Radical and ionic polymerization

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

An important difference between free radical and ionic polymerization is that a counter ion only appears in the latter case. For example, the intermediate formed from the initiation of propene with BF3-H2O could be represented as... 

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. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Copolymerization behavior can also be used to distinguish between radical and ionic polymerizations (see Chap. 6). 

Funt and Williams reported that the copolymer compositions of methyl methacrylate and acrylonitrile varied at the same electrode depending upon the salts used in the saturated dimethyl formamide solution (Table 5) (19). They believed in the simultaneous occurrence of free radical and ionic polymerization in the system. Yield of polymers also differed with a variety of salts in the polymerization of methyl methacrylate in dimethyl sulfoxide (<5). Nevertheless no pattern of correlation has been given on the aspects relating to the supporting electrolytes. 

Greater differences between the optical activity of monomers and of polymers have been recently observed by Schulz and Hartmann 133) when investigating the free-radical and ionic polymerization of a number of variously substituted N-vinyl compounds. The above authors also observed in one case a large dependence of the optical activity of the polymers on the type of solvent used. 

Delocalization of bonding electrons in the transition complex is an important event. In the absence of this far-reaching delocalization, even the most acid alkenes, ethylene and propene only undergo both radical and ionic polymerization reluctantly. 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

The formation of polymer, which is by far the most important product, has been attributed to both free-radical and ionic-polymerization reactions. 

Thus, in these cases, UV radiation provides a method for the initiation of radical or ionic polymerization. After the initiation step, chain propagation proceeds as though initiation resulted from the use of conventional catalysts for radical and ionic polymerization. 

Chain Homopolymerization Mechanism and Kinetics Free radical and ionic polymerizations proceed through this type of mechanism, such as styrene polymerization. Here one monomer molecule is added to the chain in each step. The general reaction steps and corresponding rates can be written as follows ... 

Thus NMR studies of monomers are an important tool for understanding the basic problems in radical and ionic polymerization reactions. 

Items 2 and 3 arise from the fact that both the "counterion" and the medium itself can markedly affect the nature of the growing chain end. Thus, the growing chain end may assume various forms that depend on the extent of electrical charge separation and range all the way from a polarized covalent (sigma) bond to a completely dissociated state of free ions. This characteristic presents the greatest distinction between the mechanisms of free-radical and ionic polymerization. 

Fukui et al.20), using the simple molecular orbital theory, calculated the stabilization energy arising from -conjugation in the transition state of radical and ionic polymerization of vinyl monomers. Further correlation of monomer structures with reactivities revealed that the rate constant of crosspropagation with one chosen active center increases with the basicity of the comonomer. 

A great many reactions in physics and chemistry proceed via chain mechanisms. This large family of mechanisms includes free radical and ionic polymerization, Fischer Tropsch synthesis, gas phase pyrolysis of hydrocarbons, and catalytic cracking. Nuclear reactions, of both the power generating and the explosive kind, are also chain processes. Notice that chemical chain reactions can be catalytic or non-catalytic, homogeneous or heterogeneous. One is almost tempted to say that chain reactions are the preferred route of conversion in nature. 

Compressed liquid or supercritical carbon dioxide has been recognized as a useful alternative reaction medium for radical and ionic polymerization reactions (see Chapter 4.5). Many of the benefits associated with the use of SCCO2 in these processes apply equally well to polymerizations relying on a metal complex as the chain-carrying species. However, the solubility of the metal catalyst and hence the controlled initiation of chain growth add to the complexity of the systems under study. Furthermore, many of the environmental benefits would be diminished if subsequent conventional purification steps were needed to remove the metal from the polymer. Nevertheless, the interest in metal-catalyzed polymerizations is increasing, and some promising systems have been described. 

The period following the Second World War saw the emergenee, with an accelerated speed, of new polymerization methods in 1953-1954, polymerization catalysis by coordination was developed by K. Ziegler and G. Natta (Nobel Prize, 1963), which led to for high-density polyethylene (PEHD) and polypropylene (PP). Anionic polymerization and the concept of living polymerization proposed by M. Szwarc in 1956 led to the design of blocks copolymers and the first macromolecular architectures. We then saw the emergence of catalysis by metallocene in 1980 by W. Kaminski. Radical polymerization controlled by M. Sawamoto and K. Matyjaszewski in 1994 combined the benefits of radical and ionic polymerization without the drawbacks of the former. 

---