Trialkylaluminum catalysts

A Theory of Initiation and Propagation of Carbonium Ion Polymerizations with Trialkylaluminum Catalysts. Trialkylaluminums or dialkylaluminum halides in conjunction with suitable cocatalysts in polar solvent are active polymerization catalysts. For example, when cocatalytic amounts of tert-butyl chloride are added to a quiescent mixture of trialkylaluminums or dialkylaluminum halides in methyl chloride solvent in the temperature range —30° to —100°C., immediate polymerization commences (2, 3, 4, 5, 6). 

Terpolymers form from epoxides, anhydrides, and tetrahydrofuran or oxetane with a trialkylaluminum catalyst [211] ... 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

With trialkylaluminum compounds, the addition reaction is called carboalumination. As discussed below, this reaction requires a catalyst to proceed. 

A chiral indene derivative, structure K, has been most commonly used.222 The catalyst interacts with the trialkylaluminum to generate a bimetallic species that is the active catalyst. 

Polymer Preparation. A more recent modification in the molecular structure of styrene-butadiene copolymers has been obtained with the discovery of a new catalyst system (6). The catalyst consists of a barium t-alkoxide-hydroxide salt together with a complex of dialkylmagnesium and trialkylaluminum. 

The Ziegler-Natta catalysts are prepared from transition metal halides and a reducing agent => the catalysts most commonly used are prepared from titanium tetrachloride (TiCl4) and a trialkylaluminum (R3AI). 

The use of bulky trialkylaluminum reagents, such as Al( Bu)3, with various group 4 metallocenes led to olefin-polymerization catalysts that rivaled those formed with MAO as the co-catalyst. 

Lal (8) has studied the polymerization of vinylisobutylether using a catalyst composed of titanium tetrachloride and trialkylaluminum. He concluded that this catalyst combination produced polyvinylethers by a cationic mechanism similar to that of borontrifiuoride-etherate catalysis. The stereoregular polymerization of isobutylvinylether in... 

It is also known that mildly anionic trialkylaluminums do not produce isotactic polypropylene. Only propylene dimers have been reported from this catalyst and there have been no indications of any steric control in this reaction. 

Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have shown that the hydride transfer reaction of alkylaluminum occurs much more easily with trialkylaluminum than with the more electrophilic diethylaluminum chloride. Catalysts must be more anionic in order to produce oligomers which involve large amounts of hydride transfer than with polymerization catalysts where hydride chain transfer must be minimized. Thus the oligomerization catalyst employed by Bestian and Clauss was more anionic, or less cationic, than the usual polymerization catalyst where anionic chain transfer is minimized. 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

It is known that the polymerization of ethylene by trialkyl aluminum is not a rapid reaction at normal pressures and temperatures. Ziegler, Gellert, Holzkamp, Wilke, Duck and Kroll (72) have found that ethylene was polymerized to higher trialkylaluminums only at elevated temperatures and pressures. Anionic hydride transfer commonly occured under these conditions. However, the addition of a transition metal halide such as titanium tetrachloride, the classical Ziegler catalyst, polymerized ethylene rapidly under mild conditions. 

Karapinka, Smith, Carrick (79) studied the use of methyltitanium trichloride as a catalyst for polyethylene. Alone it was inactive for the polymerization of polyethylene. It required the predecomposition to titanium trichloride at 120° or the addition of titanium trichloride to produce an active catalyst. Vanadium tetrachloride also produced an active catalyst. Aluminum bromide failed to activate the catalyst, whereas trialkylaluminum which reacts to produce alkylaluminum chlorides was effective. 

Ashikari, Kanemitsu, Yanagisawa, Nakagawa, Okomoto, Ko-bayashi and Nishioko (59) have studied the copolymerization of propylene and styrene. They found decreasing styrene content and conversion of the copolymer by increasing aluminum to titanium ratios with triisobutyl aluminum and titanium trichloride catalysts. The trialkylaluminum titanium tetrachloride catalyst had relatively low steric control on the polymerization while trialkylaluminum-titanium trichloride had higher steric control. The ionicity which is required for atactic polymerization is more cationic for styrene than for propylene which is more cationic than that for ethylene. Some of the catalyst systems for these three monomers are shown on the ionicity chart in Fig. 9. 

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic rubbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high ar-polyisoprene which duplicates the structure of natural rubber. In one method (3,4) a Ziegler-type catalyst of trialkylaluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all t /s- 1,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

Al-Tl Catalyst for cis-1,4-Poly isoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces />mf-l,4-polyisoprene. A second is a lithium alkyl which produces 90% j -l,4-p olyis oprene. Very high (99%) cis- 1,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl4, plus a trialkylaluminum, R3A1, or a combination ofTiCl4 with an alane (aluminum hydride derivative) (86—88). 

Amorphous (most likely atactic) 3,4-polyisoprene of 94—100% 3,4-microstructure was prepared with a (C2H )3A1—Ti(0— -C3H7)4 catalyst (11). Crystalline 3,4-polyisoprene containing about 70% 3,4-units and about 30% cis- 1,4-microstructure was prepared using a catalyst derived from iron acetyl acetonate, trialkylaluminum, and an amine in benzene (37). However, this polyisoprene contained gel and was obtained in poor yield. Essentially gel-free crystallizable 3,4-polyisoprene of 70—85% 3,4-microstructure with the remainder being cis-1,4 microstructure was prepared in conversions of greater than 95% with a water-modified trialkylaluminum, ferric acetyl acetonate, and 1,10-phenanthroline catalyst (38). The 3,4-polyisoprene is stereoregular and believed to be syndiotactic or isotactic. 

Thus the aluminum trialkyls act as catalyst for the alkyl exchange and indeed it has been reported that small amounts of trialkylaluminum catalyze the exchange of alkyl groups in a mixture of BR3 and BR3 (140,141). 

Table VI summarizes important homogeneous Ziegler catalysts. The best known are the systems based on bis(cyclopentadienyl)titanium(IV), titanium alcoholates, vanadium chloride, or chromium acetylacetonate with trialkylaluminum or alkylaluminum halides.

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