Ionic and coordination

Table 2 shows characteristic reactivity ratios for selected free-radical, ionic, and coordination copolymerizations. The reactivity ratios predict only tendencies some copolymerization, and hence some modification of physical properties, can occur even if and/or T2 are somewhat unfavorable. For example, despite their dissimilar reactivity ratios, ethylene and propylene can be copolymerized to a useful elastomeric product by adjusting the monomer feed or by usiag a catalyst that iacreases the reactivity of propylene relative to ethylene. 

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

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. 

In solution polymerisation, the reaction is carried out in presence of a solvent. The monomer is dissolved in a suitable inert solvent along with the chain transfer agent. A large number of initiators can be used in this process. The free radical initiator is also dissolved in the solvent. The ionic and coordination catalysts can either be dissolved or suspended in the medium. The solvent facilitates the contact of monomer and initiator and helps the process of dissipation of exothermic heat of reaction. It also helps to control viscosity increase. 

The compound Gd(H20)6 Cl3 is eight coordinate with six water molecules and two chloride ions coordinating to the metal ion 331). The third chloride remains ionic. The complex units are held together by hydrogen bonds through both the ionic and coordinated chloride ions. Each noncoordinated Cl" ion forms six Cl... H-0 bonds and each coordinated Cl- ion forms three Cl... H-0 bonds in addition to the Cl—Gd bond. The coordination geometry around Gd(III) has been described as distorted dodecahedron. In [Eu(H20)6 C12]C1, the coordination geometry has been described as distorted square antiprism 332). Similar compounds of Nd(III), Sm(III), Dy(III), Ho(III), and Er(III) are isostructural. 

There is a tendency for the formation of stereoregular sequences, particularly at low temperatures, but ionic and coordination catalysts are far superior in this aspect and are used to create stereoregular macromolecules. 

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). 

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 nitrate D3h has 4 vibrations (3 IR active 1 Raman active), coordinated nitrate C2V has 6 IR active, and 6 Raman active vibrations. The doubly degenerate V4 band of ionic nitrate (700 cm-1) is split into V3 and V4 upon coordination. Further Raman polarization studies can distinguish between monodentate from bidentate nitrate and bridging nitrate groups. As an example La(HMPA)3(N03)3 has both ionic and coordinated nitrate [161]. 

In general perchlorate is an innocent anion. However, complexes of the type La(HMPA)a (0104)3 have been isolated and their IR and Raman spectra gave evidence for the presence of both ionic and coordinated perchlorate [164], The Raman band at 436 cm 1 indicated perchlorate with C2V symmetry and as a bidentate ligand. 

It appears that careful control of reaction conditions is absolutely necessary in the synthesis of Ln HMPA complexes [218], The complex Ln(HMPA)6(C104)3 was found to contain ionic perchlorate based on infrared spectra [210]. However the complex Ln(HMPA)4(C104)3 had both ionic and coordinated perchlorate [220]. 

Complexes with 2,7-dimethyl-1,8-naphthyridine of the formula [233] Ln(2,7-dmnapy)2(N03)3 have been identified containing coordinated nitrate groups C2V symmetry. A coordination number ten for these complexes seems plausible. The IR data for ionic and coordinated nitrate groups are given below. 

With ethylenediamine complexes of the formula Ln(en)3X3 and Ln(en)4X3, where X = C1 , Br , NO, CIOJ have been characterized. IR data indicate that the tris and tetrakis complexes of the fighter lanthanides La-Sm, contain both ionic and coordinated nitrate groups. By contrast tetrakis complexes of heavier lanthanides, Eu-Yb contain ionic nitrate. This is possibly due to steric factors resulting from decreasing cationic radius that force the nitrate out of the coordination sphere of the lanthanides. A coordination number of 8 for tris complexes and a number of 9 for fighter lanthanide tetrakis complexes appears reasonable [234]. The thermodynamic parameters obtained show enthalpy stabilization for... 

The synthesis of lanthanide complexes [244] with multidentate diethylenetriamine (dien) gave rise to two types of complexes, Ln(dien)3(N03>3 for Ln = La-Gd, and Ln(dien)2(N03)3 for Ln = La-Yb. The tris complexes contain ionic nitrate while the bis complexes contain both ionic and coordinated nitrate ions. The coordination number is nine in the tris complexes while it is not known with certainty in the bis complexes. With triethylene triamine (tren) two types of complexes [Ln(tren)(N03)3] and Ln(tren)2(N03)3 have been isolated. In the bis complexes both ionic and coordinated nitrate groups are present for larger lanthanides (La-Nd) but only ionic nitrate for smaller lanthanides (Sm-Yb). When perchlorate is the anion [245] Ln(tren)(C104)3 (Ln = Pr, Gd, Er) and Ln(tren)2(C104)3 for Ln = La, Pr, Nd, Gd, Er complexes were obtained. The monocomplexes contain coordinated perchlorate ions while the bis complexes contain ionic perchlorate ions. 

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. 

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... 

The polymer-metal complexes stabihzed by the formation of intra- or in-termacromolecular ionic and coordination bonds were precipitated. The adsorption capacity of linear polybetaines decreases with PCEAC-Gly > PCEAC-Ala > PCEAC-Lys, and depends on the metal cation to be bound with the following orders ... 

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