Catalysis by metallic compounds

Self-catalyzed esterification is often too slow to be of practical use, especially because hydroxyl-terminated polymers are either sought or are a consequence of the process (aromatic polyesterifications are carried out with a large excess of hydroxyls at the beginning of the process, because of the low solubility of the diadd), and strong protic acids are not advisable, as they would catalyze polymer hydrolysis if allowed to remain with the polymer. Even volatile catalysts such as methanesul-fonic acid are avoided. Therefore, metallic salts are currently used as catalysts, both for esterification and alcoholysis. Strong bases, such as lithium hydroxide, can also be used, but for alcoholysis only (as in polycarbonate formation). 

Metals in metallic complexes can catalyze esterification and alcoholysis through two distinct mechanisms [117]  

Their activity with respect to esterification and alcoholysis has been compared by Habib and Malek [105,106] and Chung [107], who found volcano-shaped relationships with different optima of activity of the several metals in terms of metal electronegativity according to Tanaka [108] for the glycolysis of dimethyl terephthalate 

There are striking differences [109, 110] between the two groups of metals as regards sensitivity to inhibition by carboxyls (which poison the first group) or by hydroxyls (which poison the second group). Titanium has the best balance of properties, because it is little inhibited by carboxyls and hydroxyls (which poison Sb) and also efficiently catalyzes esterification. It is also active at surprisingly low concentrations [117]. 

The choice of catalyst also depends on secondary reactions (Ti causes yellowing), toxicity (a problem for Sb) and price (expensive Ge will nevertheless yield a white polymer). 

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