Cationic chain polymerization solvent effects

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The mechanism proposed by Kennedy requires that allylic or tertiary chlorines be attached to the first polymer chain. Chlorobutyl rubber, PVC, and other chlorine-containing polymers usually have 1-2% of mers with chlorines in the required reactive positions. The halogen-containing polymer is dissolved in an inert but polar solvent in the presence of an organoaluminum catalyst and a cationically polymerizable monomer, such as styrene, and polymerization is effected, usually at about — 50 C. The major point is that the second component chains can be initiated only at an active chlorine site on the first polymer. 

Cationic Poiymerizations. Homogeneous and heterogeneous cationic polymerizations of various monomers in liquid and SCCO2 have been performed (Table 2). Several different catalyst systems were employed in these polymerizations, all of which proved to be effective. The early experiments conducted using monomers such as isobutylene (67) and formaldehyde (70-72) normally resulted in a low yield of low molecular weight products. Nonetheless, proof of concept for the use of CO2 as an effective solvent for cationic chain-growth reactions was achieved. Other polymers based on different substituted alkenes (73), vinyl ethers (74,75) (eq. (4)), and oxetanes (75) (eq. (5)) have been synthesized. 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Dense carbon dioxide represents an excellent alternative reaction medium for a variety of polymerization processes. Numerous studies have confirmed that CO2 is a potential solvent for many chain growth polymerization methods, including free-radical, cationic, and ring-opening metathesis polymerizations. Carbon dioxide has also been demonstrated to be an effective solvent for step-growth polymerization techniques. Advances in the design and synthesis of surfactants for use in CO2 will allow compressed CO2 to be utilized for a wide variety of polymerization systems. These advances may enable carbon dioxide to replace hazardous VOCs and CFCs in many industrial applications, making CO2 an enviromentally responsible solvent of choice for the polymer industry. 

This marked sensitivity of the stereochemistry of anionic polymerization to the nature of the counterion and solvent can be traced to the structure of the propagating chain end. The latter involves a carbon-metal bond which can have variable characteristics, ranging all the way from highly associated species with covalent character to a variety of ionic species (Hsieh and CJuirk, 1996). The presence of a more electropositive metal and/or a cation-solvating solvents, such as ethers, can effect a variety of changes in the nature of the carbanionic chain end (a) the degree of association of the chain ends can decrease or be eliminated (b) the interaction of the cation with the anion can be decreased... 

In addition to the radical type, there is also ionic polymerization. It is initiated by ions (cations or anions), dissociation of which is naturally heavily dependent on electrostatic effects, in particular solvation by the solvent. As in radical polymerization, the ionic process consists of a chain reaction. In the start reaction, a Lewis acid or Lewis base attaches to one C atom of the double bond of a monomer. This produces a charge at the other C atom. Whether anionic or cationic polymerization takes place depends on the nature of this charge. Chain growth involves repeated attachment to a double bond, whereby the charge jumps two C atoms further. In ionic polymerization there is no chain breakage due to recombination. Termination has to be induced by adding water, alcohols, acids, or amines. If this is not done, the reaction comes to a halt when all of the monomer is used up, whereby the reactivity is maintained for some time. 

Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its constitution and substitution pattern. Also, external parameters like solvent, temperature, and pressure may also have an effect. Monomers able to grow in chain-growth polymerizations are listed in Table 2.2 of Sect. 2.1.4. 

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