Cation mechanism

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

Homopolymer. Formaldehyde polymerises by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerisation. Anionic polymerisation, which can be initiated by relatively weak bases (eg, pyridine), can be represented by the following equations Initiation... 

Methacrylate monomers do not generally polymerize by a cationic mechanism. In fact, methacrylate functionaUty is often utilized as a passive pendent group for cationicaHy polymerizable monomers. Methacrylate monomers also have been used as solvents or cosolvents for cationic polymerizations (90,91). 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

Styrene—butadiene block copolymers are made with anionic chain carriers, and low molecular weight PS is made by a cationic mechanism (110). Analytical standards are available for PS prepared by all four mechanisms (see Initiators). 

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

Several mechanisms for the polymerization of vinyl ether and epoxies have been suggested [20,22,23,25,27,28,33-35]. On irradiation with gamma rays or electrons, pure epoxies polymerize via a cationic mechanism [35]. However, this cationic polymerization is inhibited by just traces of moisture, as shown below for cyclohexene oxide in reaction 5. 

Since polymerization of epoxies proceeds in the presence of onium salts [17,33,34,29], it is most likely that the above-mentioned cationic mechanism proceeds in their presence, and that the onium salts prevent water from inhibiting the cationic polymerization. 

Lapin [33] also suggests the following cationic mechanism for the polymerization of vinyl ethers... 

The spontaneous polymerization of furan adsorbed on carbon black with or without SnCl4 vapours35 has been explained by a similar cationic mechanism. Also, the polymerization of gaseous furan on liquid acidic surfaces35 has the same origin, but in these systems the polymers suffer an acid-catalyzed hydrolysis of their tetrahydrofuran rings which produces a considerable proportion of hydroxyl and carbonyl groups. 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

The lower yield for olefin than for S02 was explained by scavenging of the formed free olefin on the cationic sites of the irradiated polymer in a homopolymerization reaction, thus reducing G(olefin). Adding cation scavengers it was found that the overall product yield was reduced with concurrent reduction of the S02/olefm ratio towards unity73. Thus it can be concluded that the homopolymerization of the olefin is occurring by a cationic mechanism. 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

An interesting feature of the ring opening polymerization of siloxanes is their ability to proceed via either anionic or cationic mechanisms depending on the type of the catalyst employed. In the anionic polymerization alkali metal hydroxides, quaternary ammonium (I NOH) and phosphonium (R POH) bases and siloxanolates (Si—Oe M ) are the most widely used catalysts 1,2-4). They are usually employed at a level of 10 2 to KT4 weight percent depending on their activities and the reaction conditions. The activity of alkali metal hydroxides and siloxanolates decrease in the following order 76 79,126). 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Polymerisation of vinyl monomers with electron-donating substituents can proceed by a cationic mechanism, while monomers with electron-withdrawing group can polymerise by an anionic pathway. 

Additional reports of multifunctional kalihinols implicate a polar and stepwise biosynthetic cationic mechanism [21, 45]. 

The G values are higher than the G values for initial species (ca 2.5), indicating a chain reaction. The much higher yield for the disappearance of a double bond indicates that cyclization occurs more frequently than cross-linking. Katzer and Heusinger concluded from the studies on the influence of dose rate, temperature and additives (air, anthracene and hydroquinone) that the chain proceeds via a cationic mechanism. 

Oxidative Alkoxylation of Nitrones to a-Alkoxy Nitrones and a-Alkoxy Substituted Nitroxyl Radicals The first direct experimental evidence of the possibility to carry out radical cation nucleophilic addition to nitrones with the formation of nitroxyl radicals has been cited in Section 2.4. Further, such a reaction route was referred to as inverted spin trapping this route is an alternative to a conventional spin trapping (508-512). Realization of either mechanism depends on the reaction conditions namely, on the strength of both nucleophile and oxidant. The use of strong oxidants in weak nucleophilic media tends to favour the radical cation mechanism... 

Under similar reaction, the action of XeF2 on PBN and DMPO led to similar spin adducts, proving the radical cation mechanism (524). Unlike this reaction, the formation of fluoro-containing spin adducts when using a weaker oxidant such as /V-lluorodibenzsulfonamide [(PhSC>2)2N-F] is believed to involve the Forrester-Hepburn mechanism (524). 

Typical monomers which polymerise through cationic mechanisms are isobutene, styrene, a-methylstyrene, vinyl ethers and vinyl carbazoles. At present, about 100 vinyl monomers are known that can be polymerised by the cationic initiators. 

There remains, of course, the question why apparently isobutene (and perhaps other aliphatic olefins) do not polymerise by the pseudo-cationic mechanism - or do so much less readily than, say, styrene. Probably the short answer lies in the relative stabilities of the esters in the polymerisation conditions, (e.g., perchlorate stabilised by co-ordination of styrene). The long answer will have to be based on a detailed understanding of all the factors which determine this stability and thus govern the equilibrium between ester and ions. 

From the fundamental reaction-mechanistic point of view, the essential difference between the cationic and the pseudo-cationic mechanisms is this the attack of an ion on the double bond of an alkene to form a carbenium ion generally involves a 3-centred transition state in... 

Extended studies later showed that the radical cation mechanism of equations (6) and (7) is prevalent in strongly oxidizing systems (Eberson, 1992 Eberson and Nilsson, 1993), especially under photolytic conditions where excited states are formed (Eberson, 1994). The latter normally can act as strong oxidants or reductants, as in equation (11), and thus can create radical ions under seemingly mild conditions. Since this type of mechanism was judged to be much more common than thought, the name inverted spin trapping was coined for it in view of the inverted situation of electron demand which appears when an electron is formally transferred between the spin trap and the nucleophile or electrophile see equations (13)-(16). 

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