Catalyst-cocatalyst complex

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

The rate of initiation (Ri) for typical cationic reactions is proportional to the concentration of the monomer [M] and the concentration of the catalyst-cocatalyst complex [C] as follows ... 

Eastham and collaborators carried out a thorough study of the isomerisation and polymerisation of butenes by the BF3-CH3OH pair They found that the initial rate of isomerisation of butene-2 was proportional to the concentration of both the catalyst-cocatalyst complex and the free catalyst. We think that these findings are... 

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... 

As a new propagating species is generated each time a growing chain is terminated by transfer to monomer, many polymer molecules can result for each molecule of catalyst-cocatalyst complex initially formed. 

Spontaneous termination, also referred to as Chain transfer to counterion, is the unimolecular rearrangement of the ion-pair which results in termination of the growing chain and simultaneous regeneration of the catalyst-cocatalyst complex. 

For the purpose of establishing the kinetics of generalized cationic polymerization, let A represent the catalyst and RH the cocatalyst, M the monomer, and the catalyst-cocatalyst complex H+ AR . Then the individual reaction steps can be represented as follows ... 

The first involves hydrogen abstraction from the growing chain to regenerate the catalyst-cocatalyst complex, whereas the second reforms a monomer-initiator... 

In very small concentrations, water acts as a co-catalyst, initiating polymerization in combination with a catalyst (e.g., SnCl4). However, in larger concentrations, it inactivates the catalyst (such as by hydrolysis of SnCU) or competes successfully with monomer for the catalyst-cocatalyst complex and inactivates the proton by forming hydronium ion [see Eqs. (P8.18.2) and (P8.18.4)] because the basicity of the carbon-carbon double bond is far less than that of water ... 

Benzoquinone acts as an inhibitor by receiving proton from the carbocation and/or catalyst-cocatalyst complex (Odian, 1991) ... 

The kinetic chain is interrupted here, but the catalyst-cocatalyst complex is regenerated and can initiate new kinetic chains. In the production of butyl rubber (as distinguished from polyisobutene) isobutene is copolyinerized with about 3%... 

J. Kress, M. Wesolek, J.-P. Le Ny, and J. A. Osborn, Molecular Complexes for Efficient Metathesis of Olefins. The Oxo-Ligand as a Catalyst-Cocatalyst Bridge and the Nature of the Active Species, J. Chem. Soc., Chem. Comm. 1981, 1039-1040. 

The use of cocatalysts is desirable and possibly absolutely neces.sary in many Lewis acid systems. The concentration of cocatalyst must be carefully controlled, however, and optimum Lewis acid/cocatalyst concentration ratios can be established for particular polymerizations. This is because the cocatalyst must be more basic than the monomer otherwise the Lewis acid would react preferentially with the monomer. If excess co-catalyst BA is present, however, it can compete with the monomer for reaction with the primary Lewis acid/cocatalyst complex. For example,... 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Species (1) and (4) in the scheme represent coordinatively unsaturated Ti(IV) complexes that are formally dP 16felectron pseudotetrahedral species species (2) represents the interacting catalyst/cocatalyst combination, while intermediate species (3) is shown with the monomer coordinated at an a molecular orbital with the three non-Cp ligands and the transition metal occupying a common equatorial plane. The growing chain is held between two lateral coordination sites accommodating an unidentified non-Cp anion (R ) and the monomer. 

With every non-carbene catalyst system there is the question as to the nature of the initiating metal carbene complex and how it is formed from the catalyst/cocatalyst/ (substrate olefin). When the cocatalyst contains an alkyl group, there is usually an obvious path whereby this becomes the source of the alkylidene group attached to the metal and there is much evidence from the initial products of reaction or from end groups in polymers formed by ROMP that can be brought to bear on this question. 

With TiCl4//-Bu3Al (2/1) as catalyst system the polymer formed shows little unsaturation, but a 1/2 catalyst/cocatalyst ratio gives ring-opened polymer. With Et3Al as cocatalyst the cationic side reactions can be suppressed by the inclusion of a tertiary amine in the reaction mixture (Saegusa 1964 Tsujino 1964, 1965 Winstein 1977). More recently, a number of titanacyclobutane complexes have... 

Complexes, catalyst- cocatalyst n. Stereospecific chemical complexes, usually derived from a transition metal halide and a metal hydride or a metal alkyl. An example is in stereospecific polymerization of propylene to crystalline polypropylene. 

Figure 4 Mechanism of the polymerization of olefins by zirconocenes. Step 1 The cocatalyst (MAO methylalumoxane) converst the catalyst after complexation into the active species that has a free coordination position for the monomer and stabilizes the latter. Step 2 The monomer (alkene) is allocated to the complex. Step 3 Insertion of the alkene into the zirconium alkyl bond and provision of a new free coordination position. Step 4 Repetition of Step 3 in a very short period of time (about 2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain.

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]. 

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