Chain polymerization other cationic polymerizations

Many other cyclic ethers have been polymerized using cationic polymerization. Ethylene oxide (also called oxirane) polymerizes forming poly(ethylene oxide) (PEO) (structure 5.24) in the presence of acids such as sulfuric acid, producing a wide range of chain-sized polymers sold under various trade names including Carbowax and Poly ox. PEO is also used in cosmetics and pharmaceuticals (as water-soluble pill coatings and capsules). 

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. 

Because strong Bronsted (proton) acids and Lewis acids can initiate styrene polymerization, other cationically polymerizable monomers can be added to the styrene-based copolymer list. Due to the facile occurrence of chain transfer processes of polymer chains with impurities, cationically prepared polystyrene-based polymers are low molecular weight materials. Nevertheless, low molecular weight polystyrenes still find important applications as additives, as tackifiers for pressure sensitive adhesives, and in hot melt adhesives. However, the market for low molecular weight polystyrene is small. 

All systems mentioned sofar are cured by radical, chain polymerization. Other monomers such as epoxides or vinyl ethers may be polymerized by cationic chain polymerization. 

Pathway (1) is mainly encountered in chain polymerization (anionic, cationic, and controlled radical polymerizations) the second one refers essentially to polycondensation and polyaddition. There is no strict distinction between these two sets of techniques, e.g., the difunctional oligoethers used in poly(ether-6-ester)s or poly(ether-6-amide)s can be prepared by ring-opening polymerization and then polycondensed with the other oligomer (Scheme 2) [39]. 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

The formation of high molecular products during the cationic polymerization depends on whether the propagation reaction, consisting of the interaction of the cationic chain end as a reactive intermediate with the monomer, reproduces the reactive intermediate (see Eq. (1)). For this reason the monomer functions as the agent and as the substrate when in the form of the cation. This means, however, the interaction between the monomer and the cationic chain end is a function of the monomer structure itself when all other conditiones remain the same. 

Such a mechanism is open to serious objections both on theoretical and experimental grounds. Cationic polymerizations usually are conducted in media of low dielectric constant in which the indicated separation of charge, and its subsequent increase as monomer adds to the chain, would require a considerable energy. Moreover, termination of chains growing in this manner would be a second-order process involving two independent centers such as occurs in free radical polymerizations. Experimental evidence indicates a termination process of lower order (see below). Finally, it appears doubtful that a halide catalyst is effective without a co-catalyst such as water, alcohol, or acetic acid. This is quite definitely true for isobutylene, and it may hold also for other monomers as well. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

Gandini and Plesch concluded that in these systems the chain-carriers are not ionic. Since they are certainly highly polar and in many respects behave as if they were ionic, we called the polymerizations propagated by them pseudo-cationic. Admittedly, in retrospect our original evidence for the non-ionic nature of the chain-carriers looks less convincing, but since that time many other phenomena have been found which support our view very forcibly [18] the case for the reality of pseudo-cationic polymerizations has been presented in detail [7], and therefore the argument need not be repeated here. 

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. 

The unexpected variety found in cationic polymerizations is well illustrated by recent findings concerning the polymerization of dioxolan by perchloric acid. Gresham91 had been unable to find end-groups in polydioxolans made under anhydrous conditions and concluded that they were cyclic. His result has been confirmed by studies made with anhydrous perchloric acid as catalyst [92]. Since it is extremely unlikely that complete absence of chain ends can be achieved by a cyclization of long chains, and from other evidence, we concluded that the polymerization takes place without formation of chain ends, i.e. by a ring expansion mechanism ... 

It is the author s hope that the foregoing detailed discussion has helped to clarify some features of cationic polymerizations. Many other aspects, such as co-polymerization and radiation polymerization, which I have not been able to discuss here, deserve equal attention. But perhaps the most urgent task, and one which is much more widely relevant, is the elucidation of details of reaction mechanism, and in particular the identification of the chain-carriers in many widely differing systems. The next problem then is to measure their concentration, its variation throughout the reaction, and, hence, the absolute rate constants. It is essential that the factors which decide whether a polymerization is ionic or pseudo-ionic be determined as soon as possible. 

The situation is quite different in chain polymerization where an initiator is used to produce an initiator species R with a reactive center. The reactive center may be either a free radical, cation, or anion. Polymerization occurs by the propagation of the reactive center by the successive additions of large numbers of monomer molecules in a chain reaction. The distinguishing characteristic of chain polymerization is that polymer growth takes place by monomer reacting only with the reactive center. Monomer does not react with monomer and the different-sized species such as dimer, trimer, tetramer, and n-trier do not react with each other. By far the most common example of chain polymerization is that of vinyl monomers. The process can be depicted as... 

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

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