Enzyme-Catalyzed Ring-Opening Polymerization of ω-Pentadecalactone

Abstract

Lipase-catalyzed ring-opening bulk polymerizations of ω-pentadecalactone (PDL) were investigated. Screening of selected commercial lipases as catalysts for PDL polymerization at 80 °C was carried out. The results of this work showed that polymerizations catalyzed by lipases PS-30, AK, Lipozyme-IM and Novozym-435 gave % PDL conversions ranging from 80 to 100% for 24 h reactions (Mn = 15 000−34 400). Lipase PS-30 both physically immobilized onto Celite-521 (I-PS-30) and in the crude powder or nonimmobilized form (NI-PS-30) was selected for further study. Comparison of % conversion vs time for bulk PDL polymerizations at 70 °C catalyzed by NI- and I-PS-30 showed that for short reaction times, the immobilized catalyst gave % conversions that were more than 10 times greater. In fact, the % monomer conversion to poly(PDL) was nearly quantitative (>98%) for 8 h polymerizations catalyzed by I-PS-30. Furthermore, for reactions conducted at 70 °C with careful removal of water, substantially greater poly(PDL) molecular weights resulted by using I-PS-30 instead of NI-PS-30 as the catalyst. Increasing the % conversion above ∼40% for PDL polymerizations at 70 °C resulted in little or no change in PDL Mn. This is consistent with chain polymerizations where the rate of propagation is much faster than initiation. The general trends observed by variation of the I-PS-30 catalyzed bulk PDL polymerization temperature were the following: (i) increased % conversion and Mn by increasing the reaction temperature from 60 to 70 °C and from 60 to 80 °C, respectively, (ii) similar polymerization rates between 70 and 90 °C, and (iii) a decrease in % monomer conversion and Mn as the reaction temperature was increased from 90 to 110 °C. It was found that water was an important factor that controls not only the rate of monomer conversion but also the polymer molecular weight. From an increase in the water content in reactions, enhanced polymerization rates were achieved while the molecular weight of poly(PDL) decreased. At low reaction water levels (0.20% w/w water), the I-PS-30 catalyzed polymerization of PDL at 70 °C gave poly(PDL) with Mn and Mw/Mn of 62 000 and 1.9, respectively. Thus far, this is the highest molecular weight polyester prepared by an enzyme-catalyzed polymerization reaction.