Maleic anhydride forming rigid polyesters

Solution As we shall see in Chapter 8, the copolymerization of maleic anhydride and styrene shows a strong tendency toward alternation (ri rj = 0.06). Consequently, a maleic anhydride-based unsaturated polyester cross-hnked with styrene forms a large number of short cross-links, resulting in a rigid polymer. With the addition of phthalic anhydride, the tendency toward alternation is reduced due to the occasional intervention of phthahc anhydride. The reduction of cross-link density results in a less rigid polymer. 

The largest single use of maleic anhydride is in the preparation of unsaturated polyester resins. It is first esterihed with a polyalcohol (two or more hydroxyls) and then the double bond is copolymerized (crosslinked) with a vinyl monomer such as styrene to form a rigid structure. Such resins are usually reinforced with hberglass (FRP). Maleic anhydride is also used to make oil additives and agricultural chemicals. 

Example 5.5 An unsaturated polyester based entirely on maleic anhydride as the dibasic acid, when cross-linked with styrene, forms a hard and rigid polymer. However, that based on maleic anhydride mixed with some phthahc anhydride forms a less rigid polymer. Explain. 

Maleination of soybean oil is interesting as it has been shown to produce two wildly different polymer structures. Maleic anhydride-modified soybean oil was polymerized with styrene to give a hard, rigid polymer modification improved modulus, strength and glass transition values when compared to the unmodified polymerized oil [45], These also showed comparable strengths to commercial unsaturated polyester resins. In contrast, by using diols as the comonomer, maleinated soybean oil was used to form soft, resilient rubbers at room temperature [46]. 

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