Iron-based catalysts for conjugated diene polymerization and the polymer properties

This review concerns the progress in the design and synthesis of iron-based catalysts for the polymerization of conjugated dienes and the properties of the produced polymers. The development of new transition metal-based catalysts for conjugated diene polymerization is a long-standing research subject since the diene polymers are the most important elastomers widely used in our modern life. As the most abundant transition metal in the earth's crust, iron is an ideal candidate for application of catalysis from the viewpoint of economic and environmental benefits. While iron-based catalysts were known to polymerize conjugated dienes even in the late 1950s, they have not been extensively studied due to the low activity and poor selectivity. The recent discovery that late-transition metal catalysts are effective for ethylene polymerization and oligomerization rejuvenated the academic and industrial interests. Many efforts have also been made in the field of conjugated diene polymerization with iron-based catalysts, and it was found that addition of appropriate electron donors could improve the catalytic performance. As a result, a variety of iron-based catalysts with nitrogen- and phosphine-containing compounds as a ligand or a catalyst component were developed, and these catalysts exhibited high activity and selectivity in the polymerization of conjugated dienes. Polymers with versatile microstructures and tacticity have been synthesized from 1,3-butadiene, isoprene, and 1,3-pentadiene. A key feature lies in the electronic and steric influences of the environment of the active center, which had a significant influence on the activity and selectivity of the catalysts. For instance, Fe(acac)3/Al(i-Bu)3/nitrogen-con- taining compound (1,10-phenanthroline and 2,20-bipyridine) are effective catalysts for 1,3-butadiene polymerization and give equivalent cis-1,4 and 1,2 polybutadiene. The catalysts with dialkylphosphite or phosphate as a donor exhibit high activity and afford syndiotactic- and atactic-1,2 polybutadienes depending the catalyst formulation. Moreover, living polymerization of 1,3-butadiene with iron(III) ethylhexanoate/Al(i-Bu)3/diethyl phosphite is achieved. In the cases of other conjugated diene polymerizations, these systems are also highly active for isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, and 3-methyl-1,3-pentadiene polymerizations, affording polyisoprene and poly(1,3-pentadiene) having predominant 3,4- or 1,2-contents and highly stereoregular polymers such as crystalline 3,4-syndiotactic polyisoprene, cis-1,4 poly(2,3-dimethyl-1,3-butadiene), and syndiotactic 1,2-poly(3-methyl-1,3-pentadiene). Some of them could not be prepared with other transition metal-based catalytic systems. The formation of stereoregular polymers indicates that the ligand structure plays an important role in determining the selectivity of iron-based catalyst. Although these catalysts have provided some valuable information of iron catalysts, the systematic studies of the structure/performance relationship are needed to develop new catalysts with desired performances. Nevertheless, taking the advantage of the commercial availability of recently developed iron-based catalysts, high catalytic activity, and high selectivity, they are promising as practically useful catalysts. Moreover, the produced polymers, atactic 1,2-polybutadiene, syndiotactic 1,2-polybutadiene, and 3,4-polyisoprene exhibited excellent wet-skid resistance, low heat build-up and rolling resistance as well as good mechanical properties, suggesting these polymers are attractive materials for high performance tires. © 2016, Science Press. All right reserved.