Anionic-cationic polymerization examples

Catalysis has also had a major impact on the functional and specialty chemicals businesses, providing lower cost routes and higher performance materials than would have otherwise been possible. Major examples are from polymer syntheses including Ziegler-Natta, anionic, cationic polymerization processes, for formation of polyolefins, ABS resins, polyesters and other synthetic materials. Future materials areas include high temperature composites, electronic materials and conducting organics. 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

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 ... 

We have seen a number of reactions in which alkene derivatives can be polymerized. Radical polymerization (see Section 9.4.2) is the usual process by which industrial polymers are produced, but we also saw the implications of cationic polymerization (see Section 8.3). Here we see how an anionic process can lead to polymerization, and that this is really an example of multiple conjugate additions. 

Macroinitiators for ATRP can be produced from polymers obtained from non-ATRP polymerization by appropriate conversion of their end groups. For example, hydroxyl-capped polymers obtained by cationic or anionic chain polymerization or even step polymerization can be esterihed with 2-bromopropanoyl bromide to yield Br-capped polymers that are ATRP macroinitiators. 

Many ionogenic monomers containing a polymerizable carbon double bond have been reported in the literature, and therefore a wide variety of anionic, cationic, and amphophilic polyelectrolytes may be synthesized using free radical polymerizations. Examples of anionic ionogenic monomers which have been used to synthesize anionic polyelectrolytes include acrylic acid [4-10], methac-rylic acid [6-8,11,12], sodium styrenesulfonate [7,13,14], p-styrene carboxylic... 

Thus the growing anionic chain can assume at least two identities the free anion and the anion-cation ion pair (several types of solvated ion-pairs can also be considered). Furthermore, the kinetics of these propagation reactions, which generally show a fractional dependency on chain-end concentration ranging from one-half to unity, can best be explained by assuming that the monomer can react with both the free anion and the ion-pair (4, 5, 60, but at different rates. Thus, for example, in the polymerization of styrene by organosodium, the rate of polymerization (Rp) can be expressed as... 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

The most recent and most puzzling example of anionic initiation is that which seems to take place when monomer is subjected to ionizing radiation at low temperatures. Acrylonitrile polymerizes rapidly at temperatures well below 0° (and even in the solid at —196°) when irradiated with X-rays or with electrons. Since it is now known that cationic polymerization may be induced by radiation, there has been speculation that similar anionic polymerization might be occurring with acrylonitrile (87, 121). This subject has been discussed recently by Magat (95) who quotes unpublished work of Bensasson and of Tabata and Hiroshi. 

Grafting from has not been a successful method in anionic polymerization because the required low molecular weight multifunctional organometallic initiators are almost always insoluble and this is also expected to be the case when dendrimers are modified. However, in cationic polymerizations the dormant species is less polar and more soluble. For example, the hexabenzyl bromide... 

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. 

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... 

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