Cationic initiators polymerization

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

The new rate-constants resulting from my analysis are listed. The lessons from these theoretical investigations will also be useful in the context of chemically initiated cationic polymerizations, especially for those in which propagation by unpaired ions is dominant. 

Protonic (Brpnsted) acids initiate cationic polymerization by protonation of the olefin. The method depends on the use of an acid that is strong enough to produce a resonable concentration of the protonated species... 

Although it Wits long believed that most Friedel-Crafts acids, particularly halides of boron, titanium, and tin. require an additional cation source to initiate polymerization, recent results show that in many systems Friedel-Crafts acids alone are able lo initiate cationic polymerization. 

Formaldehyde polymerizes by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerization and anionic polymerization is initiated by relatively weak bases (e.g., pyridine). Boron trifluoride (BF3) or other Lewis acids are used to promote polymerization where trioxane is the raw material. 

To broaden our understanding of the chemical behavior of these novel monomers, it would be appropriate to try anionic polymerizations of captodative monomers. Inasmuch as sulfur is able to stabilize adjacent carbanions, a-alkylthioacrylates and -acrylonitriles should respond well to anionic initiators. Cationic polymerization of certain captodative monomers may also be of interest as alkylthio- and cyano-substituents can stabilize a cationic propagating center. 

The D. ion-radicals may either directly initiate cationic polymerization or undergo subsequent reactions to produce the initiating species. For the VCZ/A system, the primary process of photoreaction was proposed as follows [4] ... 

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

Photo-initiated cationic polymerization combines two distinct chemical reactions. In the first, the cation for the initiator of polymerization is formed in the second, the cation initiates polymerization of the cationic monomer. The first reaction is subject to the laws of photophysics and photochemistry, whilst the second is governed by the laws of thermochemistry. 

As an example of the design of polymerization catalysts, a group of new polymerization catalysts are described here, which are activated by UV irradiation to initiate cationic polymerization. 

Use of stable organic cations to initiate cationic polymerization allows characterization of free ions and ion pairs as intermediates and in some cases facilitates measurement of their respective absolute reactivities. Further work is in progress to extend the range of catalysts and monomers which may react in this way and therefore to extend our knowledge of the absolute reactivity in cationic polymerization. 

Cationic polymerization occurs by a mechanism similar to the free-radical process, except that it involves carbocation intermediates. Strongly acidic catalysts are used to initiate cationic polymerization. BF3 is a particularly effective catalyst, requiring a trace of water or methanol as a co-catalyst. Even when the reagents are carefully dried, there is enough water present for the first initiation step of the mechanism shown in Mechanism 26-2. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

During the past two decades, considerable attention has focused on several externally stimulated initiators, for example onium salts that undergo photolysis and thermolysis to initiate cationic polymerization. The major classes of externally stimulated initiators which can suceesfully be applied to initiate cationic polymerization are collected in Table 1. 

The optical absorption spectra of characteristic radical cation, PQ+, at 630 nm was recorded [56]. It has recently been reported by Yagci and Denizligil [57] that pyridinium ions such as EMP+ also act as a powerful oxidizing agent for the hydroxybenzyl radical to yield reactive species capable of initiating cationic polymerization (Scheme 11). 

Williams et al. studied the radiation-initiated cationic polymerization of styrene. By conductivity measurements they were able to determine very small amounts of ions and thus directly determine the propagation rate constant [381] under otherwise comparable conditions, k+ is about 30times larger than k. ... 

It was demonstrated that the P-centered radical of BAPO and its derivatives is oxidized by cationic photoinitiators (onium salts in their ground state). Phosphonium ion, a BAPO fragment, can efficiently initiate cationic polymerization. Thus, under certain conditions, PI used mfree radical polymerization can initiate a cationic polymerization as well. ... 

Aromatic onium ions are a unique class of onium ions which are photoactive and decompose in the presence of light to generate radical-cations [58]. These radical-cations react with solvent or residual moisture to release protons that initiate cationic polymerization [Eq. (43)]. Thus, aromatic onium ions are used as cationic photoinitiators in both carbocati-onic olefin and ring-opening polymerizations, especially in photocuring and photolithographic processes. 

Several classes of compounds initiate cationic polymerizations of alkenes, including protonic acids, Lewis acids (usually in combination with a cation or proton source), stable carbenium ions, oxidizing reagents, and other strong electrophiles. This section attempts to explain the mechanism of initiation with quantitative information when available physical means of initiation (electric current, y-rays, field ionization and emission, nuclear chemical initiation) will not be discussed. 

Initiators based on halonium and sulfonium salts are used commercially in various microlithographic processes and in the coating industry. Onium salts were developed commercially as photoinitiators due to the lower sensitivity of cationic polymerizations to oxygen compared to radical polymerizations. Aromatic halonium and sulfonium salts with complex anions such as SbF6, AsF6 and BF4- do not initiate cationic polymerizations spontaneously, but must be activated by UV irradiation. 

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