Step Growth Thermoplastic Polymers

The functional groups that typically participate in this type of polymerization are carboxyl, amine, and alcohol groups. Examples of step growth polymers include polyesters and nylons, which are often spun into fibers used to manufacture carpeting and fabrics, and polycarbonates, which are converted into compact discs, jewel cases, and the large bottles used in water coolers. 

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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. 

Thermoplastic resins, self-reinforced, 26 Thermoplastics, preparation of, 257-258 Thermoplastic step-growth polymers, 3 Thermosetting polyester resins, 29-31 Thermosetting resins, 3-4, 19 Thermotropic compounds, 49 THF. See Tetrahydrofuran (THF) Thiobisphenol S (TBPS), 364 Thionyl chloride, 80 activation of, 111 3,3 -linked polymers, 480 Tin-amine coordination complex, 234 Tin compounds, 86, 232-233... 

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,... 

The nylon 66 molecule shown in Fig, 1,11 is a thermoplastic polymer, created by the step growth polymerization of hexamine and adipic acid. The majority of commercial polymers are thermoplastics, which permits us to readily mold them to many useful shapes. 

Aliphatic polyesters occupy a key position in the field of polymer science because they exhibit the remarkable properties of biodegradability and biocompatibihty, which opens up a wide range of applications as environmentally friendly thermoplastics and biomaterials. Three different mechanisms of polymerization can be implemented to synthesize aliphatic polyesters (1) the ring-opening polymerization (ROP) of cyclic ketene acetals, (2) the step-growth polymerization of lactones, and (3) the ROP of lactones (Fig. 1). 

Polymers are produced via polymerisation techniques, such as anionic, cationic, step-growth and so on, which result in the formation of large molecules from monomers. The use of one type of monomer results in a homopolymer, whereas the use of two or more different monomers leads to the formation of copolymers. Polymerisation is usually controlled by the addition of a catalyst or initiator, and polymers produced by these techniques are thermoplastic in nature. 

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. 

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. ... 

Solvolytic processes are mostly applicable to thermoplastic and thermoset polymers produced by step growth reactions as shown in Table 1, [1]. 

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