Polymerization of Thermosets

Thermosets differ molecularlyfrom thermoplastics in that their individual chains are anchored to one another through crosslinks. The resulting network creates cohesive materials that demonstrate better thermal stability, rigidity, and dimensional stability than thermoplastics. Some examples of traditional thermosets are melamine-formaldehyde resins, hich are used to treat fabrics to make them wrinkle-free, and Bakelite (a phenol-formaldehyde resin), a historically important polymer used in many applications, such as costume jewelry, electrical switches, and radio casings. 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network. 

Secondary processes are normally employed to crosslink chain growth polymers. In one example a linear thermoplastic, such as polyethylene, is compounded with an organic peroxide that is thermally stable at standard processing temperatures but decomposes to chemically react with the polymer chain at higher temperatures creating crosslinks. 

Traditionally, we create thermoset polymers during step growth polymerization by adding sufficient levels of a polyfiinctional monomer to the reaction mixture so that an interconnected network can form. An example of a network formed from trifunctional monomers is shown in Fig. 2.12 b). 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. 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mbcture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

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Figure 1. Bisbenzocyclobutene monomers used for the polymerization of thermoset films described in this work.

Despite the efforts that have been made in recent years, in the exploitation of kinetic models for polymerization of thermosetting resin systems (see Table 1), two main problems still arise ... 

Metal complex-assisted polymerization of thermosetting resins a convenient one-step procedure for the preparation of heterogeneous catalysts... 

Metal complex-assisted polymerization of thermosetting resin... 

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