Mechanisms, ionic initiators

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

Poly(bisphenol-A-carbonate) under pseudoideal reaction conditions was investigated, and the cyclic polycarbonate was obtained as the main product. In the system, the interface of the water/toluene mixture might have favored the cyclization reaction between the polar end groups [88]. Cyclic carbonates during the (Salen)CrCl catalyzed CCh/cyclohexene oxide copolymerization process in the presence of ionic initiators was also obtained [89]. The cyclic carbonate is produced via the backbiting mechanism, and the process is assumed to take place via a metal alkoxide (polymer chain) intermediate. Subsequent ring-opening of the cyclic carbonate with concomitant formation of polyether and CO2 was fast at the reaction temperatures from 80 to 100 °C). 

Indeed, it is not intended to discuss recent developments of conventional cationic polymerizations, i. e., polymerizations in which a cat-ionically initiable vinyl compound is attacked by a suitable (usually Friedel-Cr afts halide) catalyst and the growing ion is neutralized by the corresponding MeX type gegen-ion, etc. Rather, this review concerns unusual cationic polymerization systems and mechanisms which have not been discussed in a comprehensive manner. 

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

The most important representatives are the lowest 1-alkenes, ethylene and propene. Ethylene is not particularly easily polymerized by radical or ionic mechanisms. Its importance as a monomer was greatly enhanced by the discovery of coordination polymerizations. Propene is oligomerized by radical and ionic initiators. This explains the importance of Natta s modification [1] of Ziegler [2,3] catalysts, enabling inferior raw materials to yield high-quality polymers. 

The reaction seems to proceed by an ionic mechanism. An initial 1,4-addition can be proposed by analogy with the reaction of H+SnCl3, and this is followed by rearragement to the product of formal 1,2-addition . 

Both methods are very general. They apply to any polymer which undergoes radiolysis (i.e., "any" polymer) and the only limitation with respect to the monomer is that it polymerize by a free radical mechanism. Ionic grafting was also initiated by radiation (28,29) but the yields of this process are generally quite low. 

Plesch and collaborators have recently introduced this technique for the study of carbenium and oxonium ions and of Br nsted acid dissociation equilibria ° The ultimate aim of this work is of course to gain a better understanding of the mechanism of initiation in cationic polymerisation. For the moment however it is difficult to assess the real potential of polarography in that context and we feel that, althou it certainly possesses a hi sensitivity, the conditions required for meaningful measurements are still too distant from those employed in an actual polymerisation, in particular the high ionic strength of the medium. Its use is therefore still doubtful and one can only hope that more research will bridge the gap. 

Metal salts and complexes continue to attract interest as radical/ionic initiators. Trisoxalatoferrate/amine anion salts have been studied as initiators of the polymerization of acrylamide. Here the anion salts react with photolytically formed COa " radicals by an electron transfer mechanism to give photoactive initiating phenyl radicals by the set of reactions shown in Scheme 9. Ferric o-phenanthroline has been shown to be a good photoinitiator for... 

The polymerization of aldehydes is initiated by ionic initiators and the polymerization proceeds by ionic propagation. No radical polymerization of aldehydes has been documented yet. In the case of anionic polymerizations the growing ion is an alkoxide ion. The cationic polymerization has as the propagating species an oxonium ion. Most recent experimental results have shown that haloaldehydes, such as chloral polymerize exclusively by an anionic mechanism. 

To account for this peroxide effect, Kharasch and Mayo proposed that addition can take place by two entirely different mechanisms Markovnikov addition by the ionic mechanism that we have just discussed, and anti-Markovnikov addition by a free-radical mechanism. Peroxides initiate the free-radical reaction in their absence (or if an inhibitor, p. 189, is addedX addition follows the usual ionic path. 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

Recent ab initio calculations by Streitwieser imply that even LiCH3 is ionic Mechanism of initiation of polymerisation by alkyl lithiums in aromatic hydrocarbon is entirely different from that operating in aliphatic hydrocarbon, see p. 64... 

Initiation The mechanism of initiation of anionic polymerization of vinyl monomers with alkyllithium compounds and other organometallic compounds is complicated by association and cross-association phenomena in hydrocarbon solvents and by the presence of a variety of ionic species in polar media [3, 4, 45, 48, 55, 56]. The kinetics of initiation is complicated by competing propagation and the occurrence of cross-association of the alkyllithium initiator with the propagating organolithium [55]. Thus, only the initial rates provide reliable kinetic data. 

The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds via chain mechanism. Chain-reaction polymerization is induced by the addition of free-radical-forming reagents or by ionic initiators. Like all chain reactions, it involves three fundamental steps initiation, propagation, and termination. In addition, a fourth step called chain transfer may be involved. 

Ionic-initiated polymerizations arc much more complex than radical reactions. When the chain carrier is ionic, the reaction rates are rapid, difficult to reproduce, and yield high-molar-mass material at low temperatures by mechanisms that are often difficult to define. 

Acetylene and mono- and disubstituted acetylenes do not polymerize with conventional free-radical or ionic initiators to high molecular weight polymers in high yields. However, they polymerize via metathesis mechanism in the presence of suitable transition-metal catalysts such as Nb, Ta, Mo, W, or Ru. The polymers possess carbon-carbon alternating double bonds along the main chain and exhibit unique properties such as semimetallic or metallic conductivity, Scheme 15.1. 

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