Initiation, of cationic polymerizations

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

This reaction corresponds to the basic process during the initiation of cationic polymerizations by RX/MtXn and when reversed is the termination reaction. It will be handled more in detail in part 4.2. When X = H, the reaction enthalpy of the previous equation is equal to the hydride ion affinity (HIA) which is shown for various relevant... 

The possibility of ion formation during the interaction between two Lewis acid molecules as shown in the scheme above is important for the initiation of cationic polymerizations in the absence of cation forming additives (e.g. HX or RX)1). When aluminum-halides A1X3 (X = Cl, Br) are concerned, the ion formation in solution could be experimentally proven163). The formation of ionic species in pure SbCl5/ SbFj system has already been pointed out. 

Crivello, J. V., Lockhart, T. P., and Lee, J. L., Diaryl-iodonium salts as thermal initiators of cationic polymerization,... 

In order to assess reactivity in this context we need first to recognise that the chemical initiation of cationic polymerizations can occur in different ways. Disregarding the rarer initiations involving electron abstraction or hydride abstraction from a monomer, the most common are... 

Cationic Polymerization. The functional groups that enable initiation of cationic polymerization can be introduced on the inorganic surface. The introductions of acylium perchlorate Reaction (5), sulfonium or pyridinium salt, or active chloride... 

The generation of a protonic acid (HX) is responsible for the initiation of cationic polymerization. Various monomers are polymerized by sulfonium photoinitiators. Especially this system has been shown to be potentially useful for UV curable coatings of metal and plastics with epoxy resins. 

The photolysis of dialkylphenacylsulfonium salts 3 generates an ylid 5 and a strong acid HX (Eq. (13)), the latter being responsible for the initiation of cationic polymerization 6). 

Poly (iso butylene) glycol is a suitable rubbery segment in thermoplastic elastomers. An oligomer such as 56 with a tertiary chloro group at both ends could be employed also as the initiator of cationic polymerization of a-methylstyrene (a-MeSt) in the synthesis of a three-block copolymer of poly(a MeSt)-polyisobutylene-poly(a-MeSt)52). 

Various mechanisms for the initiation of cationic polymerization are feasible. Photosensitizer radical cations (PS+ ) may react directly with the monomer M ... 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

The initiators of cationic polymerizations must produce sufficiently reactive cations which are able to yield active centres with the monomer. It is therefore useful to discuss each of the two main initiator types separately. 

All the enumerated examples indicate the insufficiency of reactions (59)-(64) to explain completely the initiation of cationic polymerizations. An inseparable aspect of initiation is the activation of the primary products produced by ionization or dissociation. Several kinds of ion pair of various reactivities are known to exist. The solvate envelope of free ions must affect the frequency of active ion-monomer collisions, i. e. the initiation rate. In the author s opinion, the key to our understanding of some co-initiation effects in cationic polymerization is a suitable interpretation of the Winstein dissociation scheme [247]... 

Another novel class of silicon-based initiators has been described [44]. Compounds containing Si—H bond in the presence of platinum catalysts (Ptl2, H->PtBr6, PtCl2(C6HsCN)2) are effective initiators of cationic polymerization of cyclic ethers. 

Recently, Ledwith described combined systemscomposed of a radical initiator and a cationic photoinitiator, which are very suitable for hybrid systems. It is particularly convenient to employ common photochemical sources of free radicals for this purpose. Since many of them possess aromatic carbonyl groups (e.g. benzoin and acetophenone derivatives), these groups provide an extension of the absorption to longer wavelengths and promote the initiation of cationic polymerization. Figure 19 shows the proposed formation mechanism of free-radical and cationic active species by electron transfer ... 

The polymerization of tetrahydrofuran in the presence of methylenemalonitrile has already been described. Co-ordination of aluminium trichloride to the cyano groups would make methylenemalononitrile an even more effective initiator of cationic polymerization. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Irradiation of a donor monomer-electron acceptor charge transfer complex, e.g. N-vinylcarbazole-sodium chloroaurate (2), -nitrobenzene (2) or -p-chloroanil (3) and -p opiolactone-uranyl nitrate (h), results in the initiation of cationic polymerization. 

Thermally stable radical cation salts have also been used as electron-transfer initiators of cationic polymerizations. ... 

Initiation of cationic polymerization may be achieved by any proton-donating species, but the limitation in, for example, protonic acids is the nucleophilic nature of many of the counter-ions, e.g. halide, such that hydrogen halides are ineffective initiators since they produce only the addition product with vinyl compounds. Lewis acids such as BF3 and AICI3 are more widely used but require a proton donor, e.g. water, and it is this proton donor that is the actual initiator and the counter-ion is a product of the resulting reaction e.g. BF3 initiation of the polymerization of isobutylene (Odian, 1991) shown in Scheme 1.26. 

Free Radical Initiators as Reducing Agents for Dlaryllodonlua Salts. A final method by which dlaryllodonlum salts can be used as thermal Initiators of cationic polymerization has recently been reported by Ledwlth and his coworkers(14,15) and Is shown In Equations 17-19. 

Problem 8.20 Give plausible explanation for the following facts. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization of monomers such as isobutene and styrene, but t-butyl and cumyl chlorides are effective. On the other hand, triphenylmethyl chloride and cyclo-heptatrienyl (tropylium) chloride are not very efBcient in polymerizing isobutylene and styrene but produces rapid polymerization of p-methoxystyrene, vinyl ethers and N-vinylcarbazole. 

Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization, because primary and secondary carbocations are formed too slowly and/or in extremely low concentrations. However, tertiary carbocations such as t-butyl and cumyl (2-phenyl-isopropyl) are sufficiently stable to form but are not more stable than the carbocations derived from their addition to monomers such as isobutene, styrene, or N-vinylcarbazole, so that polymerizations of these monomers occur. 

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