Cocatalysts boron compounds

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

Commercially, Dow appears to have implemented activation by well-defined cocatalyst for its Insite solution-phase metallocene process. Aside from historical priority, one reason for the persistent use of MAO is that the perfluorinated boron compounds are difficult to prepare, thus raising their expense. (One of the intermediates, (CeFslLi, must be handled at very low temperatme to prevent violent decomposition.) Another is that, unlike the MAO systems, catalysts activated by discrete activators have no large excess of alkylaluminum to scavenge poisons from the reaction medium, while common scavengers may hinder activity by consuming the expensive activator. 

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

Active species are formed by combination of metallocene and cocatalyst. Examples of cocatalysts are alu-minoxane and boron compounds. 

After the discovery of MAO by Kaminsky, many chemicals had been examined as cocatalysts. For an SPS production catiyst, Campbell found that boron compounds based on tris(pentafluorophenyl)boron and its derivatives are suitable as cocatalysts in the syndiotactic polymerization of styrene [1]. 

MAO and boron compounds are expensive chemicals that increase the cost of the SPS production. However, the cocatalyst is an indispensable compound for the titanium complexes used as styrene polymerization catalysts. The roles of the cocatalyst are supposed to reduce the valence of the titanium compounds by forming the precursor of the active site, to activate the precursor, and subsequently to stabilize the active site by weak coordination. TIBA reacts with the transition metal and reduces Tl(IV) to 11(111), and this may be the precursor of the active site. Yabunouchi found that the catalytic activity in the styrene polymerization increased by the addition of a small amount of ((R )3CO)n-Al-(R )3 n to the styrene monomer, even if the amount of MAO is decreased [17]. One of these compounds is [(C6Hs)3CO](i-C4H9)2Al (Fig. 3.8). The catalytic activitiy was increased more than two times by the addition of a small amount of [(C6H5)3CO](i-C4H9)2Al to a mixture consisting of Tl MAO TIBA = 1 50 25 in molar ratio. 

AJkyl boron compounds can initiate the polymerization of vinyl monomers in the presence of a suitable common feature of the cocatalyst... 

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Discussion Point DP3 Organic derivatives of the group 13 elements aluminium and boron are needed as essential components for almost all of the insertion-catalyzed olefin polymerizations. List four such compounds of interest and describe for each of them the structural and reactivity properties relevant to its action as activator jcocatalyst. Outline some of the features of polymerization catalysts that do not require any Al- or B-containing cocatalysts. 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

Most or perhaps all of the Lewis acids are seldom effective alone as initiators or catalysts they are used in conjunction with a second compound, called a cocatalyst , which very often is water or some other proton donor protogen) such as hydrogen halide, alcohol, and carboxylic acid, or a carbocation donor cationo-gen) such as f-butyl chloride and triphenylmethyl chloride. On reaction with the Lewis acid, they form a catalyst-cocatalyst complex that initiates polymerization. For example, isobutylene is not polymerized by boron trifluoride if both are dry, but immediate polymerization takes place on adding a small amount of water. The initiation process is therefore represented by... 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

Isobutene can be copolymerized with numerous unsaturated compounds via a cationic route [577-586], Table 30 lists various comonomers and copolymerization parameters. The isobutene portion in the copolymers usually exceeds 90%. The use of aluminum organic compounds (e.g., AlEt2Cl) as opposed to aluminum trichloride permits better control of the copolymerization, as they are weaker Lewis acids. Hydrogen chloride or halogens must be added as cocatalysts that are capable of regenerating the carbocations. The organoaluminum catalysts are produced at —78 °C with boron trifluoride. 

Nickel compounds can also be employed as catalysts [161-170]. A three-component system consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoride diethyletherate is used technically. The activities are similar to those of cobalt systems. The molar Al/B ratio is on the order of 0.7 to 1.4. Polymerization temperatures range from -5 to 40 °C. On a laboratory scale the synthesis of 1,4-polybutadiene with allylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularly simple [8]. In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4) [171]. 

Cationic polymerization of unsaturated compounds proceeds through the stage of carbanion cations, called also carbocations. Typical catalysts for this reaction are strong protic acids such as sulfuric acid, perchloric and trifluoroctane or the Lewis acids, which include halides of elements III, IV and V groups of the periodic table (Friedel-Crafts catalysts), such as boron trifluoride, aluminum trichloride, tin tetrachloride and titanium tetrachloride. The activity of Friedel-Crafts catalysts increases significantly the presence of small quantities of cocatalysts, that is, ihe compounds which most often are the source of protons. 

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