Iron‐Catalyzed Polymerization of Isoprene and Other 1,3‐Dienes

Abstract

Iron is the most abundant transition metal in the Earth s crust. As such, it is potentially useful for catalysts that can be employed in high-volume processes, like the Haber–Bosch process that functions with iron oxide as the precatalyst. Maintaining the economic and environmental benefit of iron catalysis, well-defined molecular iron catalysts provide the opportunity to also control selectivity, such as stereoselectivity, during catalysis, if the ligands employed are appropriately selected. Here we report on the iron-catalyzed polymerization of 1,3-dienes to afford elastomers with catalyst content as low as 0.02 mol%. Iminopyridine ligands, as part of the catalyst and made in one step from commercially available chemicals in the case of catalyst 1, can control and invert the stereoselectivity of the polymerization. Iron complexes are suitable precatalysts for the polymerization of olefins such as ethylene to afford linear polyethylene of molar masses greater than 10 gmol . 2] Less attention has been dedicated to the iron-catalyzed polymerization of dienes such as isoprene. Polyisoprene is a naturally occurring unsaturated hydrocarbon polymer that can be refined from the latex produced by rubber trees such as Hevea brasiliensis. 5] Polymerization of isoprene can afford several isomers of polyisoprene; for example, the double bond in 1,4-polyisoprene can have either cis or trans geometry (Scheme 1). Selective polymerization is important because the identity of the isomer influences the properties of the resulting material. Natural polyisoprene can reach a cis-1,4 content exceeding 99.9% in the case of Hevea bransiliensis and a trans-1,4 content exceeding 99.9% for Gutta-percha. Natural rubber, which displays high-performance mechanical properties, is preferred over synthetic rubber in many elastomer applications, including aircraft tires and surgical gloves. As a result, more than 10 million tons of natural rubber is harvested annually from Hevea trees. Rubber trees grow only in the tropics, such as in Asia and West Africa, where they supplant food crops and are an environmental burden because of the heavy use of arsenic-based pesticides. Synthetic rubber has been introduced to replace natural rubber in less demanding applications and to reduce the extensive culturing of rubber trees. Industrial polydienes can be made by alkyllithium-based anionic polymerization. Catalysts based on titanium and, more recently, rare-earth metals such as neodymium can selectively afford high-molar-mass cis-1,4and trans-1,4polyisoprene and -polybutadiene in up to 98% yield. The molecular iron complexes we report here can provide both cis and trans isomers of polyisoprene and other 1,3dienes in greater than 99:1 selectivity, and provide new elastomer materials. Iron catalysis, if appropriately developed, could have a future impact on elastomer production because of the low cost and low environmental burden of iron compared to other transition metals. Our catalyst design was inspired by the iron bisiminopyridine complexes introduced by Gibson and Brookhart in 1998 that are used for ethylene polymerization and can also be employed for hydrosilylation reactions, as reported by Chirik et al. The iminopyridine ligands in 1 and 2 which we chose for our studies feature the redox-active behavior of the bisiminopyridines, but provide an additional available coordination site to accommodate diene coordination as opposed to alkene coordination (Scheme 1). We previScheme 1. Polymerization of isoprene using precatalysts 1 and 2. Complex 1 affords trans-1,4-polyisoprene preferentially, whereas complex 2 affords cis-1,4-polyisoprene preferentially. The 3,4-insertion motif is a minor component in both polymers (7–8% content for 1, and 15% content for 2). R= iBu for 1 and Et for 2.