3,4-selective polymerization of isoprene by iron-based system: The key role of borate salts for enhancing catalytic activities and broadening 1,10-phenanthroline ligand scope

The catalytic system of iron(III) compound/alkylaluminum/1,10-phenothroline(Phen) has been long disclosed for efficient 3,4-selective polymerization of isoprene to achieve semi-crystalline 3,4-polyisoprene product, that finds important application in the current tire industry. Nevertheless, such a system is highly sensitive to the steric nature of Phen ligands, even a subtle variation (e.g. introducing methyl group to its 2,9-position) will result in significantly decreased catalytic activities. In this research, we disclose a facile method, i.e. in-situ addition of borate salts, that can well address such a long unsolved issue. By doing such, the pristine catalytic activities were able to be greatly enhanced (ca. 4-fold higher) without damaging the selectivity of the active species. Such a method was also applicable for various substituted Phen ligands, such as Phen^{2,9-( n Bu)} ₂, Phen^{2,9-( s Bu)} ₂, Phen^{2,9-( t Bu)} ₂, Phen^{2,9-( n Hex)} ₂, etc., which greatly broadens the ligand scope of such iron(III) based catalytic system. Mechanistic studies revealed that the attraction of alkyl group by borate salts to form the cationic active species might play a governing role in enhancing the activities.