Ionic and Coordination Polymerizations

C—X, Cf, X- and C+ fX (see Fig. 2), the solvation energy increasing the driving force of these dissociations. It is possible that a coordination catalyst is not active in the C—X state but only in one or other of the ionized states. Such behavior blurs the distinction between ionic and coordination polymerization. 

Radical polymerization is the most useful method for a large-scale preparation of various kinds of vinyl polymers. More than 70 % of vinyl polymers (i. e. more than 50 % of all plastics) are produced by the radical polymerization process industrially, because this method has a large number of advantages arising from the characteristics of intermediate free-radicals for vinyl polymer synthesis beyond ionic and coordination polymerizations, e.g., high polymerization and copolymerization reactivities of many varieties of vinyl monomers, especially of the monomers with polar and unprotected functional groups, a simple procedure for polymerizations, excellent reproducibility of the polymerization reaction due to tolerance to impurities, facile prediction of the polymerization reactions from the accumulated data of the elementary reaction mechanisms and of the monomer structure-reactivity relationships, utilization of water as a reaction medium, and so on. 

The polymerization of acetylene (alkyne) monomers has received attention in terms of the potential for producing conjugated polymers with electrical conductivity. Simple alkynes such as phenylacetylene do undergo radical polymerization but the molecular weights are low (X <25) [Amdur et al., 1978]. Ionic and coordination polymerizations of alkynes result in high-molecular-weight polymers (Secs. 5-7d and 8-6c). 

Ionic and coordination polymerizations are inhibited by the presence of a certain amount of water. In this case, the amount of water means tens to hundreds parts per million (ppm). Therefore the emulsion and suspension processes in water are limited to monomer polymerizing by the radical mechanism. The most frequently used methods of liquid-phase polymerization of the more conventional monomers are summarized in Table 2. 

The solution for specific cases is greatly simplified when one of the reactions (87) or (88) is much slower than the other and thus controls the initiation rate. [In radical polymerizations, this is usually reaction (87).] We know, of course, that reaction (87) can be reversible, that R° can decay by secondary decomposition to R j (the reactivity of which generally differs from that of R°), and both reactions can only be a part of a much more complicated set of interactions, especially in ionic and coordination polymerizations. An exact kinetic analysis must be based on a proved scheme with identified intermediate transition states and products, and a knowledge of the rate constants and of the rates of various initiation stages. Such a complete and complex analysis does not yet exist. 

The radical model cannot be applied for ionic and coordination polymerizations. With a few exceptions, termination by mutual combination of active centres does not occur. The only possibility is to measure the rate of each copolymerization independently. The situation can be greatly simplified for copolymerizations in living systems. The constants ku and k22 can usually be measured easily in homopolymerizations. Also, the coaddition constants fc12 or k2] are often directly accessible when the M] and M2 active centres can be differentiated spectroscopically or when the rate of monomer M2 (M[) consumption at M] M 2 centres can be measured. Ionic equibria, association, polarity of medium and solvation must be respected, even when their quantitative effect is not known exactly. The unusual situations confronting macromolecular chemistry will be demonstrated by the example of the anionic copolymerization of styrene with butadiene initiated by lithium alkyls in hydrocarbon medium. 

Matsuzaki and Ito polymerized cis and trans dideuterated oxirane by both ionic and coordination polymerization. They observed that ring opening and chain growth proceeds almost exclusively with configuration inversion [311]... 

On the other hand, a certain dose of creative spirit is appropriate. When the requirements of modem research methods are respected, good reproducibility can be achieved, disturbing effects can be limited, and our knowledge can be promoted by a further step. The measured constants, even though only defined for a certain system, form an excellent basis for further discoveries. The values of propagation rate constants for some monomers in radical, ionic and coordination polymerizations are summarized in Table 9. 

The transformation of reactive centres to stable complexes may be of considerable practical importance. The expensive washing out of initiator residues can be substituted by their complexation. Suitable procedures and agents will also be sought for other ionic and coordination polymerizations. 

The difficulties involved in the direct determination of the momentary concentration of active centres are the most serious shortcoming in studies of termination itself. With radical polymerizations we at least know the most probable method of centre decay, and thus the molecular scheme of the termination reaction. In ionic and coordination polymerizations, the termination mechanism is mostly unknown. Quite generally we can write... 

In summary, crystallization during ionic and coordination polymerization may give rise to a large variety of morphologies. Some crystalline cyclic monomers seem to be capable of topotactic polymerization reactions which must be simultaneous polymerization and crystallization. A direct influence of nucleation and ordered structure on the polymerization rate was observed for helical polymers in solution giving an... 

In chain-growth copolymerization, the composition of the polymer depends on the concentrations and relative reactivities of the monomers. The relative reactivities can be drastically different in radical, ionic, and coordination polymerization. Three special cases... 

The mechanisms of ionic and coordination polymerizations are more complex and are not as clearly understood as those of free radical polymerization. Here, we will briefly highlight the essential features of these mechanisms, and more details will be given in Chapter 7. Initiation of ionic polymerization usually involves the transfer of an ion or an electron to or from the monomer. Many monomers can polymerize by more than one mechanism, but the most appropriate polymerization mechanism for each monomer is related to the polarity of the monomers and the Lewis acid-base strength of the ion formed. 

Solution polymerization is of limited commercial utihty in free-radical polymerization but finds ready applications when the end use of the polymer requires a solution, as in certain adhesives and coating processes [i.e., poly(viityl acetate) to be converted to poly(viityl alcohol) and some acryhc ester finishes]. Solution polymerization is used widely in ionic and coordination polymerization. High-density polyethylene, poly butadiene, and butyl rubber are produced this way. Table 10.2 shows the diversity of polymers produced by solution polymerization, while Figure 10.2 is the flow diagram for the solution polymerization of vinyl acetate. 

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