Vinyl monomers, polymerization proton transfer

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The latter mechanism is met in amine-vinyl monomer systems [41-46] (see Scheme 4). Due to the small n-acceptor ability of normal substituted vinyl monomers, an interaction in the ground-state level does not take place. The exciplexes assumed are detectable in aromatic amine-acrylonitrile (AN) systems by their emission spectra, as is shown in Fig. 1 for typical examples. The emission bands at 350 nm (by JV,JV-dimethyl-p-toluidine (DMT)) and 370 nm (by p-phenylene diamine (TMPD) result from the normal fluorescence of the isolated amine. As can be seen, the intensity of the exciplex emission is much higher in the DMT-AN system. This corresponds to the higher polymerization efficiency of that system (<)>[, by A. = 313 nm and 80 K 0.6 for DMT 0.15 for TMPD [46]). Mainly, the much higher dipole moment of DMT (1.1 D) is responsible for this result. The cation radicals [46] or neutral radicals [42] of the amines formed after PET and proton transfer have been detected by ESR measurements. As expected, the rate of photopolymerization of the systems discussed increases with increasing... 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

Proton transfer to monomer is the best known and most widely studied transfer reaction in the cationic polymerization of vinyl monomers. Cationic polymerization of styrene and isobutylene belong to the comprehensively studied examples More recently, it has been shown by H-NMR that there are two kinds of double bonds formed as result of transfer in the polymerization of isobutylene ... 

Chain transfer to vinyl monomer involves transfer of a 8-proton from the carbo-cation to a monomer molecule. Considering, for example, the polymerization of isobutylene, the chain transfer to monomer can be represented by the equation... 

Proton transfer to monomer in the polymerization of a-methyl styrene can be suppressed by the presence of 2,6-di-t-butyl pyridine. This scavenges all the protons and effectively converts the reaction from a transfer to a termination reaction. The copolymerization of crmethyl styrene with isobutyl vinyl ether - has shown how the relative reactivities of carbenium and carboxonium ions affect conversion and molecular weights. Other interesting studies include the polymerization of substituted cs-methyl styrenes - and the stereospecific polymerization of anethole. ... 

The reversibility of proton transfer limits to some extent the nature of the monomers which are subject to photopolymerization with these initiator systems, but epoxides, trioxane and vinyl ethers work very well (12,14). The sensitization of cationic polymerizations photoinitiated by compounds II and III has been reported by Crivello and Lee (13). 

Polar vinyl monomers that contain labile hydrogen such as (meth)acrylic acid, hydroxyethyl methacrylate, and (meth)acrylamide cannot be used directly for anionic polymerization as they can act as a terminator via proton transfer to the reactive anions. They can be subjected to anionic polymerization only after appropriate protection of these functional groups into nonreactive groups toward anions, for example, by esterification or silylation. " A detailed list of protected monomers is given in various reviews. 

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