Kinetics of the Ring-Opening Polymerization of 6-, 7-, 9-, 12-, 13-, 16-, and 17-Membered Lactones. Comparison of Chemical and Enzymatic Polymerizations

Abstract

The kinetics of bulk polymerization of 6-, 7-, 9-, 12-, 13-, 16-, and 17-membered lactones initiated with a zinc 2-ethylhexanoate/butyl alcohol system at 100 °C was studied and compared with that of lipase-catalyzed polymerization. Instantaneous concentrations of the lactone monomers were determined on the basis of the relative intensities of signals in the 1H NMR spectra (500 MHz, CDCl3 as a solvent, room temperature) from the ω-methylene protons (−(CH2) x - 1CH 2 OC(O)−) (where x = 4, 5, 7, 10, 11, 14, and 15) in the lactone monomer and the polyester repeating units, respectively. Linearity of the semilogarithmic kinetic dependencies (ln([lactone]0/[lactone]) vs time), revealed a first order of propagation in monomer for all of the polymerizations studied. This kinetic behavior, pointing to the constant concentration of the involved active centers and thus to the practical elimination of termination side reaction, allowed the relative polymerization rates to be determined. The following order of polymerization rates has been obtained: 2500:330:21:0.9:1.0:0.9:1.0 for the 6-, 7-, 9-, 12-, 13-, 16-, and 17-membered lactones, respectively. The order of rates of the enzymatic polymerization, determined earlier in an independent paper, shows an inverted dependence on the ring size, namely 0.10:0.13:0.19:0.74:1.0 for the 7-, 12-, 13-, 16-, and 17-membered lactones, respectively. The resulting difference in the orders of lactone reactivities in chemical and enzymatic polymerizations is explained in terms of a difference in factors controlling polymerization rates in both processes. The ring strain, which decreases with increasing lactone size, is partially released in the transition state of the elementary reaction of the polyester chain growth, which eventually leads to faster propagation for more strained monomers in chemical polymerizations. In enzymatic polymerizations, the rate-determining step involves formation of the lactone−lipase complex. The latter reaction is promoted by the hydrophobicity of the lactone monomer, which is higher for the larger lactone rings.

Recent Developments in Lipase‐Catalyzed Synthesis of Polyesters

“…[39, 40,[83][84][85] There is a tendency that, with lipase PF, larger ring-sizes have larger polymerization rates, with one order-of-magnitude differences typically seen (Table 2); that is, the higher polymerizability of the macrolides was explained by the higher rate in the formation of EM. Values of the Michaelis-Menten constant [83] These results suggest that the reaction rate is mainly governed by the larger value of V max , and much less by the binding ability.…”

“…During the propagation reaction, ring-strain of lactone monomer is operative for the rate of ROP. [84] Today, lipase CA (Novozyme 435) is widely used; its structure was determined by X-ray crystallographic analysis.…”

Controlled ring‐opening polymerization of ε‐caprolactone using polymer‐supported scandium catalyst

“…1 Metal complexes are the most commonly explored catalysts; they include tin, 2,3 aluminum, [4][5][6][7] iron, 8,9 scandium, [10][11][12] yittrium, 13,14 zinc, 15,16 bismuth, 17 ruthenium, 18 zirconocene, 19 and so on. A popular polymerization system for PCL is ring-opening polymerization (ROP) of ecaprolactone catalyzed by organotin compound.…”

Lipase-Catalyzed Ring-Opening Polymerization of Substituted Lactones

“…In the case of unsubstituted lactones with ring size larger than six, the rate constant of the medium-size lactones in the chemical polymerization catalyzed by zinc octanoate was much larger than that of the macrolides, since the macrolides have much lower ring strain, and hence, show less polymerizability. 4 In this study, the anionic polymerizability of lactones using sodium methoxide as initiator has been compared with that by the lipase catalyst. In all cases examined, the polyesters with molecular weight of several thousands were obtained and the polymerizability of α-and ω-substituted lactones was lower than that of unsubstituted ones (Figure 3).…”

“…Our recent report showed that macrolides possessed much lower reactivity in the polymerization catalyzed by zinc octanoate than δ-valerolactone (1a) or ε-caprolactone (2a). 4 In the Pseudomonas fluorescens lipase-catalyzed polymerization, on the other hand, the reverse tendency was observed. 5 This is due to the stronger recognition of the macrolides by lipase catalyst.…”