Conventional Cationic Polymerization

Cationic polymerization with Lewis acids yields resinous homopolymers containing cycHc stmctures and reduced unsaturation (58—60). Polymerization with triethyl aluminum and titanium tetrachloride gave a product thought to have a cycHc ladder stmcture (61). Anionic polymeriza tion with lithium metal initiators gave a low yield of a mbbery product. The material had good freeze resistance compared with conventional polychloroprene (62). 

Before the discovery of the pseudo-cationic reactions, one could say simply that the function of the co-catalyst is to provide cations which can initiate the polymerization [28b]. Although this is still valid for the true cationic polymerizations, it is more difficult to define the function of the co-catalyst in the pseudo-cationic reactions. Very tentatively one can suggest that the co-catalyst is the essential link in the formation of an ester which is the chain-carrier, as in the pseudo-cationic polymerizations catalysed by conventional acids in other words, the co-catalyst and catalyst combine to form an acid, but this, instead of protonating the monomer, forms an ester with it, which is then the propagating species. 

A highly obscure feature of cationic polymerization is the great phenomenological difference between aliphatic and aromatic monomers. The survey by Brown and Mathieson [84] of the behaviour of a very wide range of monomers towards trichloroacetic acid is particularly illuminating in this respect. Unfortunately, there are so few studies with aliphatic olefins that detailed comparisons must be confined to isobutene. It is well known that isobutene cannot be polymerised by conventional acids, such as sulphuric, perchloric, hydrochloric, or by salt-like catalysts such as benzoyl perchlorate, whereas all these catalysts readily give at least oligomers from aromatic olefins. Even when the same catalytic system, (e.g., titanium... 

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-... 

Unlike cationic polymerization initiated by a conventional catalyst, the propagating species in the present system would bear different type of counter-ion or would be much more free. The counter-anion obtained in this entirely organic system would be large and unstable. The problem of the counterion in charge transfer ionic polymerization certainly requires further study. 

Kennedy and Thomas (1) first reported the synthesis of a crystalline poly(3-methyl- 1-butene) by cationic polymerization at —130°C. Preliminary HNMR studies indicated that the polymer was not simply a tactic modification of the conventional 1,2-polymer but, in fact, possessed a repeat structure which resulted... 

A rare example of cationic polymerization of emulsified epoxy resins has been reported by Walker et al.973 Polymerization of water emulsion of epoxy resins with a variety of superacids (triflic acid, HCIO4, HBF4, HPF6) results in polyols with two glycidyl units (294) in contrast to commercial epoxy resins with one unit separating the aromatic moieties. The level of residual glycidyl ether and Bisphenol-A units is also much lower than in conventional epoxy resins. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

Indeed, it is not intended to discuss recent developments of conventional cationic polymerizations, i. e., polymerizations in which a cat-ionically initiable vinyl compound is attacked by a suitable (usually Friedel-Cr afts halide) catalyst and the growing ion is neutralized by the corresponding MeX type gegen-ion, etc. Rather, this review concerns unusual cationic polymerization systems and mechanisms which have not been discussed in a comprehensive manner. 

The study of cocatalysis in cationic polymerizations is extremely complicated by the fact that Lewis acids can participate in a variety of ill defined reactions (91). Satchell (91, 92) regards the hydrogen exchange reaction between Bronsted acids and aromatics catalyzed by Lewis acids as a prototype for Friedel-Crafts catalysis. He postulates that cocatalytic efficiency is determined by the stability of the complex anion B -f HX + SnCl4 - BH SnCl4Xe. A simple enhancement of conventional acidity by B + HX -> BH Xe as proposed by Plesch (93) and Russel (94) is considered to be unimportant. The stability of the complex and its catalytic activity are determined by the electron density... 

Stereospecific coordinated anionic polymerizations of unhindered monomers follow Hammett s rule (305). The negative slope indicated that the addition reaction is favored by electron-releasing substituents in the same manner as in cationic polymerizations (306). The opposite results were obtained with conventional anionic catalysts (Na-K alloy or LiC6Hs in ether) and with a free radical catalyst (benzoyl peroxide). 

The relatively low angle slope of the Hammett plot indicates that the polarization induced by complexing the monomer with the electrophilic site involves only a partial charge. Weak polarization of monomer is supported by the fact that vinylcyclopropane does not isomerize during coordinated anionic polymerization but does so during conventional cationic polymerization (307). 

An important advantage of the use of such added nucleophiles is that it allows controlled/living cationic polymerization of alkyl vinyl ethers to proceed at +50 to +70°C [101,103], relatively high temperatures at which conventional cationic polymerizations fail to produce polymers but result in ill-defined oligomers only, due to frequent chain transfer and other side reactions. Recently, initiators with functionalized pendant groups [137] and multifunctional initiators [ 138—140] have been developed for the living cationic polymerizations with added nucleophiles. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

Common for these three approaches is that living cationic polymerization permits the introduction of a variety of interesting functional groups at specific positions (outer arm layer, arm ends, central core, etc.) of the multiarmed polymer architectures. The combinations of these functional groups and the unique molecular topology would lead to physical properties and functions that cannot be found in the conventional linear counterparts. 

In the present overview, we tried to gather most of the industrial cationic polymerizations for alkenes, as well as for carbonyl and heterocyclic monomers. Our main objective was not to insist on the conventional processes which are already detailed in the specific reviews quoted all along the text, but to outline the up-to-date improvements, new products, and market trends every time the corresponding information had been made available. 

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

Cationic polymerizations differ from free-radical and homogeneous anionic syntheses of high polymers in that the cationic systems have not so far been fitted into a generally useful kinetic framework involving fundamental reactions like initiation, propagation, and so on. To explain the reasons for the peculiar problems with cationic polymerizations we will, however, postulate a conventional polymerization reaction scheme and show where its inherent assumptions are questionable in cationic systems. 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

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