Cationic polymerization protonic acid initiation

Cationic Polymerization. Protonic acids such as HBF, HPFg, HSbFg, etc., derived from the photolysis of onium salts I-III are well known initiators of cationic polymerization (9). In equations 13-15 is shown the proposed mechanism of cationic polymerization using a triarylsulfonium salt and a typical monomer, M. 

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... 

Cationic polymerization has been initiated by a variety of protonic and Lewis acids [Kubisa, 1996 Toskas et al., 2001]. The cationic process is more complicated and less understood than the anionic process. Polymerization under most reaction conditions involves the presence of a step polymerization simultaneously with ROP. This appears to be the only way to reconcile the observed (complicated) kinetics for the overall process [Chojnowski and Wilczek, 1979 Chojnowski et al., 2002 Cypryk et al., 1993 Rubinsztain et al., 1993 Sigwalt, 1987 Wilczek et al., 1986],... 

As you, by now, have doubtiess anticipated, cationic polymerizations involve an active site where there is a positive charge because, in effect, there is a deficit of one electron at the active site (Figure 3-21). Cationic polymerizations can be initiated by protonic acids (Figure 3-35) or Lewis adds (the latter sometimes combined with certain halogens). Uulike anionic polymerization, termination can occur, by anion-cat-... 

One of the few systems for which the initiation with protonic acid has been comprehensively studied is the cationic polymerization of lactams Initiation of the cationic polymerization of lactams with HCl gives predominantly a nwnomer molecule protonated at the oxygen atom but a small amount of N-protonated lactam is assumed to be present in the tautomeric equilibrium ... 

In the cationic polymerization of lactams, initiated with protonic acids, macromolecules are formed having two ends (A and B in the structure below) potentially active (cf. Sect. 3.1) ... 

Lewis acids, carbon cations, salts of oxonium ions, and strong protonic acids initiate polymerizations of tetrahydrofuran. The reactions can be conducted in solution or without a solvent. It was originally polymerized with a trialkyloxonium salt, R30-BF4 . 

Sawada [406] described the thermodynamics of cationic polymerization as follows The process of protonic acid initiation of cationic polymerization starts with heterolytic bond dissociation of the acid. The energy of this process can be designated by the quantity C2, the affinity of the proton for the olefin. The potential energy between ion pairs separated at a distance r is... 

The cationic polymerization process is initiated in the presence of an acid, which transfers a proton to the it bond of a monomer, thereby generating a carbocation. The acid catalyst most often used is formed by treating BF3 with water. 

Cationic polymerization occurs in a similar manner, except for the fact that the initiator system produces cations instead of flee radicals. Any catalyst system in cationic polymerization normally requires a cocatalyst. For example, protonic acid initiators (or catalysts) such as sulfuric acid, perchloric acid, and Irifluoro-acetic acid require a cocatalyst (e.g., acetyl perchlorate or water). Together, the two generate cations in the reaction mass. The reaction of boron Irrfluoride with water as the cocatalyst and styrene as the monomer is an example ... 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Polymerization of 33 occurred very readily with a variety of cationic initiators. Protonic acids gave high conversion to soluble polymer which was of moderate molecular weight (T jnh = 0.35), liquid (rubber), and consisted of a 60 40 mixture of iso-... 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

Another reaction that has been applied to the generation of highly functionalized polymers is cationic polymerization [12-15]. Catalysts for cationic polymerizations are aprotic acids, protic acids, or stable carbocation salts. In these processes, the catalyst generally reacts with a cocatalyst to form an active initiated species. Initiation takes place by protonation of the monomer (Fig. 2A). Monomers that possess cation stabilizing groups, such as electron rich olefins, are preferred as they more readily undergo the desired polymerization process... 

Before the discovery of the pseudo-cationic reactions, one could say simply that the function of the co-catalyst is to provide cations which can initiate the polymerization [28b]. Although this is still valid for the true cationic polymerizations, it is more difficult to define the function of the co-catalyst in the pseudo-cationic reactions. Very tentatively one can suggest that the co-catalyst is the essential link in the formation of an ester which is the chain-carrier, as in the pseudo-cationic polymerizations catalysed by conventional acids in other words, the co-catalyst and catalyst combine to form an acid, but this, instead of protonating the monomer, forms an ester with it, which is then the propagating species. 

Ionic polymerization may also occur with cationic initiations such as protonic acids like HF and H2SO4 or Lewis acids like BF3, AICI3, and SnC. The polymerization of isobutylene is a common example, shown in Fig. 14.5. Note that the two inductively donating methyl groups stabilize the carbocation intermediate. Chain termination, if it does occur, usually proceeds by loss of a proton to form a terminal double bond. This regenerates the catalyst. 

The first species produced in cationic polymerizations are carbocations, and these were unknown as such prior to World War II. It is now known that pure Lewis acids, such as boron trifluoride and aluminum chloride, are not effective as initiators. A trace of a proton-containing Lewis base, such as water, is also required. The Lewis base coordinates with the electrophilic Lewis acid, and the proton is the actual initiator. Since cations cannot exist alone, they are accompanied by a counterion, also called a gegenion. 

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... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Some common initiators for cationic polymerization reactions are protonic acids, Friedel-Crafts catalysts (Lewis acids), compounds capable of generating cations, or ionizing radiation. 

When exposed to UV radiation, these salts dissociate and react with a proton (impurities, ROH), to liberate a protonic acid that is able to initiate a cationic chain polymerization of epoxy groups. 

Note Moderately polar solvent, ethereal odor soluble in water and most organic solvents flammable moderately toxic incompatible with strong oxidizers can form potentially explosive peroxides upon long standing in air see the relevant tables in the chapter on laboratory safety commercially, it is often stabilized against peroxidation with 0.5 to 1.0% (mass/mass) p-cresol, 05 to 1.0% (mass/mass) hydroquinone, or 0.01% (mass/mass) 4,4 -thiobis(6-ferf-butyl-m-cresol) can polymerize in the presence of cationic initiators such as Lewis acids or strong proton acids. Synonyms THF, tet-ramethylene oxide, diethylene oxide, 1,4-epoxybutane, oxolane, oxacyclopentane. 

---