Protonic acids as initiators

Protonic acids as initiators of the cationic polymerization of heterocyclic monomers, should fulfil two requirements. 

Cationic polymerization of unsubstituted aziridine leads to branched polyethylene imine). This is attributed to the reaction of protonated aziridine formed either directly (protonic acids as initiators) or through proton transfer from alkylated aziridine (alkylating agents as initiators), with NH groups along the chain [162,163] ... 

Let us consider protonic acids as initiators as an example. The acid must first ionize in the medium of the reaction mass before it can protonate the monomer molecule. The overall initiation reaction for HCl, for example, consists of the following three elementary reactions ... 

Inorganic Lewis acids, specifically, Friedel-Crafts catalysts, such as SnCU, TiCl4, AICI3, AlBr3, and BF3, usually require a cocatalyst (e.g., H20 always considered to be present), and in this case, initiation is also induced by the proton formed [Eq. (13.18)]. Pioneering work on the role of proton acids as catalysts and cocatalysts in acid-catalyzed polymerization was carried out by Polanyi and... 

Catalysts for ethylene/carbon monoxide copolymerisation were initially obtained from Ni(II) derivatives, such as K2Ni(CN)4 and (w-Bu4N)2 Ni(CN)4, and Pd(II) derivatives, such as [(w-Bu3P)PdCl2]2, Pd(CN)2 and HPd(CN)3, often combined with alcohol or protonic acid as a cocatalyst [241]. It must be emphasised that, in contrast to titanium-, zirconium- or vanadium-based catalysts, nickel- and palladium-based catalysts tolerate polar functional groups (including hydroxyl, carboxylic and sulfonic groups)... 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

This strength only becomes manifest in the presence of a suitable base or proton acceptor, in which case appropriate values of such acidity parameters as Hammett s Ho can be measured. In rather simplistic terms one could draw a parallel between the Ho values and the relative ease with which a given monomer would be protonated if dissolved in the pure acids. Unfortunately, this protonation capacity is not a sufficient criterion to establish the potential usefulness of a Br nsted acid as initiator in cationic polymerisation, although it does represent an important parameter. Other factors, such as the solubility of the acid in the common solvents used, its inertness towards them, the relative stability of the ions formed in the protonation reaction against their collapse to the corresponding ester, etc., can play a major role in determining the real qualities of these possible initiators. 

The cationic polymerization of styrene has been known since the 1960s using Lewis acids or strong protic acids as initiators leading to fast, uncontrolled polymerization due to extensive proton transfer reactions to, for example, counteranion, solvent, monomer, or polymer. In addition, chain transfer occurs as a result of an intramolecular Friedel-Craft reaction of the carbocation with the penultimate monomer unit, resulting in an indane chain end and the release of a proton that can reinitiate a new polymer chain as illustrated in Scheme 8.13. 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

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). 

Initiation. A Friedel-Craft acid (hydrochloric acid, water, phenol) is used as initiator together with a proton source ( co-initiator , BF3 or AICI3 are the most common). The mixture produces a catiogen which is the true initiating species. 

Strong protonic acids can affect the polymerization of olefins (Chapter 3). Lewis acids, such as AICI3 or BF3, can also initiate polymerization. In this case, a trace amount of a proton donor (cocatalyst), such as water or methanol, is normally required. For example, water combined with BF3 forms a complex that provides the protons for the polymerization reaction. 

We can explain the observed pH dependence of imine formation by looking at the individual steps in the mechanism. As indicated in Figure 19.8, an acid catalyst is required in step 3 to protonate the intermediate carbinolamine, thereby converting the —OH into a better leaving group. Thus, reaction will be slow if not enough acid is present (that is, at high pH). On the other hand, if too much acid is present (low pH), the basic amine nucleophile is completely protonated, so the initial nucleophilic addition step can t occur. 

The reactions covered in Scheme 2 are initiated by protonation but a hydride could form on the metal as intermediate. In some instances, cationic metal hydrides have been shown to be actually involved. See, for example, the addition of [HNi (POEt)3 4+] to butadiene (54) or of [HNi(Ph3P)3(7r-C3H5)] to olefins (10c, Vol. II, p. 25). Thus the reaction of olefins or dienes with acids in the presence of zero-valent nickel may be considered proton-promoted as well as hydride-promoted. 

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