Anions radical cations

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Process Free radical Anionic Cationic Zeigler... 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

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. 

Why is it possible to manufacture polystyrene by radical, anionic, cationic and coordination polymerization methods ... 

The B state responds much less to changes in donor and acceptor properties than the TICT state, and Eq. (5.1) can often be easily fulfilled by increasing donor and/or acceptor strength. In addition to these two factors which deliver the decisive part of the reaction driving force, polar solvent stabilization SOiv and the mutual Coulombic attraction C of the linked donor and acceptor radical anion/cation also help to preferentially stabilize the TICT state with respect to the precursor B state. 

Normally, the reaction partners in PET reactions are neutral molecules. That is why a donor radical cation—acceptor radical anion pair is obtained by the PET step. These highly reactive intermediates can be used for triggering interesting reactions. Since the PET is not restricted to neutral molecules PET reactions of donor anions and neutral acceptors or neutral donors and acceptor cations resulting in radical—radical anion (cation) pairs are known as well. These reactions are also called charge shift reactions due to the fact that the overall number of charged species is kept constant throughout the PET step. Finally, a PET process of a donor anion and a acceptor cation is possible as well (Scheme 2). 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Materials that are constructed from organic polymers such as polyethylene, polystyrene, polyisoprene (natural rubber and a synthetic elastomer) and poly(vinyl chloride) are common features of our daily lives. Most of these and related organic polymers are generated from acyclic precursors by free radical, anionic, cationic or organometallic polymerisation processes or by condensation reactions. Cyclic precursors are rarely used for the production of organic polymers. 

Synthesized polymer hybrids based on polyolefins are expected to possess the properties of each polymer segment. Several methodologies for the preparation of polyolefin hybrids have been reported in recent papers [27,28]. In particular, processes such as radical, anionic, cationic polymerizations and... 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

The rate of electron transfer and its potential dependence can be described by the Butler-Volmer equation (20) (see Section 2). An electron transfer often initiates a cascade of homogeneous chemical reactions by producing a reactive radical anion/cation. The mechanism can be described mathematically by a rate equation for each species these form part of the electrochemical model. The rate law of the overall sequence is probed by the voltammetric experiment. 

Thermodynamics determines whether or not a monomer will polymerize, to what extent it polymerizes, and what conditions such as solvent, temperature, and concentrations are required. As discussed in Chapter 1, the thermodynamic polymerizability of a monomer is independent of the mechanism and is therefore identical for radical, anionic, cationic, and coordinative mechanisms if structurally identical polymers are obtained. Although this requires that both the end groups and the microstructure are the same, the influence of regioselectivity and stereoselectivity on the enthalpy and/or entropy of polymerization has not been confirmed experimentally yet. 

Kinetic aspects of step-growth copolymerization have been examined in Section 10.2.2. The principal features of chain-growth copolymerization are very different, but are alike for all types of chain growth, that is, for free-radical, anionic, cationic, and coordination polymerization. 

The reactivity ratios for pairs of given monomers can be very different for the different types of chain-growth copolymerization free-radical, anionic, cationic, and coordination copolymerization. Although the copolymer equation is valid for each of them, the copolymer composition can depend strongly on the mode of initiation (see Figure 10.8). 

Free-radical polymerization. No matter whether the propagating centers are free radicals, anionic, cationic, or coordinated, the propagation rate is equal to the sum of the consumption rates of the two monomers, given by eqns 10.93 ... 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

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