Improving the Performance of Methylalumoxane:  A Facile and Efficient Method to Trap “Free” Trimethylaluminum

Abstract

The presence of "free" trimethylaluminum (TMA) in methylalumoxane (MAO) solutions can be highly detrimental to the performance of metallocene and "post-metallocene" olefin polymerization catalysts. The most used strategy to remove "free" TMA is to evaporate MAO solutions to dryness, until a free-flowing white powder ("solid MAO") is left. This procedure is tedious and potentially hazardous, because in some cases the distillate is a concentrated hydrocarbon solution of TMA. Moreover, "solid MAO" is poorly soluble in common polymerization media, and once in solution it can regenerate TMA to some extent. This communication reports on a facile alternative, which consists in the controlled addition of a sterically hindered phenol, such as 2,6-di-tert-butylphenol, effectively trapping "free" TMA. We show here that 2,6-di-tert-butylphenol/MAO solutions activate equally well the dichloro-precursors of well-known zirconocene and bis(phenoxyimine)Ti catalysts, and that their use in propene polymerization results in a substantially higher productivity, polymer stereoregularity, and/or average molecular mass compared with activation by MAO alone.

Design of stereoselective Ziegler–Natta propene polymerization catalysts

“…The results led us to conclude that in general propene insertion with 1,2 regiochemistry is dominant, for reasons that have been described (22). They also revealed that substituent R 2 , far from the active pocket, has virtually no effect on the catalytic behavior, whereas the steric bulk of R 1 , more to the catalyst front, is the key to control at the same time the enantioselectivity of 1,2 insertion and the propensity of the growing chain undergo ␤-H elimination to the monomer, which is by far the main chain transfer pathway in the absence of Al-trialkyls (17)(18)(19)23). It is important to note that the above is at odds with what is known for C 2 -symmetric ansa-metallocene cations, whose enantioselectivity and chain transfer properties are independently affected by different substituents (7).…”

“…The experiments at lower propene concentration were carried out in a 250-ml magnetically stirred, two-necked, jacketed Pyrex reactor; those at higher concentration were carried out in a 2-liter magnetically stirred, stainlesssteel reactor (Brignole AU-2). In all cases, the precatalyst was dissolved in toluene and activated with methylalumoxane [Crompton (Middlebury, CT) 10% (wt͞wt) solution in toluene] added with 2,6-di-tert-butylphenol (Aldrich) to trap ''free'' trimethylaluminum (23). The general procedures can be found in Supporting Text.…”

Reactivity Comparison of ω-Alkenols and Higher 1-Alkenes in Copolymerization with Propylene Using An Isospecific Zirconocene-MMAO Catalyst

“…We employed BHT as the modifying reagent of trialkylaluminum in MMAO reported by Busico [30]. The results are shown in Table 1.…”

FI Catalyst for Polymerization of Olefin

“…Phenoxy-imine ligands in complexes contain alkyl substituents in the 2nd and the 4th positions of their phenyl rings, they produce moderately syndiospecific catalysts [55,56]. Detailed investigations of the Ti and Zr complexes with the same ligands showed a surprising difference in the chemistry of polymerization reactions.…”

“…The [rrrr] content in polypropylene produced with these complexes at 0-20 0 C can reach 0.95-0.96 corresponding to a very high probability of syndiotactic linking, ~0.99 [55][56][57][58]. However, a replacement of these substituents with two bromine or two iodin atoms changes the regiocontrol to primary and the stereocontrol to moderately isospecific, the [mm] values of polypropylene prepared at 0 0 C increases from 0.17 for dialkyl-substituted complexes to 0.73 for diiodo-substituted complexes [58].…”