Initiators, anionic cationic polymerization

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

According to the studies of monomers in the organic glass matrices mentioned so far, the ion radicals formed from solute monomers relate their radiation-induced ionic polymerization to the primary effect of ionizing radiations on matter. It is concievable that the initiating species in the anionic polymerization (caxbanions) are formed by the addition of the monomer molecules to the anion radicals which result from electron transfer from the matrices to the solute monomer. The formation of the cation radicals is necessary also to initiate the cationic polymerization. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. 

B-90 and B-91, respectively.390 Another route coupled with cationic ring-opening polymerizations is accomplished for polymer B-92 with the use of a hydroxyl-functionalized initiator with a C—Br terminal, where the OH group initiates the cationic polymerizations of 1,3-dioxepane in the presence of triflic acid.329 Polyethylene oxide)-based block copolymers B-93 are obtained by living anionic polymerization of ethylene oxide and the subsequent transformation of the hydroxyl terminal into a reactive C—Br terminal with 2-bromopropionyl bromide, followed by the copper-catalyzed radical polymerization of styrene.391... 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

Copolymerization of N-alkylazlrldlnes with g-proplolactone. g-proplolactone (PL) reacts with tertiary amines to form the corresponding zwltterlon (7 ). If an N-substltuted azlrldlne Is used, a zwltterlon containing an azlrldlnlum Ion and a carboxylate Ion Is formed. This zwltterlon can Initiate a cationic polymerization of the azlrldlne or an anionic polymerization of the lactone or undergo a coupling reaction with another (monomeric or polymeric) zwltterlon. 

In these cases the presence of the anion of a strong Bronsted acid as C10 has just the influence of avoiding, due to the very low basicity, any buffer effect on the proton, allowing it to initiate the cationic polymerization. 

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether in the presence of oniiim salts was induced by y-irradiation. The mechanism for the initiation process involves the reduction of onium salts either by organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. Only solvated electrons were capable of reducing the salts with reduction potentials lower than approximately -100 kJ/mol. The redox process liberates the non-nucleophilic anion from the reduced onium salt and leads to the formation of a Bronsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization in this system. The y-induced decomposition of onium salts in 2-ethoxyethyl ether was also followed by measuring the formation of protons. An ESR study of the structure of the radicals formed in the y-radiolysis of butanediol divinyl ether showed that only a-ether radicals were formed. 

In the anion-cation transformation reaction, the anionically generated living polymer chain is end-capped with a hahde, producing a chain that can be isolated for subsequent reaction. This can be used to initiate a cationic polymerization of a suitable monomer by activating the end with a silver or hthium salt according to the general scheme shown in Equation 5.20a to Equation 5.20c. 

Proton addition is one of the most simple and straightforward methods for the initiation of cationic polymerization. Bremsted adds are effective for this purpose. However, if we use a weak Bronsted acid, i.e. a conjugate acid of a strong nucleophilic anion, the addition of a Lewis acid is necessary to establish a reversible activation of a covalent end group for effective propagation. On the other hand, if we use a strong... 

Formaldehyde is a strong electrophile, allowing acetal to polymerize by nucleophilic, anionic, or cationic addition of an alcohol to ketene carbonyl groups. Relatively weak bases such as pyridine initiate anionic addition polymerization cationic addition polymerization is catalyzed by strong acids. When the cyclic trimer trioxane is used as a copolymer to polymerize acetal copolymers, Lewis acids such as boron tiifluoride promote copolymerization. A more fundamental description is polymerization of an aldehyde or ketone -l- alcohol -i- an acid or base catalyst to form hemiac-etal, which further converts to acetal. The hemiacetal reaction is reversible to aldehyde and alcohol. 

Ionic polymerizations are of two types cationic and anionic. Cationic polymerizations are initiated using electrophiles, whereas anionic polymerizations are often initiated by electron-deficient species such as organolithium compounds. The propagation steps can be summarized... 

During the early investigations, nucleophobic anions contained in onium salts were considered to be chemically inert and to act partially as stabilizing entities in ion pair formation. However, the results of later studies suggested that they might be involved to some extent and in some cases in the initiation of cationic polymerizations. For example, in the case of Ph2lPF6 dissolved in a diepoxide (Chart 5.2), the hexafluorophosphate anion was found to decompose during irradiation simultaneously with conversion of the diepoxide [23]. 

Linear polysiloxane can be synthesized by both anionic and cationic polymerizations of cyclic siloxanes such as hexamethylcyclotrisiloxane (n = 3) and octamethyl cyclotetrasiloxane (n = 4). Anionic polymerization is initiated by hydroxide, alkoxides, phenolates, silanolates and siloxoano-lates. The active species in the polymerization is the silanolate anion. Cationic polymerization is initiated by strong protonic acids such as sulfuric acid, trifluoromethane sulfonic acid and trifluoro-acetic acid (equation 53). Both the anionic and the cationic species undergo backbiting reactions during polymerization, such that an equilibrium exists between linear chains and rings. ... 

Block copolymers of methylthiirane with thiirane or with 2,2-dimethylthiirane have also been obtained by sequential polymerizations initiated in the first phase by 2-naphthylsulfonylmethylene anions.Cationic polymerization has found use in the conversion of 3,3-dimethylthietane to SH-ended oligomers which could be coupled with hydroxyl-ended POE chains by reaction with a diisocyanate, but attempts to obtain block polymers of this thietane by direct initiation with cationic polystyrene were not successful. 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

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