Copolymerization by ionic mechanism

Discuss copolymerizations by ionic mechanism. What are some of the problems that are encountered ... 

Hashimoto and coworkers smdied anionic living copolymerization by ionic mechanism of two monomers, styrene and isoprene in a dilute benzene solution with the aid of combined time-resolved... 

Many copolymers of styrene are ntanufactured on a large commercial scale. Because styrene copolymerizes readily with many other monomers, it is possible to obtain a wide distribution of properties. Random copolymers form quite readily by a free-radical mechanism. Some can also be formed by ionic mechanism. In addition, graft and block copolymers of styrene are also among commercially important materials. 

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. 

In the presence of increasing amounts of emulsifier, then, the mechanism of particle formation becomes increasingly determined by particle nucleation rather than coagulation. Similar electrostatic effects may be brought about by copolymerization of ionic monomers in the absence of conventional emulsifiers (38,... 

The number and position of substituents are important. One bulky group, e.g. naphthyl, or two substituents on a single olefinic carbon do not usually hinder polymerization. On the other hand, an ethylene derivative with a substituent on each of the two carbons does not undergo radical polymerization at conventional pressures and temperatures. Usually, however, it can be copolymerized with a suitable monomer or it can be polymerized by an ionic mechanism. With a larger number of substituents, the tendency to polymerization is further limited. The small fluorine atom represents an exception, as do the cases discussed in Sects. 1.3 and 5.2. 

Mechanistic Aspects of Cationic Copolymerizations The relative reactivities of monomers can be estimated from copolymerization reactivity ratios using the same reference active center. However, because the position of the equilibria between active and dormant species depends on solvent, temperature, activator, and structure of the active species, the reactivity ratios obtained from carbocationic copolymerizations are not very reproducible [280]. In general, it is much more difficult to randomly copolymerize a variety of monomers by an ionic mechanism than by a radical. This is because of the very strong substituent effects on the stability of carbanions and carbenium ions, and therefore on the reactivities of monomers substituents have little effect on the reactivities of relatively nonpolar propagating radicals and their corresponding monomers. The theoretical fundamentals of random carbocationic copolymerizations are discussed in detail and the available data are critically evaluated in Ref. 280. This review and additional references [281,282] indicate that only a few of the over 600 reactivity ratios reported are reliable. 

Let us consider the case for the copolymerization of two monomers Ml and M2. Although copolymerization has been more extensively studied using radical initiation, and radical copolyraerization is also more important than ionic copolyraerization, we will consider here the general case without specification as to whether polymerization occurs by a free-radical or ionic mechanism. To generalize, an asterisk( ) will be used—instead of the con-... 

Block and graft copolymerizations involve initiating polymerization reactions through active sites bound on the parent polymer molecule. Block copolymerization involves terminal active sites, whereas graft copolymerization involves active sites attached either to the backbone or to pendant side groups. Copolymerizations only by free-radical processes are discussed in this section those involving ionic mechanisms are described in Chapter 8. 

Copolymerization of ionic-type MCMs, alkali and alkaline earth metal salts, with various comonomers has been studied quite well. A detailed review of the basic studies has been given in a monograph [4] and they will not be analyzed in this book. Studies of the mechanism of copolymerization of such types of MCM were carried out in the case of organotin and -lead monomers. The widest recognition among these MCMs used as comonomers has been gained by trimethyl-, tributyl-, and triphenyltin methacrylates (Table 4-7) [4 and references therein]. 

Vinyl polymerization using metallocomplexes commonly proceeds by a radical pathway and rarely involves an ionic mechanism. For instance, metal chelates in combination with promoters (usually halogenated hydrocarbons) are known as initiators of homo- and copolymerization of vinylacetate. Similar polymer-bound systems are also known [3]. The polymerization mechanism is not well understood, but it is believed to be not exclusively radical or cationic (as polymerization proceeds in water). The macrochelate of Cu with a polymeric ether of acetoacetic acid effectively catalyzes acrylonitrile polymerization. Meanwhile, this monomer is used as an indicator for the radical mechanism of polymerization. Mixed-ligand manganese complexes bound to carboxylated (co)polymers have been used for emulsion polymerization of a series of vinyl monomers. Macromolecular complexes of Cu(N03)2 and Fe(N03)3 with diaminocellulose in combination with CCI4 are active in polymerization of MMA, etc. 

On the other hand, butyllithium-aluminum alkyl initiated polymerizations of vinyl chloride are unaffected by free-radical inhibitors. Also, the molecular weights of the resultant polymers are unaffected by additions of CCI4 that acts as a chain-transferring agent in free-radical polymerizations. This suggests an ionic mechanism of chain growth. Furthermore, the reactivity ratios in copolymerization reactions by this catalytic system differ from those in typical free-radical polymerizations An anionic mechanism was also postulated for polymerization of vinyl chloride with t-butylmag-nesium in tetrahydrofuran. ... 

Ionic copolymerizations differ characteristically from free radical copolymerizations. Random copolymers are mostly formed in free radical polymerizations alternating copolymers and block polymers are produced quite rarely. The situation is exactly the reverse for ionic copolymerizations. Thus, ionic polymerizations give rise to quite different copolymerization parameters from those of free radical copolymerizations (Table 22-15). Consequently, copolymerization experiments can be used to determine whether unknown initiators act by a free radical, a cationic, or an anionic mechanism (see also Table 22-16). From such experiments it is found that boroalkyls are free radical initiators, but lithium alkyls are anionic in the... 

Ionic mechanisms for the preparation of block copolymers are a very important tool of the synthetic polymer chemist. A feature of many homogeneous anionic polymerizations in solution is that termination can be avoided by careful control of experimental conditions. In fact, an infinite life of the active chain end is theoretically possible, and this has led to the term living polymers. Polymer carbanions can resume growth after the further addition of monomer. By changing the monomer composition, block copolymerization is readily initiated, and this process can be repeated. A major advantage of this... 

In by far the largest number of cases of free radical copolymerization, the reactivity ratios are practically independent of the nature of the starting reaction (thermal, photochemical, radical-forming type) and the site of propagation (bulk, solution, emulsion). Ionic copolymerization, by contrast, leads to quite different parameters (Table 22-13). Thus copolymerizations can be employed as a diagnostic tool for initiators whose mode of action is unknown, differentiating between free radical copolymerization and the nonradical mechanism (Table 22-14). On such evidence, boron alkyls appear to be free radical initiators in the copolymerization of methyl methacrylate with acrylonitrile, whereas lithium alkyls are anionic initiators. 

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