Polyphosphoric acid esters selective

Alkaline hydrolysis of 16 in dimethylsulfoxide occurred selectively at the halogen atom adjacent to the nitro group to yield the phenol 17 which was alkylated to 18. Reductive cyclization of 18 followed by condensation with diethylethoxymethylene malonate and heating in polyphosphoric acid at 145 °C yielded the ester 19 (Fig. 16). 

The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21-23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the ester-interchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. 

The principal phosphation reagents used to manufacture phosphate esters are phosphoric anhydride (P4O10), phosphorus oxychloride [P(0)Cl3], and polyphosphoric acid. It is important to understand the physical and chemical characteristics of each raw material. The structure of the phosphation agent affects its reactivity and selectivity, resulting in variation in the monoester, diester, and triester product distribution. 

---