Proton-transfer Polymerization

Proton-Transfer Polymerization. Proton-transfer polymerization (PTP) (Fig. 11) has been reported as a versatile route to hyperbranched polymers (56). Conceptnally, PTP is an acid-base controlled reaction where the nucleophilicity and basicity of monomer and intermediates play important roles. 

However, transfer reactions are nearly unavoidable side reactions in cationic polymerization and may involve yS-proton transfer even to rather weak bases like the monomer itself, to transfer agents and solvent as well as Friedel-Crafts alkylation of aromatic rings, if monomers like styrene are polymerized. Proton transfer yields unsaturated chains [Eq. (58)], with the double bond in either the endo or exo position. These may lead to branched polymers, if the double bonds are accessible to homo- or copolymerization. 

The chain polymerization of formaldehyde CH2O was the first example of a chemical conversion for which the low-temperature limit of the rate constant was discovered (see reviews by Goldanskii [1976, 1979]). As found by Mansueto et al. [1989] and Mansueto and Wight [1989], the chain growth is driven by proton transfer at each step of adding a new link... 

However, as can also be seen in Fig. 11, primary and secondary amines do not perform very effectively as primers, compared to tertiary amines, even though they also contain long alkyl chains. It has been demonstrated that, instead of directly initiating ECA polymerization, primary and secondary amines first form aminocyanopropionate esters, 12, because proton transfer occurs after formation of the initial zwitterionic species, as shown in Eq. 7 [8,9]. 

The proton transfer mechanism described previously was confirmed somewhat by the influence of solvent polarity on polymerization. The rate of photopo-... 

Solution of alkali metals in liquid ammonia, containing the so-called solvating electrons, may be used as an alternative homogeneous system to initiate polymerization by an electron transfer process. This system suffers, however, from complications resulting from proton transfer from ammonia leading to the formation of NH2- ions, which in turn initiate further polymerization.4... 

Acrylamides represent still another interesting class of monomers.6 Their anionic polymerization may be initiated by strong bases, like, e.g., amides. The growing chain contains the unit —CH2—CH —CO—NH2 and intramolecular proton transfer competes efficiently with its carbanionic growth. Since the rearrangement... 

For condensation polymerization, it is necessary to have two functional groups on each monomer and to mix stoichiometric amounts of the reactants. In polyamide production, the starting materials first form nylon salt by proton transfer ... 

A similar proton transfer from a growing chain end unit to give an olefinic linkage was observed in the cationic polymerization of 2-tert-butyl-7-oxabicycto[2.2.1 ]-heptane, although the proton liberated did not initiate the polymerization and hence this process was actually a termination34 . 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

The existence of chain transfer in ionic polymerizations was first found in the system isobutene-BFj at room temperature when it was discovered that very small traces of water, tert-butanol, or acetic acid would, as co-catalysts, cause the transformation of large quantities of monomer to very low unsaturated polymers [2, 5]. It was assumed that the process involved proton transfer, and there is no cause to change this view ... 

However, one of the most common mechanisms is undoubtedly proton transfer but whereas in alkene polymerizations this reaction leaves a terminal double bond, in arylene polymerizations these are generally not found. Instead the terminal group is usually a substituted indane formed by an internal Friedel-Crafts alkylation [8, 21, 23], e.g., for a-methyl styrene ... 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

In view of the fact that proton transfer to monomer is the most general and effective alternative to propagation in chemically initiated bimolecular polymerizations, it seems sensible to include here the reactions (5.V). For m < mc we need to include the normal bimolecular process,... 

Figure 8.6 Schematic representation of the newly developed proton-transfer polymerization as a route to hyperbranched polymers [27]...

Since formation of EGBs from amides, in all cases, is the result of direct reduction and H2 formation (and has to be done ex situ), the monomeric as well as the polymeric EGBs are recovered as the PB. Their reactions as bases have to be driven either by a thermodynamically favored proton transfer reaction or by a fast follow-up reaction of the depro-tonated substrate, which - particularly for (33) -is difficult, since (33) is a good nucleophile. 

Superoxide anion formed in situ in a solution exposed to air (i.e. with only a small concentration of O2) has been used as an EGB to generate nitroalkane anions that may add to activated alkenes or to carbonyl compounds [130, 131]. An example is shown in Scheme 33. The reaction is catalytic since the product anion can act as a base toward the nitroalkane. Using the nitroalkane as the solvent favors the proton transfer pathway over the competing addition of the product anion to a second molecule of activated alkene, a pathway that may lead to polymerization [130]. In some cases, better yields of the Michael addition product were obtained if a stoichiometric amount of the anion was formed ex situ (with O2 as the PB), and the activated alkene added subsequently ]130, 132]. 

Although this mechanism is an oversimplification, it does give the basic idea. Chain termination is more complicated than in free radical polymerization. Coupling and disproportionation are not possible since two negative ions cannot easily come together. Termination may result from a proton transfer from a solvent or weak acid, such as water, sometimes present in just trace amounts. 

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