Polymerization reaction free radical curing mechanisms

Resin-modified glass—ionomer lining and restorative materials add a multifunctional acidic monomer to the poly(acryhc acid) [9003-01 Hquid component of the system. Once the glass powder and Hquid are mixed, setting can proceed by the acid—glass—ionomer reaction or the added monomer can be polymerized by a free-radical mechanism to rapidly fix the material in place (74,75). The cured material stiH retains the fluoride releasing capabiHties of a glass—ionomer. 

There are essentially two different mechanisms that UV curing may occur by — free radical or cationic. Free radical polymerization is the most predominantly used route and will be discussed first. The chain reaction that occurs consists of at least four steps ... 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

Chainwise Polymerizations A typical example of a thermoset produced by a chainwise polymerizahon is the case of the cure of unsaturated polyesters with styrene by a free-radical mechanism. Styrene is a bifiinctional monomer, A2, characterized by the presence of one C=C group that is transformed into a -C-C- bond in the polymerization reaction. The unsaturated polyester is a multifunctional monomer, Aj, characterized by the presence of if 12) C=C groups in its chemical structure. The molar fraction of C=C groups belonging to the multifunctional monomer is given by ... 

It is well known that molecular oxygen inhibits free-radical polymerization by scavenging the initiator radicals, which not only reduces the polymerization rate but also affects the mechanical, optical, and structural properties of the cured systems. The mechanism of O2 inhibition is represented in Figure 6(a). As a result of a photooxidation reaction, peroxy radicals (or hydroperoxides or alcoxy radicals) are generated, which are less reactive toward monomer to initiate... 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

Anaerobic adhesives derive their name from the characteristic of requiring a relatively oxygen-free environment for proper cure, such as found in closely mating assemblies. Oxygen inhibits free radical polymerization by the mechanism shown in Eq. (1). The active radical R reacts with molecular oxygen to form an inactive hydroperoxy radical before initiation or chain propogation can occur. Anaerobics can be used as one-part systems, relying on reactions with the active metal surface of the substrate to provide the redox initiation. 

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