Polymerization by Ionic Initiators

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

Polymerization by ionic initiation is much more limited than that by free-radical initiation with vinyl monomers, but there are monomers such as carbonyl compounds that may be polymerized ionically but not through free radicals because of the high polarity. The polymerization is much more sensitive to trace impurities, especially water, and proceeds rapidly at low temperature to give polymers of narrow molar-mass distribution. The chain grows in a living way and, unlike in the case of free-radical polymerization, is generally terminated not by recombination but rather by trace impurities, solvent or, rarely, the initiator s counter-ion (Fontanille, 1989). 

Strongly electrophilic or nucleophilic monomers will polymerize exclusively by anionic or cationic mechanisms. However, monomers that are neither strongly electrophilic nor nucleophilic generally polymerize by ionic and free radical processes. The contrast between anionic, cationic, and free radical methods of addition copolymerization is clearly illustrated by the results of copolymerization utilizing the three modes of initiation (Figure 7.1). Such results illustrate the variations of reactivities and copolymer composition that are possible from employing the different initiation modes. The free radical tie-line resides near the middle since free radical polymerizations are less dependent on the electronic nature of the comonomers relative to the ionic modes of chain propagation. 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

The polymerization of aldehydes is initiated by ionic initiators and the polymerization proceeds by ionic propagation. No radical polymerization of aldehydes has been documented yet. In the case of anionic polymerizations the growing ion is an alkoxide ion. The cationic polymerization has as the propagating species an oxonium ion. Most recent experimental results have shown that haloaldehydes, such as chloral polymerize exclusively by an anionic mechanism. 

While for many alkene monomers the position of the propagation-depropagation equilibrium is far to the right under the usual reaction temperatures employed (that is, there is essentially complete conversion of monomer to polymer for all practical purposes), there are some monomers for which the equilibrium is not particularly favorable for polymerization. For example, a-methylstyrene in a 2.2 M solution will not polymerize at 25°C and pure a-methylstyrene will not polymerize at 61°C (see Table 6.14). In the case of methyl methacrylate, though the monomer can be polymerized below 220° C, the conversion will be appreciably less than complete. For example, the value of [M]g at 110°C is found to be 0.139 M [3] which corresponds to about 86% conversion of 1 M methyl methacrylate. Since Eqs. (6.195) and (6.196) contain no reference to the mode of initiation, they apply equally well to ionic and ring-opening polymerizations. Thus the lower temperatures of ionic polymerizations often offer a useful route to the polymerization of many monomers that cannot be polymerized by radical initiation because of their low ceiling temperatures. 

Polymerization by Ionic Catalysts. Attempts to polymerize monomer I with ionic catalysts such as trimethylphosphite or boron trifluoride-etherate at room temperature did not succeed. No reaction was visible in either case despite the fact that it has been reported that trialkyl phosphites react vigorously with sulfur at room temperature (21). However, even in that specific instance, no polymerization was observed. The use of an anionic initiator such as n-butyllithium did not produce polymer either. In this case, however, a reaction did occur, as evidenced by the discoloration of the monomer solution. 

The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds via chain mechanism. Chain-reaction polymerization is induced by the addition of free-radical-forming reagents or by ionic initiators. Like all chain reactions, it involves three fundamental steps initiation, propagation, and termination. In addition, a fourth step called chain transfer may be involved. 

While the lower-molecular-weight polysiloxanes can be synthesized by the hydrolytic step polymerization process, the higher-molecular-weight polymers are synthesized by ring-opening polymerization using ionic initiators ... 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

Another type of chain reaction is possible by ionic initiation. Both, positive and negative ions may start polymerization reactions (cationic and anionic polymerization). The existence of ions in solution is illustrated in Fig. 3.24 (B = base, M = metal). The interaction with the solvent plays an important role in the activity of the catalyst. In the covalent combination, no reactivity is expected. As the ions separate, they become increasingly available for reaction. While one of the ions supports the reaction, the other, the counterion, is in the vicinity and may also affect the rate of reaction. Since cationic initiators accept a pair of electrons, one finds these among the Lewis acids. Anionic initiators donate an electron pair, so they are Lewis bases. 

Ring-opening Polymerization of Cyclosiloxanes by Ionic Initiators... 

Problem 10.5 Like cyclic ethers, cyclic amines can be polymerized by ionic ring-opening method. Thus poly(ethyleneimine) can be prepared by the ring-opening polymerization of aziridine (see Table 10.1) with initiation by protonic acids followed by nucleophilic attack of the monomer. Account for the fact that the process gives rise to a branched polyamine and suggest a method by which the branching could be avoided to obtain a linear polyamine. 

The inhibiting effect of radical acceptors, however, suggests that polymerization proceeds by a radical mechanism. The yield of polymer from such reactions depends on the tendency of the different monomers to be polymerized by ionic and radical catalysts. The reactivity ratio on grinding for styrene plus methyl methacrylate is similar to that for peroxy-initiated polymerization. Nuclear magnetic resonance data, however, show a different stereochemical configuration for the copolymers [200]. 

Cationic ROP of cyclic monomers, containing trivalent phosphorus atom, °° proceeds by two mechanisms of propagation ionic and/or covalent. These both can be visualized best for the polymerization of structure 49. Monomer 49 polymerizes by CFsSOsMe initiator... 

There are many other industrial examples such as acrylic fibers made from polyacrylonitrile (with 7% vinyl acetate). The monomer is fairly water soluble at about 5% and the polymerization occurs in the aqueous phase. However, the polymer is insoluble in water. The primary particles precipitate and agglomerate, forming larger particles that are stabilized by ionic initiator end groups. Butyl rubber (isobutylene+ < 5% isoprene) is produced by cationic polymerization with aluminum trichloride catalyst in methyl chloride at about -100 °C. The polymer precipitates as fine polymer particles from the reaction medium. 

When many molecules combine the macromolecule is termed a polymer. Polymerization can be initiated by ionic or free-radical mechanisms to produce molecules of very high molecular weight. Examples are the formation of PVC (polyvinyl chloride) from vinyl chloride (the monomer), polyethylene from ethylene, or SBR synthetic rubber from styrene and butadiene. 

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