Step-growth polymerizations thermosetting polymers

Traditionally, we create thermoset polymers during step growth polymerization by adding sufficient levels of a polyfunctional monomer to the reaction mixture so that an interconnected network can form. An example of a network formed from trifimctional monomers is shown in Fig. 2.12b). Each of the functional groups can react with compatible functional groups on monomers, dimers, trimers, oligomers, and polymers to create a three-dimensional network of polymer chains. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

Abstract Polymers are macromolecules derived by the combination of one or more chemical units (monomers) that repeat themselves along the molecule. The lUPAC Gold Book defines a polymer as A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. Several ways of classification can be adopted depending on their source (natural and synthetic), their structure (linear, branched and crosslinked), the polymerization mechanism (step-growth and chain polymers) and molecular forces (Elastomers, fibres, thermoplastic and thermosetting polymers). In this chapter, the molecular mechanisms and kinetic of polymer formation reactions were explored and particular attention was devoted to the main polymerization techniques. Finally, an overview of the most employed synthetic materials in biomedical field is performed. 

Epoxy adhesives represent the most common structural adhesives and have gained wide acceptance in many diverse industries. They essentially consist of an epoxy resin, often based upon the diglycidyl ether of bisphenol A, and harden to give a thermosetting polymer by step-growth polymerization or addition polymerization. 

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... 

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. 

In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

The description of the variety of chemistries that are used to produce thermosetting polymers can be the subject of a whole book and is beyond the scope of this chapter. A description of chemistries involved in the synthesis of several families of thermosets can be found elsewhere [2]. In this section, we focus on some aspects of the chemistry of epoxy polymers because it provides examples of both step-growth and chain-growth polymerizations employed in the synthesis of polymer networks. 

The wiggly lines in the above illustration imply that the polymer extends further in their directions. The above illustration is one of a thermoset polymer that is formed by the step-growth mechanism. It is also possible to form crosslinked polymers by the chain-growth mechanism. This requires presence in the polymerization mixture of a comonomer that possesses multiple functionality. Copolymerization of styrene with a comonomer like divinyl benzene can serve as an example ... 

In this section, we first present an overview of the evolution of rheological properties during polymerization of linear chains by free-radical and step-growth mechanisms. Then, we turn our attention to thermosetting polymers that form permanently cross-linked polymer networks. 

Thermosetting polymers can be made using either step growth or chain polymerization procedures. To obtain a crosslinked polymer, at least one of the monomers used must be trifunctional. A condensation production process is used to produce phenolic polymers which have the highest volume usage of all thermosets. The reaction between phenol and formaldehyde to form a thermoset phenolic polymer is shown in Fig. 4.54. Three... 

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