Cationic mechanism heterocyclic monomers

Good evidence exists that cationic polymerization of heterocyclic monomers occurs with cyclic onium salts as the active species. Therefore it is reasonable to believe that also the degradation reactions occur via such ions, so that the most plausible reaction path is the classical back-biting mechanism ... 

We shall discuss separately the polymerization of heterocyclic monomers and vinylic monomers since the mechanism of the cationic polymerization is quite different for these two types of monomers. In contrast to some heterocyclic monomers olefinic and vinylic monomers do not produce living cationic species. Therefore, the methods for synthesizing macromonomers differ greatly. 

Heterocyclic compounds, which polymerize by cationic mechanism, contain one or more heteroaroms within a ring. Depending on the nature of heteroatoms and their arrangement, these monomers belong to the different classes of organic compounds. 

Cationic polymerization of heterocyclic monomers can proceed not only by the SjjZ mechanism involving onium ions located at the chain end, and analysed in this paper, but also by another Sj 2 mechanism involving activated monomer, adding to the neutral chain ends. For the latter no correlation is yet available and can differ from these described for onium ions in this section. 

Cationic Ring-Opening Polymerization of Heterocyclic Monomers Mechanisms... 

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. 

Although butyl rubber is by far the most important commercial elastomer to be synthesized by cationic polymerization, several heterocyclic monomers provide useful elastomeric materials via this mechanism also. Epichlorohydrin can be polymerized to high molecular weight using a complex catalyst formed from a trialkylaluminum compound and water as shown in Eq. (58) [64, 130-132], For copolymerizations with ethylene oxide, a catalyst formed from a trialkylaluminum compound, water, and acetylacetone is useful [64,130], The mechanism proposed for these polymerizations is... 

Polymers from Novel Monomers.—Novel compounds that have been polymerized by cationic mechanisms usually incorporate the reactive functional groups of more conventional monomers, and several recently developed monomers can be classified as analogs of vinyl ethers, vinyl carbazole, p-methoxystyrene, or previously studied oxygen heterocycles. 

Polymers from Novel Monomers.—Novel monomers which have been polymerized recently by cationic mechanisms may be categorized as follows oxygen heterocycles, optically active monomers, and bifunctional monomers. 

There are a number of heterocyclic monomers, for example, epoxides, cyclic sulfides, lactones and lactides, lactams, cyclic carbonates, and cydosUoxanes, which can be polymerized by ring-opening reactions many of them can he polymerized by an anionic as well as by a cationic mechanism. They cannot all be covered here, but there are a number of monographs and reviews on this subject [181-185]. 

There are many monomers that can be polymerized via a cationic mechanism (see Table 7.1), but the most important polymers from an industrial point of view are homo- and copolymers from isobutene and from some heterocyclic monomers such as trioxane, tetrahydrofuran, and epoxides. There is a detailed discussion on the mechanistic features in Refs. 1-5, 7, and 181-183. 

Cationic polymerization of heterocyclic monomers can proceed not only by the 8 2 mechanism involving onium ions... 

Penczek, S. Kubisa, P. Matyjaszewski, K. Cationic Ring-Opening Polymerization of Heterocyclic Monomers, Part I Mechanisms. Springer Berlin, Germany, 1980. 

In the following sections, a detailed overview of the respective polymerization mechanisms, that is, cationic, anionic, and catalytic ROMBPs will be treated. The respective seminal works leading to their development, specific prerequisites, synthetic principles, and peculiarities for each method will be examined and discussed for heterocyclic monomers containing different ring sizes and different heteroatoms. Selected issues treated in... 

Heterocycles form a specific class of monomers. They do not usually undergo radical polymerization, and the kind of ionic polymerization mechanism is determined by the kind of heteroatom, substituent and ring size. Oxiranes and, aziridines are polymerized by both ionic mechanisms. With the exception of lactone, four-membered and larger heterocycles with oxygen and with substituted nitrogen can only be polymerized cationically heterocycles with unsubstituted nitrogen can also be polymerized anionically. 

Lactam polymerization with anionically activated monomer has its counterpart in the cationic processes of lactam polymerization. This type of mechanism has also been observed recently in some polymerizations of oxygen-containing heterocycles (see Chap. 4, Sect. 2.3)... 

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. 

This above discussed correlations require that identical chemical mechanisms are compared. For instance, the cationic polymerizations of heterocyclics are known to proceed by S l, Sj 2, and Ac2 mechanisms. Besides, there are two different SfjZ mechanisms, and both can involve the same monomer, namely proceeding with onium ions or with actived monomer (3). 

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