Initiation mechanism, anionic

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

The polymerization of olefinic compounds like acrylonitrile, vinyl chloride, styrene, methylmethacrylate can be initiated by anion. The mechanism, in general, can be given as... 

This review aims at reporting on the synthesis of aliphatic polyesters by ROP of lactones. It is worth noting that lactones include cyclic mono- and diesters. Typical cyclic diesters are lactide and glycolide, whose polymerizations provide aliphatic polyesters widely used in the frame of biomedical applications. Nevertheless, this review will focus on the polymerization of cyclic monoesters. It will be shown that the ROP of lactones can take place by various mechanisms. The polymerization can be initiated by anions, organometallic species, cations, and nucleophiles. It can also be catalyzed by Bronsted acids, Lewis acids, enzymes, organic nucleophiles, and bases. The number of processes reported for the ROP of lactones is so huge that it is almost impossible to describe aU of them. In this review, we will focus on the more... 

In the previous communication(7a) we postulated the initiation mechanism involving the attack of BuLi on the vinyl double bond of the butyl lsopropenyl ketone formed. However, from the results mentioned above we will propose the following mechanism for the polymerization of MMA with BuLi in toluene, which is slightly different from the previous one. On mixing the monomer with BuLi the initiator reacts with both olefinic and carbonyl double bonds of the monomer. The attack on the olefinic double bond produces the MMA anions, which add the monomer to form the growing chains(E). 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

Similarly, zwitterionic tetramethylenes as initiators of anionic polymerization were also observed. For example, methyl a-cyanoacrylate polymerizes via an anionic mechanism in the presence of n-butyl vinyl ether [90]. A Diels-Alder adduct is also formed. In another example, the reaction of isobutyl vinyl ether and nitroethylene leads to an unstable adduct [91], which is capable of initiating the anionic polymerization of excess nitroethylene, and also the cationic polymerization of added VCZ. 

Application of pulse radiolysis to polymers and polymerization was motivated at first by the success of radiation-induced polymerization as a novel technique for polymer synthesis. It turned out that a variety of monomers could be polymerized by means of radiolysis, but only a little was known about the reaction mechanisms. Early studies were, therefore, devoted to searching for initiators of radiation-induced polymerization such as radicals, anions and cations derived from monomers or solvents. Transient absorption spectra of those reactive intermediates were assigned with the aid of matrix isolation technique. Thus the initiation mechanisms were successfully elucidated by this method. Propagating species also were searched for enthusiastically in some polymerization systems, but the results were rather negative, because of the low steady state concentration of the species of interest. 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

When X = OAc , the initially coordinated anion is sufficiently basic to neutralize the liberated H+ [Eq. (54)] however, X = Cl requires the presence of external (normally amine) base to effect the same transformation. In both cases the palladium is recovered as the carbonyl derivative, Pd(CO)(PPh3)3 [and/or the related Pd3(CO)3(PPh3)4]. Alkoxycarbonyl derivatives such as Pd(OAc)(COOMe)(PPh3)2 and Pd(COOMe)2(PPh3)2 have been isolated from these systems and are invoked as catalytic intermediates, the total mechanism being described in the Scheme 10. The salient... 

Perhaps the most frequently used example is the HI/ZnI2 system, where the iodine in the HI/12 counterpart is now replaced with the mild Lewis acid, zinc iodide [98,99]. A more detailed discussion of the scope and mechanism of the polymerizations by the HB/MtX systems will be given for respective monomers in the later parts of Section IV. It should be noted here that the suitable nucleophilicity of the initiator s anion B- and the mild Lewis acidity of MtX strongly depend on the nature and stability of the growing carbocations or the monomers from which they are derived. 

Reaction Mechanism The mechanism of the sodium hydroxide-catalyzed elimination of hexamethyldisiloxane may easily be understood when the reaction is compared to the well-known Peterson olefination in organic chemistry [24], Provided that an enolate anion is formed as an intermediate, either directly or via a proceeding hydrolysis of the 0-Si bond with traces of water which are always present on the hot surface of the crude catalyst, trimethylsilanolate splits off readily and thus the PsC triple bond is introduced into the molecule (Eq. 5). Subsequent attack of trimethylsilanolate at the trimethylsiloxy group of the starting compound results in a formation of hexamethyldisiloxane and the initial enolate anion so that the reaction circle is closed. 

Emulsion polymerizations normally produce polymer particles with diameters ofO.I-l pm(l pm= I micron= 10 cm), although much larger particles can be made by special techniques mentioned in Chapter 8. Tlie polymer particles made by suspension reactions have diameters in the range of 50-500 pm. Recall that free-radical initiation in suspension reactions is in the monomer phase, whereas the aqueous phase is the initiation site in emulsion polymerizations. The two processes often dilTer also in the types of stabilizers that are used. Microsuspension polymerization is an alternative technique which can yield particles in the same size range as emulsion processes. This method uses a monomer-soluble initiator and anionic emulsifiers similar in nature and concentration to those used in emulsion polymerizations. A microdispersion of the mixture of the reaction ingredients is first produced mechanically and is then polymerized to provide polymer with essentially the initial fine particle size distribution. 

Like Brjinsted acids, Lewis acids are known to homoconjugate and to heteroconjugate with anions derived from them. This point, completely neglected by researchers in the field of cationic polymerisation, is particularly relevant to direct initiation mechanisms and will be amply discussed in Chap. IV, where the evidence for the occurence of such interactions is reviewed and our views on their role in provoking the premature stoppage of a polymerisation are presented. 

These cyclizations both involve the reductive intramolecular addition of an electron deficient alkene function to an aldehyde carbonyl function, and both are effected in ca 90 % yields. The mechanism of this latter type of electrochemically induced cyclizations of carbon-carbon double bonds to carbonyl double bonds have been studied rather extensively, with especial attention to the fundamental mechanistic question of whether the cyclization step involves an anion radical, radical, or anionic mechanism [122]. The latter two mechanisms would involve the protonation of the initially formed anion radical intermediate to form a radical, which could then cyclize or, alternatively, be further reduced to an anion, which could then cyclize. Extensive and elegant electrochemical and chemical studies have led to the formulation of these reactions as involving anionic cyclization (Scheme 74). 

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. 

T]he mechanism still remains to be clarified since there is almost no definite information on the mechanism by which an alkyl or aryl group is transferred to the initially formed anion radicals. 

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