SYNOPSISThe crystallization kinetics of poly(L-lactide-co-rneso-lactide) were determined over a range of 0% to 9% mesolactide. The kinetics were fit to the nonlinear Avrami equation and then to the Hoffman-Lauritzen equation modified for optical copolymers. The theory was found to fit the data well. The crystallization half-time was found to increase about 40% for every 1 wt % rneso-lactide in the polymerization mixture. The change in crystallization rate is driven mainly by the reduction in melting point for the copolymers. The copolymer crystallization kinetics were also determined in the presence of talc, a nucleating agent for polylactide. The theory again fit the data well, using the same growth parameters and accounting for the talc only through the nucleation density term. 0 1996 John Wiley & Sons, Inc. I NTRODUCTI 0 NPolylactide is being developed as a biodegradable replacement for conventional thermoplastics. Although expensive, it has long been used as a copolymer in the medical field, providing resorbable sutures, implants, and controlled release of drugs. Recent development of a continuous process' has lowered the price of polylactide to the point where it is now competitive with other degradable polymers and potentially competitive with petroleum derived plastics.Lactide exists in three stereoisomeric forms, Llactide, D-lactide, and meso-lactide. It is prepared by depolymerization of low-molecular-weight poly-(lactic acid) and the three isomers are formed nearly in proportion to statistical expectation. Denoting the weight fraction of L-lactic acid as S and the weight fraction of D-lactic acid as R, the expected weight fractions of the lactide isomers will be S2 (L-lactide), 2RS (meso-lactide), and R2 (D-lactide). For mixtures with low values of R, the crude lactide will contain a trace of D-lactide (1% at R = 0.1) with L-lactide (81% at R = 0.1) and mesolactide (18% at R = 0.1). Control of the lactic acid optical composition gives control of the lactide composition, which in turn offers control over many of the properties of the final polymer.

The reversible kinetics of L-lactide bulk polymerization with tin(II) ethylhexanoate was determined over a wide range of temperatures, 130-220 °C, and monomer to initiator molar ratios, 1000-80 000. Both polymerization and depolymerization are accurately described by a reversible model with a propagation term that is first order in monomer and catalyst. The activation energy of propagation is 70.9 ( 1.5 kJ mol -1 . The enthalpy, entropy, and ceiling temperature of polymerization are -23.3 ( 1.5 kJ mol -1 , -22.0 ( 3.2 J mol -1 K -1 , and 786 ( 87 °C, respectively. Crystallization increases the propagation rate and decreases the apparent monomer equilibrium in proportion to the degree of crystallinity. Natural hydroxyl impurities stoichiometrically control the polymer molecular weight but do not significantly affect the propagation rate.

High-resolution 500 MHz solution-state 1H and 13C NMR spectra of various poly(lactides) indicate at least hexad stereosequence sensitivity. The poly(lactides) were prepared in vials by melt polymerization of various combinations of l-lactide, d-lactide, and meso-lactide at 180 °C for 3 h using tin(II) bis(2-ethylhexanoate) (tin(II) octoate) as the catalyst in a 1:10 000 ratio. The intensity distribution of the various stereosequence resonances in the NMR spectra indicates a preference for syndiotactic addition during the polymerization process. Minimal evidence of transesterification was observed for these polymerization conditions.

Poly(ethylene 2,5-furandicarboxylate) (PEF) is a polyester from ethylene glycol and 2,5-Furandicarboxylic acid which has gained increasing interest due to its excellent properties compared to chemically similar PET. This paper presents an estimation of the crystallization enthalpy, the crystalline and amorphous density and the crystallization kinetics of PEF. Using Avrami and the Hoffman-Lauritzen theory, HoffmanLauritzen parameters are proposed that relate crystal growth rate of catalyst-free PEF to temperature and molecular weight. Characteristic is a higher activation energy for chain diffusion (U*) for PEF compared PET, which can be attributed to more restricted chain conformational changes. Finally, the crystallization rate of PEF is shown to be significantly affected by catalyst type.

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Poly(lactide) is synthesized by ring-opening copolymerization of various combinations of l-lactide, d-lactide, and meso-lactide. The influence of the stereochemical differences of the three lactides on the kinetics of melt copolymerization was determined by monitoring the change in the polymer stereosequence distribution as a function of conversion. The living stereo-copolymerization catalyzed by Sn(II) bis2-ethylhexanoate (Sn(II) octoate) in 1:10 000 molar ratio at 180 °C was investigated by comparing to the values predicted by a reversible polymerization scheme that achieves equilibrium at ∼95% conversion. The stereosequence distribution was measured by high-resolution 500 MHz 1H NMR. A preference for syndiotactic addition was observed and is thought to be due to steric hindrance at the growing site of the polymer. Furthermore, the syndiotactic preference decreased as the polymerization proceeded in a batch process. Similar behavior was observed during lactide polymerization catalyzed by butyl Sn(IV) tris2-ethylhexanoate. The increasingly random addition during the polymerization process is due to the interplay of kinetic and equilibrium effects. Kinetic effects control the stereochemistry during the early stages of the polymerization whereas equilibrium effects dominate at later stages. Viscosity changes in the melt additionally influence the stereospecificity by increasing the residence time of the lactide at the active site. Monte Carlo calculations were used for stereosequence predictions because analytical equations are not available for these complicated kinetics.