RO-PCs of Cyclic Anhydrides

Apparently, the first polycondensation of a cyclic anhydride is described in a patent of 1916 which deals with the reaction of phthalic anhydride and glycerol. This reaction and other polycondensations of glycerol represent examples of a2 -I- ba polycondensations, which are discussed in Chap. 10. RO-PCs of phthalic anhydride with a,co-alkanediols were mentioned in papers and patents of Carothers, but a detailed description of individual examples was never provided. Homopolyesters of phthalic acid have never found any application, but phthalic anhydride plays a role as comonomer in unsaturated copolyesters based on maleic and/or fumaric acid. 

The concept of unsaturated polyesters (international abbreviation UP) which can be cured and hardened by heating with sources of radicals was developed by C. Ellis in the 1930s [1-3], and the first patent application was submitted in 1936 Ellis also discovered that dilution of the unsaturated polyesters with vinyl monomers, such as styrene (most widely used) or vinyl acetate is favorable for most applications. The dilution reduces the viscosity. Eases the dissolution of additives and catalysts and lowers the costs. The most frequently used catalysts for the curing process are peroxides. The variation of their structure allows for optimization of the cure. The most important application of UPs are  

These quite different appUcations require a broad variation of the properties. For instance, stiffness and harness can be enhanced by incorporation of rigid 

Quite recently, the author and coworkers demonstrated [15, 16] that polycondensation of alkanediols with maleic anhydride or substituted maleic anhydrides proceeds without isomerisation or other side reactions, when the temperature is limited to 100 °C Such low temperatures require metal triflates as catalysts, and the highest molar masses (Mns up to 12 kDa) were obtained with Sn triflate in the case of maleic anhydride or with Bi triflate in the case of citraconic anhydride. Unfortunately, this approach is not suited for technical production of UPs, because relatively large amounts of expensive catalysts are required. 

In various areas of step-growth polymerization, the past 20 years have seen increasing interest in monomers from renewable resources. An early example, namely a patent of 1962 [17] describes syntheses of UPs from maleic anhydride and mixtures of isosorbide and other diols. Isosorbide is synthesized from glucose and can technically be produced at relatively low costs, when quantities of 50,000 tons or more are needed. 

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