Cationic radical polymerization

Cation-Radical Polymerization 7.3.2.1 Formation of Linear Main Chains... 

The linear cation-radical polymerization of diazoacetophenone has been described (Jones 1981). The reaction proceeds with the evolution of nitrogen and attracts some interest as a route to porous plastic materials. 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

Polymers 340-343 have a deep red or brown color probably related to the presence of conjugated polynuclear units. A similar color was noted in the polymers derived from cation-radical polymerization of 1,1 -binaphthyl and o-terphenyl. In these cases the color was attributed to the presence of cation-radicals of perylene and triphenylene structural units [193,201,203]. Perylene units are possible in polymers 340-343 (Fig. 46). 

Figure 52 Mechanism of the proton transfer reactions during the cation-radical polymerization of methyl-substituted l,5-bis(phenoxy)pentane monomers. (From Ref. 214.)...

Percec and co-workers have also employed the Scholl reaction in a variety of polymerization reactions, especially for the synthesis of aromatic polyethers. The 1-naphthol derivative 42 underwent the cation-radical polymerization under Scholl conditions to afford the polyether 43 in high conversion. As a major contributor to this area, Percec has used the Scholl reaction in a similar manner in various other polymerization studies of aromatic polyethers. The Scholl reaction also been employed in polymerizations of binapthyl, 4-methyltriphenylamine, and l,3-di- -butoxybenzene -based systems. 

Ionizing radiation is capable of initiating the polymerization of vinyl ethers. The primary process [121] appears to consist of the electrolytic removal of electrons from monomer molecules with consequent formation of the corresponding cation radicals. Polymerization then proceeds by a cationic mechanism by further interaction of the cation radicals... 

Polythiophene and its derivatives can be polymerized by chemical or electrochemical techniques. In this study, the electrochemical method was utilized.The mechanism is a cationic radical polymerization 11). The polymerization pathway can be summarized in the following steps 1) oxidation of the monomer to form a radical cation, 2) dimerization of the radical cations, 3) loss of proton to yield a neutral dimer, 4) oxidation of dimer to form a radical cation, 5) reaction of dimer radical cation with another radical cation, 6) repeat of the this study, are 3-methylthiophene, tetrabutylammonium tetrafluoroborate (TBATFB), as the supporting electrolyte. The organic solvent was acetonitrile. The resulting polymer was the first conducting polymer family found to be stable in air and water in both their doped or undoped state. 

Percec V., Okita S., Wang J. H., Synthesis of aromatic polyethers by Scholl reaction. VI. Aromatic polyethers by cation-radical polymerization of 4,4 3,3 -, and 2,2 -bis -... 

My faculty colleagues of the Institute also bring great expertise in the areas of anionic, cationic, and radical polymerization to the transformation of low-molecular-weight hydrocarbons into macromole-... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Radical polymerization of diallylamine derivatives produce water-soluble polymers of low molecular weight [22,55-57]. In order to increase the molecular weight, acrylamide has been copolymerized with these diallylamine derivatives to produce cationic polymers with variable charge density depending on the content of the structural units of pyrrolidinium rings and acrylamide in polymeric chains [22,55,58-61]. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

B. Formation of MAIs by Cationic Chain Polymerization—Cation Radical Transfer... 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

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