Ionic polymerization vinyl ether

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

For the polymerization of alkyl vinyl ethers there is no direct evidence for or against the chain-carriers being ionic. Some aspects of this question are discussed in sub-section 4.4. 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

Ionic initiators are much more specific toward the type of monomer and can initiate polymerization in some cases where radical initiation is ineffective (cyclic, vinyl ether, or allylic monomers). Monomers suitable for anionic initiation are those with an electron-withdrawing substituent attached to the double bond (phenyl, carbonyl, etc.). The reaction consists... 

The evidence in the case of styrene, where both modes of radiation-induced polymerization can be conveniently studied, is quite convincing that reduction of the concentration of water changes the predominating mode of propagation from purely free radical to essentially ionic. Evidence for an ionic propagation initiated by radiation has also been obtained in pure a-methylstyrene (3, 24), isobutylene (12, 32), cyclopenta-diene (5), / -pinene (2), 1,2-cyclohexene oxide (II), isobutyl vinyl ether (6), and nitroethylene (38), although the radical process in these monomers is extremely difficult, if not impossible, to study. 

The existence of centres with non-ionic character has already been suspected in studies of polymerizations which are supposed to proceed on carbocat-ions the theory of pseudo-cationic polymerization was proposed [137] (see Chap. 3, Sect. 3.1). The transformation of an ion pair to a covalent compound will evidently be easier for acid centres with heteroatoms, i.e. in heterocycle or vinyl ether polymerizations. Propagation on covalent bonds has actually been observed, first in the studies of oxazoline polymerization [138] and later even with THF [139, 140] and with other monomers (see, for example, refs. 131, 141 and 142). 

Because the reactivities of ions and ion pairs are similar and only weakly affected by the structure of the counteranions, kp + or kp determined by either stopped-flow studies or y-radiated systems (cf., Section IV. 13) can be used in Eq. (75). The equilibrium constant of ionization can then be estimated from the apparent rate constant of propagation and the rate constant of propagation by carbenium ions [Eq. (77)]. For example, Kf 10-s mol-,L in styrene polymerizations initiated by R-Cl/SnCl4 [148]. Kt for vinyl ether polymerization catalyzed by Lewis acids can also be estimated by using the available rate constant of ionic propagation (kp- = 104 mol Lsec-1 at 0° C) [217], The kinetic data in Ref. 258 yields Kj == 10 3 mol - l L in IBVE polymerizations initiated by HI/I2 in toluene at 0° C and Kf 10-1 mol- -L initiated by HI/ZnI2/acetone can be calculated from Eq. (76). 

These salts suppress the free ionic (dissociated) growing species and accelerate the conversion of carbocations to covalent species (see Sections IV.B.3 and IV.B.3). The salt-mediated living polymerizations of vinyl ethers in polar media also provide useful systems in which polar monomers and their polymers that can be insoluble in nonpolar solvents can be polymerized successfully. 

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

Combination of a living ionic polymerization and a metal-catalyzed radical polymerization also leads to comb polymers, where both the molecular weights of the arm and main-chain polymers are well controlled. PMMA with poly(vinyl ether) arm polymers of controlled molecular weights (C-l) were prepared by the copper-catalyzed radical polymerization of methacrylate-capped macromonomers carrying a poly-(isobutyl vinyl ether), which were obtained by living cationic polymerization with a methacryloxy-capped end-functionalized initiator.428 Comb polymers with... 

It is believed that the free positive and negative species annihilate each other immediately on contact. This is borne out by the strict square root relationship which is found between the rates of polymerization and the dose rate of the radiation. The growing chain ends are therefore free in nature, i.e., with no ion-pair component. This makes radiation initiated ionic polymerization an excellent method for studying free ion polymerization examples of the power of this method have been presented for p-methoxy styrene (1 9) and the vinyl ethers (16,20,21). 

Examples of vinyl monomers for addition polymerization include acrylates, methacrylates, vinyl ethers and styrene derivatives. Radical, ionic, and group-transfer polymerizations are possible according to polymerizabil-ity of the monomers. Living polymerization is difficult because mesogenic monomers often contain bonds such as benzoate ester, which are easily attacked by growing ends. Cyclic and condensation monomers are less... 

The model is based on the proposition that all carbocationic polymerizations of olefins, alkyl vinyl ethers, etc., involve a spectrum of species with differing ionicitieS connected by equilibria formally expressed by the Winstein ionicity spectrum 1] the Winstein spectrum starts with a covalent species and progresses through increasingly polarized and ionized species, to fully ionized solvated ( free ) ion pairs ... 

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