Anionic chain-growth polymerization

Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

Chapters 5 through 7 deal with polymers formed from chain-growth polymerization. Chain-growth polymerization is also called addition polymerization and is based on free radical, cationic, anionic, and coordination reactions where a single initiating species causes the growth of a polymer chain. 

In chain-growth polymerization, propagation is caused by the direct reaction of a species bearing a suitably generated active center with a monomer molecule. The active center (a free radical, an anion, a cation, etc.) is generated chainwise by each act of growth the monomer itself constitutes the feed (reactive solvent) and is progressively converted into the polymer. 

Cationic and anionic chain-growth polymerizations occur by chain reactions similar to those for free-radical polymerizations but involving charged intermediates (14.3 14.4). 

Polyphosphazene block copolymers were synthesized by these chain-growth polymerization methods. The successive anionic polymerization of N-silylphosphoranimines 19d and 19a at 133 °C yielded the block copolymer with Mw/Mn of 1.4-2.3 (Scheme 80) [278,279]. However, due to the presence of two possible leaving groups in 19d, this approach yielded block copolymers where one of the block segments contained a mixture of side groups. On the other hand, the cationic polymerization of 19b with PCI5 was carried out at ambient temperature, followed by addition of a second phosphoranimine to yield a block copolymer with Mw/Mn of 1.1-1.4, where each block segment had one kind of side chain (Scheme 81) [280]. 

The polymerization of some monomers does not fall neatly into either of the mechanisms discussed above. We will take up a few of them (e.g., anionic and coordination polymerizations) after we further develop step-growth and chain-growth polymerizations. Some polymerizations can proceed by either mechanism, depending upon the specific monomer or the reaction conditions. The most notable examples, ring-opening polymerization and some of the newer chemistries, are presented as separate categories toward the end of the chapter. 

The process of forming an addition polymer by chain-growth polymerization involving an anion at the end of the growing chain, (p. 373)... 

Use mechanisms to show how monomers polymerize under acidic, basic, or free-radical conditions. For chain-growth polymerization, determine whether the reactive end is more stable as a cation (acidic conditions), anion (basic conditions), or free radical (radical initiator). For step-growth polymerization, consider the mechanism of the condensation. 

Lewis bases initiate anionic chain growth polymerizations, the generalized reaction being that of a propagating alkoxide anion. 

Unlike ordinary chain reactions, chain-growth polymerization need not involve free radicals. The reactive center may instead be a carbanion or carbocation generated by intermolecular transfer of a proton or electron. Depending on the sign of the ionic charge on the chain carriers, the overall reaction is called anionic or cationic polymerization. As in free-radical polymerization, initiation is required. 

Chain growth differs from step growth in that it involves initiation and usually also termination reactions in addition to actual growth. This makes its kinetic behavior similar to that of chain reactions (see Chapter 9). However, the chain carriers in chain-growth polymerization need not be free radicals, as they are in ordinary chain reactions. Instead, they could be anions, cations, or metal-complex adducts. While the general structure of kinetics is similar in all types of chain-growth polymerizations, the details differ depending on the nature of the chain carriers. 

The chain carriers in chain-growth polymerization may be anions or cations rather than free radicals. Such ionic polymerization shares many features with free-radical polymerization, but differs in one important respect Since ions of the same charge sign repel one another, spontaneous binary termination by reaction of two chain carriers with one another cannot occur. In fact, the reaction may run out of monomer with chain carriers still intact. 

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. 

Anionic chain growth polymerizations are particularly distinguished from free-radical polymerizations in the following respects. 

In summary, cationic polymerizations are much more variable and complex than homogeneous free-radical or anionic chain-growth polymerizations. No convincing general mechanism has been provided for cationic reactions, and each polymerization system is best considered as a separate case. 

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. 

Chain-growth polymerization is a chain reaction that converts an organic starting material, usually an alkene, to a polymer via a reactive intermediate—a radical, cation, or anion. 

Chain-growth polymerization can also occur by way of cationic or anionic intermediates. Cationic polymerization is an example of electrophilic addition to an alkene involving carboca-tions. Cationic polymerization occurs with alkene monomers that have substituents capable of stabilizing intermediate carbocations, such as alkyl groups or other electron-donor groups. The initiator is an electrophile such as a proton source or Lewis acid. 

Anionic polymerization (Section 30.2C) Chain-growth polymerization of alkenes substituted by electron-withdrawing groups that stabilize intermediate anions. 

Polymerization reactions require stringent operating conditions for continuous production of quality resins. In this paper the chain-growth polymerization of styrene initiated with n-butyllithium in the presence of a solvent is described. A perfectly mixed isothermal, constant volume reactor is employed. Coupled kinetic relationships descriptive of the initiator, monomer, polystyryl anion and polymer mass concentration are simulated. Trommsdorff effects (1) are incorporated. Controlled variables include number average molecular weight and production rate of total polymer. Manipulated variables are flow rate, input monomer concentration, and input initiator concentration. The... 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

Although the presence of water is generally not an issue in free-radical chain polymerization (indeed water may be a suitable medium for polymerization as in Protocols 5-7) unlike, for example, chain-growth polymerization initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of predrying. In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization... 

Addition polymerization of vinyl monomers is one of the most popular classes of chain-growth polymerization. Depending on the nature of the active species, the polymerizations are categorized as cationic, radical, or anionic. These polymerizations are usually very fast and highly exothermic. 

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