Lipase-Catalyzed Ring-Opening Polymerization of 3(S)-sec-Butylmorpholine-2,5-dione

Abstract

Lipase-catalyzed ring-opening bulk polymerizations of 3(S)-sec-butylmorpholine-2,5-dione (BMD) were investigated. Selected commercial lipases were screened as catalysts for BMD polymerization at 110°C. Polymerizations catalyzed with 10 wt.-% of lipase PPL and PC result in BMD conversions of about 70% and in molecular weights of the products ranging from 5500 to 10700. Lipases MJ, CR and ES showed lower catalytic activities for the polymerization of BMD. Poly(3-sec-butylmorpholine-2,5-dione) has a carboxylic acid group at one end and a hydroxy group at the other end. During the polymerization racemization of the isoleucine residue takes place. Lipase PPL was selected for a more detailed study. The apparent rate of polymerization increases with increasing PPL concentration when the polymerization temperature is 110°C. When the PPL concentration is 5 and 10 wt.-% with respect to the monomer, a conversion of about 70% is reached after 5 d and 3 d, respectively, while for a PPL concentration of 1 wt.-% the conversion is less than 7% even after 6 d. High concentrations of PPL (10 wt.-%) result in high M̄n values (<4 d). The highest molecular weight poly(BMD), M̄n = 19900, resulted from a polymerization conducted at 120°C with 5 wt.-% PPL for 6 d. The general trend observed by varying the polymerization temperature is as follows: (i) monomer conversion and M̄n increase with increasing reaction temperature from 110 to 125°C, (ii) monomer conversion and M̄n decrease with an increase in reaction temperature from 125 to 130°C. Water content was found to be an important factor that controls both the conversion and the molecular weight. With increasing water content, enhanced polymerization rates are achieved while the molecular weight of poly(BMD) decreases.