Styrene, chain growth polymerizations

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... 

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. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

Radical polymerization of alkenes was first discussed in Section 15.14, and is included here to emphasize its relationship to other methods of chain-growth polymerization. The initiator is often a peroxy radical (RO-), formed by cleavage of the weak 0-0 bond in an organic peroxide, ROOR. Mechanism 30.1 is written with styrene (CH2=CHPh) as the starting material. 

Problem 30.9 Explain why styrene (CH2=CHPh) can be polymerized to polystyrene by all three methods of chain-growth polymerization. 

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... 

FIGURE 3.4 Polymerization steps and kinetic equations relevant in the chain growth polymerization exemplified for the free radical polymerization of styrene initiated with azobisisobutyronitrile (AIBN) denote the kinetic constants for... 

In chain-growth polymerization, monomers can onlyjoin active chains. Monomers contain carbon-carbon double bonds (e.g., ethylene, propylene, styrene, vinyl chloride, butadiene, esters of (meth)acrylic acid). The activity of the chain is generated by either a catal) t or an initiator. Several classes of chain-growth polymerizations can be distinguished according to the type of active center ... 

Bulk polymerizations of monomers like acrylates, methacrylates, and styrene belong to the class of free-radical chain-growth polymerizations (or addition polymerizations). With this type of reaction, each polymer chain is formed in a relatively short time and subsequently excluded from further participation in the reaction process. As a consequence, at the beginning of the reaction when the monomer concentration is high, large chains are formed in a monomeric environment, and while the reaction progresses until a certain conversion, the chains formed become shorter. The chain formation can be divided into three steps ... 

Chain Growth Polymerization of Styrene and Conjugated Dienes... 

Chain-growth polymerization is perhaps the most important and complex example of a mixed series/parallel reaction. For example, polystyrene is formed by adding one styrene molecule at a time to the end of a growing polymer chain. 

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 - butadiene and styrene can form a copolymer. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. The most common unsaturated polyester is made by step growth polymerization of propylene glycol with phthalic and maleic anhydrides. Subsequent treatment with styrene and a peroxide catalyst leads to a solid, infusible thermoset. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

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