Cationic polymerization early investigations

Cationic polymerization has a history dating back to the early 1800s and has been extensively investigated by Plesch [1,2], Dainton and Sutherland [3], Evans et al. [4,5], Pepper [6,7], Evans and Meadows [8], Heiligmann [9], and others [10-13]. Whitmore [14] is credited with first recognizing that carbonium ions are intermediates in the acid-catalyzed polymerizations of olefins. The recognition of the importance of proton-donor cocatalysts for Friedel-Crafts catalysts was first reported by Evans and co-workers [4,5]... 

The problem of cocatalysis in Friedel-Crafts reactions in general and in cationic polymerizations in particular is still unresolved. In 1936 Ipatieff and Grosse (30) made the significant observation that pure ethylene and aluminum chloride will not polymerize except in the presence of added water or hydrogen chloride. This can be regarded as the discovery of cocatalysis. Ten years later British scientists started a systematic investigation in this area and proposed the concept of cocatalysis which is now generally accepted (31—34). Early development was excellently surveyed (35) and will not be discussed here. 

Early Developments till the Discovery of Controlled Initiation. Under suitable conditions any electrophilic species may induce cationic polymerizations (1). As a practical matter, the most convenient cationogens are Bronsted acids alone or in conjunction with Frie-del-Crafts acids (1). Systematic research on the initiation of carbocationic polymerization became possible by the discovery of coinitiation by British investigators (2-4). These workers found that the strong Lewis acid BF3 alone is unable to initiate isobutylene polymerization but in the presence of suitable cationogens, i.e., H2O, immediate and vigorous polymerization ensues. Their formalism ... 

The interest for such stereospecific polymerization started beginning of 50 S when crystalline polypropylene oxide of h.m.w. was synthesized. Numerous researches performed and still under investigation in the field of oxiranes were summarized in several reviews (1-5). The study of stereospecific polymerization of thi-iranes started later, early 60 S, and the main results were also recently reviewed (6-8). The comparison of different aspects of polymerization of oxiranes and thiiranes were examined in few publications (9-12). Practically no work have been succeeded in the field of coordination type polymerization of aziridines, although the cationic polymerization was extensively studied. 

Azetidines, four-membered cyclic imines and their derivatives were also investigated regarding their cationic polymerization in the early 1980s. In analogy to the polymerization of aziridine, hyperbranched structures are obtained by the same mechanism as described previously. Still, the difficult monomer preparation and the limitation of molecular weights due to rather low reactivity of the chain end compared to oxo-heterocycles limited the interest in these polymers. By using the derivative N-phenylazetidine (Scheme 3), the formation of linear chains can be favored. Branching termination reactions are suppressed because of the decreased... 

During the early investigations, nucleophobic anions contained in onium salts were considered to be chemically inert and to act partially as stabilizing entities in ion pair formation. However, the results of later studies suggested that they might be involved to some extent and in some cases in the initiation of cationic polymerizations. For example, in the case of Ph2lPF6 dissolved in a diepoxide (Chart 5.2), the hexafluorophosphate anion was found to decompose during irradiation simultaneously with conversion of the diepoxide [23]. 

During the early 1980s, the cationic polymerization of cyclic ethers in the presence of low-molecular-weight diols as chain-transfer agent was studied with the aim of preparing polyether diols [82]. A more detailed investigation of this process revealed that the addition of alcohols to the polymerization of some oxiranes reduced the proportion of cyclics which was known to be formed by back-biting. The explanation for this observation was based on the activated monomer (AM) mechanism shown in Scheme 1.5 [171, 172]. 

The antiviral properties of anionic polymers have recently received a lot of attention as agents to protect against infection with sexually transmitted diseases. Due to the cationic nature of most viruses, several anionic polymers are known to bind viruses. As early as the 1960s, researchers had studied the anti-viral properties of a variety of synthetic polymers [118]. However, not all anionic polymers inactivate viruses. Several classes of anionic polymers have been studied for their ability to inactivate the HIV virus. These polymers include poly(styrene-4-sulfonate), 2-naphthalenesulfonate-formaldehyde polymer, and acrylic acid-based polymers. Certain chemically modified natural polymers (i.e., semisynthetic) such as dextrin/dextran sulfates, cellulose sulfate, carrageenan sulfate, and cellulose acetate phthalate have also been investigated for this purpose. Of a number of such anionic polymers that have shown in-vitro and in vivo anti-HIV activity, a couple of polymeric drug candidates have proceeded to early stage human clinical trials for the evaluation of safety/tolerability [119]. While most of these have shown the desired tolerability and safety, further clinical trials are necessary to discern the therapeutic benefit and see if anionic polymers will be applicable as anti-HIV therapies. 

Though their applications as polymerization catalysts fall outside the remit of the present chapter, a general discussion of the advent of cationic aluminum complexes supported by various ligands is warranted [46]. Over the last two decades cationic aluminum complexes have been very intensively investigated and promise enhanced substrate coordination and activation by virtue of their increased electrophilicity. Early systematic works focused on the use of crown ethers and the synthesis of complexes [Cl2Al(benzo-15-crown-5)][Me2AlCl2] and... 

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