Chain copolymerization unsaturated polyester

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. The most common unsaturated polyester is made by step growth polymerization of propylene glycol with phthalic and maleic anhydrides. Subsequent treatment with styrene and a peroxide catalyst leads to a solid, infusible thermoset. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. 

Similar to unsaturated polyesters, they are copolymerized with diluents such as styrene using similar free-radical initiators. They differ from polyesters in that the unsaturation is at the end of the molecule and not along the polymer chains. Their burning behavior falls between that of polyester and epoxy resins (LOI = 20-23 vol%, Table 2.4). 

Unlike the polyurethanes and other previously discussed mortars, polyester cements are produced by a free radical chain copolymerization of a liquid unsaturated polyester and styrene. While most polyester composites are reinforced by fiber glass, polyester mortars are usually filled with silica, clay or alumina trihydrate (ATH). 

Finally, the unsaturated polyester is free-radically cross-linked by copolymerization with, for example, styrene or methyl methacrylate. Mixtures of the actual unsaturated polyester with these monomers are commercially known as unsaturated polyester resins. The properties of the thermosets can be matched to the application by variations in the acids, glycols, or vinyl monomers. Copolymerization with electronegative comonomers such as styrene or vinyl acetate leads, for example, to alternating copolymers, that is, to short cross-link bridges and therefore, to more rigid thermosets. Alternatively, electropositive comonomers such as methyl methacrylate form long methyl methacrylate bridges between the polyester chains and so produce more flexible polymerizates. 

Allyl esters, unsaturated polyesters, as well as some of what are known as vinyl or acrylic esters are cured by free radical addition polymerization. In the case of allyl esters, the monomers, themselves, are cross-linked. On the other hand, unsaturated polyesters are copolymerized with monomers such as styrene or methyl methacrylate. Since the unsaturated polyesters have many main-chain double bonds and the structure of a cross-linked network is fixed after quite low conversions, only a few double bonds actually react. These unconverted double bonds can then react later with atmospheric agents, and so produce poor weathering properties of the crosslinked networks. In addition, the polymerization produces many free chain ends that contribute nothing or even disadvantageously to the mechanical properties. The newly developed vinyl or acrylic esters avoid both of these problems in that the monomers capable of cross-linking only have unsaturated double bonds at the molecular ends (see also Section 26.4. S). 

Structopendant unsaturated polyesters, containing double bonds within the polymer chain, are produced by step-growth polycondensation reaction of an unsaturated diacid or anhydride, such as fumaric add or maleic anhydride, with a diol. Structural unsymmetry in the diol component lowers the viscosity of the prepolymer. Mostly, crosslinking of the structopendant unsaturated polyester is accomplished by copolymerization with alkene monomers such as styrene, methyl methacrylate, or others using radical initiators. 

From unsaturated polyesters with carboxylic end groups at both chain ends, after neutralization they are efficient emulsifiers for lipophilic monomers [95], and with styrene as comonomer microgels can be prepared with rather uniform diameter [96]. By using lipophilic initiators, such as 2,2 -azobis(isobutyronitrile) (AiBN), in the microemulsion copolymerization, diffusion of monomers is too slow compared with the reaction rate. Therefore, copolymerization is confined to the coherent, lipophilic phase [97,98] and very small microgel particles with a rather uniform size result. Research work by Funke and cited by Funke [76] indicates the usefulness of microemulsion copolymerization to convert unsaturated polyesters into microgels. 

As has been noted previously, the cross-linking of unsaturated linear polyesters involves the reaction of the unsaturated sites in the polymer chain with a vinyl-type monomer. This reaction is analogous to conventional vinyl copolymerization and proceeds by an essentially similar mechanism. As carried out in commercial practice, cross-linking of unsaturated polyesters is invariably a free radical reaction. Two types of initiating systems are commonly employed for this reaction, namely those effective at elevated temperatures and those effective at room temperature. 

Unsaturated polyester resins are low MW condensation polymers which are transformed to a cross-linked network via radical initiated polymerization. The double bonds in the backbone copolymerize with unsaturated monomer (reactive diluent) present in the system. During polymerization, relatively short low MW polyester chains are cross-linked by short bridges consisting of, on average, around two to three styrene units, forming a densely cross-linked polymer network (see Figure 2.17). 

UP prepolymers possess a molar mass (A/ ) ranging between 1800 and 2500 g moP and about 5 to 8 double bonds per chain. In order to form a network, they are dissolved in neat styrene whose radical polymerization is most often initiated by a peroxide. Then there is a copolymerization between the bivalent styrene (w = 2) and the plurivalent unsaturated polyester acting as a comonomer. The average valence of the latter is twice the average number of unsaturations per chain. The reaction mechanism of the process is described below ... 

After quite a short polystyrene chain is formed, the free radical end encounters polyester unsaturation and copolymerization occurs ... 

This will be recognized as the mechanism for chain polymerization which was described in Chapter 5 for the formation of polyethylene. However here, after quite a short polystyrene chain is formed, the free radical end encounters polyester unsaturation and copolymerization occurs ... 

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