Catalysts anionic initiators

In solution-based polymerisation, use of the initiating anionic species allows control over the trans /cis microstmcture of the diene portion of the copolymer. In solution SBR, the alkyUithium catalyst allows the 1,2 content to be changed with certain modifying agents such as ethers or amines. The use of anionic initiators to control the molecular weight, molecular weight distribution, and the microstmcture of the copolymer has been reviewed (15). 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Polymerization of butadiene using anionic initiators (alkyllithium) in a nonpolar solvent produces a polymer with a high cis configuration. A high cis-polybutadiene is also obtained when coordination catalysts are used. 

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is a-TiCls cocatalyzed with aluminum alkyls. The polymerization rate and cis... 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

Returning to ion-pair zirconocene catalysts, the initiation of the polymerisation process requires the displacement of the anion so that the alkene can be coordinated. The mobility of the anion is therefore an important factor and has become the focus of a number of detailed investigations. The original mechanistic scheme of alkene insertion and polymer chain growth (Scheme 8.4) implied dissociation of the anion and formation of a 14-electron cationic intermediate, which then reacted... 

Table 1.2 Kinetic Constants for Anionic Polymerization of Caprolactam with Different Catalyst and Initiator Systems...

This deficiency in the Ziegler catalyst to produce block copolymers and the abilities of anionic initiators to produce it kept the interest in anionic initiators active in many industrial laboratories. This interest in anionic research in these laboratories paid off handsomely in the areas of block and random copolymers. In this review major emphasis will be focused on the major products from both homo and block copolymers currently being manufactured by anionic technique and future trends in this area. HOMOPOLYMERIZATION... 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

The production of aldehyde groups through periodate oxidation and subsequent formation of oxime groups is being used as a basis for an anionic initiation of grafting with titanium chloride as catalyst according to a process claimed by Asahi Chemical Industries Company (135). With this method styrene was grafted onto viscose rayon. Also bi- and tri-valent vanadium salts can be used as initiators. 

Bawn and Ledwith [3) have extended the work on the solvolysis of threo-3-anisyl-2-butyl by Winstein and Robinson (5). Bawn and Ledwith showed that polymerization catalysts can be considered as having a complete range of ionic character from covalent to ionic. This spectrum can be extended both to cationic initiating species and to anionic initiating species with a complete range of degrees of freedom between the ionic species and their gegen ions. Fig. 1 shows this spectrum. In all of the subsequent discussion in this paper the shorthand notation... 

Thus the isotactic control of polystyrene requires a significantly anionic initiator system such as alfin, alkyl sodium and sodium ketyl catalysts. Ethyllithium is at the border line in ionicity to produce isotactic polystyrene. Only when a somewhat more basic component such as lithium hydroxide is present, can steric control be realized. Even in these cases the amount of steric control is not large. The less anionic... 

The fact that known anionic initiators for MMA can act as catalysts for GTP and the need for low amounts of catalysts in itself nearly puts to rest the associative mechanism. Seven of the other factors support the dissociative process. Except for the low temperature exchange studies, none supports the associative mechanism. Based on the lack of exchange of added silyl fluoride with silyl ketene acetal ends it looks like fluoride and bifluoride catalysts operate by irreversible generation of ester enolate chain ends [1] (Scheme 19b). On the other hand carboxylate catalysts appear to operate by reversible generation of ester enolate ends as evidenced by rapid exchange of silyl acetate with silyl ketene acetal ends [36] (Scheme 19c). 

Coordination polymerisations of alkyl isocyanates have not been widely studied, since these monomers could be polymerised via their C=N bond by using anionic initiators. However, such anionic polymerisations require low-temperature conditions [264]. It has been found recently [265] that alkyl isocyanates are capable of polymerisation in the presence of coordination catalysts at ambient temperature. By contrast, phenyl isocyanate appeared capable of coordination copolymerisation with oxirane [266]. 

Explain why carbon dioxide can form with propylene oxide in the presence of coordination catalysts a linear polymer, polypropylene carbonate), but with anionic initiators gives only a cyclic carbonate, propylene carbonate. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

In conclusion, in the kinetics of dioxolane polymerizations with many catalysts, the initiation mechanism is complex and inefficient. The degree of efficiency seems to be related both to the cation and to the anion. Again as in the case of cyclic ethers and cyclic sulphides, an independent measurement of the number of active sites seems essential for precise kinetics. The most probable fep for the polymerization seems to be of the order of 10—501 mole sec . With careful choice of polymerization conditions a kinetically reversible polymerization occurs, but the molecular weight of the polymer produced is not related to the initiator concentration, probably as a result of a transfer reaction. 

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