Silica gel supported ionic liquids as effective and reusable catalysts for the synthesis of highly reactive polyisobutylene in non-polar media

Abstract

Silica gel supported catalysts based on FeCl₃ and AlCl₃ as well as chloroaluminate and chloroferrate ionic liquids have been synthesized and tested in cationic polymerization of isobutylene. Among them, supported ionic liquid catalysts (SILCs) based on silica gel and ionic liquid 1-ethyl-3-methylimidazolium chloride–1.5FeCl₃ (emimC–1.5FeCl₃) of different compositions showed the best activity and regioselectivity in the synthesis of highly reactive polyisobutylene. It was shown that the SILC with a weight fraction of emimCl–1.5FeCl₃ equal to 30 wt% possessed a higher specific surface area and displayed good activity and regioselectivity in the cationic polymerization of IB in non-polar n-hexane affording highly reactive polyisobutylene with high content of exo-olefin end groups (70–90 mol%), low molecular mass (M_n < 1500 g mol⁻¹) and dispersity (Đ < 2) in a range of IB concentrations from 1 M to 5 M. The activity of the catalyst, however, gradually decreased with the increase of the number of cycles of catalyst reuse up to total loss of the activity after 3 reuse cycles. This behavior was attributed to both the decrease of the specific surface area and the chlorine content in the SILC. Therefore, an efficient approach to retain the activity of the recovered catalyst at a high level was developed, which consists in the addition of a small portion (20 wt%) of fresh SILC after each polymerization run. Using this technique, six successful polymerization cycles without any significant loss in catalytic activity have been performed.