Cocatalyst systems

To avoid high resin chloride content associated with the use of high concentrations of aluminum trichloride, a ttialhylalurninum—water cocatalyst system in a 1.0 0.5 to 1.0 mole ratio has been used in conjunction with an organic chloride for the polymerization of P-pinene (95). Softening points up to 120°C were achieved with 1—3 Gardner unit improvement in color over AlCl produced resins. 

Chlorination of OCT with chlorine at 90°C in the presence of L-type 2eohtes as catalyst reportedly gives a 56% yield of 2,5-dichlorotoluene (79). Pure 2,5-dichlorotoluene is also available from the Sandmeyer reaction on 2-amino-5-chlorotoluene. 3,4-Dichlorotoluene (l,2-dichloro-4-methylben2ene) is formed in up to 40% yield in the chlorination of PCT cataly2ed by metal sulfides or metal halide—sulfur compound cocatalyst systems (80). 

Fig. 21 Calculated structure of an active species derived from Zr-FI catalyst 33 with /-Bu,Al/ Ph CBtCy j, cocatalyst system (polymer chain model n-butyl group). Reproduced with permission from Ishii et al. [26], Copyright 2002, Wiley-VCH.

The above catalyst system is long-lived and like the Miller catalyst, it formed primarily 1,4-hexadiene, 3-methyl- 1,4-pentadiene, and 2,4-hex-adiene from ethylene and butadiene. A typical distribution of products formed by this catalyst and by the (Bu3P)2NiCl2/i-Bu2AlCl catalyst is shown in Table IX. The improved conversion and yield can be attributed to a better cocatalyst system, as shall be discussed later. Su and Collette s studies are summarized in the following discussions. 

The catalyst component consists of halides of IV-VIII group elements having transition valence and the cocatalysts are organometallic compounds like alkyls, aryls and hydrides of group I-IV metals. Although there are hundreds of such catalyst cocatalyst systems listed in table below. Systems based on the organoaluminium compounds such as triethyl aluminium (AlEt3) or diethyl aluminium chloride... 

Olefin Polymerizations and Copolymerizations with Alkylaluminum-Cocatalyst Systems... 

Anionic polymerization requires a cocatalyst system composed of an alkyllithium reagent and either N,N,N, N -tetramethylethylenediamine (TMEDA) or hexamethylphosphoramide (HMPA). 1-Methyl-l-phenyl-l-sil-acyclopent-3-ene (III), 1,1-diphenyl-l-silacyclopent-3-ene (IV), and 1,1,3-trimethyl-l-silacyclopent-3-ene (V) were polymerized under similar conditions. 

This two-step synthesis of ketones has been improved from the aldehyde into one-step synthesis with the cocatalyst system of the rhodium complex and 2-amino-3-picoline, which reacts with an aldehyde to give an aldimine in situ. The ketimine produced is easily converted to a ketone by in situ hydrolysis with H20, which is formed in the step of condensation of the aldehyde with the amine (Eq.56) [125]. 

Yamamoto et al., on the other hand, reported that a Ti-allylpalladium-TBAF cocatalyst system enables allylation of aromatic imines with allyltrimefhylsilane at room temperature [376]. As described in Scheme 10.131, this allylation would proceed via a transmetalation mechanism involving the bis-Ti-allylpalladium intermediate 121. The use of chiral 7r-allylpalladium complex 131 enables asymmetric synthesis of homoallyl amines wifh good enantioselectivity (Scheme 10.150). 

The Rp data from nonsteady-state conditions can be used to obtain the value of kp from Eq. (8.128) when [M ] is known. In polymerizations initiated by catalyst-cocatalyst systems, [M ] is taken to be the cocatalyst concentration [IB] when the initiator (catalyst) is in excess or the initiator concentration [L] when the... 

Olefin Polymerization with Homogeneous Vanadium(V), Vana[Pg.204]

Table 4. Klhylene poly meri/ation by CpVCl2(NAr). VCl2(NAr)(OAr) 25, OAr O-2,6-Me2C ,H. (a), 0-2,6- Pr2C 6Hji (b), 0-2,6-Ph2C 6H, (c) - cocatalyst system/...

For the polymerization of disubstituted acetylenes, M0CI5 and WCl6 alone are inactive, and it is necessary to use the catalyst/cocatalyst mixtures (16), which are active for sterically less crowded monomers (e.g., 2-octyne and 1-chloro-l-octyne). In contrast, NbCls and TaCls by themselves polymerize disubstituted acetylenes with bulky substituents such as 1-(trimethylsilyl)-l-propyne. Diphenylacetylene and its derivatives, however, are polymerizable only with the TaCls-cocatalyst systems. The Nb and Ta catalysts selectively afford cyclotrimers from most monosubstituted acetylenes. 

The polymer may be isotactic, syndiotactic, or atactic according to the nature of the catalyst/cocatalyst system. The Cossee-Arlman mechanism for the ZNP of propene is depicted in Scheme 4.2. 

Table 11.9 Tacticities of some polymers made using WCl6/R4Sn (1/2) as catalyst/ cocatalyst system solvent PhCl. Symbols defined in text.

Copper-catalyzed aerobic alcohol oxidation has proven to be a key step in the production of vanillin, but other commercial-scale applications have not yet been realized. Nevertheless, the industrial interest in some of the early cop-per/cocatalyst systems for aerobic oxidation bodes well for future apphcations. Recent academic studies have led to significantly improved catalyst systems... 

A telechelic polymer is defined as a relatively low-molar-mass spedes (M < 20,000), with functional end groups that can be used for further reaction to synthesize block copolymers or for network formation. Cationic polymerization methods can be used to prepare these fimctionalized polymers using the initiator-transfer, or Inifer, technique perfected by Kennedy. If the initiating catalyst-cocatalyst system is prepared from a Lewis acid and an alkyl or aryl halide, i.e.. 

Many of the uncertainties inherent in Friedel-Crafts catalyst-cocatalyst systems can be removed if stable, well-defined initiators are used. Bawn and co-workers have made use of triphenyl methyl and tropylium salts of the general formula P1 C+X and C7H7X , where X is a stable anion such as CIO4, SbClg, and PFg. 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

Even though the discussion has been mainly on homopolymerization, the same polymerization mechanism steps are valid for copolymerization with coordination catalysts. In this case, for a given catalyst/cocatalyst system, propagation and transfer rates depend not only on the type of coordinating monomer, but also on the type of the last monomer attached to the living polymer chain. It is easy to understand why the last monomer in the chain will affect the behavior of the incoming monomer as the reacting monomer coordinates with the active site, it has to be inserted into the carbon-metal bond and will interact with the last (and, less likely, next-to-last or penultimate) monomer unit inserted into the chain. This is called the terminal model for copolymerization and is also commonly used to describe free-radical copolymerization. In the next section it will be seen that, with a proper transformation, not only the same mechanism, but also the same polymerization kinetic equations for homopolymerization can be used directly to describe copolymerization. 

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