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followed by reaction with monomer:
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+ AlBr2 (AlBr4) + M
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AlBr2M +(AlBr4)
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The Lewis acid acts as both initiator and coinitiator. The second mechanism involves the addition of Lewis acid to monomer followed by reaction of the adduct with another molecule of Lewis acid:
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BF3 + M
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Most of the evidence to support the self-ionization process is indirect, consisting of kinetic, conductance, and spectrophotometric data for polymerization at different levels of dryness and purity. One concludes that self-ionization occurs if polymerization is achieved in systems subjected to the most stringent puri cation and drying procedures. The major problem in ascertaining whether self-initiation occurs and, if it does, its extent relative to the initiation process involving a coinitiator is the large effect exerted by small amount of protogens or cationogens. Thus water concentrations of 10 3 M are suf cient to increase the initiation rate by a factor of 103 for TiCl4 and AlCl3 in CH2 Cl2 [Masure et al., 1978, 1980; Sauvet et al., 1978]. Polymerizations in the presence of sterically hindered pyridines such as 2,6-di-t-butylpyridine and 2,4,6-tri-t-butylpyridine offer further evidence for the presence of a self-ionization initiation process [Gandini and Martinez, 1988]. Sterically hindered pyridines (SHP) are generally active proton scavengers but do not react with Lewis acids or carbocations; in other words, steric hindrance cuts down on the reactivity of the nitrogen toward electrophilic species larger than protons. The presence of SHP results in complete inhibition of polymerization in some systems but only lowered reaction rates in other systems. For the latter, one observes a continuous decrease in polymerization rate with increasing concentration of SHP up to some critical SHP concentration. Thereafter, there is a residual (selfinitiation) polymerization rate that is unaffected by increasing SHP concentration. The
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CATIONIC POLYMERIZATION OF THE CARBON CARBON DOUBLE BOND
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SHP method is not completely without ambiguity since SHP may not scavenge protons ef ciently in a heterogeneous reaction system or in systems where protons never appear as such but are transferred in a concerted process. The strongest evidence to support self-ionization is the detection of one boron end group per polymer molecule in the BI3 polymerization of a-methylstyrene in the presence of SHP [Koroskenyi et al., 1997]. When self-ionization occurs, its contribution to the overall initiation process is generally small in the presence of a protogen or cationogen. For many polymerizations, the moisture content (and/or level of other protogen or cationgen) is often suf cient so that self-ionization constitutes only a minor proportion of the total initiation process. (Conventional dry box conditions usually do not involve moisture levels lower than 10 3 M [Kennedy, 1976].) The activity of an initiator coinitiator complex is dependent on its ability to donate a proton or carbocation to the monomer, which, in turn, depends on the initiator, coinitiator, solvent, and monomer. The extent of formation of the initiator coinitiator complex (i.e., the value of K in Eq. 5-3c) and its rate of addition to monomer (i.e., the value of ki in Eq. 5-4c) generally increase with increasing acidity of the Lewis acid coinitiator. Lewis acidity for different metals generally increases with increasing atomic number in each group (vertical row of periodic table): Ti > Al > B; Sn > Si; Sb > As [Matyjaszewski and Pugh, 1996]. For any metal, Lewis acidity increases with increasing oxidation state, for example, TiCl4 > TiCl2 . Ligands increase Lewis acidity in the order: F > Cl > Br > I > RO > RCOO > R, Ar. The strongest Lewis acids (e.g., SbF5 ) are not always the most useful since the result may be excessively fast and uncontrolled polymerization or the reverse low rates due to the formation of excessively stable and inactive complexes between the Lewis acid and some other component of the reaction system. The activity of the initiator coinitiator complex also increases with increasing acidity of the initiator, for example, hydrogen chloride > acetic acid > nitroethane > phenol > water > methanol > acetone in the polymerization of isobutylene with tin(IV) chloride [Kennedy, 1968; Plesch, 1963]. A word of caution regarding these generalizations the order of activity of a series of initiators or coinitiators may differ depending on the identity of the other component, monomer, solvent, or the presence of competing reactions. For example, the activity of boron halides in isobutylene polymerization, BF3 > BCl3 > BBr3, with water as the initiator is the opposite of their acidities. Hydrolysis of the boron halides to inactive products, increasing in the order BBr3 > BCl3 > BF3, is responsible for the observed polymerization results [Kennedy et al., 1977, Kennedy and Feinberg, 1978]. The reactivity of organic halide cationogens in initiation depends on carbocation stability in a complex manner. Increased carbocation stability results in the formation of higher concentrations of carbocations from the cationogen but the carbocations have lower reactivity. Differences in the stability of the carbocation formed from the cationogen compared to the propagating carbocation are also important in determining the effectiveness of a cationogen. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization. Primary and secondary carbocations are formed too slowly and/or in extremely low concentrations. (There are a few reports of initiation by primary or secondary halides [Toman et al., 1989a,b], but initiation more likely involves self-ionization of the Lewis acid or the presence of adventitious water.) Tertiary carbocations such as t-butyl and 2-phenyl-isopropyl (cumyl) are suf ciently stable to form but are not more stable than the carbocations derived from their additions to monomers such as isobutylene, styrene, or N-vinylcarbazole, so that polymerizations of those monomers occur. Cumyl and t-butyl carbocations have been generated from cumyl and t-butyl esters and ethers as well as from the halides [Faust and Kennedy, 1987; Mishra and Kennedy, 1987]. Highly stable carbocations such as trityl, f3 C and cycloheptatrienyl (tropylium), C7 H are generally too 7
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