Free-radical-initiated chain polymerization polystyrene

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. 

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. 

Polymerization of styrene is carried out under free-radical conditions, often with benzoyl peroxide as the initiator. Figure 11.11 illustrates a step in the growth of a polystyrene chain by a mechanism analogous to that of the polymerization of ethylene (Section 6.21). 

Initiation of a free radical chain takes place by addition of a free radical (R ) to a vinyl monomer (Equation 6.8). Polystyrene (PS) will be used to illustrate the typical reaction sequences. (Styrene, like many aromatic compounds, is toxic, and concentrations that come into contact with us should be severely limited.) It is important to note that the free radical (R ) is a companion of all polymerizing species and is part of the polymer chain acting as an end group and hence should not be called a catalyst even though it is often referred to as such. It is most properly referred to as an initiator. 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

Free-radical polymerization results when a suitable alkene is heated with a radical initiator. For example, styrene polymerizes to polystyrene when it is heated to 100 °C in the presence of benzoyl peroxide. This chain-growth polymerization is a free-radical... 

Since larger free radicals are more stable than those with small molecules, the fragmentation in the middle of the polymeric chain is favored thermodynamically compared to the formation of small molecules. However, kinetic factors also may play a role in determining the abundance of a specific compound. The formation of small radicals from the end of a polymeric chain can be kinetically favored, and, as a result, formation of small radicals in the initiation step is more common than expected based on the thermodynamic criteria. Taking as an example polystyrene, an end chain p-scissions to the aromatic ring can be written as follows ... 

Chain growth polymerization has the characteristic of having an intermediate within the process that cannot be isolated [5], The intermediate can be a metal complex, a free radical, or an ion. These intermediates are transient to the process. The terms vinyl, olefin, and addition polymerization have been associated with this process [13], Monomer units add to a chain very rapidly once it has been initiated. Initiation is the creation of an active center such as a free radical or carbanion [13], An example is the thermal decomposition of benzoyl peroxide shown in Figure 3.4. To propagate the chain, an additional monomer is added at a very rapid rate as monomer concentration is reduced. Figure 3.5 shows the propagation of polystyrene. 

Simultaneous polymerization of two monomers by chain initiation usually results in a copolymer whose composition is different from that of the feed. This shows that different monomers have different tendencies to undergo copolymerization. These tendencies often have little or no resemblance to their behavior in homopolymerization. For example, vinyl acetate polymerizes about twenty times as fast as styrene in a free-radical reaction, but the product obtained by free-radical polymerization of a mixture of vinyl acetate and styrene is found to be almost pure polystyrene with hardly any content of vinyl acetate. By contrast, maleic anhydride, which has very little or no tendency to undergo homopolymerization with radical initiation, readily copolymerizes with styrene forming one-to-one copolymers. The composition of a copolymeir thus cannot be predicted simply from a knowledge of the polymerization rates of the different monomers individually. The simple copolymer model described below accounts for the copolymerization behavior of monomer pairs. It enables one to calculate the distribution of sequences of each monomer in the macromolecule and the drift of copolymer composition with the extent of conversion of monomers to polymer. 

Another very important elass of ehain reaetions, perhaps the most important from a commereial viewpoint, ineludes those involved in polymerization. Materials such as polyethylene and polystyrene are formed in chain reactions with free radical chain carriers. These addition polymerization ehains are similar in substance to those we have been discussing, but differ in three important respects. First, the monomer, particularly when purified, is often quite unreaetive and it is necessary to use small quantities of separate substanees (initiators) that essentially trick the monomer into... 

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