Solid-state postpolymerization of L-lactide promoted by crystallization of product polymer: An effective method for reduction of remaining monomer

Solid-state postpolymerization of L-lactide was studied by two different ways with 0.1 mol % of stannous 2-ethyl hexanoate as the catalyst. In a two-step method, the ordinary melt polymerization of L-lactide was brst performed at temperatures higher than the crystallization temperature (T-c) of poly-(L-lactide) (PLLA), and then the postpolymerization was continued around the T-c of PLLA. As PLLA crystallized in the second stage (e.g., when the temperature was changed from 140 to 120 degrees C), the monomer consumption was found to reach 100% because the monomer and catalyst could be concentrated in the amorphous part. Without the crystallization of PLLA occurring in the postpolymerization, a homogeneous supercooling state was formed to have a remaining monomer ratio exceeding 5 wt %. In the alternative one-step method where the polymerization was continued around the T-c of PLLA, the polymer crystallization was induced during the polymerization to promote the monomer consumption to reach 100%. The kinetic analysis of this polymerization revealed that the rate of monomer consumption is inversely proportional to the square of the amorphous ratio of PLLA, which is opposite to the crystal ratio. However, the molecular weight did not increase with the monomer consumption. This should be because various oligomers are formed in the postpolymerization stage by the ester interchange reaction.