Radical cation polymerization system

While the detailed mechanistic understanding of many cationic polymerization systems has not as yet been clearly established, there are some that are reasonably well understood. Let us consider the detailed progression of the cationic polymerization of isobutylene using a TiCl4/H20 catalyst system as an example. This system cannot be used as a general model for all cationic polymerizations as is possible for free radical-initiated systems, since there is a wider variety of mechanisms seen for cationic, than for free radical, systems. 

The systems under investigation include the radical cationic polymerization of olefin and diolefin monomers. In order to establish the mechanistic features... 

Concurrent Radical-Cationic Polymerization—Hybrid Systems... 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

Just as anionic polymerizations show certain parallels with the free-radical mechanism, so too can cationic polymerization be discussed in terms of the same broad outline. There are some differences from the anionic systems, however, so the fact that both proceed through ionic intermediates should not be overextended. 

On the basis of these observations, criticize or defend the following proposition Regardless of the monomer used, zero-order Markov (Bernoulli) statistics apply to all free radical, anionic, and cationic polymerizations, but not to Ziegler-Natta catalyzed systems. 

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. 

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. 

For SCVP of styrenic inimers, the mechanism includes cationic (14 [18], 19 [29]), atom transfer radical (15 [22, 27]), nitroxide-mediated radical (16 [21]), anionic (20 [19]), photo-initiated radical (17 [2], 18 [52-55]), and ruthenium-catalyzed coordinative (21 [56]) polymerization systems. Another example in-... 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

Cx is a charge transfer complex the position of the equilibria, and, hence, the importance of Cx, and the concentration of the radical ions, may differ greatly from one system to another. The radical cation then probably reacts in most systems in such a way that the radical function is rapidly inactivated and the cationic function then propagates a quite normal cationic polymerization. 

Once a compound has been shown to polymerise, the most interesting question for me is What is stopping the chains from growing When that question has been answered we must know much about the kinetics of the system and at least a little about its chemistry. Before entering into an account of the reactions which stop chains from growing, it is important to make once again a clear distinction between termination and transfer reactions. There is no reason for not adhering to the radical chemist s definition of termination a reaction in which the chain-carrier is destroyed. In cationic polymerizations there are two main types of termination reaction ... 

In the present context the word termination is applied not to the breaking-off of a physical chain, i.e., the cessation of growth of a particular molecule, but to the complete destruction of a kinetic unit, which means the irreversible annihilation of one ion pair. This kinetic termination, which is a well-understood feature of radical polymerizations, is a comparatively rare event in cationic polymerizations it may occur in several different ways and in some systems not at all. 

The mechanism of electropolymerization is still not fully understood. The one certainty is that in the very first step the neutral monomer is oxidized to a radical cation. It must have an oxidation potential that is accessible via a suitable solvent-electrolyte system and should react more quickly with identical species than with nucleophiles in the electrolyte solution. Therefore, as a general rule, polymerization without defects becomes less successful with increasing oxidation potential of the starting monomer, for example, in... 

Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media in ionic polymerizations is often not clear since heterogeneous inorganic initiators are often involved. Further, it is extremely difficult in most instances to obtain reproducible kinetic data because ionic polymerizations proceed at very rapid rates and are extremely sensitive to the presence of small concentrations of impurities and other adventitious materials. The rates of ionic polymerizations are usually greater than those of radical polymerizations. These comments generally apply more to cationic than anionic polymerizations. Anionic systems are more reproducible because the reaction components are better defined and more easily purified. 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

The evidence for this mechanism is based on mass spectroscopy of the gas-phase radiolysis of isobutylene, which may not be applicable to the typical liquid-phase polymerization system. Initiation in condensed systems may follow the same course as electroinitiation— coupling of radical-cations to form dicarbocations. 

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