A comprehensive single‐particle model for solid‐state polymerization of poly(L‐lactic acid)

Abstract

We propose here, a comprehensive model for the solid-state polymerization (SSP) of a low to moderate molecular weight (MW) prepolymer of lactic acid, to produce high MW poly(L-lactic acid) (PLLA). The reactions are rationally assumed to occur only in the amorphous region, and effective concentrations of end groups, vary with crystalinity, X c , during SSP. We estimate byproduct diffusivities, D, using free volume theory. The effects of various parameters on the SSP of PLLA prepolymer have been examined with respect to the optimum MW, X c and D. We introduce self-consistently, scaling factors of $ 0.27, in the experimental procedure, to determine via 19 F-NMR, concentrations of the end groups, after converting them to fluorinated ester groups. The relevant reaction rate constants are obtained by fitting to early time data from representative SSP experiments at 150 C, under high vacuum, on PLLA prepolymer powder (i.e., spherical geometry) of number average MW, M n0 $ 10,200 Da, which attains M n $ 150,000 Da, via SSP. The subsequent successful comparison of the model predictions with experimental data throughout the entire SSP duration indicates that the model is comprehensive and accounts for all the relevant phenomena occurring during the SSP to synthesize high MW PLLA.

Direct condensation of lactic acid in the presence or absence of supported zirconium sulfate

“…The decrease in the dispersity can be explained by statistical decomposition of polymer chains since the final dispersity would be equal to 1. The effect of slower kinetics at high crystallinity has been discussed in the literature . A linear decrease in M n as a result of PLA degradation is consistent with the statistical decomposition model .…”

“…It is known that the SSP rate increases in the first few hours of SSP due to the increase in the effective concentration of functional end groups . On the basis of the experimental results of Katiyar et al, we assume that an initial crystallinity of 50% results in a crystallinity of 70% after 20 h of SSP. This effect is also observed in the current study.…”

“…Some of the results are consistent with those obtained by other authors. The presence of this work is still justified in terms of Brønsted acid catalysis and different reaction conditions since Lewis acids have been used in other experiments . Supported ZS is found to be an efficient and enantioselective catalyst for the SSP of short‐chain prepolymers.…”

“…reported SSP of PLA reaching high MW PLA for the first time in 2001. Today, several SSP studies of PLA exists including the synthesis of nanocomposite PLA . Most SSP articles refer exclusively to weight average MWs based on polystyrene standards (PS).…”

“…Generally, the time of post‐condensation is much longer than in the melt since the mobility of end groups is restricted and slower compared to higher reaction temperatures . It is surprising that a 14–25 times higher MW after 10–20 h of SSP has been described in the literature …”