Ionic polymerization coordination

The kinetics of chain-reaction polymerization is illustrated in Fig. 3.28 for a free radical process. Analogous equations, except for termination, can be written for ionic polymerizations. Coordination reactions are more difficult to describe since they may involve solid surfaces, adsorption, and desorption. Even the crystallization of the macromolecule after polymerization may be able to influence the reaction kinetics. The rate expressions, as given in Appendix 7, Fig. A7.1, are easily written under the assumption that the chemical equations represent the actual reaction path. Most important is to derive an equation for the kinetic chain length, v, which is equal to the ratio of propagation to termination-reaction rates. This equation permits computation of the molar mass distribution (see also Sect. 1.3). The concentration of the active species is very small and usually not known. First one must, thus, ehminate [M ] from the rate expression, as shown in the figure. The boxed equation is the important equation for v. 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

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. 

All of these examples explain why such a variety of phenomena are observed in ionic or coordination polymerization. What we need to understand is the cause which gives to a particular center this or other properties, e.g., why dissociation into isolated ions leads to one and not another change in the reactivity and the specificity, how changes in solvation shell change the behavior of the growing center. This whole field is still uncharted, and calls for a thorough academic research. 

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

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. 

Triphenylaluminum is useful as a component of catalyst systems for ionic or coordination polymerization of vinyl compounds. This preparation of the material in solid form enables the purity of the compound to be easily determined. The availability of solid triphenylaluminum permits the user a choice of solvents for a reaction, and a variety of concentrations of the reagent. Storage and dispensation of the reagent are more convenient in the solid form. 

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophiUc to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

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). 

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. 

Ethylene is conveniently polymerized in the laboratory at atmospheric pressure using a titanium-based coordination catalyst [34]. It may also be polymerized less conveniently in the laboratory under high pressures using free radical catalysts at high and low temperatures [35-37]. Other olefins such as propylene, 1-butene, or 1-pentene homopolymerize free radically only to low molecular weight polymers and require ionic or coordination catalysts to afford high molecu-... 

A rather special case of bimolecular termination was described recently in the literature by Chien (7). It concerns the polymerization of ethylene initiated by a soluble biscyclopentadienyl titanium dichloride-dimethyl aluminium chloride complex. Such a polymerization should be classified as a coordination polymerization and not as an ionic polymerization. Nevertheless, some similarity to anionic polymerization justifies its discussion at this place. It was shown that the termination is kineti-cally bimolecular, and it is postulated that it involves the reduction of two TiIV+ to TiIII+ complexes, proceeding simultaneously with the disproportion of 2 polymeric chains,... 

Coordinative initiation differs from ionic polymerization in that the propagating species consists of a covalent bond species. This generally reduces the reactivity and the polymerization rate. Decreased reactivity also leads to fewer amounts of side reactions and the often-living ROP of lactones may take place under these conditions. Chedron, in the early 1960s, showed that some Lewis acids, such as triethylaluminum and water or ethanolate of diethylaluminum, were effective initiators for lactone polymerizations. Tin(IV) alkoxides and phenox-ides, [92,93] aluminum alkoxides, mainly aluminum / so-propoxide, and soluble... 

Much has been written about polymerization kinetics and the essential steps are shown in Table III for the three principal types of mechanisms (free radical, ionic and coordination, and... 

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. 

Strictly speaking, some kind of coordination is a prerequisite for any ionic polymerization. Some active centres can bind the monomer prior to its controlled attachment to the end of a propagating macromolecule. Chains of a regular or tactic polymer are thus formed. Such processes are designated as coordination polymerizations proper. At the present time, the centres of alkene coordination polymerizations and the precursors of such centres are of greatest importance. 

Radical propagations proceed smoothly in non-polar and also in highly polar media. This is understandable as radical solvation is weaker than ion solvation by one or two orders of magnitude. Propagation on coordination polymerization catalysts is only possible in non-polar media which do not interfere with monomer (usually hydrocarbon) coordination on the transition metal atom. Ionic polymerizations also proceed in non-polar media and they are accelerated with increasing medium polarity. 

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. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

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. 

A relatively high-molecular-weight, crystalline polyphenylacetylene of molecular mass of the order of 104 is formed by ionic or coordination polymerization (with WCI6 or (acac),Fe) [77]. In these polymerizations, termination is also a function of chain length [78]. 

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. 

Decay of ionic and coordination centres always leads to the formation of some end groups and centre residues. The centres usually lose their polymerizing activity on contact with atmospheric humidity. A residue of very active centres, which are rare, is usually not removed from the polymer (e.g. of the order of one ppm of the transition metal in low-pressure polyethylene). Larger residues have to be washed out (some types of polypropylene are still washed at the present time). 

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... 

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