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

Guangqian Zhu,Qinggang Wang,Peng Wang,Xianhui Zhang,Liang Wang,Chuyang Jing,Mengmeng Zhao,Xiaowu Wang

doi:10.3390/polym10090934

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 3a, 2b and 3b were determined by X-ray diffraction analysis. Complex 3a contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas 2b adopted a chloro-bridged dimer, and 3b featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes 2b and 3b 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 1a 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 (Mn = 3.5 × 104 g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of cis-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 (Mn < 2000) with high trans-1,4-unit (>95%).

Highlights

Synthetic polyisoprene displays significant interests in material application, which has been introduced to replace the natural rubber in some extent [1]
Since the discovery of late transition metal catalysts for olefin polymerization reported by Gibson [14] and Brookhart [15] in the late 1990s, a plethora of catalysts have been developed for the polymerization of various type of monomers
The corresponding iminopyridine Fe(II) and Co(II) dichloride complexes containing fluorine substituents were synthesized by a reaction of the resulting iminopyridine ligand with one equivalent of anhydrous FeCl2 in CH2 Cl bond distances ranging from 2.233(8) (Cl2) or anhydrous CoCl2 in THF, respectively (Scheme 2)

Summary

Introduction

Synthetic polyisoprene displays significant interests in material application, which has been introduced to replace the natural rubber in some extent [1]. Cis-1,4-polyisoprene shows similar characteristics stereo and chemical compositions and thereby mechanical properties with the natural rubber which has been largely applied to tires [2]. Various selective diene polymerization catalysts were investigated using Ziegler-Natta Ti [6] or rare earth metal catalysts [3,7,8,9,10,11,12,13]. Due to the lower Lewis acidic nature and higher functional group tolerance, late transition metal, especially cheap and earth-abundant iron- and cobalt-based catalysts, have attracted extensive investigations in both α-olefin and diene polymerization [16,17,18,19,20,21,22,23,24,25,26]

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Results

Conclusion