Novel Aluminum-Based, Transition Metal-Free, Catalytic Systems for Homo- and Copolymerization of Alkenes

Abstract

Since the initial reports by Ziegler 1 on the "aufbau" reaction, or stepwise insertion of ethene into the aluminum-carbon bond of alkylaluminum compounds, it has been widely believed that because of the accompanying displacement reaction the products of this reaction were limited to oligoethenes, with few reports of the preparation of high molecular weight polyethene at an aluminum center. Martin has described the preparation of polyethene by exposing ethene to a heptane solution of either bis-(dichloroaluminum)ethane or trialkylaluminum over several days. 2 Recently, Jordan and Gibson have reported the polymerization of ethene using chelated alkylaluminum complexes activated by a Lewis acid. 3 On the other hand, the polymerization of higher alkenes, such as propene, by an aluminum-based system has neWer been reported. Herein, we report that high molecular weight, linear homo-and copolymers of ethene and propene can be prepared, in the absence of any transition metal species, via a catalyst system consisting of simple alkylaluminum compounds activated by Lewis acids.The homo-and copolymerization of ethene and propene were carried out in 125 mL glass-lined reactors and our results are summarized in Tables 1-3. As can be seen, several systems based on the combination of an alkylaluminum compound and a Lewis acid are effective. Of note is the observation that methylaluminoxane (MAO) can act as either one of the two components (presumably because of the presence of small amounts of trimethylaluminum in the commercial sample). Several features of these systems are of interest. First, the molecular weights are high and 1 H and 13 C NMR spectroscopy indicates that the polymers are highly linear. The linearity of the polyethene formed was further supported by its high melting point (T m > 135°C). The polypropene is atactic. For the ethene-propene copolymer, the melting point was found to decrease with increasing propene content in the copolymer.Narrow polydispersities (approximately 2) were observed for the polyethene and polypropene formed, suggesting a single-site catalyst. Finally, in the polymerization of ethene, the polymer molecular weight was found to increase with increasing reaction time (first entry in Table 1), indicating some degree of "livingness" to the system. This may be due to the lack of d-orbitals on aluminum that are necessary for chain transfer through facile -hydrogen abstraction ( -hydrogen abstraction from neutral aluminum alkyls occurs only at elevated temperatures). The transfer of a growing polymer chain from a transition metal center to aluminum to form a stable aluminum-terminated polymer has been reported. 4A critically important issue that must be addressed for all transition metal-free polymerization systems is whether trace amounts of transition metal impurities are actually responsible for the polymerization. For reasons given below, this appears to be unlikely for the present systems. First, the two components used in our systems were analyzed for Ti, Zr, and V by AA spectroscopy and we...

Et2Al+ Alumenium Ion-like Chemistry. Synthesis and Reactivity toward Alkenes and Alkene Oxides

“…Mechanistic studies should lead to insight into the challenging problem of finding effective non-transition-metal catalysts for alkene polymerization and understanding the factors that control alkene insertion into an aluminum alkyl bond. [2][3][4]20 …”

Homogeneous Vanadium‐Based Catalysts for the Ziegler—Natta Polymerization of α‐Olefins

“…Alkylation of the dichloride was only possible in the case of ligand b with R = allyl. Complex L(a) 2 VMe turned out to be a well-defined catalyst. It oligomerized ethene at 80 °C in C 6 D 6 to yield a Flory-Schultz distribution of linear 1-and 2-alkenes.…”

“…Motevalli 2 ]. 25 It was tested in the polymerization of both ethene and propene with 10 equivalents of AlMe 3 as cocatalyst.…”

“…15). 32 Both ligands a and b were used to synthesize mono-and dichloro vanadium(III) compounds, LVCl 2 (THF) 2 and L 2 VCl. The monochlorides could be alkylated to yield L(a) 2 VR (R = Me, Et, allyl) and L(b) 2 VR (R = Me, allyl).…”

“…1 Even the main-group metal aluminium has been shown to produce polyethene under mild conditions. 2 The important lesson which can be learned from these findings is that it is not so much the metal, but the ligand system in combination with the metal which determines the outcome of the polymerization reaction. This was also the theme of our recent studies, which were directed to develop well-defined vanadium-based catalysts.…”