Chain Growth Polymerization of Thermoplastics

Chain growth polymers comprise most of the commodity polymers found in consumer products. Common examples include the polyethylene used in trash can liners, the polyvinyl chloride used as wire insulation, and the polypropylene used in food storage containers. - 

Chain growth 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. 

The choice of one polymerization method over another is defined by the type of monomer and the desired properties of the polymer. Table 2.1 lists advantages and disadvantages of the different chain gro vth mechanisms. Table 2.2 summarizes some well known addition polymers and the methods by which they can be polymerized. 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required 

Anionic polymerization Narrow molecular weight distribution Limited chain transfer reactions Predictable molecular weight average Possibility of forming living polymers End groups can be tailored for further reactivity Solvent-sensitive due to the possibility of chain transfer to the solvent Can be slow Sensitive to trace impurities Narrow molecular weight distribution 

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We can create thermoplastic polymers by chain growth or step growth reactions. In either case the polymer chains consist of a string of monomer residues, each of which is attached to two other monomer residues. The polyethylene molecule shown in Fig. 1.1 is an example of a thermoplastic polymer made via chain growth polymerization, as shown in Fig. 1.7,... 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

The two principal types of polymerization for thermoplastics including engineering thermoplastics are polycondensation polymerization and chain-growth polymerization. Both types can usually produce hnear, branched, crosslinked amorphous and semicrystalline aromatic and aliphatic polymers [14]. 

The term solution acrylics refers to acrylic resins prepared by chain-growth polymerization using a solutionbased polymerization process. Here, acrylic monomers and initiators are slowly added to an organic solvent and polymerization is carried out at a predetermined temperature and inert atmosphere with efficient stirring. Both monomers and the polymer formed are miscible in the selected solvent. With the progress of polymerization, the solution viscosity will Increase and heat transfer becomes difficult, limiting the solid content of the final solution. Both thermoplastic and thermosetting solution acrylics can be prepared by this technique. 

Typical epoxy resins used to formulate epoxy adhesives have at least two epoxy rings, usually at the ends of a relatively short-chain prepolymer. The epoxy groups then are reacted with other epoxy groups in a chain-growth polymerization or with another curative in a step-growth polymerization to produce a polymer network, which can be either thermoplastic or thermoset, The polymer linkages created by reaction of the epoxy ring are polar... 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

Step-growth polymerizations in extruders, both polycondensations and polyadditions, are far less investigated than chain growth reactions. Because the polymer has to remain thermoplastic, only bifunctional monomers should be used, and the molecular weight can be controlled by the addition of a small amount of monofunctional monomers. For both polycondensation and poly-addition reactions the feeding should be very accurate and stochiometrically correct, because otherwise the conversion and therefore pressure built up will be seriously restricted. 

Anionic polymerization requires a type of monomer that contains an electron-withdrawing substituent such as phenyl, carboxyl, nitrile, and diene. Successful industrial examples are some styrenic products such as styrene-butadiene mbber (SBR) and styrene-butadiene-styrene (SBS) thermoplastic elastomer resins. Commonly used industrial catalysts are ethyl lithium (C2H5li) and sodium naphthalide (CioHgNa), which quickly dissolves and dissociates in a proper solvent. The primary anion R" reacts with monomer and initiates chain growth through successive propagation steps ... 

Several families of polymers constitute this category of thermoplastics, but two are particularly important polyesters and polyamides. These polymers are generally obtained by step growth polymerization using reactions whose mechanism was described in Chapter 7. Polyesters and polyamides obtained from chain polymerization by ring opening of heterocycles will also be presented in this chapter, with the two methods used to obtain the same material. 

The most important chain-growth polymers are polyolefins and other vinyl polymers. Examples of the former are polyethylene, and polypropylene, and examples of the latter are poly(vinyl chloride), polystyrene, poly(vinyl alcohol), polyacrylonitrile, and poly(methyl acrylates). The most common stepwise reactions are condensation polymerizations. Polyamides, such as nylon 6-6, which is poly(hexamethylene adipamide), and polyesters, such as poly(ethylene terephthalate), are the most important commercial condensation polymers. These polymers were originally developed for use in fiber manufacture because of their high melting points but are now used also as thermoplastics. Polycarbonate is an engineering plastic that is made from bisphenol A and phosgene by a stepwise reaction. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

The product is an ABA-type triblock thermoplastic elastomer. Styrene is polymerized first since styryl initiation of isoprene is faster than the reverse reaction. The reaction is carried out in a nonpolar solvent with Li as the counterion to enable a block of cis-1,4-polyisoprene to be formed in the second growth stage. The living polystyrene-6-polyisoprene AB di-block copolymer thus formed is then coupled by a double nucleophilic displacement of Cl ions from dichloromethane to give a polystyrene-Z -polyisoprene- i-polystyrene triblock copolymer. (Note that the mole ratio of living diblock chain to dichloromethane is 2 1.)... 

Bio-based monomers with more complex chemical structure and multiple functionalities suitable for step-growth or for ring-opening chain polymerization expand the scope of macromolecular engineering based on glucidic feedstock. Lactide monomers, obtained by the cyclodimerization of lactic acid produced by bacterial fermentation of carbohydrates, is chemically polymerized into renewable, biocompatible and biodegradable thermoplastics, poly(L-lactic acid) and related polymers, well-suited for a broad range of commercial uses. ... 

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