Cationic Polymerization of Vinyl Monomers

Cationic polymerization of vinyl monomers is by electrophilic addition of the monomer to a growing carbenium ion. The reactivity of vinyl monomers in cati- 

In cationic polymerization of alkenes, there is an equilibrium between active ions or ion pairs and inactive covalent species, where the ionization constant is rather low (10 in the isobutene/BCI3 system). The dynamics of this equilib- 

Bimodal distributions may result, if free ions and ion pairs have the same reactivity but different lifetimes [218]. 

However, transfer reactions are nearly unavoidable side reactions in cationic polymerization and may involve yS-proton transfer even to rather weak bases like the monomer itself, to transfer agents and solvent as well as Friedel-Crafts alkylation of aromatic rings, if monomers like styrene are polymerized. Proton transfer yields unsaturated chains [Eq. (58)], with the double bond in either the endo or exo position. These may lead to branched polymers, if the double bonds are accessible to homo- or copolymerization. 

As proton transfer usually has the higher activation energy, the molecular veight of the polymer decreases with temperature. Tabulated transfer coefficients may be taken from Refs. 219 and 220. 

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The latter, used to investigate the cationic polymerization of vinyl monomers, is the subject of the presented article. The central point is not to collect all quantum chemical calculations carried out but to interprete some of this calculations in a reaction theoretical manner. 

The initiation of the cationic polymerization of vinyl monomers by a Lewis acid MtXn in the absence or presence of a coinitiator can proceed in a number of ways depending on the concrete reaction conditions 1,2). 

Dunn, D. J., The Cationic Polymerization of Vinyl Monomers, Chap. 2 in Developments in Polymerization-1, R. N. Haward, ed., Applied Science, London, 1979. 

Carbon blacks have been reported to be capable of initiating the cationic polymerization of vinyl monomers such as vinyl ethers, indene, and acenaphthylene. The grafting sites of the polymer were based on carboxyl groups present on the surface [88]. The polymerization was inhibited by treatment of the carbon blacks with NaHCOs, CH2N2, pyridine, and DMF. Also, the degree of conversion was found to be dependent on temperature and time of polymerization [89]. 

Higashimura, T., and N. Kanoh Studies on monomer transfer and termination constants in the cationic polymerization of vinyl monomers catalyzed by iodine. Kobunshi Kagaku 23, 114 (1966). 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

The cationic polymerization of vinyl monomers such as isobutene, styrene, ot-methylstyrene, indene, and vinyl ethers is generally impeded by several transfer processes65 . The molecular weight of the polymer is not determined by the molar ratio of monomer to initiator. Therefore, the methods that are based on the long lifetime of the active sites at the chain ends cannot be applied here. 

Nevertheless, a few years ago, Kennedy 66 69) developed a method yielding co-functional polymers by cationic polymerization of vinyl monomers. The principle of the socalled inifer method is to kinetically favor transfer to the initiating species with respect to all other kinds of transfer reactions (especially the transfer to monomer). A typical initiating system is composed of an allyl or benzyl halide and boron trichloride BCl3. This mixture behaves like an alkenium tetrachloro-borate and readily initiates the polymerization of monomers such as isobutene or a-methylstyrene. The efficiency of the halide as a transfer agent depends on the lability of the C—Cl bond and on the molar ratio [RC1]/[BC13],... 

In 1982 Higashimura et al. [54] began studies focused on the development of living cationic polymerizations of vinyl monomers. They decided to use IBVE and related alkyl vinyl ethers as monomers because they form the alkoxy-stabilized growing carbocations, along with iodine as the initia-... 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

For efficient cationic polymerization of vinyl monomers, it is necessary that the carbon-carbon double bond be the strongest nucleophile in the molecule. Thus vinyl acetate would be classed as an electron-donor-type monomer (Section 7.10.2) but it cannot be polymerized cationically because the carbonyl group complexes the active center. (It is polymerized only by free radicals anionic initiators attack the ester linkage.)... 

Cationic polymerizations of vinyl monomers differ from other chain-growth polymerizations particularly as follows ... 

