co(II) complexes bearing fluorinated aryl substituents were synthesized for the polymerization of isoprene. The structures of complexes <b>3a</b>, <b>2b</b> and <b>3b</b> were determined by X-ray diffraction analysis. Complex <b>3a</b> contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas <b>2b</b> adopted a chloro-bridged dimer, and <b>3b</b> featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes <b>2b</b> and <b>3b</b> represented rare examples of chlorine bridged bimetallic Co(II) complexes. The fluorine substituents effects, particularly on catalytic activity and polymer properties such as molecular weight and regio-/stereo-selectivity were investigated when these complexes were employed for isoprene polymerization. Among the Fe(II)/methylaluminoxane (MAO) systems, the 4-CF₃ substituted iminopyridine Fe(II) complex <b>1a</b> was found as a highly active isoprene polymerization catalyst exhibiting the highest activity of 10⁶ g·(mol of Fe)^-1·h^-1. The resultant polymer displayed lower molecular weight (<i>M</i>_n = 3.5 × 10⁴ g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of <i>cis</i>-1,4-/3,4 was not affected by the F substituents. In the series of Co(II)/AlEt₂Cl binary systems, complexes containing electron-withdrawing N-aryl substituents (R = 4-CF₃, 2,6-2F) afforded higher molecular weights polyisoprene than that was obtained by the complex containing electron-donating N-alkyl substituents (R = octyl). However, ternary components system, complex/MAO/[Ph₃C][B(C₆F₅)₄] resulted in low molecular weight polyisoprene (<i>M</i>_n < 2000) with high <i>trans</i>-1,4-unit (>95%).