Carbocations polymerization initiators

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Polymerizations initiated by ionizing radiation or stable carbocation salts such as trityl or tropylium hexachloroantimonate are useful for evaluating the free-ion propagation rate constant. Ionizing radiation yields free ions (in the absence of ion pairs) whose concentrations... 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

G. Sauvet and P. Sigwalt, Carbocation polymerization general aspects and initiation, in Comprehensive polymer science, Vol. 3, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds., Pergamon, Oxford, 1989, ISBN 0080325157, Chapter 39. 

For obtaining a cationic polymerization, the new carbocation generated between R and the monomer should have enough stability to be relatively easily formed and to continue the polymerization (for example, CH2=CH2 is not polymerized using a cationic initiator, while (CH3)2C=CH2 can be polymerized because the species RCH2-C(CH3)2 is stable enough to be formed). The stability of the carbocation increases as the chain length increases. Chain transfer reactions are common in carbocation polymerization. The termination reactions typically occur because of the combination of the cationic component with a counterion. 

Because it is the proton or carbocation that initiates the polymerization reaction, the compounds that give rise to them are correctly referred to as the initiators, and the Lewis acids as coinitiators [11] (not the other way around, as is commonly done in the polymer literature). The combination of Lewis acid and proton or cation source is the initiating system. The initiation steps described above can thus be generalized as... 

Figure 8.6 illustrates, for example, the complicated equilibria [19] in initiation by alkyl halides which are widely used as initiators in combination with coinitiator such as aluminum alkyl halides or aluminum halide Lewis acids. Each carbocation can initiate polymerization or remove an alkyl (ethyl) group from the counterion to produce a saturated hydrocarbon, REt, and a more acidic Lewis acid. The propagating cation can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the coinitiator used is diethylaluminum chloride there may be major contributions to the polymerization from ethylaluminum dichloride or aluminum chloride. 

Similarly if Kd is less than 0.1, the system consists of more than 90% free ions and less than 10% less reactive ion pairs see Problem 8.28) and hence one can safely equate fc with k. Such conditions may be obtained in polymerizations initiated by stable carbocation salts such as hexachloroantimonate (SbCl ) salts of triphenyl methyl [(C6H5)3C" "] and cycloheptatrienyl (CtH ) carbocations, which are thus useful for evaluating the free-ion propagation rate constant [24]. However, since these cations are stable, their use is limited to the initiation of the more reactive monomers like N-vinylcarbazole and aUcyl vinyl ethers. The dissociation constant Kd)... 

In some special cases, a Brpnsted acid does not need to be the only initiating species. If heterolytical cleavage of one of the alkyl groups results with a stable carbocation, polymerization can possibly be initiated by this intermediate structure (Scheme 11.5). 

Indeed this reaction can be viewed as a model for the initiation and termination reactions for the carbocation polymerizations induced by alkylaluminum/coinitiator systems. In the absence of monomer, alkylation of the carbocation occurs exclusively however, in the presence of a suitable polymerizable olefin this nucleophile successfully competes for the cation and initiation ensues. In this latter manner and depending on the reaction conditions (structure of the monomer and its... 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

The open transition state—-hybridized carbocations— has low stereoselectivity, thus mostly atactic polymers are formed (94). At the same time, reportedly the very first synthetic stereoregular polymer was an isotactic poly(isobutyl vinyl ether) prepared by carbocationic polymerization initiated with BF3/0(C2115)2 at -78°C (95). Other examples of stereoregular crystalline polymers synthesized by homogeneous and heterogeneous carbocationic polymerization are polymers of vinyl ether derivatives, and 2,5-dimethyl styrene or a-methylstyrene (95-100). Stereoregular vinyl ether polymers have also been synthesized using... 

Vinyl Ethers and Vinyl Carbazoles.—With few exceptions, most recent kinetic studies of the cationic polymerizations of vinyl ethers and vinyl carbazoles have been conducted with the use of stable carbocation salt initiators, which facilitates analysis of the data. 

In cationic polymerization, the initiator is an electrophile (generally a proton) that adds to the monomer, causing it to become a carbocation. The initiator cannot be an acid such as HCl because d will be able to react with the carbocation. Thus, the initiator most often used in cationic polymerization is a Lewis acid, such as BF3, together with a proton-donating Lewis base, such as water. Notice that the reaction follows the rule that governs electrophilic addition reactions—that is, the electrophile (the initiator) adds to the sp carbon bonded to the most hydrogens (Section 6.4). 

As will be shown later in this review, there are many other differences between these two classes of cationic initiators. For example, polymerizations with an MX -type counteranion proceed quantitatively, unless the cocatalyst employed is consumed by some reaction the MX initiator is not depleted. Polymerizations with a non-MX counteranion, on the other hand, terminate before the entire available monomer has been polymerized initiators of this type, except for superacids (strong protonic acids), are usually consumed during the reaction. The termination process is considered to be the combination of the propagating carbocation with its counteranion, as exemplified in Eq. (3) for the polymerization by hydrogen chloride ... 

When a counteranion is not so nucleophilic as the iodide anion, the propagating carbocation may be stabilized instead by adding an base (Z) so that living polymerization proceeds (Eq. 9 see also section 1.1) (4). This method is particularly effective for the polymerization initiated with ethylaluminum dichloride (EtAlCl2) (13) and typically, the bases may be 1,4-dioxane and related ethers that form a "base-stabilized" carbocationic species like where Z is an ether oxygen. We have recently synthesized end-functionalized polymers via these base-stabilized living species (14). 

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. 

Cationic polymerizations work better when the monomers possess an electron-donating group that stabilizes the intermediate carbocation. For example, isobutylene produces a stable carbocation, and usually copolymerizes with a small amount of isoprene using cationic initiators. The product polymer is a synthetic rubber widely used for tire inner tubes ... 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

---