We pause here to note that the steady-state a.ssumption that is so helpful in simplifying the analysis of free-radical kinetics (Section 6.3.4) will not apply to many cationic polymerizations of vinyl monomers, because propagation through free carbenium ions is so much faster than any of the other reactions in the kinetic chain. 

Proton transfer to monomer is the best known and most widely studied transfer reaction in the cationic polymerization of vinyl monomers. Cationic polymerization of styrene and isobutylene belong to the comprehensively studied examples More recently, it has been shown by H-NMR that there are two kinds of double bonds formed as result of transfer in the polymerization of isobutylene ... 

Recently, Higashimura [7] has reviewed the data on elementary rate coefficients (fej, fep, fet and fej) in cationic polymerization of vinyl monomers. Information available on initiation and termination reactions is extremely limited, and virtually no attempt [50] has been made to elucidate, either qualitatively or quantitatively the role of free ions and ion pairs in these processes. Numerical data on the separate contributions to propagation by free ions and ion pairs is slowly becoming available, though in a less ordered fashion than in the case of anionic systems. It seems likely that the most fruitful approach to the problem of absolute reactivity, in initiation processes at least, will be an examination of reactions of non-polymerizable monomer models, where electronic factors... 

Lewis Acids. The consensus of opinion now seems to be emerging that strong Lewis acids such as AICI3, TiCU, SnCU, ZrCU, and BF3 are capable of initiating cationic polymerization of vinyl monomers without the assistance of a cocatalyst capable of ionizing to form a proton or carbocation. However, the presence of such a co-initiator, as either an adventitious impurity or a purposefully added component, enhances the initiation process. The mechanism of initiation may well vary with each Lewis acid employed. 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers.A general scheme for the cationic polymerization of vinyl monomers having a cation stabilizing group (R) is shown in Scheme 9.4. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions are used as... 

For efBdent cationic polymerization of vinyl monomers it is necessary that the carbon-carbon double bond be the strongest nucleophile (electron donor) in the molecule. If more than one nucleophilic site exists in a monomer and the rr electron system of the double bond does not represent the most nucleophilic site, the other site(s) may complex with the electrophile (proton, cation, Lewis acid). Thus, for methyl methacrylate ... 

Following an interesting article on the influence of electric fields on the cationic polymerization of vinyl monomers by iodine in 1,2-dichloroethane solutions116, Giusti and coworkers published a basic research on the electrochemical activation of tetraalkylammonium iodide and triiodide in chlorinated organic solvents117, in the absence of the monomer. The electrodic reactions are ... 

A. Kikkawa, T. Takata, and T. Endo, Cationic polymerization of vinyl monomers with latent initiators. 3. l-(Para-methoxybenzyl)tetrahydrothiophenium hexafluoroantimonate as a new potent cationic initiator. Makromol. Chem. Macromol. Chem. Phys. 1991,192(3), 655-662. 

Cationic polymerization of vinyl monomers involves the highly reactive carbenium ions, which can only be formed when they are stabilized by the substituents on the vinyl group. Stabilization of the carbenium ion can be achieved by resonance structures, as is the case for vinyl ether and styrenic monomers, or by an inductive effect, which is the stabilizing mechanism for 1,1-disubstituted alkenes. Despite this stabilization, the reactivity of such carbenium ions is still very high, giving rise to the occurrence of side reactions, such as isomerization, hydride abstraction, as well as various transfer reactions. Moreover, carbocationic polymerizations are very sensitive to minor nucleophilic impurities. Nonetheless, in recent years a range of less sensitive polymerization initiators have been developed that can be used in aqueous environment [20]. 

Basic Principles of Cationic Polymerization of Vinyl Monomers... 

Scheme 2 Chain transfer as a side reaction in cationic polymerization of vinyl monomers...

Aoshima S, Higashimura T (1989) Living cationic polymerization of vinyl monomers by organoaluminum halides. Living polymerization of isobutyl vinyl ether by EtAlCl2 in the presence of ester additives. Macromolecules 22 1009-1013... 

The synthesis of branched polymers by cationic polymerization of vinyl monomers has been reviewed recently [219] therefore, these will be briefly considered here. Star-shaped or multiarm star (co) polymers can be prepared by three general methods ... 

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