Other Cationic Polymerizations Heterocyclic Monomers

The most commonly used catalyst for the commercial polymerization of tetrahydrofuran is fluorosulfuric acid as shown in Eq. (2.60) (Dreyfuss et al., 1989). 

The mechanism of this cationic polymerization is quite different from the polymerization of isobutene (Eqs. (2.43)-(2.46)) in that the growing chain end is an oxonium ion intermediate in which the positive charge is located on oxygen 

Another interesting aspect of this polymerization is the observation that the covalent ester is in equilibrium with the oxonium ion (Eq. (2.63)) and that both of these species can participate in propagation by reaction with monomer (Matyjaszewski et al., 1975). 

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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). 

The virtues of photoinitiated cationic polymerization are rapid polymerization without oxygen inhibition, minimal sensitivity to water, and the ability to polymerize vinyl ethers, oxiranes (epoxides), and other heterocyclic monomers (see Table 10.7) that do not polymerize by a free radical mechanism. 

Cationic and anionic polymerizations of heterocyclic monomers provide many examples in which the concurrent formation of cyclics of various sizes is observed during the ring-opening polymerization. As illustrated in Scheme 1, in these systems active species follow three pathways they can react with a functional group of the monomer, of its own polymer chain, or of other chains. When the function / involved belongs to a linear polymer chain, intramolecular chain saambling or intermolecular macrocycle formation takes place, as observed in the cationic polymerization of cyclic ethers, acetals, esters, amides, siloxanes, and so forth. 

In the cationic polymerization of THF, the concentration of active species was measured by capping the growing chain with sodium phenoxide and the determination of phenoxy groups in polymers by UV spectroscopy. More general methods have been developed in the Lodz group. Active species of cationic polymerization of cyclic ethers (and other heterocyclic monomers) were trapped by reaction with tertiary phosphine. It was shown that oxonium ions are fast and irreversibly converted to corresponding phosphonium ions that could be quantitatively analyzed by NMR using a known excess of phosphine as an internal standard without the need for polymer isolation. The principle of the method, which allows determination not only of concentration but also of the structure of active species, is outlined in Scheme 12. 

Ionic Polymerization Ionic chain polymerizations can also be initiated with the aid of Vis/UV light although, as in the case of free-radical photopolymerization, the light serves only as an initiating tool. Most studies on ionic photopolymerization have focused on cationic polymerization, which proceeds rapidly and is not inhibited by oxygen [18,21-24]. Moreover, cationic photopolymerization is apt to polymerize monomers such as vinyl ethers, oxiranes (epoxides), and other heterocyclic compounds that do not polymerize via a free-radical mechanism (Table 3.9). 

Cationic ring-opening polymerizations are also nucleophilic substitution reactions e.g. equation (7). Two experiments are frequently employed to follow the cationic polymerization of heterocyclic monomers. The first is based on the polymerization of a low molar ratio of monomer and initiator directly in an NMR sample tube. This experiment requires a low rate of polymerization and allows direct identification of all growing species and determination of the rate constants of all individual reaction steps by HNMR spectroscopy. Cyclic imino ethers were the first to be studied by this method. Subsequently, cationic polymerization of other heterocyclic monomers has been investigated." ... 

These are much slower than to the preceding group of monomers, evidently because of the lower reactivity of oxonium, sulphonium, ammonium, phos-phonium and siloxonium, ions. Moreover, monomers with these heteroatoms are strongly basic, and therefore cations are preferentially solvated by the monomers. This reduces the probability of other kinds of transfer to solvent, impurities, etc. Many heterocycles, e. g. A-substituted aziridines, thiethanes [62], tetrahydrofuran [63], under suitable conditions polymerize by a living mechanism, i. e. without transfer. In situations where transfer does occur, it is assumed to proceed by the mechanism disscussed previously, for example by transfer to the counter-ion. With regard to transfer intensity, vinyl ethers can be ordered between the hydrocarbon monomers and the heterocycles. The mechanism of transfer in their polymerization has yet to be studied. 

By far the most commonly exploited polymerization of heterocycles is the oxidative pol5rmerization, which can be carried out using chemical or electrochemical oxidation conditions. Chemical oxidative polymerization is advantageous in that the reactions are fast and simple, using relatively mild conditions (94), and polymers could presumably be mass-produced at a reasonable cost (95). Oxidation potentials depend upon the electron density of the monomers the more electron-rich a monomer is, the easier it is to oxidize. The oxidative polymerization of thiophene is shown in Figure 3 this mechanism is equally applicable to other heterocycles. The mechanism is thought to involve a one-electron oxidation of the monomer to form a resonance-stabilized radical cation. This can couple with a molecule of starting material to form a radical cation dimer, which loses another electron to form the dicationic dimer, or the radical cation can couple with another radical cation to form a dicationic dimer. The dicationic dimer then loses two protons to form the neutral dimer, and the entire process is repeated to form poisoner. The fimdamental polsonerization mechanism is the same for both chemical and... 

Figures shows the potential range where some heterocycles polymerizeThe cathodic cutoff for the polymerization (around 1.2 V) occurs when the stability of the radical cation is enhanced (intrinsically or via a substituent). When becomes greater than kp - - k ([S] + PC ]) diffusion of R+ from the electrode results in the production of soluble products. The anodic cutoff (around 2.1 V) occurs when k,([S] -t- [X ]) > (kp -t- k ). Then R" becomes unstable and reacts with the solvent or anions. Between around 1.2 and 2.1 V good conditions for the electropolymerization of such monomers exists where kp > k q- k ([S] -f- [X ]). The influence of substituents in pyrroles, thiophenes, indoles, azulenes, fluorenes, and pyrenes on whether electropolymerization of the monomers or other reactions can occur has been discussed in detail including consideration of electronic or steric effects...

Besides the oxiranes, the respective four-membered heterocyclic oxetanes have been studied as monomers in ROPs. Vandenberg et al obtained a linear and highly crystalline polymer from oxetanes and other authors detailed the synthesis of hyperbranched polyethers from hydroxyl-functional oxetanes.Mostly cationic initiators have been used in the ROP of oxetanes, primarily because of the higher basicity compared to three-membered oxiranes, which are prevalently polymerized by anionic techniques. 

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