Читать книгу Poly(lactic acid) - Группа авторов - Страница 44

Carboxylic acids are poor initiators, but they are believed to interfere with the commonly used Sn(II) polymerization catalyst. According to Kowalski, carboxylic acids may suppress the rate of polymerization by shifting the equilibrium between ROH and Sn(Oct)₂ to the inactive Sn(Oct)₂ side [83, 84]. Consequently, longer polymerization times are needed to achieve the desired molecular weight, accompanied by unavoidable degradation caused by the extra residence time at high temperature in the presence of a catalyst [84].

The effect of carboxylic acids on lactide polymerization rate was published in 1993 in patents by Ford and O'Brien [78, 85]. The results clearly show the dramatic rate‐decreasing effect of organic acids: according to O'Brien, melt polymerization slows down by a factor of 2 upon increasing free acidity from less than 2 to between 2 and 4 meq/kg [85].

Witzke, however, states that the presence of lactic acid did not negatively influence polymerization rate [4, 6]. Lactic acid is therefore a practically used initiator that is already present in lactide as an impurity.

Lactic acid and its oligomers have a hydroxyl group and a carboxylic acid group. Consequently, a free acidity of 10 meq/kg—that is, 900 ppm expressed as lactic acid equivalents— in lactide corresponds to a hydroxyl concentration that limits M _n to 100 kg/mol. Free acidity of 4 meq/kg sets a theoretical limit of 250 kg/mol to M _n.

Free acid and water content specifications are essential for any lactide grade; the lower the amount of hydroxyl impurities, the better the storage stability and product properties of the lactide.